Methodology
How We Calculate AC Sizing
Updated July 2026 · by Murugan Vellaichamy · reviewed against current standards
Every calculator on this site applies ACCA Manual J–style load factors — floor area adjusted for climate, ceiling height, sun, occupants and insulation — to estimate cooling capacity in BTU/hr and tons. These are fast planning estimates, not a stamped Manual J; for final central-system selection a licensed contractor’s full load calculation remains the standard.
The core load model
Each room’s cooling load is built from a climate baseline and a set of multipliers:
Load (BTU/hr) = area (sq ft) × climate BTU/sq ft × ceiling × sun × roof + occupant heat + window heat
Tons = load ÷ 12,000, since one refrigeration ton = 12,000 BTU/hr.
Climate baselines
The base BTU per square foot is set by climate, not a single number:
| Market / zone | Base BTU per sq ft |
|---|
| US Zone 1 — hot (FL, S. TX, AZ) | ~28 |
| US Zone 3 — moderate | ~22 |
| US Zone 5 — cool (Upper Midwest, NE) | ~18 |
| India — single-room split (all climates) | ~90–120 |
India runs far higher per sq ft because sizing is per room, not per whole house, and solar loads are harsher — hence the “1 ton per 100 sq ft” rule.
Adjustment factors
- Ceiling height — +10% per foot above 8 ft (you cool volume, not floor).
- Sun exposure — ×0.90 shaded, ×1.00 average, ×1.10 sunny.
- Top floor / roof — ×1.12 for roof heat gain.
- Occupants — +400–600 BTU per person past the first two.
- Windows & appliances — added for large glass, kitchens and equipment.
US central vs India per-room
- US mode sizes one central system to the whole envelope (or splits above 5 tons).
- India mode sizes a separate split per room. Switch with the US/India toggle in the header.
Efficiency & running cost
The bill calculator converts capacity to power using the efficiency rating: kW = (tons × 3.517) ÷ ISEER, or (tons × 12,000) ÷ SEER2 ÷ 1,000. Energy = kW × compressor-hours; cost = energy × your marginal tariff.
Electrical sizing
The ampere calculator uses running amps = kW × 1,000 ÷ voltage, with breakers at ≥125% of running current. These are indicative — the unit’s nameplate MCA / MOCP and a licensed electrician are the authority.
What these tools don’t do
- They are not a stamped Manual J; US code requires a full load calculation before equipment selection.
- They assume typical construction — unusual insulation, glazing or occupancy needs a detailed calculation.
- They estimate cooling load, not duct design (Manual D) or final refrigerant charge.
Sources & standards
Our method is grounded in recognised standards:
- ANSI/ACCA 2 Manual J — residential load calculation (ACCA).
- ANSI/ACCA 3 Manual S — equipment selection (ACCA).
- ASHRAE Handbook — Fundamentals — heat-transfer & load principles (ASHRAE).
- AHRI Standard 210/240 — SEER2 / EER2 / HSPF2 ratings (AHRI).
- BEE Star Labelling Programme (ISEER) — India efficiency ratings (BEE).
- NFPA 70 (NEC) Art. 440 & IS 732 — electrical sizing (NFPA, BIS).
Standards evolve; always follow the edition current in your jurisdiction and a licensed professional’s judgement.