Boiler Size Calculator

This boiler size calculator finds a home's hydronic heat loss in BTU per hour, matches it to a standard boiler input and output, and sizes the baseboard or radiators.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size a boiler, multiply the heated square footage by a climate heat loss factor of 25 to 65 BTU per square foot and adjust for insulation. Then divide by the seasonal efficiency the emitters allow, which gives the input rating boilers are labeled by. A 2,000 sq ft house in IECC zone 5 with average insulation loses 100,000 BTU per hour, which takes a 120,000 BTU input boiler and 176 ft of fin-tube baseboard.

Your measurements

A rectangular volume: length times width times depth.

Heated floor areasq ftOnly the rooms that have an emitter in them. A basement with two loops of baseboard counts; an unheated attic or an attached garage does not.Climate zoneThe 2021 IECC map. These are rule-of-thumb heat loss figures for the design day, and they run high on any house built or insulated after about 1990.Insulation and windowsMost boiler houses are old. If the walls are empty balloon frame with storm windows over single glazing, use poor even where the attic has been topped up.Emitter typeThis sets the supply water temperature the system needs, which is what decides whether a condensing boiler ever gets to condense.Domestic hot waterA combi has to make hot water on demand, so the hot water side usually picks the boiler size instead of the heating load.Existing baseboard already installedftMeasure the finned element inside the covers, not the covers. Blank sections at the ends and behind doors put out almost nothing. Enter 0 if there is none yet.

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Results

Design heat loss: 100,000 BTU per hour; 50 BTU per sq ft across 2,000 sq ft in zone 5
Boiler input and output: 116,279 BTU in, 100,000 BTU out; Boilers are labeled by input, and output is input times efficiency. At 180 F supply water expect about 86% seasonal, because the return water never falls low enough for a condensing boiler to condense
Closest standard boiler: 120,000 BTU input; Delivers 103,200 BTU per hour of output at 86% seasonal efficiency, which is 103% of the load
Fin-tube baseboard needed: 176 ft; At 570 BTU per hour per linear foot with 180 F water and 1 gal per minute. That is the ceiling on what the house can absorb, so a boiler bigger than the baseboard is wasted. Warning: the 140 ft already installed delivers only 79,800 BTU per hour at 180 F, so the house cannot absorb the calculated load and the real heat loss is almost certainly lower
Domestic hot water: Priority zone; An indirect tank runs on a priority zone, so the heating zones pause while it recovers and the tank does not add to the boiler size. A 40 gal indirect wants roughly 100,000 BTU per hour of boiler output to handle two showers back to back
Before you buy: Room by room heat loss; Planning estimate only. IRC M1401.3 wants the equipment sized from a load calculation, and a licensed installer pulls the mechanical and gas permits and sets the near-boiler piping

Boiler Size formula

Heat loss = Area (sq ft) x zone factor (25 to 65) x insulation factor (1.25, 1, 0.8)
Boiler input = Heat loss / seasonal efficiency  (0.86 on 180 F water, 0.95 on 110 F radiant)
Combi hot water = GPM x 500 x temperature rise, so 3.5 GPM at a 70 F rise is 122,500 BTU per hour
Baseboard feet = Heat loss / 570 BTU per hour per linear foot at 180 F water

Worked example

A 1,400 sq ft house in IECC zone 4 with empty balloon-frame walls and storm windows, heated by cast iron radiators, with an indirect tank for hot water. The zone 4 factor is 45 BTU per square foot and the poor insulation factor is 1.25, so the heat loss is 1,400 x 45 x 1.25 = 78,750 BTU per hour.

Cast iron runs on 170 F water, so a condensing boiler cannot condense and the realistic seasonal efficiency is about 87 percent. The input is 78,750 / 0.87 = 90,517 BTU per hour, and the next standard size is a 100,000 BTU input boiler delivering 87,000 BTU of output, 110 percent of the load. The emitters have to absorb it: 78,750 / 150 = 525 square feet of radiator EDR at 170 F, or 139 feet of fin-tube baseboard if the radiators are being replaced.

