diff --git a/construction-installment-calculator/index.html b/construction-installment-calculator/index.html
new file mode 100644
index 0000000..c2a0e09
--- /dev/null
+++ b/construction-installment-calculator/index.html
@@ -0,0 +1,495 @@
+<!DOCTYPE html>
+<html lang="th">
+<head>
+<meta charset="UTF-8">
+<meta name="viewport" content="width=device-width, initial-scale=1.0">
+<title>คำนวณงวดงานสิ่งก่อสร้าง</title>
+<script src="https://unpkg.com/xlsx/dist/xlsx.full.min.js"></script>
+<style>
+* { box-sizing: border-box; margin: 0; padding: 0; }
+body {
+  font-family: 'Sarabun', 'Segoe UI', sans-serif;
+  background: #f0f2f5;
+  padding: 20px;
+  font-size: 14px;
+  color: #222;
+}
+.page {
+  max-width: 1200px;
+  margin: auto;
+  background: #fff;
+  border-radius: 10px;
+  box-shadow: 0 2px 12px rgba(0,0,0,0.1);
+  overflow: hidden;
+}
+.page-header {
+  background: #1a56a0;
+  color: white;
+  padding: 18px 28px;
+}
+.page-header h1 { font-size: 18px; font-weight: 700; margin-bottom: 4px; }
+.page-header p { font-size: 13px; opacity: 0.85; }
+
+/* Form */
+.form-section {
+  background: #f7f9fc;
+  border-bottom: 1px solid #e0e6ef;
+  padding: 18px 28px;
+}
+.form-row { display: flex; gap: 20px; align-items: flex-end; flex-wrap: wrap; }
+.form-group { display: flex; flex-direction: column; gap: 5px; }
+.form-group label {
+  font-size: 11px; color: #555; font-weight: 700;
+  text-transform: uppercase; letter-spacing: 0.5px;
+}
+.form-group input[type="text"],
+.form-group input[type="number"],
+.form-group input[type="date"] {
+  border: 1.5px solid #c5d3e8;
+  border-radius: 6px;
+  padding: 8px 12px;
+  font-size: 14px;
+  background: white;
+  color: #333;
+  font-family: inherit;
+}
+.form-group input[type="number"] { width: 90px; }
+.form-group input[type="date"] { width: 160px; }
+.form-group input#contractAmount { width: 180px; }
+.ratio-group { display: flex; align-items: center; gap: 6px; }
+.ratio-group span { font-size: 12px; color: #666; }
+
+/* Buttons */
+.btn {
+  border: none; border-radius: 6px; padding: 9px 20px;
+  font-size: 14px; font-weight: 600; cursor: pointer; font-family: inherit;
+}
+.btn-calc { background: #1a56a0; color: white; }
+.btn-export { background: #217346; color: white; }
+.btn-export:disabled { background: #aaa; cursor: not-allowed; }
+.btn-add {
+  background: #e8f0fb; color: #1a56a0;
+  border: 1.5px solid #c5d3e8; border-radius: 5px;
+  padding: 5px 14px; font-size: 13px; cursor: pointer;
+  font-family: inherit; font-weight: 600;
+}
+.btn-del {
+  background: none; border: none; color: #cc3333;
+  font-size: 16px; cursor: pointer; padding: 0 6px; line-height: 1;
+}
+
+/* Summary cards */
+.table-section { padding: 20px 28px 28px; }
+.summary-row { display: flex; gap: 12px; margin-bottom: 16px; flex-wrap: wrap; }
+.summary-card {
+  background: #f0f6ff; border: 1px solid #c5d3e8;
+  border-radius: 8px; padding: 10px 16px; min-width: 150px;
+}
+.summary-card .lbl { font-size: 11px; color: #666; font-weight: 600; }
+.summary-card .val { font-size: 15px; font-weight: 700; color: #1a56a0; margin-top: 3px; }
+.summary-card.warn .val { color: #c0392b; }
+.summary-card.ok .val { color: #217346; }
+
+/* Table toolbar */
+.table-toolbar {
+  display: flex; justify-content: space-between;
+  align-items: center; margin-bottom: 12px;
+}
+.legend { display: flex; gap: 14px; font-size: 12px; color: #666; align-items: center; }
+.legend-box {
+  width: 13px; height: 13px; border-radius: 3px;
+  display: inline-block; vertical-align: middle; margin-right: 4px;
+}
+
+/* Table */
