diff --git a/newtab.html b/newtab.html
index ae73e13..aa1eac3 100644
--- a/newtab.html
+++ b/newtab.html
@@ -508,6 +508,7 @@
   <script src="src/js/notes.js"></script>
   <script src="src/js/calculator.js"></script>
   <script src="src/js/calc-scientific.js"></script>
+  <script src="src/js/calc-converter.js"></script>
   <script src="src/js/timer.js"></script>
   <script src="src/js/image-ref.js"></script>
   <script src="src/js/bookmark-import.js"></script>
diff --git a/src/css/main.css b/src/css/main.css
index e622a4f..bc0c866 100644
--- a/src/css/main.css
+++ b/src/css/main.css
@@ -1391,6 +1391,67 @@ body.show-desc .bm-desc { display: block; }
   padding: 2px 0;
 }
 
+/* Calculator Converter Mode */
+.calc-conv-select {
+  width: 100%;
+  padding: 6px 8px;
+  background: rgba(0,0,0,0.3);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  color: var(--text-primary);
+  font-size: 13px;
+  font-family: 'Rajdhani', sans-serif;
+}
+.calc-conv-row {
+  display: flex;
+  gap: 6px;
+}
+.calc-conv-input {
+  flex: 1;
+  padding: 8px;
+  background: rgba(0,0,0,0.3);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  color: var(--text-primary);
+  font-size: 16px;
+  font-family: 'Rajdhani', monospace;
+}
+.calc-conv-input:read-only {
+  color: var(--accent);
+  font-weight: 600;
+}
+.calc-conv-unit {
+  width: 80px;
+  padding: 4px 6px;
+  background: rgba(0,0,0,0.3);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  color: var(--text-primary);
+  font-size: 12px;
+  font-family: 'Rajdhani', sans-serif;
+}
+.calc-conv-swap {
+  align-self: center;
+  width: 36px;
+  height: 28px;
+  background: rgba(255,255,255,0.04);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  color: var(--accent);
+  font-size: 16px;
+  cursor: pointer;
+  transition: all 0.15s;
+}
+.calc-conv-swap:hover {
+  background: var(--accent-dim);
+}
+.calc-conv-ref {
+  font-size: 11px;
+  color: var(--text-muted);
+  padding: 4px 0;
+  border-top: 1px solid var(--border);
+}
+
 /* ============================================
    TIMER WIDGET
    ============================================ */
diff --git a/src/js/calc-converter.js b/src/js/calc-converter.js
new file mode 100644
index 0000000..4083bd7
--- /dev/null
+++ b/src/js/calc-converter.js
@@ -0,0 +1,344 @@
+/* =============================================
+   HELLION NEWTAB — calc-converter.js
+   Unit-Converter Modus für Calculator Widget
+   ============================================= */
+
+(function() {
+  'use strict';
+
+  const CATEGORIES = {
+    length: {
+      titleKey: 'calculator.conv.cat.length',
+      baseUnit: 'm',
+      units: {
+        mm:  { toBase: v => v / 1000,       fromBase: v => v * 1000 },
+        cm:  { toBase: v => v / 100,        fromBase: v => v * 100 },
+        m:   { toBase: v => v,              fromBase: v => v },
+        km:  { toBase: v => v * 1000,       fromBase: v => v / 1000 },
+        in:  { toBase: v => v * 0.0254,     fromBase: v => v / 0.0254 },
+        ft:  { toBase: v => v * 0.3048,     fromBase: v => v / 0.3048 },
+        yd:  { toBase: v => v * 0.9144,     fromBase: v => v / 0.9144 },
+        mi:  { toBase: v => v * 1609.344,   fromBase: v => v / 1609.344 }
+      }
+    },
+    weight: {
+      titleKey: 'calculator.conv.cat.weight',
+      baseUnit: 'g',
+      units: {
+        mg:  { toBase: v => v / 1000,       fromBase: v => v * 1000 },
+        g:   { toBase: v => v,              fromBase: v => v },
+        kg:  { toBase: v => v * 1000,       fromBase: v => v / 1000 },
+        t:   { toBase: v => v * 1000000,    fromBase: v => v / 1000000 },
+        oz:  { toBase: v => v * 28.3495,    fromBase: v => v / 28.3495 },
+        lb:  { toBase: v => v * 453.592,    fromBase: v => v / 453.592 }
+      }
+    },
+    temperature: {
+      titleKey: 'calculator.conv.cat.temperature',
