diff --git a/bases/behave_components/resources/public/css/styles.css b/bases/behave_components/resources/public/css/styles.css
index 64bc47b45..a3e40a17e 100644
--- a/bases/behave_components/resources/public/css/styles.css
+++ b/bases/behave_components/resources/public/css/styles.css
@@ -1345,6 +1345,7 @@ p {
 .input-number__error {
   color: var(--error);
   font-size: var(--font-size-10);
+  margin-top: 3px;
 }
 
 .input-number--disabled .input-number__input,
diff --git a/bases/behave_components/src/cljs/behave/components/inputs.cljs b/bases/behave_components/src/cljs/behave/components/inputs.cljs
index fa06265e0..17639170a 100644
--- a/bases/behave_components/src/cljs/behave/components/inputs.cljs
+++ b/bases/behave_components/src/cljs/behave/components/inputs.cljs
@@ -62,7 +62,7 @@
 ;;==============================================================================
 
 #_{:clj-kondo/ignore [:shadowed-var]}
-(defn number-input [{:keys [label id name on-change on-blur disabled? error? error-msg min max value value-atom default-value step]}]
+(defn number-input [{:keys [label id name on-change on-blur disabled? error? error-msg min max value value-atom default-value step placeholder]}]
   [:div {:class ["input-number " (when error? "input-number--error")]}
    [:label
     {:class "input-number__label" :for id}
@@ -78,6 +78,7 @@
              :min       min
              :max       max}
       step          (assoc :step step)
+      placeholder   (assoc :placeholder placeholder)
       value         (assoc :value value)
       value-atom    (assoc :value @value-atom)
       default-value (assoc :default-value default-value))]
diff --git a/development/migrations/2026_05_18_set_calculator_variable_min_max.clj b/development/migrations/2026_05_18_set_calculator_variable_min_max.clj
new file mode 100644
index 000000000..d342fd034
--- /dev/null
+++ b/development/migrations/2026_05_18_set_calculator_variable_min_max.clj
@@ -0,0 +1,114 @@
+(ns migrations.2026-05-18-set-calculator-variable-min-max
+  (:require [behave-cms.server :as cms]
+            [behave-cms.store :refer [default-conn]]
+            [datomic.api :as d]
+            [schema-migrate.interface :as sm]))
+
+;; ===========================================================================================================
+;; Overview
+;; ===========================================================================================================
+;; Sets :variable/minimum and :variable/maximum for calculator (tool) variables.
+;;
+;; All values are in native (English) units. The app handles unit conversion
+;; for display when the user selects metric units.
+;;
+;; Probability of Ignition:
+;;   1-h Fuel Moisture:          1 - 60
+;;   Air Temperature:            -40 - 120 (F)
+;;   Fuel Shading from the Sun:  0 - 100
+;;
+;; Vapor Pressure Deficit:
+;;   Air Temperature:            -40 - 120 (F)
+;;   Relative Humidity:          0 - 100
+;;
+;; Relative Humidity:
+;;   Dry Bulb Temperature:       -40 - 120 (F)
+;;   Wet Bulb Temperature:       -40 - 120 (F)
+;;   Site Elevation:             0 - 10,000
+;;
+;; SSD / SZS:
+;;   Vegetation Height:          0 - 300 (ft)
+
+;; ===========================================================================================================
+;; Initialize
+;; ===========================================================================================================
+
+(cms/init-db!)
+
+#_{:clj-kondo/ignore [:missing-docstring]}
+(def conn (default-conn))
+
+;; ===========================================================================================================
+;; Payload
+;; ===========================================================================================================
+
+#_{:clj-kondo/ignore [:missing-docstring]}
+(def payload
+  [{:db/id             (sm/name->eid conn :variable/name "1-h Fuel Moisture")
+    :variable/minimum  1.0
+    :variable/maximum  60.0}
+
+   {:db/id             (sm/name->eid conn :variable/name "Air Temperature")
+    :variable/minimum  -40.0
+    :variable/maximum  120.0}
+
+   {:db/id             (sm/name->eid conn :variable/name "Fuel Shading from the Sun")
+    :variable/minimum  0.0
+    :variable/maximum  100.0}
+
+   {:db/id             (sm/name->eid conn :variable/name "Relative Humidity")
+    :variable/minimum  0.0
+    :variable/maximum  100.0}
+
+   {:db/id             (sm/name->eid conn :variable/name "Dry Bulb Temperature")
+    :variable/minimum  -40.0
+    :variable/maximum  120.0}
+
+   {:db/id             (sm/name->eid conn :variable/name "Wet Bulb Temperature")
+    :variable/minimum  -40.0
+    :variable/maximum  120.0}
+
+   {:db/id             (sm/name->eid conn :variable/name "Site Elevation")
+    :variable/minimum  0.0
+    :variable/maximum  10000.0}
+
+   {:db/id             (sm/name->eid conn :variable/name "Vegetation Height")
+    :variable/minimum  0.0
+    :variable/maximum  300.0}])
+
+;; ===========================================================================================================
+;; Transact Payload
+;; ===========================================================================================================
+
+(comment
+  #_{:clj-kondo/ignore [:missing-docstring]}
+  (try (def tx-data @(d/transact conn payload))
+       (catch Exception e (str "caught exception: " (.getMessage e)))))
+
+;; ===========================================================================================================
+;; Verify
+;; ===========================================================================================================
+
+(comment
+  (let [variable-names ["1-h Fuel Moisture"
+                        "Air Temperature"
+                        "Fuel Shading from the Sun"
+                        "Relative Humidity"
+                        "Dry Bulb Temperature"
+                        "Wet Bulb Temperature"
+                        "Site Elevation"
+                        "Vegetation Height"]]
+    (doseq [vname variable-names]
+      (let [eid (sm/name->eid conn :variable/name vname)
+            e   (d/entity (d/db conn) eid)]
+        (println (format "%-30s min: %-8s max: %s"
+                         vname
+                         (:variable/minimum e)
+                         (:variable/maximum e)))))))
+
+;; ===========================================================================================================
+;; In case we need to rollback.
+;; ===========================================================================================================
+
+(comment
+  (sm/rollback-tx! conn tx-data))
diff --git a/projects/behave/resources/public/css/app-style.css b/projects/behave/resources/public/css/app-style.css
index 62c22d5b9..7cca245d7 100644
--- a/projects/behave/resources/public/css/app-style.css
+++ b/projects/behave/resources/public/css/app-style.css
@@ -1223,8 +1223,9 @@ body {
 