+.table-wrapper { overflow-x: auto; }
+table { width: 100%; border-collapse: collapse; font-size: 13px; }
+th {
+  background: #1a56a0; color: white;
+  padding: 8px 8px; text-align: center;
+  white-space: nowrap; font-size: 12px;
+}
+th.sub { background: #2a6abf; }
+th.editable-col { background: #c0832a; }
+td {
+  padding: 6px 8px;
+  border-bottom: 1px solid #e8edf5;
+  text-align: right; white-space: nowrap;
+}
+td.center { text-align: center; }
+td.editable-cell { background: #fffbf0; text-align: center; padding: 3px 5px; }
+td.editable-cell input {
+  border: 1.5px solid #f0c040; border-radius: 4px;
+  padding: 4px 5px; width: 62px; text-align: center;
+  font-size: 13px; font-family: inherit;
+}
+td.editable-cell input:focus { outline: none; border-color: #e67e22; }
+td.acc { background: #f0f6ff; font-weight: 600; color: #1a56a0; }
+td.calc { color: #333; }
+tr:nth-child(even) td { background: #fafcff; }
+tr:nth-child(even) td.editable-cell { background: #fffdf0; }
+tr:nth-child(even) td.acc { background: #e8f0fb; }
+tr:hover td { background: #f0f6ff !important; }
+tr.total-row td {
+  background: #dce8f8 !important;
+  font-weight: 700; color: #1a56a0;
+  border-top: 2px solid #1a56a0;
+}
+.pct-status { font-size: 11px; font-weight: 700; margin-left: 6px; }
+.pct-warn { color: #c0392b; }
+.pct-ok { color: #217346; }
+</style>
+</head>
+<body>
+<div class="page">
+  <div class="page-header">
+    <h1>การคำนวณงวดงานสิ่งก่อสร้าง</h1>
+    <p>กรอกข้อมูล กำหนดจำนวนงวด แล้วแก้ไขวันและร้อยละในตาราง ระบบจะคำนวณให้อัตโนมัติ</p>
+  </div>
+
+  <div class="form-section">
+    <div class="form-row">
+      <div class="form-group">
+        <label>วงเงินสัญญา (บาท)</label>
+        <input type="text" id="contractAmount" placeholder="เช่น 120000000" value="">
+      </div>
+      <div class="form-group">
+        <label>วันเริ่มก่อสร้าง</label>
+        <input type="date" id="startDate" value="">
+      </div>
+      <div class="form-group">
+        <label>สัดส่วน งปม. / เงินนอก (%)</label>
+        <div class="ratio-group">
+          <input type="number" id="ngpmPct" value="95" min="0" max="100" style="width:70px">
+          <span>งปม. /</span>
+          <input type="number" id="nookPct" value="5" min="0" max="100" style="width:70px" readonly>
+          <span>นอก</span>
+        </div>
+      </div>
+      <div class="form-group">
+        <label>จำนวนงวด</label>
+        <input type="number" id="installmentCount" value="27" min="1" max="100" style="width:75px">
+      </div>
+      <div class="form-group" style="flex-direction:row;gap:10px;align-items:flex-end">
+        <button class="btn btn-export" id="btnExport" onclick="exportExcel()" disabled>⬇ Export Excel</button>
+      </div>
+    </div>
+  </div>
+
+  <div class="table-section">
+    <div class="summary-row" id="summaryRow">
+      <div class="summary-card">
+        <div class="lbl">วงเงินสัญญา</div>
+        <div class="val" id="s_contract">—</div>
+      </div>
+      <div class="summary-card">
+        <div class="lbl">เงิน งปม.</div>
+        <div class="val" id="s_ngpm">—</div>
+      </div>
+      <div class="summary-card">
+        <div class="lbl">เงินนอก</div>
+        <div class="val" id="s_nook">—</div>
+      </div>
+      <div class="summary-card" id="s_pct_card">
+        <div class="lbl">รวมร้อยละทุกงวด</div>
+        <div class="val" id="s_pct">0.00%</div>
+      </div>
+      <div class="summary-card">
+        <div class="lbl">วันสิ้นสุดสัญญา</div>
+        <div class="val" id="s_endDate">—</div>
+      </div>
+    </div>
+
