BTU Calculator

Estimate the cooling load for a single room. Enter area, ceiling height, insulation quality, climate zone, window area, sun exposure, and occupants. Returns BTU/h, equivalent tons, and BTU per square foot.

1 ton of cooling

12,000BTU/h

Per occupant load

600BTU/h

Per sq ft window

50BTU/h

Zone 4 base load

26BTU/sf

Units Switches inputs and outputs

Room area (sq ft) Finished floor area of the conditioned space

Ceiling height (ft) Standard 8 ft baseline; add 2% per foot over 8

Insulation quality Wall + ceiling assembly R-value

IECC climate zone See IECC zone map

Window area (sq ft) Total glazing facing the sun

Sun exposure Dominant glazing orientation

Occupants Typical daytime occupancy, 600 BTU/h each

Cooling load

16,200BTU/h

Sized for Zone 4, average insulation

Equivalent tons

1.35tons

BTU per sq ft

32.4BTU/sf

Sensible (estimate)

12,150BTU/h

Metric

4.75kW

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Tip Round up to the next standard capacity (e.g., 1.35 tons → 1.5 ton unit), but do not jump two sizes. Oversized AC short-cycles and leaves the room humid.

Where the per-sqft constants come from The 22–28 BTU/sf base loads are not magic numbers. ACCA Manual J cooling load decomposes into U·A·ΔT envelope conduction through walls/ceiling/glass + solar gain through fenestration + internal gain from people, lights, and appliances (sensible) plus moisture removal (latent). For a typical single-zone room with R-13 to R-19 walls, 8 ft ceiling, and a 10–15% window-to-floor ratio, that full expression collapses into a BTU-per-sqft heuristic that varies almost entirely with outdoor design temperature — i.e. climate zone. So the zone-4 = 26 BTU/sf, zone-2 = 28 BTU/sf, zone-6 = 22 BTU/sf rules of thumb are simplified forms of the same Manual J decomposition. The window (50 BTU/sf) and occupant (600 BTU/h) line items add the load components most sensitive to the inputs a homeowner can observe directly.

Valid range Good to ±15% for 200–1,500 sq ft single-zone, balanced glazing, ducted central system. For multi-zone homes, areas >2,000 sq ft, vaulted ceilings, oversized west glazing, slab-on-grade in heating-dominant climates, or unusual orientations, run a full ACCA Manual J 8th edition — this tool is for screening and quick estimates, not load letters submitted to a permitting authority.

Worked example

500 sq ft bedroom in IECC Zone 4 (Nashville) with average insulation, 8 ft ceiling, 40 sq ft of east-facing windows, and 2 occupants.

1. Base load from area × zone base = 500 × 26 = 13,000 BTU/h 2. Adjust for insulation and sun × 1.0 (average) × 1.0 (medium sun) = 13,000 BTU/h 3. Ceiling adjustment (8 ft baseline) × 1.00 (no change) = 13,000 BTU/h 4. Window + occupant gain windows: 40 × 50 = 2,000 BTU/h people: 2 × 600 = 1,200 BTU/h 5. Total cooling load Q = 16,200 BTU/h ≈ 1.35 tons (32.4 BTU/sf)

BTU reference by room type and climate

Room	Area (sf)	Zone 2 hot	Zone 4 mixed	Zone 6 cold
Small bedroom	150	4,500	4,000	3,400
Master bedroom	300	9,000	8,000	6,800
Living room	500	16,000	13,500	11,500
Open great room	800	25,000	21,500	18,000
Garage (conditioned)	400	14,500	12,000	10,000
Kitchen (high load)	200	8,500	7,500	6,500
Attic bonus room	350	13,500	11,000	9,500

Values assume average insulation, 8 ft ceilings, 2 occupants, and 10–15% window-to-floor ratio. Use the calculator above for your exact inputs.

Common mistakes

Using area alone. A 500 sq ft sunroom in Phoenix and a 500 sq ft basement in Minneapolis need different BTU. Climate and insulation change the answer by 30–50%.
Oversizing "to be safe." A 3-ton unit in a 2-ton space cools fast, stops, and leaves humidity. The room feels cold and clammy.
Ignoring the kitchen. A stove adds 1,200 BTU/h when running. Open kitchens in the conditioned zone need a sensible-load bump.
Skipping Manual J for whole-house jobs. This tool is a single-room estimator. Permits and HVAC equipment selection use Manual J.

Warn Rule-of-thumb BTU sizing is not a substitute for ACCA Manual J on installed systems. Underwriters, utility rebates, and code officials require Manual J output. Use this calculator for scoping and sanity checks.

FAQ

How many BTUs do I need per square foot?

The rule of thumb is 20 BTU per sq ft in cool climates (Zone 5–6) and up to 30 BTU per sq ft in hot climates (Zone 1–2). Adjust for insulation quality, ceiling height above 8 ft, sun exposure, windows, and occupants as the calculator does above.

What size AC do I need for a 500 sq ft room?

A 500 sq ft room in IECC Zone 4 with average insulation needs about 14,000–16,000 BTU/h, which is just over 1 ton. The same room in hot humid Zone 2 climbs to about 18,000 BTU/h; in cool Zone 6 it drops closer to 11,000 BTU/h.

Is bigger always better?

No. An oversized AC short-cycles: it cools the air quickly but does not run long enough to remove humidity. The result is a cold, clammy room with poor dehumidification and higher utility bills. Manual J sizing keeps latent and sensible load in balance.

Does this replace Manual J?

No. This calculator is a single-room scoping estimator. ACCA Manual J is the ANSI-standard whole-house load calculation used for permits, utility rebates, and equipment selection. Use our Manual J calculator for that.

What is a ton of cooling?

One ton of cooling equals 12,000 BTU/h. The unit comes from the amount of heat needed to melt one ton of ice in 24 hours — a relic from pre-mechanical ice-based cooling.

Sources

ACCA Manual J 8th Edition — Residential Load Calculation ASHRAE Handbook — Fundamentals, Ch. 17 (2021) DOE Energy Saver — central AC sizing guide IECC Climate zone definitions

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