BTU Calculator: Size a Furnace or Air Conditioner

BTU Calculator

This BTU calculator sizes heating and cooling equipment from your floor area, in both directions. For cooling, it starts from the standard planning rate of 20 BTU per square foot, one ton of cooling (12,000 BTU) per 600 sq ft, then adjusts for ceiling height, insulation, sun exposure and climate, and reports the size as a range (±10%). For heating, it applies the furnace rule of 30–60 BTU of input per square foot, a base rate of 35 in a mild climate, 45 in a temperate one, 55 in a very cold one, adjusted for insulation and held inside that published 30–60 band. The same calculator is embedded in our Furnace Replacement Cost and AC Replacement Cost guides; this page is where the sizing maths behind both guides lives, in full.

The formula, in full

Cooling mode

BTU needed = Area (sq ft) × 20 × (Ceiling height ÷ 8) × Insulation factor × Sun factor × Climate factor

  • Area, the floor area you want to cool, in square feet.
  • 20, the base rate: 20 BTU per square foot at standard conditions (8 ft ceilings, average insulation, average sun, temperate climate). In tons, that is one ton (12,000 BTU) per 600 sq ft.
  • Ceiling height ÷ 8, cooling load scales with the volume of air, so the calculator scales the result by your ceiling height against the standard 8 ft: 7 ft ceilings multiply by 0.875, 9 ft by 1.125, 10 ft by 1.25.
  • Insulation factor, Poor ×1.15 · Average ×1.00 · Good ×0.90. A poorly insulated home leaks cooled air and needs a bigger unit to hold temperature.
  • Sun factor, Shady ×0.95 · Average ×1.00 · Sunny ×1.10. A sun-baked home gains heat through glass and roof all afternoon.
  • Climate factor, Mild ×0.85 · Temperate ×1.00 · Hot ×1.20. At the hot setting the effective rate is 24 BTU per sq ft, one ton per 500 sq ft, the hot-climate end of the rule of thumb in our AC guide. (The climate menu’s labels follow the mode you pick: in cooling they read Mild / Temperate / Hot, as here; in heating they read Mild (warm winters) / Temperate / Very cold, matching the base rates below.)

The calculator then reports a range: the result ×0.9 and ×1.1, rounded to the nearest 100 BTU, plus the tonnage (result ÷ 12,000, to one decimal place).

Heating mode

Furnace size (input BTU) = Area (sq ft) × Rate, where Rate = Climate base × Insulation factor, held between 30 and 60

  • Climate base, the climate answer sets the base rate: Mild 35 · Temperate 45 · Very cold 55 BTU of input per square foot, the furnace guide’s 30–60 BTU-per-sq-ft rule, coldest climates at the top.
  • Insulation factor, Poor ×1.15 · Average ×1.00 · Good ×0.90, applied to the base rate.
  • The clamp, after the insulation adjustment, the rate is held inside the published 30–60 band: a very cold climate with poor insulation computes 55 × 1.15 = 63.25 and is reported at 60; a mild climate with good insulation computes 35 × 0.90 = 31.5 and stays 31.5.

The result is reported as a range (±10%, rounded to the nearest 100) in input BTU, the rating furnaces are sold by. What your home actually receives is input × AFUE: a 100,000 BTU furnace at 80% AFUE delivers 80,000 BTU of heat; at 96% AFUE it delivers 96,000, which is why a high-efficiency replacement is sometimes correctly sized smaller than the unit it replaces.

Two honest notes about heating mode: the sun-exposure answer does not change a heating result (winter sizing turns on climate and insulation, not afternoon sun), and neither does the ceiling-height answer, the furnace rule of thumb is a floor-area rule, and the calculator applies the ceiling factor in cooling mode only. Both are stated here because this page is the specification of the tool, not just its advertisement.

The numbers underneath: rates and results tables

Cooling at standard settings (8 ft ceilings, average insulation, average sun, temperate climate)

Area to cool Base load (area × 20) Calculator’s reported range Tonnage shown
600 sq ft 12,000 BTU 10,800–13,200 BTU about 1.0 tons
1,000 sq ft 20,000 BTU 18,000–22,000 BTU about 1.7 tons
1,300 sq ft 26,000 BTU 23,400–28,600 BTU about 2.2 tons
1,600 sq ft 32,000 BTU 28,800–35,200 BTU about 2.7 tons
2,000 sq ft 40,000 BTU 36,000–44,000 BTU about 3.3 tons
2,300 sq ft 46,000 BTU 41,400–50,600 BTU about 3.8 tons
2,700 sq ft 54,000 BTU 48,600–59,400 BTU about 4.5 tons
3,200 sq ft 64,000 BTU 57,600–70,400 BTU about 5.3 tons

Computed with the cooling formula above at standard settings; every row is exactly what the calculator returns for those answers.

One presentation difference to understand: our AC guide pairs home sizes with nominal equipment sizes, 1.5 ton for 600–1,000 sq ft, 2 ton for 1,000–1,300, 3 ton for 1,600–2,000, 4 ton for 2,200–2,700, 5 ton above that. The calculator reports the load itself, so its tonnage is a decimal, 1,000 sq ft computes to 20,000 BTU ≈ 1.7 tons, and real equipment is then chosen at the nominal size that covers it (a 2-ton unit), confirmed by the installer’s Manual J. The two presentations come from the same 20 BTU-per-sq-ft rule; neither is a different answer.

