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BTU Calculator for Heating & Cooling

A 400 ft² very sunny room with three occupants in climate zone 4 needs about 10,500 BTU/h of cooling — 0.88 tons, so a 1.0-ton unit. That starts from the ENERGY STAR table figure of 9,000 BTU and adjusts for sun and occupancy. This is a screening estimate, not an ACCA Manual J load calculation. Table verified 2026-08-04.

Reference only

Independent, third-party reference tool — not affiliated with, endorsed by, or operated on behalf of any manufacturer, retailer, standards body or building-code authority. Every result is a researched estimate for planning, not a final verdict. Full disclaimer.

Your measurements

ft
ft
ft
Tables assume 8 ft
Sun exposure
ENERGY STAR: ±10% capacity
+600 BTU each above two

Results update as you type. The button is there for touch keyboards that hide the page while you enter numbers.

Results

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BTU Calculator — materials list

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Prepared with MaterialMath — https://estimatormath.com. Quantities are estimates based on the dimensions entered and the published figures cited on the tool page (verified 2026-08-04). Verify against your own measurements and your supplier's actual pack sizes before ordering. Not professional engineering, construction or financial advice.

How is BTU capacity estimated?

Cooling capacity starts from a published table rather than a formula, because the relationship between floor area and load is not linear — a big room has proportionally less envelope than a small one. The adjustments that follow are the four the EPA publishes alongside that table, plus two this calculator adds for ceiling height and climate.

# Start from the ENERGY STAR table (area → BTU) base = lookup(floor area) # You condition AIR, not floor — the table assumes 8 ft × height = ceiling height ÷ 8 ft # The four published ENERGY STAR adjustments + sun = ±10% for very sunny or heavily shaded + people = 600 BTU per occupant above two + kitchen = 4,000 BTU # Then climate and envelope × zone = 0.85 (zone 7) to 1.15 (zone 1) × envelope = 0.65 (deep retrofit) to 1.30 (pre-1980) # Heating is a much rougher figure heating = area × BTU/ft² for the zone × envelope # One ton = 12,000 BTU/h

Note what the tool deliberately does not do: it does not round the answer up to the next thousand BTU. Rounding up is how a 7,700 BTU/h room quietly becomes a 9,000 BTU/h purchase, and oversizing is the failure mode everything else on this page warns about.

A worked example: a sunny 20 × 20 ft living room

400 ft², 8 ft ceiling, large west-facing windows, three people, climate zone 4, average insulation.

Step by step

Base from the ENERGY STAR table (400 ft²)
9,000 BTU/h
Ceiling height factor
8 ÷ 8 = 1.00 — no change
Very sunny room
+900 BTU/h
1 occupant above two
+600 BTU/h
Zone 4 multiplier
× 1.00
Average envelope
× 1.00
Cooling capacity
10,500 BTU/h = 0.88 tons
Practical equipment size
1.0 tons
Heating estimate
16,000 BTU/h output

One detail on the heating figure: furnaces are rated on input, not output. At 95% AFUE you need roughly 17,000 BTU/h of input to deliver 16,000 of output. At 80% AFUE it is 20,000. Comparing a nameplate input figure against a calculated output figure is a common and expensive mistake.

The ENERGY STAR sizing table

Floor area up to Capacity Tons Implied BTU/ft²
150 ft² 5,000 BTU/h 0.42 33.3
250 ft² 6,000 BTU/h 0.50 24.0
300 ft² 7,000 BTU/h 0.58 23.3
350 ft² 8,000 BTU/h 0.67 22.9
400 ft² 9,000 BTU/h 0.75 22.5
450 ft² 10,000 BTU/h 0.83 22.2
550 ft² 12,000 BTU/h 1.00 21.8
700 ft² 14,000 BTU/h 1.17 20.0
1,000 ft² 18,000 BTU/h 1.50 18.0
1,200 ft² 21,000 BTU/h 1.75 17.5
1,400 ft² 23,000 BTU/h 1.92 16.4
1,500 ft² 24,000 BTU/h 2.00 16.0
2,000 ft² 30,000 BTU/h 2.50 15.0
2,500 ft² 34,000 BTU/h 2.83 13.6

The right-hand column is why per-square-foot rules of thumb fail: the implied rate more than halves across the table. Adjustments per EPA ENERGY STAR guidance — +10% very sunny, −10% heavily shaded, +600 BTU per occupant above two, +4,000 BTU for a kitchen.

Who this calculator is for

  • Anyone buying a window or portable AC, where the decision is which box to take off the shelf and a Manual J would be absurd overkill.
  • People sizing a mini-split for a single room, a converted garage or an addition.
  • Homeowners sanity-checking a contractor's proposal. If a contractor proposes five tons for a 1,600 ft² house without doing a load calculation, that is worth questioning — and this gives you the ballpark to question it with.
  • Anyone weighing insulation against equipment. Change the envelope quality and watch the tonnage move.

