diff --git a/app.js b/app.js
index cb86587..7ee2e53 100644
--- a/app.js
+++ b/app.js
@@ -6,6 +6,7 @@
   var endInput = document.getElementById("end-date");
   var hoursInput = document.getElementById("worked-h");
   var minutesInput = document.getElementById("worked-m");
+  var vacationInput = document.getElementById("unused-vacation");
   var errorEl = document.getElementById("form-error");
   var daysEl = document.getElementById("working-days");
   var expectedEl = document.getElementById("expected-hours");
@@ -29,6 +30,7 @@
     if (!endInput.value) endInput.value = todayIso();
     if (!hoursInput.value) hoursInput.value = "0";
     if (!minutesInput.value) minutesInput.value = "0";
+    if (!vacationInput.value) vacationInput.value = "0";
   }
 
   function showError(message) {
@@ -38,10 +40,13 @@
 
   function render() {
     var parts = Worktime.parseDurationParts(hoursInput.value, minutesInput.value);
+    var vacation = Worktime.parseNonNegativeInt(vacationInput.value);
     var result = Worktime.calculate(
       startInput.value,
       endInput.value,
-      parts ? parts.hmm : "invalid"
+      parts ? parts.hmm : "invalid",
+      undefined,
+      vacation == null ? "invalid" : vacation
     );
 
     if (!result.ok && result.error === "range") {
@@ -64,6 +69,16 @@
       return;
     }
 
+    if (vacation == null || (!result.ok && result.error === "vacation")) {
+      showError("Usa un número entero de días, sin negativos.");
+      daysEl.textContent = result.days != null ? String(result.days) : "—";
+      expectedEl.textContent = Worktime.formatHmm(result.expected);
+      differenceEl.textContent = "—";
+      differenceEl.classList.remove("is-behind");
+      dailyEl.textContent = "—";
+      return;
+    }
+
     showError("");
     daysEl.textContent = String(result.days);
     expectedEl.textContent = Worktime.formatHmm(result.expected);
@@ -86,4 +101,6 @@
   hoursInput.addEventListener("change", render);
   minutesInput.addEventListener("input", render);
   minutesInput.addEventListener("change", render);
+  vacationInput.addEventListener("input", render);
+  vacationInput.addEventListener("change", render);
 })();
diff --git a/index.html b/index.html
index 3e6466e..301b936 100644
--- a/index.html
+++ b/index.html
@@ -59,6 +59,18 @@ <h1>Horas laborales</h1>
             </div>
           </div>
         </div>
+        <div class="field">
+          <label for="unused-vacation">Vacaciones no disfrutadas (días)</label>
+          <input
+            id="unused-vacation"
+            name="unusedVacation"
+            type="number"
+            inputmode="numeric"
+            min="0"
+            step="1"
+            autocomplete="off"
+          />
+        </div>
         <button type="submit">Calcular</button>
         <p id="form-error" class="error" hidden></p>
       </form>
diff --git a/worktime.js b/worktime.js
index 64a5b01..80143e6 100644
--- a/worktime.js
+++ b/worktime.js
@@ -70,6 +70,19 @@
     return days * perDay;
   }
 
+  /** Non-negative integer field. Empty → 0. Rejects decimals and negatives. */
+  function parseNonNegativeInt(raw) {
+    if (raw == null || raw === "") return 0;
+    if (typeof raw === "number") {
+      if (!isFinite(raw) || raw < 0 || Math.floor(raw) !== raw) return null;
+      return raw;
+    }
+    var s = String(raw).trim();
+    if (s === "") return 0;
+    if (!/^\d+$/.test(s)) return null;
+    return Number(s);
+  }
+
   /** Hours + minutes fields (Android numeric keypad). Empty → 0. Minutes must be 0–59. */
   function parseDurationParts(hoursRaw, minutesRaw) {
     function asInt(raw) {
@@ -106,18 +119,23 @@
     return sign + h + ":" + String(min).padStart(2, "0");
   }
 
-  function calculate(startIso, endIso, workedText, holidays) {
+  function calculate(startIso, endIso, workedText, holidays, unusedVacationDays) {
     var days = countWorkingDays(startIso, endIso, holidays);
     var worked = parseHmm(workedText);
+    var vacation = parseNonNegativeInt(unusedVacationDays);
 
     if (days == null) {
       return { ok: false, error: "range", days: 0, expected: null, worked: worked, difference: null, daily: null };
     }
     if (worked == null) {
-      return { ok: false, error: "hours", days: days, expected: days * HOURS_PER_DAY, worked: null, difference: null, daily: null };
+      var expectedOnHoursError = vacation == null ? days * HOURS_PER_DAY : (days - vacation) * HOURS_PER_DAY;
+      return { ok: false, error: "hours", days: days, expected: expectedOnHoursError, worked: null, difference: null, daily: null };
+    }
+    if (vacation == null) {
+      return { ok: false, error: "vacation", days: days, expected: days * HOURS_PER_DAY, worked: worked, difference: null, daily: null };
     }
 
-    var expected = days * HOURS_PER_DAY;
+    var expected = (days - vacation) * HOURS_PER_DAY;
     var difference = worked - expected;
     var daily = days === 0 ? null : worked / days;
     return { ok: true, error: null, days: days, expected: expected, worked: worked, difference: difference, daily: daily };
@@ -131,6 +149,7 @@
     expectedHours: expectedHours,
     parseHmm: parseHmm,
     parseDurationParts: parseDurationParts,
+    parseNonNegativeInt: parseNonNegativeInt,
     formatHmm: formatHmm,
     calculate: calculate,
   };
diff --git a/worktime.test.js b/worktime.test.js
index e0f271a..924c961 100644
--- a/worktime.test.js
+++ b/worktime.test.js
@@ -97,3 +97,35 @@ test("zero working days does not divide by zero", () => {
   assert.equal(r.days, 0);
   assert.equal(r.daily, null);
 });
+
+test("unused vacation days default to 0 and subtract 8 h each from expected", () => {
+  const none = W.calculate("2025-01-02", "2025-01-03", "16:00");
+  assert.equal(none.ok, true);
+  assert.equal(none.expected, 16);
+  assert.equal(none.difference, 0);
+
+  const one = W.calculate("2025-01-02", "2025-01-03", "16:00", undefined, 1);
+  assert.equal(one.ok, true);
+  assert.equal(one.days, 2);
+  assert.equal(one.expected, 8);
+  assert.equal(one.difference, 8);
+  assert.equal(one.daily, 8);
+
+  const empty = W.calculate("2025-01-02", "2025-01-03", "16:00", undefined, "");
+  assert.equal(empty.ok, true);
+  assert.equal(empty.expected, 16);
+});
+
+test("unused vacation days reject negatives and non-integers", () => {
+  assert.equal(W.parseNonNegativeInt(""), 0);
+  assert.equal(W.parseNonNegativeInt("0"), 0);
+  assert.equal(W.parseNonNegativeInt("3"), 3);
+  assert.equal(W.parseNonNegativeInt(-1), null);
+  assert.equal(W.parseNonNegativeInt("-1"), null);
+  assert.equal(W.parseNonNegativeInt("1.5"), null);
+  assert.equal(W.parseNonNegativeInt("abc"), null);
+
+  const r = W.calculate("2025-01-02", "2025-01-03", "16:00", undefined, "-1");
+  assert.equal(r.ok, false);
+  assert.equal(r.error, "vacation");
+});