Quick reference

Boiler size and baseboard needed by house area (zone 5, average insulation, 180 F water, 86% seasonal)
House area	Heat loss	Input needed	Standard boiler	Baseboard at 180 F
1,000 sq ft	50,000 BTU	58,140 BTU	70,000 BTU	88 ft
1,200 sq ft	60,000 BTU	69,767 BTU	70,000 BTU	106 ft
1,500 sq ft	75,000 BTU	87,209 BTU	100,000 BTU	132 ft
1,800 sq ft	90,000 BTU	104,651 BTU	120,000 BTU	158 ft
2,000 sq ft	100,000 BTU	116,279 BTU	120,000 BTU	176 ft
2,500 sq ft	125,000 BTU	145,349 BTU	150,000 BTU	220 ft
3,000 sq ft	150,000 BTU	174,419 BTU	175,000 BTU	264 ft

Size from heat loss, not from the old nameplate

Boilers outlive houses. The unit coming out was sized in 1965 for a building with no wall insulation, single glazing and 180 degree water, and it was probably padded on top of that. Insulation, storm windows and air sealing since then have often cut the real load in half. Matching the old input is the single most common way a hydronic job goes wrong, and it is why so many replacement boilers short cycle.

Start with area times a climate factor, adjust for the shell, and treat the answer as an upper bound. Then get the real number a second way: count the emitters. Fin-tube baseboard delivers about 570 BTU per hour per linear foot at 180 degree water. Cast iron gives 150 BTU per hour per square foot of EDR at 170 degrees, and radiant floor gives about 30 BTU per hour per square foot. Whatever the emitters can put into the rooms is all the heat the house can absorb, no matter what the boiler is rated at.

The water temperature sets the efficiency you actually get

A 95 percent AFUE boiler earns that number on a test stand with cool return water. In the house, the emitters decide the water temperature. Fin-tube baseboard was designed around 180 degree supply and about 160 degree return, and flue gas only starts condensing below roughly 130. On 180 degree water a modulating condensing boiler runs closer to 86 percent, which is a real gain over an old atmospheric unit but nowhere near the sticker. Radiant floor at 110 degrees, or panel radiators at 150, keep the return cold enough to condense most of the season.

There is a way to get the low water temperature on existing baseboard: add element. Output per foot falls with temperature, to roughly 300 BTU per hour per foot at 140 degrees, so covering the same room at 140 takes about twice the length. Doubling the baseboard in the two or three coldest rooms and running outdoor reset is what turns a mod-con into a condensing boiler for most of the winter. Oversizing hurts more here than on a furnace, because a boiler also has to heat its own water mass. A big cast iron block that fires for four minutes in October sends most of that heat up the flue.

What this calculator leaves out

This is a whole-house figure, so it cannot balance the zones or tell you which room is short. It does not size the circulator, the expansion tank, the air separator or the zone valves, and it does not model outdoor reset. Not included: near-boiler piping, a low-loss header or primary-secondary loop, the chimney liner or the new PVC vent and its termination clearances. Nor combustion air, the condensate drain and neutralizer, a gas line upsize, the electrical circuit, permits, or the cost of taking a cast iron sectional boiler out of a basement in pieces. Systems with steam radiators are a different calculation entirely and are sized by connected EDR, not by heat loss.

Code limits that apply

IRC M2001.1
Boilers installed to the manufacturer's instructions and the terms of the listing
If you miss it: Near-boiler piping that ignores the manual starves the heat exchanger of flow and voids the warranty on the first fault
IRC M2002.1
A pressure relief valve on every boiler, set at or below the nameplate working pressure
If you miss it: A stuck control with no relief path pressurizes the system until a fitting or the heat exchanger lets go
IRC M2003.1
An expansion tank on every closed hydronic system, sized to the system volume
If you miss it: A waterlogged or missing tank makes the relief valve weep every time the boiler fires, and the fill valve keeps topping it up
IRC M1411.3
Condensate piped to an approved point, sloped and sized so it cannot back up
If you miss it: Condensate off a condensing boiler runs near pH 3 and eats a cast iron stack or a concrete floor without a neutralizer
IRC G2413
Gas pipe sized from the developed length and the total input of every appliance
If you miss it: A 199,000 BTU combi on a line sized for an 80,000 BTU boiler drops pressure and locks out at high fire
IRC M1401.3checked
Equipment sized from a heat loss calculation, not from the old boiler's nameplate
If you miss it: An oversized boiler short cycles through the whole shoulder season, which is where most of the heating hours are

