diff --git a/README.md b/README.md
index 99cddc8..5717f80 100644
--- a/README.md
+++ b/README.md
@@ -135,8 +135,9 @@ SECRET_KEY=your-secure-random-string
 # Database location (optional, defaults to SQLite in the app's data folder)
 # Note the slashes: sqlite:///path is relative, sqlite:////path is absolute.
 DATABASE_URL=sqlite:////srv/may/data/may.db
-# PostgreSQL is also supported:
+# PostgreSQL, MySQL, and MariaDB are also supported:
 # DATABASE_URL=postgresql://user:password@host:5432/may
+# DATABASE_URL=mysql+pymysql://user:password@host:3306/may
 
 # Upload folder for attachments (optional)
 UPLOAD_FOLDER=/srv/may/data/uploads
diff --git a/app/routes/fuel.py b/app/routes/fuel.py
index 18a390b..e2ba640 100644
--- a/app/routes/fuel.py
+++ b/app/routes/fuel.py
@@ -130,6 +130,18 @@ def new():
                 flash(err, 'error')
                 return redirect(url_for('fuel.new', vehicle_id=vehicle_id))
 
+        # Derive the unit price from the amount paid when it was omitted.
+        # Add the discount back because total_cost represents the amount paid
+        # after the per-unit discount has been applied (#209).
+        if price_per_unit is None and volume and total_cost is not None:
+            price_per_unit = round(total_cost / volume + (discount_per_unit or 0), 3)
+            price_per_unit, err = validate_positive_number(
+                price_per_unit, 'Price per unit', max_value=1000
+            )
+            if err:
+                flash(err, 'error')
+                return redirect(url_for('fuel.new'))
+
         log = FuelLog(
             vehicle_id=vehicle_id,
             user_id=current_user.id,
diff --git a/app/security.py b/app/security.py
index 3d68f68..257d26c 100644
--- a/app/security.py
+++ b/app/security.py
@@ -7,6 +7,7 @@
 from functools import wraps
 from flask import request, redirect, url_for, flash
 from flask_login import current_user
+from app.utils import parse_decimal
 
 # File signature (magic bytes) mappings
 FILE_SIGNATURES = {
@@ -180,9 +181,14 @@ def validate_positive_number(value, field_name, max_value=None, allow_zero=True)
         return None, None  # Empty is OK
 
     try:
-        num = float(value)
+        num = parse_decimal(value)
     except (ValueError, TypeError):
         return None, f"{field_name} must be a valid number"
+    if num is None:
+        # parse_decimal returned its default/None for inputs like the
+        # literal string "None" or other absent-value markers. Treat this
+        # as an invalid number rather than silently accepting it.
+        return None, f"{field_name} must be a valid number"
 
     if not allow_zero and num == 0:
         return None, f"{field_name} cannot be zero"
diff --git a/app/templates/fuel/form.html b/app/templates/fuel/form.html
index f8abff5..20156fe 100644
--- a/app/templates/fuel/form.html
+++ b/app/templates/fuel/form.html
@@ -68,7 +68,7 @@ <h1 class="text-2xl font-bold text-gray-900 dark:text-white mt-2">{% if log %}{{
                     <label for="volume" class="block text-sm font-medium text-gray-700 dark:text-gray-300">{{ _('Volume') }} ({{ current_user.volume_unit }})</label>
                     <input type="number" name="volume" id="volume" step="0.001"
                            value="{{ log.volume if log else '' }}"
-                           onchange="calculateTotal()"
+                           onchange="calculateFuelAmounts()"
                            class="mt-1 block w-full rounded-md border border-gray-300 dark:border-gray-600 px-3 py-2 focus:border-primary-500 focus:outline-none focus:ring-1 focus:ring-primary-500">
                 </div>
 
@@ -76,7 +76,7 @@ <h1 class="text-2xl font-bold text-gray-900 dark:text-white mt-2">{% if log %}{{
                     <label for="price_per_unit" class="block text-sm font-medium text-gray-700 dark:text-gray-300">{{ _('Price per') }} {{ current_user.volume_unit }} ({{ current_user.currency }})</label>
                     <input type="number" name="price_per_unit" id="price_per_unit" step="0.001"
                            value="{{ log.price_per_unit if log else '' }}"
-                           onchange="calculateTotal()"
+                           onchange="calculateFuelAmounts()"
                            class="mt-1 block w-full rounded-md border border-gray-300 dark:border-gray-600 px-3 py-2 focus:border-primary-500 focus:outline-none focus:ring-1 focus:ring-primary-500">
                 </div>
 
