How many BTUs do you need?
A quick cooling estimate from ENERGY STAR's sizing chart and its adjustments, shown in BTU per hour and tons. Good for a first conversation with a customer; size the real system from a Manual J.
| Step | Change | BTU/h |
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This is a rule-of-thumb estimate from ENERGY STAR's room air conditioner chart, not an ACCA Manual J load calculation. Size central systems from a Manual J done for the house; an oversized system short-cycles and dehumidifies poorly.
By ENERGY STAR's room air conditioner chart, 450 to 550 square feet needs about 12,000 BTU per hour, or one ton. Add 10% for a very sunny room or take off 10% if it's heavily shaded, add 600 BTU/h for each regular occupant over two and 4,000 BTU/h for a kitchen. This is a rule of thumb; size central systems with an ACCA Manual J load calculation.
How to use the BTU calculator
Enter the area
Square feet of the room or zone to cool, and the ceiling height.
Set the conditions
Sun exposure, how many people regularly use the space, and whether it includes a kitchen.
Read BTU/h and tons
The estimate, the closest half-ton size and each step that built it.
What the BTU calculator does
ENERGY STAR basis
Uses ENERGY STAR's area-to-capacity chart, from 100 to 2,500 sq ft, based on an 8 ft ceiling.
Published adjustments
Shade −10%, sun +10%, +600 BTU/h per extra person over two, +4,000 BTU/h for a kitchen.
Ceiling height
Scales the chart figure by ceiling height over 8 ft, a common volume rule of thumb.
Tons and steps
Converts to tons at 12,000 BTU/h per ton and shows every step in a table.
- Basis
- ENERGY STAR room air conditioner sizing chart (8 ft ceiling)
- Adjustments
- Shaded −10%, very sunny +10%, +600 BTU/h per person over 2, kitchen +4,000 BTU/h
- Above 2,500 sq ft
- Extrapolated at the chart's last ratio, 13.6 BTU/h per sq ft, and flagged
- Not included
- Manual J inputs: orientation, window areas, infiltration, duct losses, local design temperatures
- Cost and privacy
- Free, no sign-up; runs in your browser
HVAC BTU Calculator FAQ
How many BTUs do I need per square foot?
There's no single figure. ENERGY STAR's chart works out to about 22 to 27 BTU/h per square foot for a 450 to 550 sq ft room (12,000 BTU/h), more for small rooms and about 14 to 20 for 1,500 to 2,500 sq ft. Sun, people and kitchens move it from there; insulation, windows and climate need a Manual J.
How many tons of AC do I need for my house?
Divide the BTU/h load by 12,000 to get tons. A rule of thumb gives a starting point, but the right size for a whole house comes from an ACCA Manual J load calculation: the International Residential Code (M1401.3) has equipment sized with Manual S from Manual J loads or another approved method.
Is this a Manual J calculation?
No. Manual J works room by room from the house's walls, windows, insulation, air leakage, ducts, orientation and local design temperatures. This calculator uses floor area and a few adjustments, so treat it as an estimate.
Why not just oversize to be safe?
An oversized air conditioner cools the air quickly and shuts off before it removes much moisture, so the house feels clammy, and the short cycles add wear. That's why ACCA's Manual S matches equipment to the Manual J load instead of rounding up.
Does a kitchen really add 4,000 BTUs?
That's ENERGY STAR's adjustment for a room air conditioner used in a kitchen, to cover heat from cooking. Apply it once per kitchen, not per appliance.
Where the numbers come from
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