▸ Worked examples (hand-verified)
Three real scenarios with the full math shown, each independently verified against the tool's engine.
1 · 2,000 sq ft home, zone 5, average insulation
Heat loss = 2,000 × 40 (zone 5 factor) × 1.00 (average) × 8/8 × 1.00 = 80,000 BTU/hr. Recommended output = 80,000 × 1.15 = 92,000 BTU/hr = 26.96 kW = 2.75 boiler hp. Standard gas (82%): input = 92,000 ÷ 0.82 = 112,195 BTU/hr.
→ Shop for ≈112,200 BTU/hr input on standard gas, or a 92,000 BTU/hr (≈27 kW) electric boiler.
2 · 1,200 sq ft home, zone 3, good insulation
Heat loss = 1,200 × 30 (zone 3) × 0.80 (good) = 28,800 BTU/hr. Recommended output = 28,800 × 1.15 = 33,120 BTU/hr = 9.71 kW = 0.99 boiler hp. Oil (85%): input = 33,120 ÷ 0.85 = 38,965 BTU/hr.
→ A small ~39,000 BTU/hr-input oil boiler (or ~9.7 kW electric) covers it.
3 · 3,500 sq ft home, zone 7, poor insulation, 9 ft ceilings, large windows
Heat loss = 3,500 × 50 × 1.25 × 9/8 × 1.08 = 265,781 BTU/hr. Recommended output = 265,781 × 1.15 = 305,648 BTU/hr = 89.58 kW = 9.13 boiler hp.
→ A large boiler around 306,000 BTU/hr output. Note how insulation alone swings this by tens of thousands of BTU/hr — upgrading insulation first can shrink the boiler you need.
▸ Methodology — the exact math
Heat loss
heat loss = area × zone factor × insulation × (ceiling ÷ 8) × windows
Zone factors run 20 BTU/hr per sq ft in zone 1 up to 55 in zone 8 (see the reference table above), for average insulation and 8 ft ceilings. Insulation multiplies by 1.25 (poor), 1.00 (average) or 0.80 (good); window coverage multiplies by 0.95, 1.00 or 1.08.
Recommended boiler output
recommended output = heat loss × 1.15 — the standard piping and pickup allowance for hydronic (hot-water) systems, so the boiler can bring cold pipes and radiators up to temperature.
Fuel adjustment and conversions
rated input = output ÷ efficiency (gas condensing 0.95, standard gas 0.82, older gas 0.75, oil 0.85, electric 1.00). kW = BTU/hr ÷ 3,412.142. boiler hp = BTU/hr ÷ 33,475 (output basis, the standard definition).
Oversize check
ratio = existing input × efficiency ÷ heat loss. Below 90% is undersized, 90–100% is a tight fit, 100–125% is a good fit, and above 125% is flagged oversized (short-cycling risk).
Honest limitations
- Not a Manual J. This is a whole-house screening estimate; a professional room-by-room Manual J is the standard for final sizing.
- Hot-water systems. The 1.15 pickup factor suits hydronic heating; steam systems traditionally use 1.33 (see FAQ).
- Space heating only. Domestic hot water (combi boilers, indirect cylinders) is a separate requirement — see FAQ.
- Climate zones are US DOE. Outside the US, pick the zone whose winter best matches your area.
- No infiltration modeling. Very leaky old houses may exceed the "poor insulation" multiplier — treat the result as a floor, not a ceiling.
▸ Frequently asked questions
What size boiler do I need for a 2,000 sq ft house?
A 2,000 sq ft home in climate zone 5 (e.g. Chicago) with average insulation and 8 ft ceilings has a design heat loss of about 80,000 BTU/hr (2,000 x 40 BTU per sq ft). With the standard 1.15 piping and pickup allowance, that means a boiler rated for 92,000 BTU/hr of output — roughly 27.0 kW or 2.75 boiler horsepower. If you buy a standard gas boiler at 82% efficiency, look for about 112,200 BTU/hr of input rating (92,000 / 0.82). Enter your own numbers in the calculator above for an exact size.
How many BTUs per square foot do I need for a boiler?
It depends on your climate zone. This tool uses 20 BTU/hr per sq ft in zone 1 (hot, e.g. Miami), 25 in zone 2, 30 in zone 3, 35 in zone 4, 40 in zone 5 (e.g. Chicago), 45 in zone 6, 50 in zone 7, and 55 in zone 8 (subarctic, e.g. Fairbanks) — for average insulation and standard 8 ft ceilings. Poor insulation multiplies the result by 1.25 and good insulation by 0.80, and taller ceilings or lots of glass add more. Example: 2,000 sq ft x 40 = 80,000 BTU/hr heat loss in zone 5.
What is boiler horsepower, and how do I convert BTU to boiler hp?
One boiler horsepower is defined as 33,475 BTU/hr of heat output — it is not the same as mechanical horsepower. Divide the boiler's output rating by 33,475: a 92,000 BTU/hr boiler is 92,000 / 33,475 = 2.75 boiler hp, and a large 305,648 BTU/hr unit is 9.13 boiler hp. Residential boilers are usually sold by BTU/hr input, so this conversion matters most when comparing commercial or steam boiler specs.
How do I convert boiler size from BTU/hr to kW?