+      baseUnit: null,
+      units: { '\u00B0C': null, '\u00B0F': null, 'K': null },
+      convert(value, from, to) {
+        if (from === to) return value;
+        const key = from + '_' + to;
+        const conversions = {
+          '\u00B0C_\u00B0F': v => (v * 9 / 5) + 32,
+          '\u00B0C_K':  v => v + 273.15,
+          '\u00B0F_\u00B0C': v => (v - 32) * 5 / 9,
+          '\u00B0F_K':  v => (v - 32) * 5 / 9 + 273.15,
+          'K_\u00B0C':  v => v - 273.15,
+          'K_\u00B0F':  v => (v - 273.15) * 9 / 5 + 32
+        };
+        const fn = conversions[key];
+        return fn ? fn(value) : null;
+      }
+    },
+    volume: {
+      titleKey: 'calculator.conv.cat.volume',
+      baseUnit: 'ml',
+      units: {
+        ml:       { toBase: v => v,               fromBase: v => v },
+        L:        { toBase: v => v * 1000,         fromBase: v => v / 1000 },
+        'm\u00B3':{ toBase: v => v * 1000000,      fromBase: v => v / 1000000 },
+        'gal(US)':{ toBase: v => v * 3785.41,      fromBase: v => v / 3785.41 },
+        'gal(UK)':{ toBase: v => v * 4546.09,      fromBase: v => v / 4546.09 },
+        'ft\u00B3':{ toBase: v => v * 28316.8,     fromBase: v => v / 28316.8 }
+      }
+    },
+    speed: {
+      titleKey: 'calculator.conv.cat.speed',
+      baseUnit: 'm/s',
+      units: {
+        'm/s':  { toBase: v => v,             fromBase: v => v },
+        'km/h': { toBase: v => v / 3.6,      fromBase: v => v * 3.6 },
+        'mph':  { toBase: v => v * 0.44704,   fromBase: v => v / 0.44704 },
+        'kn':   { toBase: v => v * 0.514444,  fromBase: v => v / 0.514444 }
+      }
+    },
+    area: {
+      titleKey: 'calculator.conv.cat.area',
+      baseUnit: 'm\u00B2',
+      units: {
+        'mm\u00B2': { toBase: v => v / 1000000,     fromBase: v => v * 1000000 },
+        'cm\u00B2': { toBase: v => v / 10000,        fromBase: v => v * 10000 },
+        'm\u00B2':  { toBase: v => v,                fromBase: v => v },
+        'km\u00B2': { toBase: v => v * 1000000,      fromBase: v => v / 1000000 },
+        'ha':       { toBase: v => v * 10000,         fromBase: v => v / 10000 },
+        'acre':     { toBase: v => v * 4046.86,       fromBase: v => v / 4046.86 },
+        'ft\u00B2': { toBase: v => v * 0.092903,     fromBase: v => v / 0.092903 },
+        'in\u00B2': { toBase: v => v * 0.00064516,   fromBase: v => v / 0.00064516 }
+      }
+    }
+  };
+
+  const CATEGORY_ORDER = ['length', 'weight', 'temperature', 'volume', 'speed', 'area'];
+
+  let _currentCategory = 'length';
+  let _fromUnit = 'cm';
+  let _toUnit = 'in';
+  let _fromInput = null;
+  let _toInput = null;
+  let _refEl = null;
+
+  /**
+   * Converts a value from one unit to another within the current category.
+   * @param {number} value
+   * @param {string} from
+   * @param {string} to
+   * @returns {number|null}
+   */
+  function convert(value, from, to) {
+    const cat = CATEGORIES[_currentCategory];
+    if (!cat) return null;
+    if (cat.convert) return cat.convert(value, from, to);
+    const fromDef = cat.units[from];
+    const toDef = cat.units[to];
+    if (!fromDef || !toDef) return null;
+    const base = fromDef.toBase(value);
+    return toDef.fromBase(base);
+  }
+
+  /**
+   * Recalculates the output field and reference lines based on current input.
+   */
+  function recalc() {
+    if (!_fromInput || !_toInput) return;
+    const val = parseFloat(_fromInput.value);
+    if (isNaN(val)) {
+      _toInput.value = '';
+      updateReference();
+      return;
+    }
+    const result = convert(val, _fromUnit, _toUnit);
+    if (result === null) {
+      _toInput.value = '';
+    } else {
+      _toInput.value = Calculator._formatResult(result);
+    }
+    updateReference();
+  }
+
+  /**
+   * Updates the reference conversion lines below the inputs.