@@ -84,7 +84,7 @@ <h1 class="text-2xl font-bold text-gray-900 dark:text-white mt-2">{% if log %}{{
                     <label for="discount_per_unit" class="block text-sm font-medium text-gray-700 dark:text-gray-300">{{ _('Discount per') }} {{ current_user.volume_unit }} ({{ current_user.currency }})</label>
                     <input type="number" name="discount_per_unit" id="discount_per_unit" step="0.001" min="0"
                            value="{{ log.discount_per_unit if log and log.discount_per_unit is not none else '' }}"
-                           onchange="calculateTotal()"
+                           onchange="calculateFuelAmounts()"
                            placeholder="0.000"
                            class="mt-1 block w-full rounded-md border border-gray-300 dark:border-gray-600 px-3 py-2 focus:border-primary-500 focus:outline-none focus:ring-1 focus:ring-primary-500">
                     <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">{{ _('Optional loyalty discount, subtracted from the price per unit.') }}</p>
@@ -94,6 +94,7 @@ <h1 class="text-2xl font-bold text-gray-900 dark:text-white mt-2">{% if log %}{{
                     <label for="total_cost" class="block text-sm font-medium text-gray-700 dark:text-gray-300">{{ _('Total Cost') }} ({{ current_user.currency }})</label>
                     <input type="number" name="total_cost" id="total_cost" step="0.01"
                            value="{{ log.total_cost if log else '' }}"
+                           onchange="calculateFuelAmounts()"
                            class="mt-1 block w-full rounded-md border border-gray-300 dark:border-gray-600 px-3 py-2 focus:border-primary-500 focus:outline-none focus:ring-1 focus:ring-primary-500">
                 </div>
 
@@ -211,6 +212,8 @@ <h1 class="text-2xl font-bold text-gray-900 dark:text-white mt-2">{% if log %}{{
 <script>
 const T_LAST_ODOMETER = {{ _('Last odometer')|tojson }};
 const T_LAST_HOURS = {{ _('Last engine hours')|tojson }};
+let pricePerUnitWasDerived = false;
+
 function updateVehicleOdometer(vehicleId) {
     const select = document.getElementById('vehicle_id');
     const selectedOption = select.options[select.selectedIndex];
@@ -308,13 +311,32 @@ <h1 class="text-2xl font-bold text-gray-900 dark:text-white mt-2">{% if log %}{{
     }
 }
 
-function calculateTotal() {
+function calculateFuelAmounts() {
     const volume = parseDecimal(document.getElementById('volume').value) || 0;
-    const pricePerUnit = parseDecimal(document.getElementById('price_per_unit').value) || 0;
+    const priceInput = document.getElementById('price_per_unit');
+    const totalInput = document.getElementById('total_cost');
     const discountPerUnit = parseDecimal(document.getElementById('discount_per_unit').value) || 0;
+
+    // Preserve an explicitly entered total cost when the unit price is derived from it.
+    // Keep the derived-state flag until the user edits the price manually.
+    if ((pricePerUnitWasDerived || !priceInput.value) && volume && totalInput.value) {
+        const totalCost = parseDecimal(totalInput.value);
+        if (totalCost !== null) {
+            const pricePerUnit = (totalCost / volume) + discountPerUnit;
+            priceInput.value = pricePerUnit.toFixed(3);
+            pricePerUnitWasDerived = true;
+            return;
+        }
+    }
+
+    let pricePerUnit = parseDecimal(priceInput.value) || 0;
+    if (pricePerUnitWasDerived && !priceInput.value) {
+        pricePerUnitWasDerived = false;
+    }
+
     if (volume && pricePerUnit) {
         const effectivePrice = Math.max(pricePerUnit - discountPerUnit, 0);
-        document.getElementById('total_cost').value = (volume * effectivePrice).toFixed(2);
+        totalInput.value = (volume * effectivePrice).toFixed(2);
     }
 }
 
