Boiler Sizing Calculator

Work out boiler sizing instantly with clear inputs, formula shown and shareable results.

About Boiler Sizing Calculator

Boiler duty is the space heating load plus a domestic hot water allowance, uplifted by a small margin for recovery after setback. Dividing that output by seasonal efficiency gives the gross input rating used to check gas or oil supply capacity and flue sizing.

Formulas

Boiler output

Output kW = (space load + DHW allowance) x (1 + margin)

Gross input

Input kW = output kW / efficiency

Worked example

With the default values below, Boiler Sizing Calculator returns Gross input rating of 18.33. Change any input above and every figure here is recalculated in your browser.

Inputs used

Space heating loadminimum 0.5 kW: 12 kW
Hot water allowanceminimum 0 kW: 3 kW
System marginaccepts 0–50 %: 10 %
Seasonal efficiencyaccepts 50–100 %: 90 %

Results

Boiler output required: 16.50 kW
Gross input rating: 18.33 kW
Output in BTU/h: 56,300 BTU/h
Share taken by hot water: 20.0% %

Step by step

1Values used
2Boiler output
3Gross input
4Gross input rating= 18.33 kW
5Boiler output required= 16.50 kW
6Output in BTU/h= 56,300 BTU/h
7Share taken by hot water= 20.0 %

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Boiler selection, flue routing and gas supply must be verified by a qualified heating engineer against local regulations.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Boiler selection, flue routing and gas supply must be verified by a qualified heating engineer against local regulations.

Frequently Asked Questions

How is the Boiler Sizing figure worked out?

Output kW = (space load + DHW allowance) x (1 + margin). Boiler duty is the space heating load plus a domestic hot water allowance, uplifted by a small margin for recovery after setback.

Should hot water always be added on top?

For a combination boiler the hot water duty usually dominates and is not added to space heating, because the two are not served simultaneously. For a stored-cylinder system the cylinder reheat allowance is added as shown.

What values do I need to enter?

This calculator takes 4 inputs: Space heating load, Hot water allowance, System margin, Seasonal efficiency. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.