+   */
+  function updateReference() {
+    if (!_refEl) return;
+    _refEl.textContent = '';
+    const r1 = convert(1, _fromUnit, _toUnit);
+    const r2 = convert(1, _toUnit, _fromUnit);
+    if (r1 !== null) {
+      const line1 = document.createElement('div');
+      line1.textContent = '1 ' + _fromUnit + ' = ' + Calculator._formatResult(r1) + ' ' + _toUnit;
+      _refEl.appendChild(line1);
+    }
+    if (r2 !== null) {
+      const line2 = document.createElement('div');
+      line2.textContent = '1 ' + _toUnit + ' = ' + Calculator._formatResult(r2) + ' ' + _fromUnit;
+      _refEl.appendChild(line2);
+    }
+  }
+
+  /**
+   * Populates a unit <select> element with options for the current category.
+   * @param {HTMLSelectElement} selectEl
+   * @param {string} selectedUnit
+   */
+  function populateUnitSelect(selectEl, selectedUnit) {
+    while (selectEl.firstChild) {
+      selectEl.removeChild(selectEl.firstChild);
+    }
+    const cat = CATEGORIES[_currentCategory];
+    if (!cat) return;
+    const units = Object.keys(cat.units);
+    units.forEach(unit => {
+      const opt = document.createElement('option');
+      opt.value = unit;
+      opt.textContent = unit;
+      if (unit === selectedUnit) opt.selected = true;
+      selectEl.appendChild(opt);
+    });
+  }
+
+  /**
+   * Returns sensible default from/to units for a given category key.
+   * @param {string} catKey
+   * @returns {{ from: string, to: string }}
+   */
+  function getDefaultUnits(catKey) {
+    const defaults = {
+      length: { from: 'cm', to: 'in' },
+      weight: { from: 'kg', to: 'lb' },
+      temperature: { from: '\u00B0C', to: '\u00B0F' },
+      volume: { from: 'L', to: 'gal(US)' },
+      speed: { from: 'km/h', to: 'mph' },
+      area: { from: 'm\u00B2', to: 'ft\u00B2' }
+    };
+    return defaults[catKey] || { from: Object.keys(CATEGORIES[catKey].units)[0], to: Object.keys(CATEGORIES[catKey].units)[1] };
+  }
+
+  /**
+   * Loads persisted converter state from storage.
+   */
+  async function loadState() {
+    const data = await Store.get(Calculator.STORAGE_KEY);
+    if (data && data.calculator && data.calculator.converter) {
+      const s = data.calculator.converter;
+      if (s.lastCategory && CATEGORIES[s.lastCategory]) _currentCategory = s.lastCategory;
+      if (s.fromUnit) _fromUnit = s.fromUnit;
+      if (s.toUnit) _toUnit = s.toUnit;
+    }
+  }
+
+  /**
+   * Persists current converter state to storage (read-before-write).
+   */
+  async function saveState() {
+    const data = await Store.get(Calculator.STORAGE_KEY) || {};
+    if (!data.calculator) data.calculator = {};
+    data.calculator.converter = {
+      lastCategory: _currentCategory,
+      fromUnit: _fromUnit,
+      toUnit: _toUnit
+    };
+    await Store.set(Calculator.STORAGE_KEY, data);
+  }
+
+  /**
+   * Builds the converter UI and appends it to the widget body element.
