BTU Calculator
Enter the floor area, ceiling height, climate, sun exposure, occupants and insulation. You get the cooling load in BTU/h and tons from the ENERGY STAR chart with each adjustment itemised, the heating load for your climate, and what the unit will draw while running.
For the power-draw estimate. 14-16 typical, 20+ premium mini splits.
Cooling needed
10,000 BTU/h
0.83 tons
Chart base
10,000 BTU/h
ENERGY STAR for 400 sq ft
Adjustments
+0
none
Draw while running
667 W
at SEER2 15
Heating needed
18,000 BTU/h
45 BTU/sq ft × 400
Furnace input at 95% AFUE
18,947 BTU/h
what the nameplate should read
Electric resistance equivalent
5.3 kW
1 kW = 3,412 BTU/h
Rule-of-thumb sizing from the ENERGY STAR chart and contractor BTU-per-square-foot ranges. A Manual J load calculation, which accounts for windows, orientation, infiltration and duct losses, is what a contractor should use before you buy equipment.
ENERGY STAR room air-conditioner chart
The base the calculator starts from, before adjustments. Running watts assume a SEER2 of 15.
| Area (sq ft) | BTU/h | Tons | BTU per sq ft | Watts running |
|---|---|---|---|---|
| 100–150 | 5,000 | 0.42 | 40 | 333 |
| 150–250 | 6,000 | 0.50 | 30 | 400 |
| 250–300 | 7,000 | 0.58 | 25 | 467 |
| 300–350 | 8,000 | 0.67 | 25 | 533 |
| 350–400 | 9,000 | 0.75 | 24 | 600 |
| 400–450 | 10,000 | 0.83 | 24 | 667 |
| 450–550 | 12,000 | 1.00 | 24 | 800 |
| 550–700 | 14,000 | 1.17 | 22 | 933 |
| 700–1000 | 18,000 | 1.50 | 21 | 1,200 |
| 1000–1200 | 21,000 | 1.75 | 19 | 1,400 |
| 1200–1400 | 23,000 | 1.92 | 18 | 1,533 |
| 1400–1500 | 24,000 | 2.00 | 17 | 1,600 |
| 1500–2000 | 30,000 | 2.50 | 17 | 2,000 |
| 2000–2500 | 34,000 | 2.83 | 15 | 2,267 |
Heating BTUs by house size and climate
Average insulation. Furnace output, not input; divide by the AFUE for the nameplate.
| Climate | BTU/sq ft | 1,000 sq ft | 1,500 sq ft | 2,000 sq ft | 2,500 sq ft | 3,000 sq ft |
|---|---|---|---|---|---|---|
| Hot (Florida) | 30 | 30K | 45K | 60K | 75K | 90K |
| Warm (Georgia) | 35 | 35K | 53K | 70K | 88K | 105K |
| Moderate (Virginia) | 45 | 45K | 68K | 90K | 113K | 135K |
| Cool (Ohio) | 50 | 50K | 75K | 100K | 125K | 150K |
| Cold (Minnesota) | 60 | 60K | 90K | 120K | 150K | 180K |
The adjustments and where they come from
- Ceiling height: the chart assumes 8 ft. The load scales with volume, so a 10 ft ceiling adds 25%.
- Sun: ENERGY STAR's own adjustment, +10% for a very sunny room, −10% for heavily shaded.
- Occupants: +600 BTU/h per person beyond two, from the same chart. A resting adult gives off roughly 400 BTU/h; the extra covers activity.
- Kitchen: +4,000 BTU/h for cooking heat, again per ENERGY STAR.
- Insulation and climate: contractor rules of thumb: ±10% for poor or good insulation, and up to ±15% for climate zone, which reflects the outdoor design temperature the equipment has to fight.
Frequently Asked Questions
How many BTUs do I need per square foot?
For cooling, about 20 BTU/h per square foot is the common rule, and the ENERGY STAR chart works out to 18-25 depending on room size. For heating, 30 to 60 BTU/h per square foot depending on climate: 30 in Florida, 45 in the mid-Atlantic, 60 in Minnesota. Ceiling height, sun, insulation and occupants move those by ±30%, which is what the calculator adjusts for.
What size AC for a 12×12 room? A 500 sq ft room?
A 12×12 room is 144 sq ft: 5,000 BTU/h from the chart, so a 5,000-6,000 BTU window unit. A 500 sq ft room is 12,000 BTU/h, one ton, which is a large window unit or a 12K mini split. Add 10% if it faces south or west with big windows, and 4,000 BTU if it is a kitchen.
What is a ton of cooling?
12,000 BTU/h -- historically the cooling delivered by melting one ton of ice over a day. Central systems come in 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons; a 2,000 sq ft house in a moderate climate is usually 3 to 3.5 tons.
Is a bigger unit better?
No. An oversized air conditioner cools the air quickly and shuts off before it has removed humidity, so the room feels clammy at 72°F and the compressor short-cycles, which wears it out. Matching the load matters more for cooling than for heating. If you are between sizes, size down for cooling and up for heating.
What is a Manual J and do I need one?
The ACCA Manual J is the room-by-room heat-load calculation that accounts for window area and orientation, wall and attic insulation values, air leakage, duct location and local design temperatures. Contractors are supposed to run it before selling a central system; many size by square footage instead. For a window unit or a single mini split, the chart is fine; for a whole-house system, insist on the Manual J.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.