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Boiler Sizing and Efficiency Calculator

Size a boiler from the building’s heat loss, see the input rating that matches, and check how efficiently a running boiler turns fuel into hot water.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

From a room-by-room heat-loss calculation. Do not use the old boiler’s rating — it is often oversized.

Only if your designer specifies one, for example for piping in unheated space or an indirect water heater served by the boiler. 0 % sizes to the heat loss.

From the boiler’s rating plate or literature. 84 % is the federal minimum for new gas hot-water boilers; condensing boilers are rated higher. Electric boilers: 100 %.

Result

Boiler output (heating capacity) needed
60,000BTU/h
Matching input rating
71,429BTU/h

A boiler with about 60,000 BTU/h of output — about 71,429 BTU/h input at 84 % AFUE — covers this heat loss.

  • That boiler is 33 % larger than the heat loss. Oversized boilers short-cycle; a modulating boiler with a low minimum firing rate handles mild days better.
  • Boilers are listed by heating capacity (output) and input. Match the output to the heat loss, and check that the radiators or baseboard can emit that heat at the water temperature you plan to run.
Boiler output ÷ heat loss
133%
Show the working
  1. Output = heat loss × (1 + allowance)
    60,000 × 1 = 60,000 BTU/h
  2. Input = output ÷ AFUE
    60,000 ÷ 0.84 = 71,429 BTU/h
  3. Oversizing = boiler output ÷ heat loss
    80,000 ÷ 60,000 = 133 %

Sizing from the heat loss

Boilers are listed by both output (heating capacity) and input. Match the output to the building’s design heat loss from a room-by-room calculation; the matching input is output ÷ AFUE. Do not copy the old boiler’s size — older boilers were very often oversized, and an oversized boiler short-cycles.

The extra allowance field is there only for a specific, designed reason, such as an indirect water heater on the boiler or long piping through unheated space. Leave it at zero otherwise.

Federal minimum efficiency

Residential boilers made since January 15, 2021 must meet minimum AFUE ratings set in 10 CFR 430.32: 84 % for gas hot-water boilers, 82 % for gas steam, 86 % for oil hot-water and 85 % for oil steam. Electric boilers have no AFUE standard. Condensing boilers are rated well above these minimums when return water is cool enough for them to condense.

Measuring efficiency on site

With the boiler firing steadily, the heat going into the water is 500 × GPM × (supply − return temperature). Divide by the fuel input — measured by clocking the gas meter, not read from the rating plate — to get the steady-state thermal efficiency. It is a snapshot, not AFUE, which includes cycling and standby losses over a season. Fuel-burning appliances should be tested and adjusted only by a qualified technician.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate
Boiler size for a heat loss
Design heat loss of the building
60,000 BTU/h
Extra allowance (optional)
0 %
Boiler type
Gas-fired hot-water boiler
Boiler AFUE
84 %
Output (heating capacity) of the boiler you have or are offered (optional)
80,000 BTU/h

Result

Boiler output (heating capacity) needed
60,000 BTU/h
Matching input rating
71,429 BTU/h
Boiler output ÷ heat loss
133 %

A boiler with about 60,000 BTU/h of output — about 71,429 BTU/h input at 84 % AFUE — covers this heat loss.

Assumptions

  • Output to match = design heat loss (plus any allowance you enter); input = output ÷ AFUE.
  • Measured output = 500 × GPM × ΔT (8.33 lb/gal × 60 min/h × 1 BTU/lb·°F, rounded), for water.

Limitations

  • AFUE is a seasonal rating; steady-state efficiency at a given moment differs.
  • The 500 factor is for plain water; glycol mixtures carry less heat per gallon.
  • Steam systems are sized from the connected radiation, not only the heat loss; follow the boiler maker’s steam sizing method.

Frequently asked questions

What size boiler do I need for 2,000 square feet?

Floor area alone cannot say: insulation, windows, air leakage and climate change the heat loss of the same-sized house several-fold. Calculate the heat loss first, then size the boiler to it.

Is the BTU on my boiler input or output?

Rating plates and literature usually give both. The model number often reflects the input; the heating capacity (output) is the figure to compare with your heat loss.

Why does my calculated efficiency look too high?

Flow is the hardest number to measure. If the result is above the boiler’s rating — or above 100 %, which the tool refuses — recheck the flow and temperatures.

Sources

  1. 10 CFR 430.32(e)(2) — residential boiler AFUE standards — U.S. Code of Federal Regulations (Cornell LII). Boilers manufactured on or after January 15, 2021: AFUE at least 84 % gas-fired hot water, 82 % gas-fired steam, 86 % oil-fired hot water, 85 % oil-fired steam; no AFUE standard for electric boilers.
  2. What is AFUE? — Carrier. AFUE is the share of fuel energy delivered as heat over a season.
  3. Water Density — U.S. Geological Survey Water Science School. Water’s density is roughly 1 g/mL (about 62.4 lb/ft³) and decreases as it warms; the page tabulates density by temperature.
  4. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.

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