How many BTU do I need per square foot?

A common rule of thumb is 20 BTU per square foot for average ceilings (8 ft) and average insulation. This calculator adjusts that baseline for ceiling height, insulation quality, climate zone, and window count to give a more accurate number.

How do I convert BTU to watts or kilowatts?

1 BTU/hr equals approximately 0.293 watts. To convert, divide the BTU value by 3.412 to get watts, then divide by 1,000 for kilowatts. The calculator shows all three values automatically.

What climate zone should I choose?

Mild climates (Zone 1) cover southern US, coastal areas, and similar regions where winter lows stay above 30°F. Cold (Zone 2) is most of the northern US and Canada. Very cold (Zone 3) covers extreme northern regions, mountain areas, and anywhere with sustained sub-zero winters.

Does insulation really affect heater size that much?

Yes. Poor insulation can increase BTU requirements by 25–50% compared to well-insulated spaces. Upgrading insulation is often more cost-effective long-term than buying a larger heater.

Should I buy a heater exactly at the calculated BTU?

Add a 10–20% safety buffer to the calculated number. A slightly oversized heater reaches temperature faster; an undersized one runs continuously without ever heating the room adequately. Many heaters also have adjustable output, so rounding up to the next standard size is usually fine.

Heater Size Calculator

Name: Heater Size Calculator
Availability: InStock
Author: Nham Vu

Enter your room dimensions, insulation level, and climate to find the exact BTU or wattage your heater needs.

Room Details

Room Dimensions

Length (ft)

Width (ft)

Ceiling (ft)

Insulation Quality

Climate Zone

Number of Windows

Each window adds ~1,000 BTU to the requirement.

Room Type

Fill in the room details and click Calculate

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Summary

Enter your room dimensions, insulation level, and climate to find the exact BTU or wattage your heater needs.

How it works

Enter the room length, width, and ceiling height in feet.
Select your insulation quality (poor, average, or good).
Choose your climate zone based on your region.
Optionally add window count and room type for a more precise result.
The calculator applies the BTU formula with correction factors and shows BTU and wattage side by side.

Use cases

Size a space heater for a bedroom or home office.
Determine the heater output needed before buying a new unit.
Compare BTU requirements across different room configurations.
Convert BTU results to kilowatts for electric heater shopping.
Check if an existing heater is undersized for a renovated room.
Plan heating for a garage, workshop, or sunroom.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu