BTU Heating Calculator
Calculate heating requirements for any room or space
Room Heating Analysis
BTU Requirements Breakdown
Visual breakdown of heating requirements: base load, temperature difference, insulation, windows, and doors.
Your BTU Calculation
Room Size: 0 m³
Temperature Diff: 0°C
Required BTU: 0 BTU/hr
Annual Cost: £0
Calculation Steps
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Room volume: 0 m³ (L × W × H)
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Temperature difference: 0°C
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Base BTU: 0 BTU/hr
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With adjustments: 0 BTU/hr
Heater Size Recommendations
Small Room (10 m³)
5,000 BTU
Bedroom, small office
Medium Room (25 m³)
12,000 BTU
Living room, kitchen
Large Room (50 m³)
24,000 BTU
Open-plan area
Very Large (100 m³)
50,000 BTU
Large warehouse/hall
Frequently Asked Questions
BTU stands for British Thermal Unit—a standard measure of heating power. One BTU is the energy needed to raise 1 pound of water by 1°F. Knowing your room’s BTU requirement ensures you buy a heater that’s correctly sized: too small won’t heat the space, too large wastes energy and costs more.
Basic formula: BTU = (Room Volume × Temperature Difference) × Heat Loss Factor. This calculator uses: Volume (L×W×H) × 35 BTU/cubic meter/°C × Insulation Multiplier + Window/Door Penalties. The 35 BTU constant accounts for standard air density and heat capacity. Multipliers adjust for poor to excellent insulation.
Insulation directly reduces heating requirements. Poor insulation (×1.5) needs 50% more BTU; average (×1.2) needs 20% more. Good insulation (×1.0 baseline) is standard. Excellent insulation (×0.8) cuts needs by 20%. Passive house standards (×0.6) reduce BTU needs by 40%. Better insulation = lower heater size and operating costs.
Windows and doors are thermal weak points—they lose heat much faster than insulated walls. Each window adds ~500 BTU/hr, each door adds ~1,000 BTU/hr in heating demand. Double/triple-glazed windows lose less heat than single-pane. Proper weatherstripping around doors reduces their heating penalty.
Cost = Required kW × Annual Hours × Electricity Rate. Example: 5 kW heater running 2,500 hours/year at £0.25/kWh costs £3,125/year. Reducing heating hours (better insulation, lower setpoint) or using off-peak rates saves money. Heat pump heating is typically 300-400% efficient, costing 3-4× less than electric resistance.
No—oversizing is inefficient. A heater 20-30% larger than calculated provides slight comfort margin but wastes energy cycling on/off. A correctly-sized heater runs steadier and costs less. If warmup speed matters, improve insulation first. For peak-load heating, match calculated BTU; for supplemental heating, go 10-15% under requirement.