This calculator gives an indicative estimate of the heat loss of a single rectangular room, in watts (W) and BTU/h. It is meant to help you plan a radiator purchase: you can take the figure and compare it with radiator outputs quoted at ΔT50 under BS EN 442, which is the basis UK catalogues use.
What it does
You enter the room size, room type, external walls and glazing, plus any U-values and temperatures you know in Advanced mode. The calculator then:
- works out fabric heat loss through walls, windows, floor and ceiling as area × U-value × temperature difference;
- adds ventilation heat loss using 0.33 × room volume × air changes per hour × temperature difference;
- applies a planning allowance (10% by default, adjustable from 0% to 25%);
- rounds the total up to the nearest 50 W, with no rounding of the steps in between;
- converts the rounded watt figure to BTU/h using 1 W = 3.412142 BTU/h (NIST) and shows it as a whole number.
The breakdown shows every component. Try it on the calculator page. The full method, with sources, is set out on the Methodology and assumptions page.
What it does not do
The calculator is a simplified planning tool. It does not carry out a full BS EN 12831 heat-load calculation, it does not size a heat source or a whole heating system, and it does not handle rooms that are not rectangular. It does not count heat gains from people, appliances or sunshine. The default U-values for walls, floors and ceilings are indicative averages, and the default air-change rates and ground temperature are conservative estimates. Better values, for example from an EPC, can be entered in Advanced mode.
Where the figures come from
Formulas and defaults are documented in the project specification and are based on CIBSE guidance (Guide A and the Domestic Heating Design Guide), BRE RdSAP 10 Table 24 as the reference for window U-values (2.8 is the table's value for older 12 mm double glazing; that row also covers an unknown installation date), MCS design-condition references for temperatures, and NIST SP 811 for the watt to BTU/h conversion. Radiator output correction for other temperature differences follows the BS EN 442 relationship. More detail is in the guides: how to size a radiator in BTU, what affects a room's heat loss and watts, BTU/h and ΔT50.
Who runs this site
This site is operated by {{OPERATOR_NAME}}. For questions, corrections or feedback about the calculator, please write to {{CONTACT_EMAIL}} or see the Contact page. If a figure looks wrong, please include the inputs you used.
The calculator result does not depend on any commercial relationship. How the site handles affiliate links is explained in the Affiliate disclosure, and how it handles personal data in the Privacy Policy.
Disclaimer
This calculator provides an indicative room heat-loss estimate for planning radiator purchases. It is not a substitute for a qualified heating engineer’s design, a Gas Safe registered installer’s advice, or an MCS-compliant heat-pump system design. Radiator outputs should be compared at ΔT50 (BS EN 442).