 .tool-output,
 .tool-input {
-  display: flex;
-  align-items: center;
+  display: grid;
+  grid-template-columns: auto 1fr;
+  align-items: start;
   min-height: 40px;
   padding-bottom: 5px;
 }
@@ -1236,12 +1237,12 @@ body {
   flex-direction: row;
 }
 
+
 .tool-output .input-text__label,
 .tool-input .input-radio-group__label,
 .tool-input__input > .input-dropdown > .input-dropdown__label,
 .tool-input .input-number__label {
   min-width: 250px;
-  padding: 0px;
 }
 
 .tool-input .input-dropdown__select-wrapper {
@@ -1250,12 +1251,18 @@ body {
 
 .tool-input .input-number__label,
 .tool-input .input-text__label,
-.tool-input .input-dropdown__label,
+.tool-input .input-dropdown__label {
+  padding: 7px 10px 0px 0px;
+  font-size: var(--font-size-15);
+  font-weight: 400;
+  color: var(--gray-5);
+}
+
 .tool-input .input-radio-group__label {
   padding: 0px 10px 0px 0px;
-  font-size: var(--font-size-14);
+  font-size: var(--font-size-15);
   font-weight: 400;
-  color: var(--gray-6);
+  color: var(--gray-5);
 }
 
 .tool-input + .tool-input:last-child {
@@ -1302,8 +1309,13 @@ body {
   background-color: var(--white);
 }
 
+.tool-input .wizard-input__units {
+  padding-top: 12px;
+}
+
 .tool__units-system-selector {
   padding-left: 5px;
+  margin-bottom: 15px;
 }
 