@@ -329,6 +351,19 @@ <h1 class="text-2xl font-bold text-gray-900 dark:text-white mt-2">{% if log %}{{
 // Initialize odometer and fuel type for selected vehicle
 document.addEventListener('DOMContentLoaded', function() {
     const vehicleSelect = document.getElementById('vehicle_id');
+    const priceInput = document.getElementById('price_per_unit');
+    const totalInput = document.getElementById('total_cost');
+
+    priceInput.addEventListener('change', function() {
+        pricePerUnitWasDerived = false;
+    });
+
+    totalInput.addEventListener('change', function() {
+        if (!priceInput.value || pricePerUnitWasDerived) {
+            calculateFuelAmounts();
+        }
+    });
+
     updateVehicleOdometer(vehicleSelect.value);
     const selectedOption = vehicleSelect.options[vehicleSelect.selectedIndex];
     updateFuelTypeSelector(selectedOption);
diff --git a/app/utils.py b/app/utils.py
index 17ae282..3f02e7c 100644
--- a/app/utils.py
+++ b/app/utils.py
@@ -20,7 +20,7 @@ def parse_decimal(value, default=None):
     Empty / missing input returns ``default`` (``None`` by default) so callers can
     drop the ``... if request.form.get('x') else None`` guards.
 
-    Genuinely non-numeric input raises ``ValueError`` (matching ``float()``), so
+    Non-numeric or malformed grouped input raises ``ValueError`` (matching ``float()``), so
     existing error handling in the routes continues to work.
     """
     if value is None:
@@ -44,6 +44,21 @@ def parse_decimal(value, default=None):
     has_dot = '.' in s
     has_comma = ',' in s
 
+    # If both separators are present, ensure the layout is a valid mixed
+    # grouping (either dot as thousands + comma as decimal, or comma as
+    # thousands + dot as decimal). Reject malformed mixes such as "1,23.45".
+    if has_dot and has_comma and not (
+        re.fullmatch(r'[+-]?\d{1,3}(?:\.\d{3})+,\d+', s)
+        or re.fullmatch(r'[+-]?\d{1,3}(?:,\d{3})+\.\d+', s)
+    ):
+        raise ValueError(f"Cannot parse malformed grouped decimal {value!r}")
+    # Legacy check: multiple commas without a dot must follow comma-thousands
+    # grouping (e.g. "1,234,567" or "1,234,567.89").
+    elif s.count(',') > 1 and not re.fullmatch(
+        r'[+-]?\d{1,3}(?:,\d{3})+(?:\.\d+)?', s
+    ):
+        raise ValueError(f"Cannot parse malformed grouped decimal {value!r}")
+
     if has_dot and has_comma:
         # Both separators present: the right-most one is the decimal separator,
         # the other is a thousands separator to be removed.
diff --git a/requirements.txt b/requirements.txt
index 55ae913..ce97145 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -12,6 +12,7 @@ weasyprint>=69.0
 requests>=2.34.2
 python-dateutil>=2.9.0.post0
 psycopg2-binary>=2.9.12
+pymysql>=1.2.0
 pytest>=9.1.1
 pytest-cov>=7.1.0
 coverage>=7.15.4
diff --git a/tests/test_fuel.py b/tests/test_fuel.py
index 8321ad5..aa8f13d 100644
--- a/tests/test_fuel.py
+++ b/tests/test_fuel.py
@@ -100,6 +100,51 @@ def test_no_discount_is_none(self, auth_client, sample_vehicle):
         assert log.discount_per_unit is None
         assert log.total_cost == 60.0
 