What this calculator does not account for

This is a screening estimate. The list below is exactly what separates it from a real load calculation, and each item routinely moves the answer by more than 10%.

  • Windows. Area, orientation, U-factor and solar heat gain coefficient. A wall of west-facing single glazing and a wall of north-facing triple glazing are not the same room, and the "very sunny" checkbox is a crude stand-in for a real calculation.
  • Air leakage. Measured by blower door, in air changes per hour. Infiltration can be a third of the load in an old house and nearly nothing in a new one.
  • Ductwork. Ducts in an unconditioned attic can lose 20–30% of the capacity before it reaches a room. Duct sizing and static pressure are also why a correctly sized replacement sometimes still underperforms — the old ducts cannot carry the airflow.
  • Latent versus sensible load. Humidity removal is a separate capacity from temperature reduction, and in humid climates it is the one that matters. This gives a single combined figure.
  • Zoning and multi-storey. One correctly sized unit for a two-storey house typically overcools the ground floor while the upper floor stays warm.
  • Internal gains. Appliances, electronics and lighting all add heat. A home office with three monitors is a meaningful load.

Heating factors by climate zone

Zone Heating BTU/ft² Cooling multiplier
Zone 1 — south Florida, Hawaii 25 × 1.15
Zone 2 — Gulf Coast, Phoenix, Orlando 30 × 1.1
Zone 3 — Atlanta, Dallas, Los Angeles 35 × 1.05
Zone 4 — Washington DC, St Louis, Portland 40 × 1
Zone 5 — Chicago, Denver, Boston 45 × 0.95
Zone 6 — Minneapolis, Burlington 50 × 0.9
Zone 7 — northern Minnesota, Maine 55 × 0.85

Heating factors are rules of thumb indexed to IECC climate zones and are a much rougher figure than the cooling table. Cooling multipliers run the other way from heating — hotter zones need more cooling and less heat.

Common questions

How many BTU do I need per square foot?

There is no single number, which is why the ENERGY STAR table this calculator uses is indexed by area band rather than by a per-square-foot rate. Across the table the implied rate runs from about 33 BTU/ft² at 150 ft² down to about 14 BTU/ft² at 2,500 ft², because larger spaces have less envelope per unit of floor.

Anyone quoting you a flat "20 BTU per square foot" is using a rule of thumb that is wrong at both ends of the range. Use the table, then adjust for sun, occupancy, ceiling height and climate.

Is it better to oversize or undersize an air conditioner?

Undersize, if you must err. This is counterintuitive and it is the most important thing on this page.

An oversized air conditioner cools the air to the thermostat setpoint quickly and shuts off — before it has run long enough to pull moisture out. The result is a house that is cold and clammy at the same time, which feels worse than a house that is slightly warm and dry. It also short-cycles the compressor, which is the fastest way to kill it. A slightly undersized unit runs longer, dehumidifies properly, and simply takes a bit more time on the hottest afternoons.

What is a Manual J load calculation and do I need one?

Manual J is the ACCA procedure for calculating a building's actual heating and cooling load. It accounts for window area, glazing U-factor and solar heat gain coefficient, orientation, measured air leakage, duct location and duct losses, and internal gains — none of which this calculator knows about.

You need one before buying equipment. The figure here is a screening estimate to tell you roughly what size class you are in and whether a contractor's proposal is in the right ballpark. Those two figures can differ by 30% in either direction, and on a well-sealed modern house the Manual J number is usually much lower.

How do I convert BTU to tons?

Divide by 12,000. One ton of cooling is 12,000 BTU per hour — the rate at which melting one ton of ice over 24 hours absorbs heat, which is where the unit comes from.

So 10,500 BTU/h is 0.88 tons. Residential equipment is sold in half-ton steps, so the practical choice is 1.0 tons.

Does ceiling height affect BTU requirements?

Yes, and the standard tables ignore it. The ENERGY STAR chart assumes an 8 ft ceiling, because you condition air by volume rather than floor by area.

A room with a 12 ft ceiling has 50% more air in it than the same floor plan at 8 ft, and this calculator scales capacity by that volume ratio. It also has more wall area losing heat, so if anything the scaling is conservative.

Should I insulate first or buy a bigger unit?

Insulate and air-seal first, almost always. On a leaky, poorly insulated house the envelope work usually costs less than the extra half-ton of equipment it saves you from buying — and unlike the equipment, it keeps paying every month afterwards.

Switch the envelope quality from "poor" to "good" in this calculator and watch the required capacity drop. That difference is a real purchasing decision. The insulation calculator will tell you what your climate zone now requires.

Sources for the figures used

Every rate, coverage figure and code limit in this calculator comes from a published source. Where a figure is a convention rather than a standard, it says so.

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