 .tool__units-system-selector .input-toggle__label {
diff --git a/projects/behave/src/cljs/behave/tool/subs.cljs b/projects/behave/src/cljs/behave/tool/subs.cljs
index 9f9568474..57b9f9d2d 100644
--- a/projects/behave/src/cljs/behave/tool/subs.cljs
+++ b/projects/behave/src/cljs/behave/tool/subs.cljs
@@ -1,9 +1,60 @@
 (ns behave.tool.subs
-  (:require [behave.vms.store       :as s]
-            [clojure.set            :refer [rename-keys]]
+  (:require [behave.lib.units       :refer [convert]]
             [behave.translate       :refer [<t]]
+            [behave.vms.store       :as s]
+            [clojure.set            :refer [rename-keys]]
             [datascript.core        :as d]
-            [re-frame.core          :refer [reg-sub path] :as rf]))
+            [goog.string            :as gstring]
+            [number-utils.interface :refer [parse-float]]
+            [re-frame.core          :refer [reg-sub] :as rf]))
+
+;;; Helpers
+
+(defn- in-range?
+  "True when v falls within [v-min v-max]. Nil bounds are open."
+  [v-min v-max v]
+  (cond
+    (and (some? v-min) (some? v-max)) (<= v-min v v-max)
+    (some? v-min)                     (<= v-min v)
+    (some? v-max)                     (<= v v-max)
+    :else                             true))
+
+(defn- converted-bounds
+  "Converts var-min/var-max from native to selected units (floored)."
+  [native-unit-uuid selected-unit-uuid var-min var-max]
+  (let [from-sc (when native-unit-uuid
+                  (:unit/short-code (d/entity @@s/vms-conn [:bp/uuid native-unit-uuid])))
+        to-sc   (when selected-unit-uuid
+                  (:unit/short-code (d/entity @@s/vms-conn [:bp/uuid selected-unit-uuid])))]
+    (if (and from-sc to-sc (not= from-sc to-sc))
+      [(when var-min (convert var-min from-sc to-sc 0))
+       (when var-max (convert var-max from-sc to-sc 0))]
+      [var-min var-max])))
+
+(defn- outside-range?
+  [native-unit-uuid selected-unit-uuid var-min var-max value]
+  (when (and (or var-min var-max) value (seq (str value)))
+    (when-let [v (parse-float (str value))]
+      (let [[adj-min adj-max] (converted-bounds native-unit-uuid selected-unit-uuid var-min var-max)]
+        (not (in-range? adj-min adj-max v))))))
+
+(defn- range-placeholder
+  "Returns a range string like '1 - 60' with bounds converted to selected units."
+  [native-unit-uuid selected-unit-uuid var-min var-max]
+  (when (and var-min var-max)
+    (let [[adj-min adj-max] (converted-bounds native-unit-uuid selected-unit-uuid var-min var-max)]
+      (str (int adj-min) " - " (int adj-max)))))
+
+(defn- outside-range-error-msg
+  "Returns an error message when value is outside the converted range."
+  [native-unit-uuid selected-unit-uuid var-min var-max value]
+  (when (and (or var-min var-max) value (seq (str value)))
+    (when-let [v (parse-float (str value))]
+      (let [[adj-min adj-max] (converted-bounds native-unit-uuid selected-unit-uuid var-min var-max)]
+        (when-not (in-range? adj-min adj-max v)
+          (gstring/format "Error: Not within range (min: %2f, max: %2f)" adj-min adj-max))))))
+
+;;; Subscriptions
 
 (reg-sub
  :tool/show-tool-selector?
@@ -146,9 +197,54 @@
                                    :subtool-variable/translation-key)]
      @(<t translation-key))))
 
-(comment
-  (rf/subscribe [:tool/all-inputs
-                 "64e5023e-1b70-4b55-8312-6578fc9c64a9"
-                 "64e5024c-1841-4fd6-ba62-e6f983608793"])
+(reg-sub
+ :tool/input-range-placeholder
+ (fn [_ [_ native-unit-uuid var-min var-max effective-unit-uuid]]
+   (range-placeholder native-unit-uuid effective-unit-uuid var-min var-max)))
+
+(reg-sub
+ :tool/input-error-msg
+ (fn [_ [_ _ _ _ native-unit-uuid var-min var-max effective-unit-uuid value]]
+   (outside-range-error-msg native-unit-uuid effective-unit-uuid var-min var-max value)))
+
+(reg-sub
+ :tool/all-inputs-filled?
+ (fn [[_ tool-uuid subtool-uuid]]
+   [(rf/subscribe [:subtool/input-variables subtool-uuid])
+    (rf/subscribe [:tool/all-inputs tool-uuid subtool-uuid])])
+ (fn [[input-variables all-inputs] _]
+   (every? (fn [{sv-uuid :bp/uuid}]
+             (some? (:input/value (get all-inputs sv-uuid))))
+           input-variables)))
 