+    <div class="table-toolbar">
+      <div class="legend">
+        <span>
+          <span class="legend-box" style="background:#fffbf0;border:1.5px solid #f0c040"></span>
+          แก้ไขได้
+        </span>
+        <span>
+          <span class="legend-box" style="background:#f0f6ff;border:1px solid #c5d3e8"></span>
+          คำนวณอัตโนมัติ
+        </span>
+      </div>
+      <button class="btn-add" onclick="addRow()">+ เพิ่มงวด</button>
+    </div>
+
+    <div class="table-wrapper">
+      <table id="mainTable">
+        <thead>
+          <tr>
+            <th rowspan="2">งวดที่</th>
+            <th class="editable-col" rowspan="2">จำนวนวัน<br><small>(แก้ไขได้)</small></th>
+            <th class="editable-col" rowspan="2">ร้อยละ (%)<br><small>(แก้ไขได้)</small></th>
+            <th rowspan="2">วันส่งมอบงาน</th>
+            <th colspan="3">จำนวนเงินงวดนั้น (บาท)</th>
+            <th colspan="3">จำนวนเงินสะสม (บาท)</th>
+            <th rowspan="2"></th>
+          </tr>
+          <tr>
+            <th class="sub">รวม</th>
+            <th class="sub">เงิน งปม.</th>
+            <th class="sub">เงินนอก</th>
+            <th class="sub">รวม</th>
+            <th class="sub">เงิน งปม.</th>
+            <th class="sub">เงินนอก</th>
+          </tr>
+        </thead>
+        <tbody id="tableBody"></tbody>
+      </table>
+    </div>
+  </div>
+</div>
+
+<script>
+// State
+let installments = [];
+
+// Template data (27 งวด from Excel)
+const TEMPLATE_27 = [
+  {days:20, percent:3.80},
+  {days:20, percent:3.70},{days:20, percent:3.70},{days:20, percent:3.70},{days:20, percent:3.70},
+  {days:30, percent:3.70},{days:30, percent:3.70},{days:30, percent:3.70},{days:30, percent:3.70},
+  {days:30, percent:3.70},{days:30, percent:3.70},{days:30, percent:3.70},{days:30, percent:3.70},
+  {days:30, percent:3.70},{days:30, percent:3.70},{days:30, percent:3.70},{days:30, percent:3.70},
+  {days:30, percent:3.70},{days:30, percent:3.70},{days:30, percent:3.70},{days:30, percent:3.70},
+  {days:30, percent:3.70},{days:30, percent:3.70},{days:30, percent:3.70},
+  {days:40, percent:3.70},{days:40, percent:3.70},{days:40, percent:3.70}
+];
+
+function parseAmount(str) {
+  return parseFloat(String(str).replace(/,/g, '')) || 0;
+}
+
+function formatNumber(n) {
+  if (!n && n !== 0) return '—';
+  return Math.round(n).toLocaleString('th-TH');
+}
+
+function addDays(dateStr, days) {
+  const d = new Date(dateStr);
+  d.setDate(d.getDate() + days);
+  return d;
+}
+
+function toThaiDate(date) {
+  if (!(date instanceof Date) || isNaN(date)) return '—';
+  const y = date.getFullYear() + 543;
+  const m = String(date.getMonth() + 1).padStart(2, '0');
+  const d = String(date.getDate()).padStart(2, '0');
+  return `${d}/${m}/${y}`;
+}
+
+function getInputs() {
+  return {
+    contract: parseAmount(document.getElementById('contractAmount').value),
+    startDate: document.getElementById('startDate').value,
+    ngpmPct: parseFloat(document.getElementById('ngpmPct').value) || 0,
+    nookPct: parseFloat(document.getElementById('nookPct').value) || 0,
+  };
+}
+
+function recalcAll() {
+  const {contract, startDate, ngpmPct, nookPct} = getInputs();
+  const hasStart = startDate && contract > 0;
+
+  let cumDays = 0, cumTotal = 0, cumNgpm = 0, cumNook = 0;
+  let sumPct = 0;
+
+  installments.forEach((inst, i) => {
+    const days = parseFloat(inst.days) || 0;
+    const pct  = parseFloat(inst.percent) || 0;
+    cumDays += days;
+    sumPct  += pct;
+
+    const total = contract > 0 ? Math.round(contract * pct / 100) : 0;
