How do I calculate room heat loss?

Sum fabric losses (U × A × ΔT) on each surface plus ventilation loss from air change rate and room volume. Use design internal and external temperatures for your location.

What design temperatures should I use?

Internal setpoints depend on room type (typically 18–21°C occupied). External winter design temperature comes from CIBSE Guide A for your UK location.

When is a full heat loss survey needed?

For issued calculations, SAP, or complex buildings use accredited software. This calculator is for early-stage screening and single-room checks.

BuildFlo Design Guide · Thermal · CIBSE Guide A / Part L

Heat load calculator for space heating

Estimate room or building heat loss and heating plant duty. Free UK heat load calculator for first-pass boiler and emitter sizing.

What this calculator does

Heat load helps UK building services engineers with heating demand and boiler/heat pump sizing. The calculation follows CIBSE Guide A / Part L methods used in everyday MEP design — enter your project inputs below for a first-pass result.

Whether you searched for "heat load", "heat load calculator", or "how to heat load", this page gives you a working tool plus the context to use it on a real job.

When to use it

Use this for early-stage sizing and sanity checks on UK building services work. It is not a substitute for issued calculations — verify against the project specification, drawings, and relevant standard before sign-off.

How to use it

1. Fill in the inputs for your scenario (defaults are starting points only). 2. Click Calculate — results include the method and assumptions where applicable. 3. Verify against the project specification and relevant standard before issuing.

Typical output unit: kW.

Tips

Defaults are indicative — replace with project-specific values from the architect, structural, or MEP spec.
For issued calculations, save results in your BuildFlo project and attach to the calculation folder.
Need the regulation citation? Open BuildFlo chat in the app and ask about heat load under UK standards.

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Frequently asked questions

How do I calculate room heat loss?: Sum fabric losses (U × A × ΔT) on each surface plus ventilation loss from air change rate and room volume. Use design internal and external temperatures for your location.
What design temperatures should I use?: Internal setpoints depend on room type (typically 18–21°C occupied). External winter design temperature comes from CIBSE Guide A for your UK location.
When is a full heat loss survey needed?: For issued calculations, SAP, or complex buildings use accredited software. This calculator is for early-stage screening and single-room checks.