-  (rf/subscribe [:tool/all-output-uuids "64e5024c-1841-4fd6-ba62-e6f983608793"]))
+(reg-sub
+ :tool/any-input-outside-range?
+ (fn [[_ tool-uuid subtool-uuid]]
+   [(rf/subscribe [:subtool/input-variables subtool-uuid])
+    (rf/subscribe [:tool/all-inputs tool-uuid subtool-uuid])
+    (rf/subscribe [:settings/tool-units-system])])
+ (fn [[input-variables all-inputs units-system] _]
+   (boolean
+    (some (fn [{var-min      :variable/minimum
+                var-max      :variable/maximum
+                sv-uuid      :bp/uuid
+                kind         :variable/kind
+                nat-uuid     :variable/native-unit-uuid
+                eng-uuid     :variable/english-unit-uuid
+                met-uuid     :variable/metric-unit-uuid
+                domain-uuid  :variable/domain-uuid}]
+            (when (= kind :continuous)
+              (let [domain         (when domain-uuid (d/entity @@s/vms-conn [:bp/uuid domain-uuid]))
+                    native-uuid    (or (:domain/native-unit-uuid domain) nat-uuid)
+                    english-uuid   (or (:domain/english-unit-uuid domain) eng-uuid)
+                    metric-uuid    (or (:domain/metric-unit-uuid domain) met-uuid)
+                    input          (get all-inputs sv-uuid)
+                    value          (:input/value input)
+                    per-input-uuid (:input/units-uuid input)
+                    effective-uuid (or per-input-uuid
+                                       (case units-system
+                                         :english english-uuid
+                                         :metric  metric-uuid
+                                         native-uuid))]
+                (outside-range? native-uuid effective-uuid var-min var-max value))))
+          input-variables))))
diff --git a/projects/behave/src/cljs/behave/tool/views.cljs b/projects/behave/src/cljs/behave/tool/views.cljs
index 1da096c78..b8da48ef0 100644
--- a/projects/behave/src/cljs/behave/tool/views.cljs
+++ b/projects/behave/src/cljs/behave/tool/views.cljs
@@ -23,8 +23,8 @@
        :options       (->> units
                            (map (fn [unit]
                                   (cond->
-                                      {:label (:unit/name unit)
-                                       :value (:bp/uuid unit)}
+                                   {:label (:unit/name unit)
+                                    :value (:bp/uuid unit)}
                                     (= @*unit-uuid (:bp/uuid unit))
                                     (assoc :selected? true))))
                            (sort-by :label))}]]))
@@ -123,6 +123,8 @@
   (let [{sv-uuid           :bp/uuid
          domain-uuid       :variable/domain-uuid
          var-name          :variable/name
+         var-min           :variable/minimum
+         var-max           :variable/maximum
          dimension-uuid    :variable/dimension-uuid
          native-unit-uuid  :variable/native-unit-uuid
          english-unit-uuid :variable/english-unit-uuid
@@ -131,16 +133,35 @@
          help-key          :subtool-variable/help-key} variable
         translated-name                                @(rf/subscribe [:tool/sv->translated-name sv-uuid])
         domain                                         @(rf/subscribe [:vms/entity-from-uuid domain-uuid])
+        resolved-native-uuid                           (or (:domain/native-unit-uuid domain) native-unit-uuid)
+        resolved-english-uuid                          (or (:domain/english-unit-uuid domain) english-unit-uuid)
+        resolved-metric-uuid                           (or (:domain/metric-unit-uuid domain) metric-unit-uuid)
         unit-uuid                                      @(rf/subscribe [:tool/input-units tool-uuid subtool-uuid sv-uuid])
+        units-system                                   (rf/subscribe [:settings/tool-units-system])
+        effective-unit-uuid                            (or unit-uuid
+                                                           (case @units-system
+                                                             :english resolved-english-uuid
+                                                             :metric  resolved-metric-uuid
+                                                             resolved-native-uuid))
         value                                          @(rf/subscribe [:tool/input-value tool-uuid subtool-uuid sv-uuid])
-        value-atom                                     (r/atom value)]
+        value-atom                                     (r/atom value)
+        error-msg                                      @(rf/subscribe [:tool/input-error-msg
+                                                                       tool-uuid subtool-uuid sv-uuid
+                                                                       resolved-native-uuid var-min var-max
+                                                                       effective-unit-uuid value])
+        range-placeholder                              @(rf/subscribe [:tool/input-range-placeholder
+                                                                       resolved-native-uuid var-min var-max
+                                                                       effective-unit-uuid])]
     [:div.tool-input
      [:div.tool-input__input
       {:on-mouse-over #(rf/dispatch [:help/highlight-section help-key])}
-      [c/number-input {:id         sv-uuid
-                       :label      (or translated-name var-name)
-                       :value-atom value-atom
-                       :required?  true
+      [c/number-input {:id          sv-uuid
+                       :label       (or translated-name var-name)
+                       :value-atom  value-atom
+                       :required?   true
+                       :placeholder range-placeholder
+                       :error?      (some? error-msg)
+                       :error-msg   error-msg
                        :on-change  #(reset! value-atom (input-value %))
                        :on-blur    #(rf/dispatch [:tool/upsert-input-value
                                                   tool-uuid
@@ -268,13 +289,13 @@
 