+    def test_price_per_unit_is_calculated_from_total_cost(self, auth_client, sample_vehicle):
+        resp = auth_client.post('/fuel/new', data={
+            'vehicle_id': str(sample_vehicle.id),
+            'date': '2024-03-05',
+            'odometer': '15400',
+            'volume': '40.0',
+            'total_cost': '64.0',
+            'is_full_tank': 'on',
+        }, follow_redirects=True)
+        assert resp.status_code == 200
+        log = FuelLog.query.filter_by(vehicle_id=sample_vehicle.id, odometer=15400.0).first()
+        assert log is not None
+        assert log.price_per_unit == 1.6
+
+    def test_calculated_price_respects_maximum(self, auth_client, sample_vehicle, sample_station):
+        resp = auth_client.post('/fuel/new', data={
+            'vehicle_id': str(sample_vehicle.id),
+            'date': '2024-03-05',
+            'odometer': '15401',
+            'volume': '1',
+            'total_cost': '2000',
+            'station_id': str(sample_station.id),
+        }, follow_redirects=True)
+        assert resp.status_code == 200
+        assert FuelLog.query.filter_by(
+            vehicle_id=sample_vehicle.id, odometer=15401.0
+        ).first() is None
+        assert FuelPriceHistory.query.filter_by(
+            station_id=sample_station.id
+        ).first() is None
+
+    def test_calculated_price_includes_discount(self, auth_client, sample_vehicle):
+        auth_client.post('/fuel/new', data={
+            'vehicle_id': str(sample_vehicle.id),
+            'date': '2024-03-06',
+            'odometer': '15500',
+            'volume': '40.0',
+            'discount_per_unit': '0.10',
+            'total_cost': '60.0',
+            'is_full_tank': 'on',
+        }, follow_redirects=True)
+        log = FuelLog.query.filter_by(vehicle_id=sample_vehicle.id, odometer=15500.0).first()
+        assert log is not None
+        assert log.price_per_unit == 1.6
+
     def test_new_redirects_to_vehicles_if_none(self, auth_client):
         # No vehicles exist for this user
         resp = auth_client.get('/fuel/new', follow_redirects=False)
diff --git a/tests/test_security.py b/tests/test_security.py
index 3b54584..4718a42 100644
--- a/tests/test_security.py
+++ b/tests/test_security.py
@@ -155,6 +155,46 @@ def test_string_number(self):
         assert abs(val - 10.5) < 0.001
         assert error is None
 
+    def test_comma_decimal(self):
+        val, error = validate_positive_number('9,99', 'price')
+        assert val == 9.99
+        assert error is None
+
+    def test_grouped_number(self):
+        val, error = validate_positive_number('1,234,567.89', 'price')
+        assert val == 1234567.89
+        assert error is None
+
+    def test_malformed_grouping_is_rejected(self):
+        val, error = validate_positive_number('1,2,3', 'price')
+        assert val is None
+        assert error is not None
+
+    def test_malformed_mixed_grouping_is_rejected(self):
+        # Ensure validator rejects malformed mixed separators
+        val, error = validate_positive_number('1,23.45', 'price')
+        assert val is None
+        assert error is not None
+
+    def test_validator_accepts_valid_comma_and_grouped(self):
+        # Validator should accept comma-decimal and comma-thousands formats
+        val, error = validate_positive_number('9,99', 'price')
+        assert val == 9.99
+        assert error is None
+
+        val, error = validate_positive_number('1,234,567', 'price')
+        assert val == 1234567.0
+        assert error is None
+
+        val, error = validate_positive_number('9.99', 'price')
+        assert val == 9.99
+        assert error is None
+ 
+        # Single comma is treated as decimal separator, not thousands
+        val, error = validate_positive_number('1,000', 'price')
+        assert val == 1.0
+        assert error is None
+
     def test_invalid_string(self):
         val, error = validate_positive_number('abc', 'cost')
         assert val is None
diff --git a/tests/test_utils.py b/tests/test_utils.py
index c94f6ec..fe70073 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -31,6 +31,21 @@ def test_grouped_with_decimal_period(self):
     def test_multiple_commas_are_thousands_separators(self):
         assert parse_decimal('1,234,567') == 1234567.0
 
+    def test_malformed_comma_grouping_is_rejected(self):
+        with pytest.raises(ValueError):
+            parse_decimal('1,2,3')
+
+    def test_malformed_mixed_grouping_is_rejected(self):
+        # Mixed separators with malformed grouping should be rejected
+        with pytest.raises(ValueError):
+            parse_decimal('1,23.45')
+
+    def test_valid_comma_and_grouped_numbers(self):
+        # Single comma as decimal and grouped thousands should parse
+        assert parse_decimal('9,99') == 9.99
+        assert parse_decimal('1,000') == 1.0
+        assert parse_decimal('1,234,567') == 1234567.0
+
     def test_negative_comma_decimal(self):
         assert parse_decimal('-3,5') == -3.5