Provision	Limit	If you miss it
IRC M2001.1	Boilers installed to the manufacturer's instructions and the terms of the listing	Near-boiler piping that ignores the manual starves the heat exchanger of flow and voids the warranty on the first fault
IRC M2002.1	A pressure relief valve on every boiler, set at or below the nameplate working pressure	A stuck control with no relief path pressurizes the system until a fitting or the heat exchanger lets go
IRC M2003.1	An expansion tank on every closed hydronic system, sized to the system volume	A waterlogged or missing tank makes the relief valve weep every time the boiler fires, and the fill valve keeps topping it up
IRC M1411.3	Condensate piped to an approved point, sloped and sized so it cannot back up	Condensate off a condensing boiler runs near pH 3 and eats a cast iron stack or a concrete floor without a neutralizer
IRC G2413	Gas pipe sized from the developed length and the total input of every appliance	A 199,000 BTU combi on a line sized for an 80,000 BTU boiler drops pressure and locks out at high fire
IRC M1401.3checked	Equipment sized from a heat loss calculation, not from the old boiler's nameplate	An oversized boiler short cycles through the whole shoulder season, which is where most of the heating hours are

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Replacing a 1960s boiler with the same input. Those were sized for 180 F water and an uninsulated house, usually double the real load.
Fitting a condensing boiler to 180 F baseboard and expecting 95 percent. Return water never drops below 160 F, so it never condenses.
No primary-secondary loop or low-loss header on a mod-con. Flow through the heat exchanger falls and the boiler faults on high limit.
A 199,000 BTU combi on the 1/2 in gas line that fed an 80,000 BTU boiler. It starves at high fire and locks out (IRC G2413).
A combi in a house with 1.5 GPM showerheads. The burner cycles on every hand wash and the outlet temperature never settles.
Relief valve discharge left unpiped. It has to run full size to a safe point near the floor, with no valve in the line.

Key facts

Fin-tube baseboard puts out roughly 550 to 600 BTU per hour per linear foot with 180 F supply water at 1 GPM, and manufacturer ratings for 3/4 in element reach about 640.
One square foot of cast iron radiator EDR delivers 150 BTU per hour with 170 F water, which is the standard basis old radiators were rated on.
Water carries 500 BTU per hour for every gallon per minute and every degree F of temperature drop, so 1 GPM across a 20 F drop moves 10,000 BTU per hour.
A condensing boiler only condenses when the return water is below about 130 F, which a system designed around 180 F baseboard water never reaches.
A combi boiler delivering 3.5 gallons per minute at a 70 F rise has to produce 122,500 BTU per hour, more heat than most houses need on the coldest day.

Frequently asked questions

What size boiler do I need for a 2,000 sq ft house?

In IECC zone 5 with average insulation, about 100,000 BTU per hour of heat loss, which is a 120,000 BTU input boiler at 86 percent seasonal efficiency. That load also needs 176 feet of fin-tube baseboard at 180 degree water to get it into the rooms.

How many BTU does baseboard heat put out per foot?

Roughly 550 to 600 BTU per hour per linear foot with 180 degree supply water at 1 GPM. Output falls with water temperature, to about 300 BTU per hour per foot at 140 degrees, which is why low-temperature systems need longer runs.

Is a combi boiler worth it?

It saves the floor space and the standby loss of a tank, but the hot water demand picks the boiler size. Making 3.5 gallons per minute at a 70 degree rise takes 122,500 BTU per hour, so the unit is often twice the heating load and needs a wide turndown burner to behave.

Why is my new high efficiency boiler not saving what I expected?

Almost always because the emitters need hot water. A condensing boiler has to see return water below about 130 degrees, and 180 degree baseboard never gives it that. Adding baseboard length and running outdoor reset is what unlocks the efficiency.

Sources and references

Next steps for this project

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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.