 (defmethod tool-output :discrete
   [{:keys [variable tool-uuid subtool-uuid]}]
-  (let [{sv-uuid  :bp/uuid
-         var-name :variable/name
-         list     :variable/list
-         help-key :subtool-variable/help-key} variable
-        translated-name                       @(rf/subscribe [:tool/sv->translated-name sv-uuid])
-        value                                 @(rf/subscribe [:tool/output-value tool-uuid subtool-uuid sv-uuid])
-        list-options                          (index-by :list-option/value (:list/options list))
+  (let [{sv-uuid   :bp/uuid
+         var-name  :variable/name
+         vlist     :variable/list
+         help-key  :subtool-variable/help-key} variable
+        translated-name                        @(rf/subscribe [:tool/sv->translated-name sv-uuid])
+        value                                  @(rf/subscribe [:tool/output-value tool-uuid subtool-uuid sv-uuid])
+        list-options                           (index-by :list-option/value (:list/options vlist))
         matching-option                       (get list-options (str value))
         background                            (get-in matching-option [:list-option/color-tag-ref :tag/color] "#FFFFFF")
         lum-bg                                (apply luminance (hex-to-rgb background))
@@ -296,28 +317,33 @@
 (defn- auto-compute-subtool
   "Renders a subtool that automatically computes outputs as inputs change."
   [tool-uuid subtool-uuid]
-  (rf/dispatch [:tool/solve tool-uuid subtool-uuid])
-  (let [variables @(rf/subscribe [:subtool/encriched-subtool-variables subtool-uuid])]
-    [:div
-     (for [{io :subtool-variable/io :as variable} variables
-           :let                                   [params {:variable      variable
-                                                           :tool-uuid     tool-uuid
-                                                           :subtool-uuid  subtool-uuid
-                                                           :auto-compute? true}]]
-       (if (= io :input)
-         [tool-input params]
-         [tool-output params]))
-     [c/button {:label         @(<t (bp "close_calculator"))
-                :variant       "secondary"
-                :icon-name     "close"
-                :icon-position "right"
-                :on-click      #(rf/dispatch [:tool/close-tool])}]]))
+  (let [any-error?  @(rf/subscribe [:tool/any-input-outside-range? tool-uuid subtool-uuid])
+        all-filled? @(rf/subscribe [:tool/all-inputs-filled? tool-uuid subtool-uuid])]
+    (when (and (not any-error?) all-filled?)
+      (rf/dispatch [:tool/solve tool-uuid subtool-uuid]))
+    (let [variables @(rf/subscribe [:subtool/encriched-subtool-variables subtool-uuid])]
+      [:div
+       (for [{io :subtool-variable/io :as variable} variables
+             :let                                   [params {:variable      variable
+                                                             :tool-uuid     tool-uuid
+                                                             :subtool-uuid  subtool-uuid
+                                                             :auto-compute? true}]]
+         (if (= io :input)
+           [tool-input params]
+           [tool-output params]))
+       [c/button {:label         @(<t (bp "close_calculator"))
+                  :variant       "secondary"
+                  :icon-name     "close"
+                  :icon-position "right"
+                  :on-click      #(rf/dispatch [:tool/close-tool])}]])))
 
 (defn- manual-subtool
   "Renders a subtool that requires manual computation via a compute button."
   [tool-uuid subtool-uuid]
   (let [input-variables  @(rf/subscribe [:subtool/input-variables subtool-uuid])
-        output-variables @(rf/subscribe [:subtool/output-variables subtool-uuid])]
+        output-variables @(rf/subscribe [:subtool/output-variables subtool-uuid])
+        any-error?       @(rf/subscribe [:tool/any-input-outside-range? tool-uuid subtool-uuid])
+        all-filled?      @(rf/subscribe [:tool/all-inputs-filled? tool-uuid subtool-uuid])]
     [:div
      (for [variable input-variables]
        [tool-input {:variable     variable
@@ -328,6 +354,7 @@
                  :variant       "highlight"
                  :icon-name     "arrow2"
                  :icon-position "right"
+                 :disabled?     (or any-error? (not all-filled?))
                  :on-click      #(rf/dispatch [:tool/solve tool-uuid subtool-uuid])}]]
      (for [variable output-variables]
        [tool-output