Replacement Window Fit and Clearance Calculator
ANA›Life Services Authority›National Calculator Authority›Replacement Window Fit and Clearance Calculator
.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }
Replacement Window Fit and Clearance Calculator
Enter your rough opening dimensions to calculate the correct replacement window size with proper clearance on all sides.
Rough Opening Width (inches)
Rough Opening Height (inches)
Side Clearance Each Side (inches)
Top Clearance (inches)
Bottom Clearance (inches)
Display Units
Decimal Inches Fractional Inches (1/8") Millimeters
Calculate
function repToFraction(val) { var whole = Math.floor(val); var frac = val - whole; var eighths = Math.round(frac * 8); if (eighths === 0) return whole + '"'; if (eighths === 8) return (whole + 1) + '"'; var gcd = function(a, b) { return b === 0 ? a : gcd(b, a % b); }; var g = gcd(eighths, 8); var num = eighths / g; var den = 8 / g; return (whole > 0 ? whole + ' ' : '') + num + '/' + den + '"'; }
function repFormat(val, unit) { if (unit === 'fraction') return repToFraction(val); if (unit === 'mm') return (val * 25.4).toFixed(1) + ' mm'; return val.toFixed(4) + '"'; }
function repCalc() { var ow = parseFloat(document.getElementById('rep-opening-width').value); var oh = parseFloat(document.getElementById('rep-opening-height').value); var cs = parseFloat(document.getElementById('rep-clearance-side').value); var ct = parseFloat(document.getElementById('rep-clearance-top').value); var cb = parseFloat(document.getElementById('rep-clearance-bottom').value); var unit = document.getElementById('rep-unit').value; var res = document.getElementById('rep-result');
// Validation if (isNaN(ow) || ow Please enter a valid Rough Opening Width.'; return; } if (isNaN(oh) || oh Please enter a valid Rough Opening Height.'; return; } if (isNaN(cs) || cs Side clearance must be at least 1/16".'; return; } if (isNaN(ct) || ct Top clearance must be at least 1/16".'; return; } if (isNaN(cb) || cb Bottom clearance must be at least 1/16".'; return; } if (cs > 1 || ct > 1 || cb > 1) { res.innerHTML = 'Clearance values should not exceed 1 inch. Verify your inputs.'; return; }
// Core formulas // Window Width = Rough Opening Width - (2 × Side Clearance) // Window Height = Rough Opening Height - Top Clearance - Bottom Clearance var winWidth = ow - (2 * cs); var winHeight = oh - ct - cb; var totalHorizClear = 2 * cs; var totalVertClear = ct + cb;
if (winWidth Clearance values exceed the opening size. Please reduce clearances.'; return; }
// Shim space (gap available for shimming/insulation each side) // Typically the clearance gap is split: ~1/2 for shimming, rest for insulation/foam var shimSide = cs / 2; var shimTop = ct / 2; var shimBottom = cb / 2;
// Rough opening area vs window area var roArea = ow * oh; var winArea = winWidth * winHeight;
res.innerHTML = '### Results ' + '' + 'MeasurementValue' + 'Recommended Window Width' + repFormat(winWidth, unit) + '' + 'Recommended Window Height' + repFormat(winHeight, unit) + '' + 'Total Horizontal Clearance (both sides)' + repFormat(totalHorizClear, unit) + '' + 'Total Vertical Clearance (top + bottom)' + repFormat(totalVertClear, unit) + '' + 'Shim Space Per Side (≈ ½ of side clearance)' + repFormat(shimSide, unit) + '' + 'Shim Space Top (≈ ½ of top clearance)' + repFormat(shimTop, unit) + '' + 'Shim Space Bottom (≈ ½ of bottom clearance)' + repFormat(shimBottom, unit) + '' + 'Rough Opening Area' + (unit === 'mm' ? (roArea * 645.16).toFixed(0) + ' mm²' : roArea.toFixed(3) + ' in²') + '' + 'Window Unit Area' + (unit === 'mm' ? (winArea * 645.16).toFixed(0) + ' mm²' : winArea.toFixed(3) + ' in²') + '' + '' + 'Order a window sized ' + repFormat(winWidth, unit) + ' W × ' + repFormat(winHeight, unit) + ' H. ' + 'Fill remaining gaps with low-expansion foam and cover with interior/exterior trim.
'; }
#### Formulas Used
Window Width = Rough Opening Width − (2 × Side Clearance)
Window Height = Rough Opening Height − Top Clearance − Bottom Clearance
Shim Space (each side) ≈ Side Clearance ÷ 2
The remaining clearance gap (after shimming) is filled with low-expansion spray foam for insulation and air sealing.
#### Assumptions & References
- Standard industry practice recommends ¼" (0.25") clearance on each side, top, and bottom for replacement windows (Andersen, Pella, JELD-WEN installation guides).
- Minimum clearance of 1/16" is required to allow for shimming and leveling.
- Maximum recommended clearance is 1"; larger gaps require additional framing or blocking.
- Remaining gap after shimming should be filled with low-expansion polyurethane foam (e.g., Great Stuff Window & Door) per AAMA 2400 installation standards.
- Rough opening dimensions should be measured at multiple points (top, middle, bottom for width; left, center, right for height); use the smallest measurement.
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