+    const ngpm  = Math.round(total * ngpmPct / 100);
+    const nook  = total - ngpm;
+    cumTotal += total; cumNgpm += ngpm; cumNook += nook;
+
+    const deliveryDate = hasStart ? toThaiDate(addDays(startDate, cumDays)) : '—';
+
+    const row = document.getElementById(`row_${i}`);
+    if (!row) return;
+    row.querySelector('.cell-delivery').textContent = deliveryDate;
+    row.querySelector('.cell-total').textContent    = formatNumber(total);
+    row.querySelector('.cell-ngpm').textContent     = formatNumber(ngpm);
+    row.querySelector('.cell-nook').textContent     = formatNumber(nook);
+    row.querySelector('.cell-acc-total').textContent = formatNumber(cumTotal);
+    row.querySelector('.cell-acc-ngpm').textContent  = formatNumber(cumNgpm);
+    row.querySelector('.cell-acc-nook').textContent  = formatNumber(cumNook);
+  });
+
+  const totalRow = document.getElementById('totalRow');
+  if (totalRow) {
+    totalRow.querySelector('.cell-total').textContent = formatNumber(contract);
+    totalRow.querySelector('.cell-ngpm').textContent  = formatNumber(Math.round(contract * ngpmPct / 100));
+    totalRow.querySelector('.cell-nook').textContent  = formatNumber(Math.round(contract * nookPct / 100));
+    totalRow.querySelector('.cell-pct').textContent   = sumPct.toFixed(2) + '%';
+  }
+
+  document.getElementById('s_contract').textContent = contract > 0 ? formatNumber(contract) + ' ฿' : '—';
+  document.getElementById('s_ngpm').textContent = contract > 0 ? formatNumber(Math.round(contract * ngpmPct / 100)) + ' ฿' : '—';
+  document.getElementById('s_nook').textContent = contract > 0 ? formatNumber(Math.round(contract * nookPct / 100)) + ' ฿' : '—';
+
+  const pctOk = Math.abs(sumPct - 100) < 0.001;
+  const pctCard = document.getElementById('s_pct_card');
+  pctCard.className = 'summary-card ' + (pctOk ? 'ok' : 'warn');
+  document.getElementById('s_pct').innerHTML =
+    sumPct.toFixed(2) + '%' +
+    (pctOk
+      ? '<span class="pct-status pct-ok">✓ ครบ 100%</span>'
+      : '<span class="pct-status pct-warn">⚠ ยังไม่ครบ 100%</span>');
+
+  if (hasStart) {
+    document.getElementById('s_endDate').textContent = toThaiDate(addDays(startDate, cumDays));
+  } else {
+    document.getElementById('s_endDate').textContent = '—';
+  }
+
+  document.getElementById('btnExport').disabled = !(pctOk && contract > 0 && startDate);
+}
+
+function renderTable() {
+  const tbody = document.getElementById('tableBody');
+  tbody.innerHTML = '';
+
+  installments.forEach((inst, i) => {
+    const tr = document.createElement('tr');
+    tr.id = `row_${i}`;
+    tr.innerHTML = `
+      <td class="center">${i + 1}</td>
+      <td class="editable-cell">
+        <input type="number" value="${inst.days}" min="0"
+          onchange="installments[${i}].days=this.value; recalcAll()"
+          oninput="installments[${i}].days=this.value; recalcAll()">
+      </td>
+      <td class="editable-cell">
+        <input type="number" value="${inst.percent}" min="0" max="100" step="0.01"
+          onchange="installments[${i}].percent=this.value; recalcAll()"
+          oninput="installments[${i}].percent=this.value; recalcAll()">
+      </td>
+      <td class="center calc cell-delivery">—</td>
+      <td class="calc cell-total">—</td>
+      <td class="calc cell-ngpm">—</td>
+      <td class="calc cell-nook">—</td>
+      <td class="acc cell-acc-total">—</td>
+      <td class="acc cell-acc-ngpm">—</td>
+      <td class="acc cell-acc-nook">—</td>
+      <td class="center">
+        <button class="btn-del" onclick="deleteRow(${i})" title="ลบงวดนี้">✕</button>
