BTU Calculator
Estimate the heating and cooling BTUs a room needs from its size, insulation, sun exposure, climate zone and occupancy, with the tonnage and every adjustment shown.
BTU Calculator: with the default inputs, cooling needed is 6,000 BTU/h.
The rules of thumb assume 8 ft; taller ceilings scale the load in proportion.
ENERGY STAR's adjustment: ±10% for a room that is heavily shaded or very sunny.
Sets the heating band. The cooling rule of thumb is a single nationwide figure.
Above two, ENERGY STAR adds 600 BTU/h per extra person.
ENERGY STAR adds 4,000 BTU/h for cooking appliances.
- Equivalent tonnage
- 0.51 ton of cooling = 12,000 BTU/h, the rate that melts a ton of ice in a day.
- Heating needed
- 14,250 BTU/h
- Floor area
- 300 ft²
- Cooling per square foot
- 20BTU/h per ft².
- Heating per square foot
- 47.5BTU/h per ft².
- Nearest standard unit
- 6,000 BTU/h (0.5 tons) — the next standard size up
Assumptions
- Rule-of-thumb sizing, not an ACCA Manual J load calculation — expect it to run high.
- The 20 BTU/h per ft² cooling base, the ±10% sun adjustment, the 600 BTU/h per extra occupant and the 4,000 BTU/h kitchen allowance are ENERGY STAR's published room-air-conditioner guidance.
- The heating bands by climate zone and the insulation factors are the common contractor rules of thumb, not a published standard.
- No allowance for window area or orientation, air leakage, duct losses or local design temperatures.
| Contribution | BTU/h | Source |
|---|---|---|
| Floor area at 20 BTU/h per ft² | 6,000 | ENERGY STAR base rule, scaled for ceiling height |
| Average exposure (no change) | 0 | ENERGY STAR ±10% |
| average insulation | 0 | contractor rule of thumb |
| 0 people above two | 0 | ENERGY STAR 600 BTU/h each |
| Kitchen appliances | 0 | ENERGY STAR 4,000 BTU/h |
| Total | 6,000 | 0.5 tons |
| Room area (ft²) | Capacity (BTU/h) |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
| 1,000–1,200 | 21,000 |
| 1,200–1,400 | 23,000 |
| 1,500–2,000 | 30,000 |
The published chart works out at roughly 17–24 BTU/h per ft² depending on room size; the flat 20 used above sits in the middle of it.
How this is worked out
The formula
Cooling (BTU/h)
= floor area (ft²) × 20 × (ceiling height ÷ 8 ft)
× sun factor (0.9 shaded / 1.0 average / 1.1 sunny)
× insulation factor (0.9 good / 1.0 average / 1.15 poor)
+ 600 × (occupants − 2, if positive)
+ 4,000 if it is a kitchen
Heating (BTU/h)
= floor area (ft²) × band (BTU/h per ft², from the climate zone and insulation)
× (ceiling height ÷ 8 ft)
Zone 1 30–35 · Zone 2 35–40 · Zone 3 40–45 · Zone 4 45–50 · Zone 5 50–60
Tonnage = BTU/h ÷ 12,000. 1 ton is the rate that melts one short ton of ice in 24 hours.Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Room length
- A number.in m, cm, ft, in, yd · from 0 to 500 · defaults to 20
- Room width
- A number.in m, cm, ft, in, yd · from 0 to 500 · defaults to 15
- Ceiling height
- The rules of thumb assume 8 ft; taller ceilings scale the load in proportion.in m, cm, ft, in, yd · from 0 to 40 · defaults to 8
- Insulation quality
- Choose one of 3 options.Good — modern, well sealed, double glazed · Average — typical existing house · Poor — draughty, single glazed, little insulation
- Sun exposure
- ENERGY STAR's adjustment: ±10% for a room that is heavily shaded or very sunny.Heavily shaded · Average · Very sunny
- Climate zone
- Sets the heating band. The cooling rule of thumb is a single nationwide figure.1 — hot (south Florida, south Texas, Hawaii) · 2 — warm (Gulf coast, southern California, Arizona) · 3 — mild (Carolinas, Tennessee, coastal Pacific) · 4 — cold (mid-Atlantic, Midwest, Pacific Northwest) · 5 — very cold (New England, upper Midwest, mountains)
- People normally in the room
- Above two, ENERGY STAR adds 600 BTU/h per extra person.from 0 to 40 · whole numbers only · defaults to 2
- It is a kitchen
- ENERGY STAR adds 4,000 BTU/h for cooking appliances.defaults to off
What you get back
- Cooling neededmain answer
- Equivalent tonnage
- 1 ton of cooling = 12,000 BTU/h, the rate that melts a ton of ice in a day.
- Heating needed
- Floor area
- Cooling per square foot
- BTU/h per ft².
- Heating per square foot
- BTU/h per ft².
- Nearest standard unit
What this assumes
- Rule-of-thumb sizing, not an ACCA Manual J load calculation — expect it to run high.
- The 20 BTU/h per ft² cooling base, the ±10% sun adjustment, the 600 BTU/h per extra occupant and the 4,000 BTU/h kitchen allowance are ENERGY STAR's published room-air-conditioner guidance.
- The heating bands by climate zone and the insulation factors are the common contractor rules of thumb, not a published standard.
- No allowance for window area or orientation, air leakage, duct losses or local design temperatures.
About this calculator
A BTU — British thermal unit — is the heat needed to raise one pound of water by one degree Fahrenheit. Heating and cooling equipment is rated in BTUs per hour, a rate of heat moved. This calculator estimates that rate for a room from its size and the handful of things that most change the answer.
What it is doing
Cooling starts from the widely quoted 20 BTU/h per square foot, then applies the four adjustments ENERGY STAR publishes for room air conditioners: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU/h for each occupant beyond two, and +4,000 BTU/h for a kitchen. Ceiling height scales the whole thing against the 8-foot assumption baked into per-square-foot rules, and an insulation factor adjusts for how leaky the envelope is.
Heating uses the long-published contractor band table: 30–35 BTU/h per ft² in the hottest US climate zone rising to 50–60 in the coldest, with insulation quality picking the bottom, middle or top of the band. Heating loads are climate-driven in a way cooling loads are not, which is why the zone matters here and not there.
Tonnage
Air-conditioning capacity is often quoted in "tons" for historical reasons: one ton is 12,000 BTU/h, the rate at which a ton of ice melts over a day, from the era when buildings were cooled with actual ice. A 3-ton unit moves 36,000 BTU/h. The calculator gives both, plus the nearest standard size up, because equipment comes in fixed steps.
The honest caveat
This is a rule of thumb, and the professional answer is an ACCA Manual J load calculation. Manual J works from the actual construction: every wall area and its R-value, every window's orientation, size and glazing, infiltration measured or estimated, duct losses, internal gains from appliances and people, and the local design temperatures. It routinely lands 20–40% below what per-square-foot rules suggest, because those rules were built with a large safety margin.
That margin is not harmless. Oversized cooling equipment is worse than slightly undersized. An oversized air conditioner satisfies the thermostat quickly and shuts off — short-cycling — before it has run long enough to condense moisture out of the air. The result is a room that is cold and clammy, with more wear on the compressor and worse efficiency than the label promises. Oversized heating short-cycles too, and swings the temperature about.
Use this figure to know whether a quote is in the right neighbourhood, to choose between a 6,000 and an 8,000 BTU window unit, or to sanity-check a contractor who has clearly guessed. For anything that gets installed and lived with, pay for the Manual J.
Things this cannot see
Window area and orientation, which can easily be the largest single load in a sunny room. Air leakage, which dominates in older buildings. Duct runs through unconditioned attics. Number and type of appliances. Local design temperature and humidity. Whether the room is above a garage or under a black roof. All of these are exactly what Manual J is for.
Frequently asked questions
▸How many BTU do I need per square foot?
About 20 BTU/h per ft² for cooling as a nationwide rule of thumb, and 30 to 60 BTU/h per ft² for heating depending on climate zone and insulation. Both are starting points, not designs.
▸What is a ton of air conditioning?
12,000 BTU/h — the rate at which one short ton of ice melts in 24 hours, a unit inherited from the days when buildings were cooled with ice. A 2.5-ton unit is 30,000 BTU/h.
▸Is it better to oversize or undersize an air conditioner?
Slightly undersized. An oversized unit cools the air quickly, shuts off before it has dehumidified, and leaves the room cold and damp while short-cycling the compressor. Running longer at lower capacity is both more comfortable and more efficient.
▸Why does the climate zone not change the cooling figure?
Because the 20 BTU/h per ft² rule ENERGY STAR's guidance is built on is a single nationwide number for room air conditioners. Heating loads vary far more with climate, so the zone drives the heating band instead. In the hottest zone treat the cooling figure as a floor.
▸Do I still need a Manual J calculation?
For anything you are installing, yes. Manual J measures the actual walls, windows, infiltration and ducts and typically comes in well below rule-of-thumb sizing. Use this page to check whether a quote is plausible.
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