Heater Calculator
Calculate the heating power required for any room. Enter room dimensions, insulation, windows and climate to get your recommended heater size in Watts and BTUs.
How the heater calculator works
This calculator estimates the heating power (Watts) needed to maintain a comfortable temperature in your room. It uses your room volume, insulation quality, number of windows, external walls, and climate zone to calculate heat loss. The result is the minimum heater output required to keep your space warm.
Enter your room dimensions, insulation level, window count, and climate below. The calculator instantly shows your recommended heater wattage, BTU rating, estimated running cost, and a comparison across different insulation levels.
Step 1: Room Information
Understanding Heat Loss
Heat loss depends on room volume, insulation, windows and climate. The calculator uses a heat loss coefficient (W/m³K) to determine the power needed to maintain your desired temperature.
Step 2: Windows, Walls & Climate
Heater Sizing Tips
For most rooms, choose a heater with 10–20% more output than the calculated minimum to allow for cold snaps and faster warm-up times.
Heater Size Results
Recommended Heater Output
Insulation Level Comparison
| Insulation Level | Coefficient (W/m³K) | Required Watts | Required BTUs | Description |
|---|
Heat Loss Breakdown
Heating Power Formula
P = V × ΔT × U
P = Heating power (Watts) · V = Room volume (m³) · ΔT = Temperature difference (°C) · U = Heat loss coefficient (W/m³K)
The heat loss coefficient accounts for insulation, windows, and external walls. A higher U-value means more heat loss and a larger heater required.
Frequently Asked Questions
What size heater do I need for my room?
The heater size depends on room volume, insulation, windows, and climate. As a rough guide, allow 30–50 W per m³ in a well-insulated room, and 50–80 W per m³ in a poorly insulated room. Use this calculator for a precise recommendation.
How is heating power calculated?
Heating power is calculated using: Room volume (m³) × Temperature difference (°C) × Heat loss coefficient (W/m³K). The heat loss coefficient depends on insulation quality, window area, and climate zone.
What is the difference between Watts and BTUs?
Watts (W) and British Thermal Units (BTUs) are both units of power. 1 Watt = 3.412 BTUs per hour. Heater outputs are often given in BTUs in the US and Watts in Europe/UK.
What insulation level is best for heating efficiency?
Better insulation reduces heat loss, meaning you need a smaller heater and pay less to run it. For best efficiency, aim for cavity wall insulation, double glazing, and good roof insulation.
How many windows affect heating requirements?
Windows are a major source of heat loss. Single-glazed windows lose about 5–6 times more heat than insulated walls. Each additional window increases the required heating power by roughly 150–300 W depending on size and glazing type.
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