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BTU calculator for room heating

Output = volume × G coefficient × (room temperature − outdoor design temperature). G sums up the home's heat loss, in watts per m³ and per degree of difference.

Presets

The outdoor design temperature is the cold a heating system is sized for, reached only a few days a year. The presets above are rough figures by climate, not the official value for your town: ask a heating professional or check your local design data.

Rooms

Estimated heating output —
How it is worked out

    A ballpark figure, nothing more. Proper sizing needs a heat loss calculation, wall by wall and window by window, by a heating professional, especially before choosing a heat pump, furnace or boiler.

    Yearly energy use is not estimated here: it depends too much on the weather, your habits and the thermostat setting. For the energy use and running cost of a given appliance, use the electricity cost calculator on this site.

    Everything is calculated in your browser; your latest values stay on this device.

    How many BTUs do you need to heat a room?

    This BTU calculator estimates the heating output a room needs, then adds up several rooms for a whole home. It works in feet, square feet and °F by default (switch "Units" to metres and °C if you prefer) and gives the answer in BTU per hour, watts and kilowatts. It also suggests how many heaters to fit. Treat it as a ballpark figure, not a professional heat loss calculation.

    How to use it

    1. Pick a method: "By volume (G coefficient)", the more complete one, or "By floor area (rougher)".
    2. Choose the "Home insulation" level and set the "Outdoor design temperature", by hand or with one of the "Presets".
    3. For each room, enter the length and width (or the floor area), the "Ceiling height", the "Room temperature" and the number of "Walls facing outside". Tick "Room under the roof" or "Large windows or patio doors" when they apply.
    4. Use "Add a room" to total the whole home and "Copy link" to share or save the calculation.

    The volume method, step by step

    A simple method, widely used for quick estimates:

    Output (W) = volume (m³) × G (W/m³·°C) × (room temperature − outdoor design temperature)

    Exact example: a 12 × 14 ft bedroom with an 8 ft ceiling holds 1,344 ft³, or 38.06 m³. You want 68 °F inside and the design temperature is 14 °F, a gap of 54 °F (30 °C). With average insulation (G = 1): 38.06 × 1 × 30 = 1,142 W. Multiply by 3.412 to get BTU per hour, since 1 W = 3.412 BTU/h: about 3,896 BTU/h, or 1.14 kW. A single 1,500 W electric heater covers it.

    InsulationG coefficient12 × 14 × 8 ft room, 54 °F gap
    Recent, well insulated0.72,727 BTU/h (799 W)
    Average insulation13,896 BTU/h (1,142 W)
    Older, poorly insulated1.55,843 BTU/h (1,713 W)

    These G values are rough orders of magnitude. If an energy assessor gave you a figure for your home, choose "Custom value". The tick boxes add simple adjustments: +20% for a room under the roof, +15% for large windows, +10% or +20% for two or three outside walls, −10% for a room with no outside wall.

    Why insulation matters so much

    An older house with solid uninsulated walls and single glazing can lose roughly twice as much heat as a recent one, so the same room needs about twice the output. Insulating the loft or attic, often does more than buying a bigger heater.

    Outdoor design temperature

    This is the cold a heating system is sized for, reached only a few days a year. It varies a lot: just below freezing on a mild coast, well below 0 °F in the northern plains or in the mountains. The "Presets" are general figures by climate, not the official design value for your town: a heating contractor can give you that one.

    The floor area method and its limits

    A common rule of thumb is about 20 BTU per square foot. The calculator's presets go from about 19 BTU/h per ft² (well insulated) to about 35 (poorly insulated). With average insulation, the same 168 ft² room gives about 4,260 BTU/h (1,249 W), which is higher than the volume method here. This method assumes an 8 ft ceiling (the tool corrects for other heights) and ignores your climate and target temperature.

    Oversizing or undersizing

    An undersized heater runs flat out and the room stays chilly on the coldest days. An oversized electric heater costs more to buy but does not use more energy, because its thermostat cuts it off. An oversized furnace, boiler or heat pump, on the other hand, short-cycles: it switches on and off constantly and wears out sooner.

    The calculator does not estimate yearly energy use. For the running cost of a given heater, use the electricity cost calculator on this site.

    When to call a professional

    Before choosing a heat pump, furnace or boiler, or for a full renovation, ask for a room-by-room heat loss calculation. A heating professional works out the losses through each wall, window and ceiling, using the local design temperature.

    Frequently asked questions

    How many BTUs per square foot do I need?

    Roughly 19 to 35 BTU/h per square foot depending on insulation, for an 8 ft ceiling. The volume method also takes your climate into account.

    How do I convert watts to BTU per hour?

    Multiply by 3.412: 1,500 W is 5,118 BTU/h.

    Can I add several rooms?

    Yes. Each room has its own size and adjustments; the table shows the home's total.

    Is this enough to size a heat pump?

    No. It gives an order of magnitude; sizing a heat pump or furnace calls for a heat loss calculation by a professional.