+      </td>
+    `;
+    tbody.appendChild(tr);
+  });
+
+  const totalTr = document.createElement('tr');
+  totalTr.id = 'totalRow';
+  totalTr.className = 'total-row';
+  totalTr.innerHTML = `
+    <td colspan="2" class="center">รวมทั้งสิ้น</td>
+    <td class="center cell-pct">0.00%</td>
+    <td class="center">—</td>
+    <td class="cell-total">—</td>
+    <td class="cell-ngpm">—</td>
+    <td class="cell-nook">—</td>
+    <td colspan="3" class="center">—</td>
+    <td></td>
+  `;
+  tbody.appendChild(totalTr);
+
+  recalcAll();
+}
+
+function addRow() {
+  installments.push({days: 0, percent: 0});
+  document.getElementById('installmentCount').value = installments.length;
+  renderTable();
+}
+
+function deleteRow(i) {
+  installments.splice(i, 1);
+  document.getElementById('installmentCount').value = installments.length;
+  renderTable();
+}
+
+function onCountChange(n) {
+  n = parseInt(n) || 1;
+  if (n < 1) n = 1;
+  if (installments.length > 0) {
+    if (!confirm(`การเปลี่ยนจำนวนงวดเป็น ${n} งวด จะล้างข้อมูลเดิม ต้องการดำเนินการต่อไหม?`)) {
+      document.getElementById('installmentCount').value = installments.length;
+      return;
+    }
+  }
+  if (n === 27) {
+    installments = TEMPLATE_27.map(t => ({...t}));
+  } else {
+    installments = Array.from({length: n}, () => ({days: 0, percent: 0}));
+  }
+  renderTable();
+}
+
+function exportExcel() {
+  const {contract, startDate, ngpmPct, nookPct} = getInputs();
+  const wb = XLSX.utils.book_new();
+
+  const headers = [
+    'งวดที่','จำนวนวัน','ร้อยละ (%)','วันส่งมอบงาน',
+    'รวม (บาท)','เงิน งปม. (บาท)','เงินนอก (บาท)',
+    'รวมสะสม (บาท)','งปม. สะสม (บาท)','เงินนอก สะสม (บาท)'
+  ];
+
+  const rows = [headers];
+  let cumDays = 0, cumTotal = 0, cumNgpm = 0, cumNook = 0;
+
+  installments.forEach((inst, i) => {
+    const days   = parseFloat(inst.days) || 0;
+    const pct    = parseFloat(inst.percent) || 0;
+    cumDays += days;
+    const total  = Math.round(contract * pct / 100);
+    const ngpm   = Math.round(total * ngpmPct / 100);
+    const nook   = total - ngpm;
+    cumTotal += total; cumNgpm += ngpm; cumNook += nook;
+    const delivery = toThaiDate(addDays(startDate, cumDays));
+    rows.push([i+1, days, pct, delivery, total, ngpm, nook, cumTotal, cumNgpm, cumNook]);
+  });
+
+  rows.push([
+    'รวม', '', installments.reduce((s,r) => s + (parseFloat(r.percent)||0), 0).toFixed(2),
+    '',
+    Math.round(contract), Math.round(contract*ngpmPct/100), Math.round(contract*nookPct/100),
+    '', '', ''
+  ]);
+
+  const ws = XLSX.utils.aoa_to_sheet(rows);
+  ws['!cols'] = [
+    {wch:8},{wch:12},{wch:12},{wch:18},
+    {wch:18},{wch:18},{wch:16},
+    {wch:18},{wch:18},{wch:18}
+  ];
+
+  XLSX.utils.book_append_sheet(wb, ws, 'งวดงาน');
+
+  const today = new Date();
+  const fname = `งวดงาน_${today.getFullYear()}${String(today.getMonth()+1).padStart(2,'0')}${String(today.getDate()).padStart(2,'0')}.xlsx`;
+  XLSX.writeFile(wb, fname);
+}
+
+document.getElementById('contractAmount').addEventListener('input', recalcAll);
+document.getElementById('startDate').addEventListener('change', recalcAll);
+document.getElementById('ngpmPct').addEventListener('input', function() {
+  const v = parseFloat(this.value) || 0;
+  document.getElementById('nookPct').value = (100 - v).toFixed(2);
+  recalcAll();
+});
+document.getElementById('installmentCount').addEventListener('change', function() {
+  onCountChange(this.value);
+});
+
+// Init
+installments = TEMPLATE_27.map(t => ({...t}));
+renderTable();
+</script>
+</body>
+</html>
