OneTripDIY
BTU Calculator — What Size AC or Heater Do I Need?

BTU Calculator — Sizing an Air Conditioner or Heater

Sizing matters more than power. An oversized unit cools fast, shuts off, and leaves the room clammy because it never runs long enough to pull moisture out. Enter the room and get the range to shop in.

By The OneTripDIY Team · Updated

Measure the space

L-shaped room or more than one area? Add a section for each rectangle and we'll total them.

ft

Above 8 ft, the extra volume needs extra capacity.

no.

Each person past two adds about 600 BTU of cooling load.

Used for heating. Cooling starts from 20 BTU per sq ft everywhere.

Size to look for —
Sensible minimum — BTU
Do not exceed — BTU
In tons (cooling) —
Room area — sq ft

Estimates only. Check against your supplier's stated coverage before you buy.

Take this list to the store

We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.

We send the list you asked for. Anything else only if you tick the box, and you can unsubscribe in one click. What we do with it.

What decides the BTU you needArea, height, climate and exposure all feed the figure.What decides the BTU you needRoom areaCeiling heightClimate zoneSun and insulationBTU per hour12,000Area alone is not enough — the same room needs different output in different places.

How to work it out

  1. Measure the room, not the house

    Size the unit to the space it actually serves. A window unit cools one room; sizing it for the whole floor gives you a unit that short-cycles and never dehumidifies.

  2. Account for the ceiling

    Air conditioning cools volume, not floor area. A 10 ft ceiling is 25% more air than an 8 ft one, and the calculator scales for it.

  3. Adjust for sun, insulation and use

    A west-facing room with big windows takes about 10% more. A well-insulated modern room takes about 10% less. A kitchen adds 4,000 BTU because the cooking does. Each person past two adds roughly 600.

  4. Buy inside the range, not above it

    Use the figure as the target and stay under the “do not exceed” number. Oversizing is the most common and most expensive mistake in this whole category.

Starting figures by room size

Before adjusting for ceiling height, sun, insulation and occupancy.

Room areaCoolingHeating — mixed climateHeating — very cold
150 sq ft3,000 BTU5,250 BTU7,500 BTU
250 sq ft5,000 BTU8,750 BTU12,500 BTU
350 sq ft7,000 BTU12,250 BTU17,500 BTU
450 sq ft9,000 BTU15,750 BTU22,500 BTU
550 sq ft11,000 BTU19,250 BTU27,500 BTU
700 sq ft14,000 BTU24,500 BTU35,000 BTU
1000 sq ft20,000 BTU35,000 BTU50,000 BTU
1500 sq ft30,000 BTU52,500 BTU75,000 BTU

Cooling starts at 20 BTU per sq ft. Heating varies far more with climate, from about 32 to 58 BTU per sq ft.

Cooling capacity in tons

BTUTonsTypically suits
5,000 BTU0.4A bedroom up to about 250 sq ft
8,000 BTU0.7A living room around 350 sq ft
12,000 BTU1.0Around 550 sq ft, or a small open plan
18,000 BTU1.5Around 800 sq ft
24,000 BTU2.0Around 1,100 sq ft
36,000 BTU3.0Around 1,600 sq ft

One ton equals 12,000 BTU per hour. The term comes from the cooling a ton of melting ice once provided.

What size air conditioner for a 500 square foot room?

Five hundred square feet at 20 BTU per square foot is 10,000 BTU as a starting point. With an 8 ft ceiling, average sun and average insulation, that is the figure to shop for.

Make it a sunny west-facing room with poor insulation and it rises to about 13,000. Make it a well-insulated north-facing bedroom and it falls to around 8,000.

Why bigger is worse, not better

This is the thing most people get wrong. An oversized air conditioner cools the air quickly, hits the thermostat setting, and switches off — before it has run long enough to condense moisture out of the air.

The result is a room that is cold and clammy rather than cool and dry, a compressor that short-cycles and wears out early, and a higher bill than a correctly sized unit would give you. If you are between two sizes, take the smaller one.

How many BTU to heat 1,000 square feet?

It depends far more on where you live than on the room. A thousand square feet needs roughly 32,000 BTU on the Gulf coast, 45,000 in a mixed climate like Virginia, and 58,000 in Minnesota.

Insulation shifts it just as much. A well-sealed modern house at the cold end can need less heat than a drafty old one in a mild climate.

When to get a professional load calculation

This calculator is a rule-of-thumb estimate, good for choosing a window unit or a portable heater. For a whole-house system — a furnace, a heat pump, central air — get a Manual J load calculation from an HVAC contractor.

