HVAC System Sizing Calculator (Tonnage)
ANA›Life Services Authority›National Calculator Authority›HVAC System Sizing Calculator (Tonnage)
.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; } }
HVAC System Sizing Calculator (Tonnage)
Estimate the required HVAC system size in tons of cooling/heating capacity using a simplified Manual J-style load calculation based on square footage, climate zone, insulation quality, occupancy, and window area.
Conditioned Floor Area (sq ft)
Ceiling Height (ft)
Climate Zone
Hot & Humid (Zone 1 – Miami, Houston) Hot & Dry (Zone 2 – Phoenix, Las Vegas) Mixed / Moderate (Zone 3–4 – Atlanta, Dallas) Cool (Zone 5 – Chicago, Denver) Cold (Zone 6–7 – Minneapolis, Anchorage)
Insulation Quality
Poor (older home, minimal insulation) Average (code-minimum insulation) Good (well-insulated, double-pane windows) Excellent (high-performance, triple-pane)
Number of Occupants
Total Window Area (sq ft)
Predominant Window Orientation
North (least solar gain) East / West South (moderate solar gain) West-facing majority (highest afternoon gain)
Number of Stories
1 Story (more roof exposure) 2 Stories 3+ Stories
Calculate Tonnage
function hvaCalc() { var resultDiv = document.getElementById('hva-result');
var area = parseFloat(document.getElementById('hva-area').value); var ceiling = parseFloat(document.getElementById('hva-ceiling').value); var climate = parseFloat(document.getElementById('hva-climate').value); var insulation = parseFloat(document.getElementById('hva-insulation').value); var occupants = parseFloat(document.getElementById('hva-occupants').value); var windows = parseFloat(document.getElementById('hva-windows').value); var orient = parseFloat(document.getElementById('hva-orientation').value); var stories = parseFloat(document.getElementById('hva-stories').value);
// --- Validation --- if (isNaN(area) || area 50000) { resultDiv.innerHTML = '⚠ Please enter a valid floor area between 100 and 50,000 sq ft.'; resultDiv.style.display = 'block'; return; } if (isNaN(ceiling) || ceiling 20) { resultDiv.innerHTML = '⚠ Please enter a ceiling height between 7 and 20 ft.'; resultDiv.style.display = 'block'; return; } if (isNaN(occupants) || occupants 100) { resultDiv.innerHTML = '⚠ Please enter a valid number of occupants (1–100).'; resultDiv.style.display = 'block'; return; } if (isNaN(windows) || windows 5000) { resultDiv.innerHTML = '⚠ Please enter a valid window area between 0 and 5,000 sq ft.'; resultDiv.style.display = 'block'; return; } if (windows > area * 0.5) { resultDiv.innerHTML = '⚠ Window area seems unusually large relative to floor area. Please verify.'; resultDiv.style.display = 'block'; return; }
// --------------------------------------------------------------- // LOAD CALCULATIONS (BTU/hr) // ---------------------------------------------------------------
// 1. Envelope / Fabric Load // Base: 25 BTU/hr per sq ft at standard 8-ft ceiling, adjusted for // ceiling height (volume ratio), climate, insulation, and stories. var ceilingFactor = ceiling / 8.0; var envelopeLoad = area * 25 * ceilingFactor * climate * insulation * stories;
// 2. Solar / Window Load // Average solar heat gain: ~200 BTU/hr per sq ft of window (SHGC ~0.4, // peak irradiance ~500 BTU/hr·ft²), adjusted for orientation. var windowLoad = windows * 200 * orient * climate;
