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Construction & Home
BTU Calculator — What Size AC or Heater Do I Need?
Match your AC or heater to the room using the U.S. Department of Energy 20-BTU-per-square-foot rule
Buy an air conditioner that's too small and it runs nonstop on a hot day without ever cooling the room. Buy one that's too big and it short-cycles — blasting cold air, snapping off, and leaving the air clammy because it never runs long enough to pull humidity out. Either way you waste money and comfort. The fix is to size the unit in BTU (British Thermal Units, the standard rating on every AC and heater) to match the actual room, and that's exactly what this calculator does.
The method comes straight from the U.S. Department of Energy and ENERGY STAR sizing guidance. For cooling, the baseline is simple:
Base cooling BTU = room area (sq ft) × 20.
A 300-square-foot living room needs roughly 300 × 20 = 6,000 BTU before adjustments. From there you tune for the things that change the heat load:
• +10% if the room is very sunny (large south/west windows, top floor). • −10% if the room is heavily shaded. • +600 BTU for each person beyond two who regularly occupies the room. • +4,000 BTU if the space is a kitchen (the stove and oven dump a lot of heat).
So that same 300 sq ft room, if it's very sunny, needs 300 × 20 × 1.10 = 6,600 BTU — which rounds up to a standard 8,000 BTU window unit, since you always round up to the next size you can actually buy.
Heating works a little differently because it depends heavily on how cold your winters get. Instead of a flat 20, you multiply the area by a BTU-per-square-foot factor for your climate zone:
• 30 BTU/sq ft — mild (Gulf Coast, Florida, Southern California) • 40 BTU/sq ft — moderate (mid-Atlantic, Pacific Northwest) • 50 BTU/sq ft — cold (Midwest, Northeast) • 60 BTU/sq ft — very cold (northern plains, mountain states)
A 1,000 sq ft space in a cold zone needs about 1,000 × 50 = 50,000 BTU of heating. These are quick, reliable estimates for window ACs, portable units, mini-splits and space heaters. For a whole-house central system a contractor should run a full Manual J load calculation that also accounts for insulation, ceiling height and air sealing — but for picking a room unit, the DOE rule of thumb gets you the right size fast.
📖 See also: How to Calculate a Tip (and Split the Bill)
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
Cooling: • Base cooling BTU = area (sq ft) × 20 • Very sunny room: × 1.10 (+10%) • Heavily shaded room: × 0.90 (−10%) • Add 600 BTU for each person beyond 2 • Add 4,000 BTU if the room is a kitchen • Round UP to the next standard unit size Heating: • Heating BTU = area (sq ft) × climate factor • Climate factor (BTU per sq ft): mild = 30, moderate = 40, cold = 50, very cold = 60
📰 Formula
Cooling: • Base cooling BTU = area (sq ft) × 20 • Very sunny room: × 1.10 (+10%) • Heavily shaded room: × 0.90 (−10%) • Add 600 BTU for each person beyond 2 • Add 4,000 BTU if the room is a kitchen • Round UP to the next standard unit size Heating: • Heating BTU = area (sq ft) × climate factor • Climate factor (BTU per sq ft): mild = 30, moderate = 40, cold = 50, very cold = 60
🧪 Worked examples
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Example 4
⚠️ Common mistakes
- Rounding the BTU figure DOWN to a smaller unit — always round up so the AC can keep up on the hottest day.
- Forgetting the +4,000 BTU kitchen bump, which leaves a kitchen unit badly undersized.
- Using cooling math (× 20) for a heater instead of the climate-zone factor (× 30–60).
- Counting every occupant instead of only the people beyond the first two.
- Assuming bigger is always better — an oversized AC short-cycles and leaves the room humid and clammy.
💡 Tips
- Measure the room's square footage first (length × width); that single number drives the whole estimate.
- Pick 'very sunny' for top-floor or large west/south-facing-window rooms, 'shaded' for north-facing or tree-covered rooms.
- When the result lands between two standard sizes, choose the larger one — it's far better than running short.
- For a whole house, add the rooms up but have an HVAC pro run a full Manual J load calculation before buying central equipment.
- High ceilings, poor insulation or lots of glass push you toward the larger unit; tight, well-insulated rooms toward the smaller.
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❓ Frequently asked questions
How many BTU do I need to cool a room?
Start with the room's square footage times 20. A 300 sq ft room needs about 6,000 BTU. Then adjust for sun, extra people and whether it's a kitchen, and round up to the next standard unit size.
What size air conditioner do I need for a 500 square foot room?
About 500 × 20 = 10,000 BTU for a normal room. A very sunny 500 sq ft room needs roughly 11,000 BTU, while a heavily shaded one needs around 9,000 BTU. Round up to the nearest size you can buy.
Is it bad to buy an air conditioner that's too big?
Yes. An oversized AC cools the air fast and shuts off before it removes humidity, so it short-cycles and leaves the room cold but clammy. It also wastes energy and wears the compressor out faster, which is why matching BTU to the room matters.
How many BTU do I need to heat 1,000 square feet?
Multiply the area by your climate factor. In a mild zone that's 1,000 × 30 = 30,000 BTU; in a cold zone it's 1,000 × 50 = 50,000 BTU; in a very cold zone about 60,000 BTU. Colder winters need more BTU per square foot.
Why does a kitchen need extra BTU?
Stoves, ovens and the refrigerator add a steady heat load that other rooms don't have, so the DOE rule adds a flat 4,000 BTU for kitchens. Skip that bump and a kitchen AC will struggle on the hottest days.
How do extra people change the BTU I need?
Each person gives off body heat, so the rule adds 600 BTU for every occupant beyond the first two. A room normally used by four people needs about 1,200 BTU more than the same room used by two.
What's the difference between BTU and BTU per hour?
On air conditioners and heaters, 'BTU' is shorthand for BTU per hour — the amount of heat the unit can move in one hour. A 6,000 BTU AC removes 6,000 BTU of heat per hour, so the two terms mean the same thing on a spec sheet.
Does this BTU calculator work for central air or a furnace?
It's a fast, accurate estimate for room units, mini-splits, portables and space heaters. For whole-house central air or a furnace, have an HVAC contractor run a full Manual J load calculation that also factors in insulation, ductwork, ceiling height and air sealing.
How do I convert BTU to tons of cooling?
One ton of air conditioning equals 12,000 BTU per hour. So a 6,000 BTU unit is about half a ton, and a 24,000 BTU unit is 2 tons. Central systems are usually rated in tons, while window and portable units are rated in BTU.