Manual J accounts for every window's orientation and glazing, actual insulation values, air leakage, duct losses and local design temperatures. On a system costing thousands, a proper calculation pays for itself by stopping you buying two sizes too big.

Sizing a mini-split is a different calculation

Ductless mini-splits are sized the same way for capacity, but two things change.

First, there are no duct losses. A ducted system loses 20 to 30% of its output into an unconditioned loft or crawl space, and sizing usually accounts for it. A mini-split delivers into the room, so the figure above is closer to what you actually need — do not add a margin on top.

Second, most mini-splits are inverter-driven, so they vary output continuously instead of switching on and off. That makes them more forgiving of slight oversizing than a single-stage unit — but only slightly. A badly oversized inverter still spends its life at minimum output, and the humidity problem returns.

For a multi-zone system, size each head to its own room and check the outdoor unit's total capacity against the sum. Outdoor units are usually rated below the sum of their heads, on the assumption not every room peaks at once.

What the calculator cannot see

Rule-of-thumb sizing gets you close for a window unit or a space heater. It misses several things that a full load calculation catches:

  • Air leakage. A drafty house can need half again what a sealed one does at the same size and insulation level.
  • Window orientation and glazing. Sixty square feet of single-pane west-facing glass is a completely different load from the same area facing north, double-glazed.
  • Duct losses. Ducts running through an uninsulated loft can lose a quarter of the output before it reaches a room.
  • Local design temperatures. Sizing is done against the temperature your area actually reaches, not an average.

For a window unit, none of that justifies the cost of a professional calculation. For a whole-house system costing thousands, all of it does.

How this is worked out

No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.

  1. cooling base = area × 20 BTU The standard starting figure for residential cooling.
  2. heating base = area × climate factor 32 BTU per sq ft in the hot south, up to 58 in the very cold north.
  3. × ceiling height ÷ 8 Conditioning air is about volume, not floor area.
  4. × sun factor (0.9 – 1.1) Big west-facing glass gains heat all afternoon.
  5. × insulation factor (0.9 – 1.2) A drafty single-glazed room loses what you put into it.
  6. + 600 per person over two, + 4,000 for a kitchen People and cooking are real heat sources, for cooling only.
  7. rounded to the nearest 500 BTU Units are sold in steps, not to the exact figure.

Common questions

How many BTU do I need per square foot?

About 20 BTU per sq ft for cooling almost anywhere in the US. Heating varies with climate — roughly 32 BTU per sq ft in the hot south up to 58 in the coldest states.

What size AC for a 12x12 bedroom?

144 sq ft at 20 BTU is about 2,900 BTU, so the smallest common unit — 5,000 BTU — is plenty. Do not go larger; it will short-cycle and leave the room damp.

Is it bad to buy an air conditioner that is too big?

Yes, and it is the most common mistake here. An oversized unit cools the air fast, shuts off before it has removed any humidity, and leaves the room cold and clammy. It also wears the compressor out faster and costs more to run.

How many tons is 12,000 BTU?

One ton. The unit dates from the days of ice cooling — one ton of melting ice absorbs about 12,000 BTU per hour. So 24,000 BTU is two tons, 36,000 is three.

Does ceiling height change the BTU I need?

Yes. Air conditioning works on volume, so a 10 ft ceiling needs about 25% more capacity than an 8 ft one over the same floor area. The calculator scales for it automatically.

Do I need more BTU for a kitchen?

Add about 4,000 BTU. Ovens, hobs and dishwashers put a lot of heat into the room, and no amount of extractor fan removes all of it.

Should I size a furnace this way?

For a space heater, yes. For a whole-house furnace or heat pump, get a Manual J load calculation from an HVAC contractor — it accounts for your actual windows, insulation, air leakage and duct losses, and on a system that expensive it is worth doing properly.

Does insulation really change the answer that much?

Substantially. A well-insulated, double-glazed room needs around 10% less than average; an old drafty single-glazed one around 20% more. That is the difference between an 8,000 and a 12,000 BTU unit on the same floor area.

Where these figures come from

Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.

  • ENERGY STAR room air conditioner sizing guide — the 20 BTU per sq ft baseline and the occupancy and kitchen adjustments
  • ACCA Manual J — the professional residential load calculation standard
  • US Department of Energy climate zone map — heating BTU per square foot by region
  • AHRI capacity ratings — the 12,000 BTU per ton relationship

Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 11, 2026.