Air conditioner capacity is sold in tons, where one ton is the cooling power of melting one ton of ice in a day – 12,000 BTU per hour. An oversized unit short-cycles and leaves the air clammy; an undersized one runs constantly and still cannot keep up. A full Manual J calculation by an HVAC contractor is the gold standard, but these estimates are a good starting point.
How to use the AC Tonnage Calculator
- Method
- Floor area
- Ceiling height
- Occupants
- Windows
- Exterior doors
- Includes a kitchen
- Sun exposure
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose a method, enter the floor area and ceiling height, the number of people and, for the Manual J method, windows and exterior doors. The calculator adds up the loads, applies the sun and kitchen adjustments, converts to tons and rounds up to the next half-ton unit.
Tips for an accurate estimate
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
Inputs explained
| Field | What to enter |
|---|---|
| Method | Choose one: Quick estimate (20 BTU/ft²), Simplified Manual J |
| Floor area | Units: ft², m², yd², acre |
| Ceiling height | Units: ft, m, in, yd, cm |
| Occupants | A number |
| Windows | A number |
| Exterior doors | A number |
| Includes a kitchen | Choose one: No, Yes |
| Sun exposure | Choose one: Normal, Very sunny (+10%), Heavily shaded (−10%) |
What each result means
- Tonnage = [Cooling load]÷12000
- Nearest unit size (round up to ½ ton) = ceil([Tonnage]×2 – 0.000001)÷2
- Cooling capacity = [Cooling load]
Example values
How the main result changes when Floor area changes and everything else stays at the example values above:
| Floor area (ft²) | Cooling load (BTU/h) |
|---|---|
| 750 | 20,200 |
| 1,125 | 27,700 |
| 1,500 | 35,200 |
| 1,875 | 42,700 |
| 2,250 | 50,200 |
| 3,000 | 65,200 |
Formula
Quick: BTU/h = area × 20 × (ceiling ÷ 8 ft) + 600 × (people − 2) + 4,000 kitchen, × sun factorSimplified Manual J: BTU/h = area × ceiling height + 100 × people + 1,000 × windows + 1,000 × doorsTons = BTU/h ÷ 12,000Worked example
Inputs — Method: Quick estimate (20 BTU/ft²); Floor area: 1,500 ft²; Ceiling height: 8 ft; Occupants: 4; Windows: 8; Exterior doors: 2; Includes a kitchen: Yes; Sun exposure: Normal.
Results — Cooling load = 35,200 BTU/h; Tonnage = 2.93; Nearest unit size (round up to ½ ton) = 3 tons; Cooling capacity = 10.32 kW.
Key terms
- BTU
- British thermal unit, the heat needed to raise 1 lb of water by 1 °F (1,055 J).
Frequently asked questions
How many tons of AC do I need for 1,500 ft²?
Roughly 2.5 tons by the 20 BTU/ft² rule – more in hot, sunny climates or with high ceilings.
What is one ton of air conditioning?
12,000 BTU/h, or about 3.52 kW of cooling.
Is a bigger air conditioner better?
No. Oversized units cycle on and off quickly, waste energy and do not dehumidify well.
Why get a Manual J load calculation?
It accounts for insulation, window orientation, air leakage and local climate, which rules of thumb cannot.
What formula does the AC Tonnage Calculator use?
Quick: BTU/h = area × 20 × (ceiling ÷ 8 ft) + 600 × (people − 2) + 4,000 kitchen, × sun factor.