Adjustment factors (cooling)

Answer Factor Effect on a 1,000 sq ft home (base 20,000 BTU)
Ceiling 7 ft / 9 ft / 10 ft ×0.875 / ×1.125 / ×1.25 17,500 / 22,500 / 25,000 BTU before other factors
Insulation: Poor / Average / Good ×1.15 / ×1.00 / ×0.90 23,000 / 20,000 / 18,000 BTU
Sun: Shady / Average / Sunny ×0.95 / ×1.00 / ×1.10 19,000 / 20,000 / 22,000 BTU
Climate: Mild / Temperate / Hot ×0.85 / ×1.00 / ×1.20 17,000 / 20,000 / 24,000 BTU

Heating rates (BTU of input per sq ft, after insulation adjustment and the 30–60 clamp)

Climate base Poor insulation (×1.15) Average (×1.00) Good (×0.90) Input per 1,000 sq ft at the average rate
Mild, 35 40.25 35 31.5 35,000 BTU
Temperate, 45 51.75 45 40.5 45,000 BTU
Very cold, 55 60 (clamped from 63.25) 55 49.5 55,000 BTU

Rates from the furnace guide’s published 30–60 BTU-per-sq-ft rule (checked October 2026); the calculator’s output for a given home is the rate × area, reported ±10%.

Worked example: cooling a 1,800 sq ft home

Answers: 1,800 sq ft · 9 ft ceilings · average insulation · sunny exposure · temperate climate.

  1. Base load: 1,800 × 20 = 36,000 BTU.
  2. Ceiling adjustment: 36,000 × (9 ÷ 8) = 36,000 × 1.125 = 40,500 BTU.
  3. Insulation: average, ×1.00 → 40,500 BTU (unchanged).
  4. Sun: sunny, ×1.10 → 44,550 BTU.
  5. Climate: temperate, ×1.00 → 44,550 BTU, the central estimate.
  6. Reported range (±10%, rounded to the nearest 100): 40,100–49,000 BTU.
  7. Tonnage: 44,550 ÷ 12,000 = about 3.7 tons, in practice a 3.5- or 4-ton nominal unit, with the Manual J making the final call.

That is exactly what the calculator returns for these answers. The same home in heating mode (temperate climate, average insulation) runs at the 45 rate: 1,800 × 45 = 81,000 BTU of input, reported as 72,900–89,100 BTU, a standard 80,000 BTU furnace class.

FAQ

How many BTU do I need per square foot?
For cooling, the planning rate is 20 BTU per square foot at standard conditions, rising to an effective 24 in a hot climate (the calculator’s ×1.20 climate factor), which is one ton per 500 sq ft instead of one per 600. For heating, the furnace rule is 30–60 BTU of input per square foot: base rates of 35 (mild), 45 (temperate) and 55 (very cold), adjusted for insulation and held inside the 30–60 band. Both rules are the ones our furnace and AC guides publish (checked October 2026).

How many square feet does a 3-ton air conditioner cool?
A 3-ton unit delivers 36,000 BTU, which at the standard 20 BTU per sq ft covers about 1,800 sq ft; in a hot climate, at 24 BTU per sq ft, about 1,500 sq ft. Our AC guide’s nominal pairing puts 3-ton systems in 1,600–2,000 sq ft homes (standard efficiency, existing ductwork, AC replacement typically $5,000–$9,200 installed in that band, costs checked October 2026). Ceiling height, insulation and sun move the answer inside those spans, which is what the calculator’s factors are for.

Is a bigger furnace or air conditioner better?
No, matched is better. An oversized AC cools the air fast and shuts off before removing humidity, leaving the house cool but clammy and cycling wear onto the compressor; an undersized one runs constantly on the hottest days and wears out early. The calculator gives the size class to discuss; the deciding step before you buy is a Manual J load calculation by an installer, which accounts for your windows, layout and duct condition. The rule-of-thumb range on this page is a planning estimate, not a substitute for that calculation.

What is the difference between input and output BTU on a furnace?
Input is the fuel the furnace burns; output is the heat your home gets, input × AFUE. A 100,000 BTU furnace delivers 80,000 BTU at 80% AFUE and 96,000 BTU at 96% AFUE. The calculator’s heating mode reports input BTU, because that is the rating furnaces are sold and sized by (our furnace guide’s 30–60 BTU-per-sq-ft rule is an input rule).

Why does the calculator report a range instead of one size?
Because a rule of thumb is honest only as a band. The calculator reports its central estimate ±10% (rounded to the nearest 100 BTU) to mark the planning uncertainty in any area-based method: two homes with identical floor plans can land a nominal size apart once windows, ducts and layout are counted. Treat the range as the size class, typically one or two nominal equipment sizes, and let the Manual J settle which one.

Does ceiling height change the result?
In cooling mode, yes: the load scales by ceiling height ÷ 8, so 10 ft ceilings add 25% against the standard 8 ft. In heating mode, no: the furnace rule of thumb is a floor-area rule (30–60 BTU of input per sq ft), and the calculator does not apply the ceiling factor there. If your home has very tall ceilings and a heating decision turns on it, that is one more reason the installer’s load calculation, not any online calculator, makes the final call.

How this estimate works

This page’s method is the planning rule of thumb the HVAC trade itself starts from, 20 BTU per sq ft for cooling (one ton per 600 sq ft, one per 500 in hot climates) and 30–60 BTU of input per sq ft for heating, with the standard adjustments for ceiling height, insulation, sun and climate shown as explicit factors above, so any result on this page can be reproduced by hand. It is a planning estimate, not a quote and not a load calculation: a Manual J by an installer is the deciding step before any equipment is bought. The rates and factors are re-checked quarterly with the cost guides that rely on them. Full method: How We Estimate.

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