---
title: "Room Heater BTU Calculator: Watts, kW & BTU/h | Warm Window"
description: "Size a room heater from floor area, ceiling height, insulation and the outdoor design temperature. Get watts, kW and BTU/h as a range, formula shown."
url: https://warmwindow.homes/
---

Room heater size

# Room heater BTU calculator

How big a heater does your room need? Enter the room, how well it holds heat and how cold it gets outside. You get a rough range in watts, kilowatts and BTU per hour, with the working shown.

## How to use the room heater calculator

1. Choose feet and °F or metres and °C, then give the floor area (or length and width, or the volume) and the ceiling height.
2. Pick the insulation level that sounds most like the room, count its outside walls and tick a roof or cold floor if there is one.
3. Set the temperature you want indoors and the outdoor design temperature: the cold spell the heater should still cope with.

### What the result means

The figure is the heat the room loses on that cold day, which is the output a heater must match to hold your temperature. It is a range because two rooms with the same description can leak very differently. The lower end assumes the tighter, better-glazed end of your insulation level; the upper end assumes more glass and more draughts. If you want the room warm on the coldest days, size for the upper figure.

Electric heaters are rated by the power they draw, and all of it ends up as heat in the room, so a 1.5 kW heater gives 1.5 kW, about 5,100 BTU/h. Fuel-burning and heat-pump appliances list their heat output separately; compare that figure.

### Worked example

A 200 ft² living room with 8 ft ceilings has 1,600 ft³ of air. With average insulation, two outside walls and a lift from 20 °F to 70 °F (50 °F), the calculator finds about 3,100 to 4,500 BTU/h, or 0.9 to 1.3 kW. Its heat-loss factor works out at 0.039 to 0.057 BTU per hour per cubic foot per °F, and 1,600 × 50 × 0.057 ≈ 4,500.

## How the estimate is worked out

Behind the quick inputs is a small heat-loss model of a square room. Each outside wall is the room's width times the ceiling height; windows take up 15% (low estimate) to 25% (high estimate) of the floor area and sit in those walls. Every surface loses heat at its U-value × area × temperature difference, thermal bridges add a little per square metre of outside surface, and air leaking in and out takes 0.33 W for every cubic metre per hour per °C. The total, divided by the room volume and the temperature difference, is the heat-loss factor `f` shown with your result.

| Level | Walls | Windows | Roof or loft | Floor | Air changes per hour |
| --- | --- | --- | --- | --- | --- |
| Poor | 1.5–1.7 | 4.8 | 0.68–2.3 | 0.7–1.2 | 1–2 |
| Average | 0.6–0.7 | 2.7–3.1 | 0.3–0.4 | 0.5–0.75 | 0.75–1.25 |
| Good | 0.28–0.45 | 1.6–2.0 | 0.16–0.21 | 0.22–0.45 | 0.5–1 |
| Very good | 0.18–0.26 | 1.3–1.6 | 0.11–0.16 | 0.15–0.18 | 0.5–0.75 |

The wall, roof and window figures are default values from the UK's RdSAP 10 and SAP 10.2 tables and the limits in Approved Document L 2021; the floor figures are ground-floor U-values we worked out with the ISO 13370 method for typical house sizes. Air change rates are our own rough bands, with 0.5 per hour the minimum the CIBSE Domestic Heating Design Guide assumes for a habitable room. The thermal-bridging allowance follows RdSAP 10: 0.15 W/m²K of exposed area for poor and average rooms, 0.11 for good, 0.08 for very good.

### Outdoor design temperatures

Heating engineers size for a temperature that is only beaten on the coldest few hours of a typical year, often the "99.6%" value: 99.6% of the hours in the year are warmer. CIBSE Guide A lists these for UK weather stations, for example London (Heathrow) −3.0 °C, Birmingham −5.1 °C, Manchester −4.5 °C, Edinburgh −5.4 °C, Glasgow −5.6 °C, Cardiff −3.1 °C, Belfast −3.2 °C and Plymouth −1.5 °C, reduced by 0.6 °C for every 100 m your home sits above the station. Elsewhere, the ASHRAE Handbook of Fundamentals publishes the same 99.6% heating design temperature for thousands of stations worldwide, and local building codes often state one.

