What external design temperature should I use?

CIBSE Guide A provides external design temperatures for UK locations based on the 97.5 percentile winter temperature. London/South East = -1°C, Midlands/East Anglia = -2°C, North East/Southern Scotland = -3°C, Northern Scotland = -4°C. These represent conditions exceeded for only about 1% of the heating season.

What internal design temperatures should I use?

CIBSE recommended internal design temperatures: Living room = 21°C, Bedroom = 18°C, Bathroom = 22°C, Kitchen = 18°C (heat from cooking), Hallway = 18°C, Office/study = 21°C. These are the temperatures the heating system should achieve at external design conditions.

How does altitude affect design temperature?

Temperature drops by approximately 0.5°C for every 100m increase in altitude above sea level. A property at 300m above sea level in the Midlands would use a design temperature of -2°C minus 1.5°C = -3.5°C. This correction is important for properties in hilly or upland areas.

What is ΔT in heat loss calculations?

ΔT (delta-T) is the temperature difference between inside and outside design conditions. It drives the heat loss through the building fabric and ventilation. For example: 21°C internal minus -2°C external = 23°C ΔT. The higher the ΔT, the greater the heat loss and the larger the heating system needed.

How do I calculate room heat loss?

Total heat loss = fabric loss + ventilation loss. Fabric loss = U-value × area × ΔT for each building element (walls, floor, ceiling, windows). Ventilation loss = 0.33 × room volume × air changes per hour × ΔT. The total determines the radiator/heat emitter size needed.

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Design Temperature Calculator — UK Regions

Determine external design temperatures by UK region (-1 to -4°C), internal target temperatures by room type, and calculate ΔT for heat loss calculations.

UK Region

Location for external design temperature

Room Type

Room type for internal design temp

Floor Area

Room floor area

Ceiling Height

Floor to ceiling height

How We Calculate This

This calculator provides UK external design temperatures by region and internal target temperatures by room type. It then calculates the ΔT for heat loss estimation.

CIBSE method

External temp: Regional design temp − altitude correction
ΔT: Internal target − external design temp
Fabric loss: U-value × area × ΔT (watts)
Ventilation loss: 0.33 × volume × ACH × ΔT
Total loss: Fabric + ventilation = watts needed

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.