+   * @param {HTMLElement} bodyEl
+   */
+  function buildUI(bodyEl) {
+    const catSelect = document.createElement('select');
+    catSelect.className = 'calc-conv-select';
+
+    CATEGORY_ORDER.forEach(catKey => {
+      const opt = document.createElement('option');
+      opt.value = catKey;
+      opt.textContent = t(CATEGORIES[catKey].titleKey);
+      if (catKey === _currentCategory) opt.selected = true;
+      catSelect.appendChild(opt);
+    });
+
+    const fromRow = document.createElement('div');
+    fromRow.className = 'calc-conv-row';
+
+    _fromInput = document.createElement('input');
+    _fromInput.type = 'number';
+    _fromInput.className = 'calc-conv-input';
+    _fromInput.placeholder = '0';
+    _fromInput.step = 'any';
+
+    const fromSelect = document.createElement('select');
+    fromSelect.className = 'calc-conv-unit';
+    populateUnitSelect(fromSelect, _fromUnit);
+
+    fromRow.append(_fromInput, fromSelect);
+
+    const swapBtn = document.createElement('button');
+    swapBtn.type = 'button';
+    swapBtn.className = 'calc-conv-swap';
+    swapBtn.textContent = '\u21C5';
+    swapBtn.title = t('calculator.conv.swap');
+
+    const toRow = document.createElement('div');
+    toRow.className = 'calc-conv-row';
+
+    _toInput = document.createElement('input');
+    _toInput.type = 'text';
+    _toInput.className = 'calc-conv-input';
+    _toInput.readOnly = true;
+    _toInput.placeholder = '0';
+
+    const toSelect = document.createElement('select');
+    toSelect.className = 'calc-conv-unit';
+    populateUnitSelect(toSelect, _toUnit);
+
+    toRow.append(_toInput, toSelect);
+
+    _refEl = document.createElement('div');
+    _refEl.className = 'calc-conv-ref';
+
+    _fromInput.addEventListener('input', () => recalc());
+    fromSelect.addEventListener('change', () => {
+      _fromUnit = fromSelect.value;
+      recalc();
+      saveState();
+    });
+    toSelect.addEventListener('change', () => {
+      _toUnit = toSelect.value;
+      recalc();
+      saveState();
+    });
+    swapBtn.addEventListener('click', () => {
+      const tmpUnit = _fromUnit;
+      _fromUnit = _toUnit;
+      _toUnit = tmpUnit;
+      populateUnitSelect(fromSelect, _fromUnit);
+      populateUnitSelect(toSelect, _toUnit);
+      const currentVal = _toInput.value;
+      if (currentVal) {
+        _fromInput.value = currentVal;
+      }
+      recalc();
+      saveState();
+    });
+    catSelect.addEventListener('change', () => {
+      _currentCategory = catSelect.value;
+      const defaults = getDefaultUnits(_currentCategory);
+      _fromUnit = defaults.from;
+      _toUnit = defaults.to;
+      populateUnitSelect(fromSelect, _fromUnit);
+      populateUnitSelect(toSelect, _toUnit);
+      _fromInput.value = '';
+      _toInput.value = '';
+      updateReference();
+      saveState();
+    });
+
+    bodyEl.append(catSelect, fromRow, swapBtn, toRow, _refEl);
+    updateReference();
+  }
+
+  Calculator.registerMode('converter', {
+    label: '⚖️',
+    shortName: 'Unit',
+    titleKey: 'calculator.tab.converter',
+
+    render(bodyEl) {
+      bodyEl.style.padding = '8px';
+      bodyEl.style.display = 'flex';
+      bodyEl.style.flexDirection = 'column';
+      bodyEl.style.gap = '8px';
+
+      loadState().then(() => {
+        buildUI(bodyEl);
+      });
+    },
+
+    destroy() {
+      _fromInput = null;
+      _toInput = null;
+      _refEl = null;
+      saveState();
+    }
+  });
+})();
diff --git a/src/js/i18n.js b/src/js/i18n.js
index 0039cb7..59a1e13 100644
--- a/src/js/i18n.js
+++ b/src/js/i18n.js
@@ -99,6 +99,14 @@ const STRINGS = {
     'calculator.sci.field.b':            'Seite b',
     'calculator.sci.field.value':        'Wert',
     'calculator.sci.field.percent':      'Prozent',
+    'calculator.tab.converter':          'Umrechner',
+    'calculator.conv.swap':              'Einheiten tauschen',
+    'calculator.conv.cat.length':        'Länge',
+    'calculator.conv.cat.weight':        'Gewicht',
+    'calculator.conv.cat.temperature':   'Temperatur',
+    'calculator.conv.cat.volume':        'Volumen',
+    'calculator.conv.cat.speed':         'Geschwindigkeit',
+    'calculator.conv.cat.area':          'Fläche',
 
     // Timer
     'timer.title':             'Timer',
@@ -423,6 +431,14 @@ const STRINGS = {
     'calculator.sci.field.b':            'Side b',
     'calculator.sci.field.value':        'Value',
     'calculator.sci.field.percent':      'Percent',
+    'calculator.tab.converter':          'Converter',
+    'calculator.conv.swap':              'Swap units',
+    'calculator.conv.cat.length':        'Length',
+    'calculator.conv.cat.weight':        'Weight',
+    'calculator.conv.cat.temperature':   'Temperature',
+    'calculator.conv.cat.volume':        'Volume',
+    'calculator.conv.cat.speed':         'Speed',
+    'calculator.conv.cat.area':          'Area',
 
     // Timer
     'timer.title':             'Timer',