For indoor temperatures, the CIBSE guide behind UK heat-pump sizing uses 21 °C (70 °F) for living rooms, 22 °C for bathrooms and, in older homes, 18 °C for bedrooms and halls.

## Sources

- BRE, [*RdSAP 10 Specification*](https://bregroup.com/documents/d/bre-group/rdsap-10-specification-10-06-2025) (9 June 2025): default wall U-values (Tables 6 and 13), roof U-values by loft insulation thickness (Table 16), exposed floors (Table 20), thermal bridging (Table 21), doors (Table 26) and the ISO 13370 ground-floor method (section 5.12).
- BRE, [*SAP 10.2*](https://files.bregroup.com/SAP/SAP%2010.2%20-%2011-04-2023.pdf) (11 April 2023), Table 6e: default U-values for windows and doors.
- HM Government, [*Approved Document L, Volume 1: Dwellings*](https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l) (2021 edition incorporating 2023 amendments), Tables 4.1 and 4.2: limiting U-values for new homes and for new elements in existing homes.
- MCS, [Heat Load Calculator: design conditions](https://heatloadcalculator.mcscertified.com/docs/reference-sources/design-conditions), reproducing the 99.6% external design temperatures of CIBSE Guide A (2015) Table 2.5 and the room temperatures of the CIBSE Domestic Heating Design Guide; and [ventilation rates](https://heatloadcalculator.mcscertified.com/docs/reference-sources/ventilation-rates) (minimum 0.5 air changes per hour).

## More heating and cooling calculators

Go from a quick heater size to a proper heat-loss sum, a condensation check and a yearly bill.

- [Air conditioner BTU](https://warmwindow.homes/air-conditioner-btu-calculator/): ENERGY STAR room AC sizing with sun, people and kitchen adjustments.
- [Heat loss calculator](https://warmwindow.homes/heat-loss-calculator/): Walls, windows, roof, floor and air, element by element, with typical U-values.
- [Dew point & condensation](https://warmwindow.homes/dew-point-calculator/): Will your windows run wet? Dew point, surface humidity and mould risk.
- [Heating cost calculator](https://warmwindow.homes/heating-cost-calculator/): Electric heaters, heat pump, gas, oil, propane and wood at your own prices.

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## Questions

**How many BTU do I need to heat a room?**

It depends on the room's volume, how well it holds heat and how cold it gets outside. A 200 square foot room with 8 foot ceilings, average insulation and two outside walls needs roughly 3,100 to 4,500 BTU/h to stay at 70 °F when it is 20 °F outside. The same room with single glazing and draughts needs roughly 5,600 to 7,900 BTU/h.

**How many watts per square metre does a room need?**

There is no single figure, because ceiling height, insulation and climate all matter. For a 16 m² room with 2.4 m ceilings and two outside walls, heated to 21 °C when it is −3 °C outside, this calculator gives about 45 to 65 W/m² with average insulation, 80 to 110 W/m² with poor insulation and 20 to 30 W/m² in a very well insulated room.

**Why do I get a range instead of one number?**

A short description cannot pin down how much glass a room has or how draughty it is, and those two things change the heat loss more than anything else. The range covers both ends of your insulation level. For a single figure, enter your own walls, windows and U-values in the heat loss calculator.

**Is a bigger heater better?**

A heater somewhat above the upper figure warms the room faster after it has been off, then its thermostat cycles it on and off. An electric heater turns all the power it draws into heat, so a larger one costs no more per unit of heat. A heater below the lower figure will not keep the room at temperature in a cold spell.

**What outdoor temperature should I enter?**

Use your area's heating design temperature, which only the coldest hours of a typical year fall below. CIBSE gives −3.0 °C for London Heathrow and −5.4 °C for Edinburgh, for example; ASHRAE publishes values worldwide. A milder figure gives a smaller heater that may struggle in a hard frost.

**Can I use this to size a boiler or heat pump?**

Only as a first guess. Whole-home systems are sized room by room from measured walls, windows and ventilation. Our heat loss calculator does that for one room or a whole house, and an installer's survey to EN 12831 or ACCA Manual J should set the final size.