This tool estimates the steady-state heat loss of a single rectangular room so you can plan radiator purchases. Fabric losses use Q = A × U × ΔT. Ventilation / infiltration uses Qv = 0.33 × V × N × ΔT, where the factor 0.33 approximates cpρ/3600 (CIBSE Guide A / common UK practice). A configurable planning allowance (default 10%) is then applied. The total is calculated without intermediate rounding, then rounded up to the nearest 50 W; a value of exactly 500 W remains 500 W. BTU/h is calculated only from the already-rounded watt figure using 1 W = 3.412142 BTU/h (NIST), then shown as a whole number.
Conservative and indicative defaults
- Air changes per hour (ACH) defaults by room type are conservative estimates for planning (not a measured airtightness or Part F design rate). Primary reference for the widely cited minimum of 0.5 ACH for most heated rooms (living, bedroom, bathroom, kitchen, hall, etc.): CIBSE Domestic Heating Design Guide (2026) Table 2-18, as documented by the MCS Heat Load Calculator — Ventilation Rates. Product defaults are higher than that minimum. Use Advanced mode to override.
- Under-floor / ground temperature defaults to 10 °C — a conservative estimate for simplified planning (lower assumed ground temperature → larger floor ΔT → higher heat-loss estimate). MCS treats the mean annual external air temperature as the assumed ground temperature for solid ground floors; values are site/region-specific from MCS MGD 007 Table B1 (MET Office 1981–2010), published in the MCS Heat Load Calculator — Design Conditions. Across the 18 UK regions in that table the annual mean ranges from 8.5 °C (North East Scotland, Dyce) to 11.3 °C (Thames Valley, Heathrow); example: Heathrow = 11.3 °C. The 10 °C default is a conservative planning value. Not a site survey result — change in Advanced mode if you have better data.
- Floor and roof/ceiling U-values (and the simplified wall default) are indicative averages for a mid-performance UK dwelling. They are not derived from an RdSAP age-band selector in this version. If you have EPC or manufacturer U-values, enter them in Advanced mode.
Radiator comparison
Radiator catalogue outputs should be compared at ΔT50 under BS EN 442. Correcting a catalogue figure to another water temperature first computes an intermediate continuous watt value via Q = Q50×(ΔT/50)n, then applies the same R-01 rule (round up to the nearest 50 W) for the displayed result; BTU/h is taken only from that rounded watt figure. The intermediate value is not shown as the final answer. Educational only — not an MCS-certified system design.
Limits
This estimate does not replace a qualified heating engineer, a Gas Safe registered installer, or a full BS EN 12831 / MCS heat-load calculation.
Primary references
- BS EN 442-1 / EN 442-2 — radiators and convectors (ΔT50, characteristic exponent n)
- CIBSE Guide A — Environmental design (fabric and ventilation heat loss principles)
- CIBSE Domestic Heating Design Guide (2026) Table 2-18 — minimum room ACH (via MCS Heat Load Calculator)
- MCS MGD 007 Table B1 / MCS Heat Load Calculator Design Conditions — mean annual external air temperature used as assumed ground temperature (e.g. Heathrow / Thames Valley 11.3 °C)
- NIST SP 811 — BtuIT/h ↔ watt conversion
- RdSAP 10 — window/door U-value tables (where cited)