Radiator BTU / Room Heat Calculator (UK)

Room details

Length (m)

Width (m)

Ceiling height (m)

Room type

External walls

Glazing

Air-change defaults are conservative estimates (CIBSE DHDG 2026 Table 2-18 minimum for most heated rooms is 0.5 ACH; product defaults are higher). Under-floor default 10 °C is a conservative estimate vs MCS regional ground means (e.g. Heathrow 11.3 °C). Floor / wall U-values are indicative averages, not an RdSAP age-band assessment. Use Advanced to override.

Advanced inputs

Length (m)

Width (m)

Height (m)

External wall length (m)

Glazing / openings (m²)

Room type (defaults)

Indoor design T (°C)

Outdoor design T (°C)

Air changes ACH (1/h)

Conservative estimate — adjust if you know your ventilation rate.

Above

Below

T under floor (°C)

Default 10 °C is an indicative average, not a measured site value.

T above if unheated (°C)

Planning allowance (%)

Glazing preset

U wall

Indicative average — not RdSAP age-banded.

U glazing

U floor

Indicative average for simplified planning.

U ceiling / roof

Indicative average — enter known U-values when available.

Estimated room heat requirement

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Compare radiators at ΔT50 (BS EN 442)

ACH and fabric U defaults used above are conservative / indicative averages for planning only. See Methodology.

Where the number comes from

Component	Power

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).

W ↔ BTU/h converter

Direction

Value

Educational: catalogue output at another ΔT

Does not change the room heat-loss result. Uses Q = Q₅₀ × (ΔT/50)ⁿ per BS EN 442 characteristic equation.

Catalogue Q at ΔT50 (W)

Flow (°C)

Return (°C)

Room (°C)

Exponent n

Use n from the manufacturer datasheet when available; 1.3 is a typical panel-radiator assumption only.