Divide BTU/hr by 3,412.142. A 92,000 BTU/hr boiler is 26.96 kW of output, and a 305,648 BTU/hr boiler is 89.58 kW. Electric boilers are often rated in kW directly — a 27 kW electric boiler delivers about 92,000 BTU/hr (27 x 3,412.142 = 92,128). Note that gas and oil boilers are usually rated by input, so divide by efficiency first: the 92,000 BTU/hr of output from a standard 82%-efficient gas boiler needs 112,195 BTU/hr of input.
Is it bad to oversize a boiler? What is short cycling?
Yes — oversizing is the most common boiler mistake. A boiler much larger than the heat loss satisfies the thermostat in minutes, shuts off, then fires again minutes later: this is short cycling. It wastes fuel (a cold boiler is least efficient at startup), wears burners and controls, and can leave rooms with uneven heat. A safe rule: the boiler's effective output should not exceed 125% of the design heat loss. Example: a 150,000 BTU/hr-input gas boiler at 82% efficiency delivers 123,000 BTU/hr of output — on an 80,000 BTU/hr house that is 154% of the load, clearly oversized. This tool's oversize checker flags anything above 125%.
Does the fuel type (gas, oil, electric) change the boiler size I need?
The heat load of the house never changes with fuel — only the input rating you shop for does, because efficiency differs. For the same 92,000 BTU/hr of required output: a condensing gas boiler (95% efficient) needs 96,842 BTU/hr of input, a standard gas boiler (82%) needs 112,195 BTU/hr, an oil boiler (85%) needs 108,235 BTU/hr, and an electric boiler (100%) needs exactly 92,000 BTU/hr. Always compare boilers by output (or by input divided by efficiency), never by input rating alone.
What climate zone am I in for boiler sizing?
This calculator uses the eight US DOE climate zones: zone 1 is hot (Miami, Honolulu), zone 2 is hot (Houston, Phoenix), zone 3 is warm (Atlanta, Dallas), zone 4 is mixed (Washington DC, Seattle, St. Louis), zone 5 is cool (Chicago, New York, Denver), zone 6 is cold (Minneapolis, Burlington), zone 7 is very cold (Duluth, Fargo), and zone 8 is subarctic (Fairbanks). Outside the US, pick the zone whose winter matches your area — southern England is roughly zone 4, Toronto is roughly zone 6.
How much does insulation change the boiler size?
Dramatically. Take the same 2,000 sq ft zone 5 house: average insulation gives an 80,000 BTU/hr heat loss, poor insulation (drafty, uninsulated walls or attic) multiplies that by 1.25 to 100,000 BTU/hr, and good insulation (well sealed, modern windows) multiplies by 0.80 to 64,000 BTU/hr. That is the difference between sizing for 115,000 BTU/hr of output versus 73,600 BTU/hr of output — at 82% gas efficiency, roughly 140,200 versus 89,800 BTU/hr of input rating — often a bigger swing than the difference between two climate zones.
Do high ceilings mean I need a bigger boiler?
Yes, because there is more air volume to heat. The adjustment is ceiling height divided by 8 ft. A house with 10 ft ceilings needs 25% more capacity than the same footprint with 8 ft ceilings: the 2,000 sq ft zone 5 example rises from 80,000 to 100,000 BTU/hr of heat loss. Vaulted or cathedral ceilings count at their average height.
Do I need a bigger boiler if it also heats my domestic hot water?
It depends on the setup. A combi (combination) boiler is usually sized by the hot-water flow rate, not the heating load — heating a shower at 2.5 gallons per minute needs far more instantaneous power than heating the house, so combis are often 'oversized' for space heating by design. With an indirect water-heater cylinder fed by the boiler, add roughly 30,000-40,000 BTU/hr to the heating load or follow the cylinder manufacturer's recovery requirement. This calculator sizes space heating only; treat domestic hot water as a separate requirement.
Should I just replace my boiler with the same size as the old one?
Usually not. Old boilers were routinely oversized — installers added generous safety margins, and the house has often gained insulation, new windows, or an extension since. Measure your actual heat loss instead: if your old 150,000 BTU/hr-input boiler (about 123,000 BTU/hr output at 82% efficiency) served a house that only loses 80,000 BTU/hr, it was running at 154% of the load its whole life. Sizing from the heat loss, like this tool does, typically lets you buy a smaller, cheaper, more efficient boiler.
Is steam boiler sizing different from hot water boiler sizing?
Yes. Steam systems need a larger pickup factor because the boiler must first heat the water to boiling and fill the pipes with steam — installers traditionally use 1.33 times the connected radiator load for steam, versus 1.15 for hot water. This calculator models hot-water (hydronic) systems with the 1.15 factor. For steam, multiply this tool's heat-loss result by 1.33 instead, and confirm the size against your radiators' total EDR (equivalent direct radiation).
Is this the same as a Manual J heat-loss calculation?
No. A Manual J (ACCA) calculation measures every wall, window, door, and infiltration path room by room — it is the professional standard and what a contractor should do before installing. This tool is a screening method: it estimates whole-house heat loss from floor area, climate zone, insulation, ceiling height, and windows. It is accurate enough to sanity-check a quote or compare bids (typically within about 15-25% of a Manual J), but it is not a substitute for one on a new build or a major system change.
Does this calculator send my data anywhere?
No. It runs entirely in your browser — every calculation happens in JavaScript on this page, with zero network requests. There is no account, no analytics payload carrying your inputs, and nothing to intercept. You can verify this yourself: disconnect from the internet after the page loads and the calculator keeps working.