// 3. Occupant Load // Sensible heat per person ≈ 250 BTU/hr (ASHRAE 62.1 seated/light activity) var occupantLoad = occupants * 250;
// 4. Lighting & Appliance Load // Residential estimate: ~3 W/sq ft → 3 × 3.412 BTU/hr·W ≈ 10.24 BTU/hr/sq ft var internalLoad = area * 10.24;
// 5. Ventilation / Infiltration Load // ACH (air changes per hour) ≈ 0.35 for average construction // Q_infiltration = Volume × ACH × ΔT × 0.018 (BTU/hr·°F·ft³/min factor) // Simplified: use volume × 0.35 / 60 × 1.1 × ΔT (ΔT = 20°F typical) var volume = area * ceiling; var infiltrationLoad = volume * 0.35 / 60 * 1.1 * 20;
// Total BTU/hr var totalBTU = envelopeLoad + windowLoad + occupantLoad + internalLoad + infiltrationLoad;
// Convert to Tons: 1 ton = 12,000 BTU/hr var rawTons = totalBTU / 12000;
// Round up to nearest 0.5-ton increment (standard equipment sizing) var recommendedTons = Math.ceil(rawTons * 2) / 2;
// Efficiency range: ±10% for low/high estimates var lowTons = Math.max(0.5, Math.ceil((rawTons * 0.90) * 2) / 2); var highTons = Math.ceil((rawTons * 1.10) * 2) / 2;
// BTU/hr per sq ft (sanity check metric) var btuPerSqFt = totalBTU / area;
// --------------------------------------------------------------- // OUTPUT // --------------------------------------------------------------- var html = '### ✓ Estimated HVAC Sizing Result '; html += ''; html += 'Load ComponentBTU/hr'; html += 'Envelope / Fabric Load' + Math.round(envelopeLoad).toLocaleString() + ''; html += 'Solar / Window Load' + Math.round(windowLoad).toLocaleString() + ''; html += 'Occupant Load' + Math.round(occupantLoad).toLocaleString() + ''; html += 'Lighting & Appliance Load' + Math.round(internalLoad).toLocaleString() + ''; html += 'Infiltration / Ventilation Load' + Math.round(infiltrationLoad).toLocaleString() + ''; html += 'Total Cooling Load' + Math.round(totalBTU).toLocaleString() + ' BTU/hr'; html += '';
html += ''; html += 'Calculated Load: ' + rawTons.toFixed(2) + ' tons (' + Math.round(totalBTU).toLocaleString() + ' BTU/hr)
'; html += '★ Recommended System Size: ' + recommendedTons.toFixed(1) + ' Tons
'; html += 'Typical Range: ' + lowTons.toFixed(1) + ' – ' + highTons.toFixed(1) + ' tons (±10% for site conditions)
'; html += 'Load Density: ' + btuPerSqFt.toFixed(1) + ' BTU/hr per sq ft';
if (btuPerSqFt 45) { html += ' (High – consider additional insulation or shading)'; } else { html += ' (Within typical residential range of 15–45 BTU/hr·ft²)'; } html += '
';
html += 'ⓘ This is a simplified estimate. A full ACCA Manual J calculation by a licensed HVAC engineer is recommended before equipment purchase.
';
resultDiv.innerHTML = html; resultDiv.style.display = 'block'; }
#### Formulas Used
Total Cooling Load (BTU/hr) = Envelope Load + Window Load + Occupant Load + Internal Load + Infiltration Load
- Envelope Load = Area × 25 BTU/hr·ft² × (Ceiling Height / 8) × Climate Factor × Insulation Factor × Stories Factor
- Window Load = Window Area × 200 BTU/hr·ft² × Orientation Factor × Climate Factor
- Occupant Load = Occupants × 250 BTU/hr per person (ASHRAE sensible heat, light activity)
- Internal Load = Area × 10.24 BTU/hr·ft² (≈ 3 W/ft² lighting & appliances × 3.412 BTU/W)
- Infiltration Load = Volume × 0.35 ACH / 60 × 1.1 BTU·min/(ft³·°F) × 20°F ΔT
- Tonnage = Total BTU/hr ÷ 12,000 BTU/hr per ton, rounded up to nearest 0.5-ton increment
#### Assumptions & References
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