Tell us how big the space is and where you'll use the heater. We'll estimate the BTU output you need, suggest the right type of portable propane heater, and show roughly how long it runs on a 1 lb cylinder and a 20 lb tank. It's an estimate to get you in the right range — always confirm against the manufacturer's rated coverage.
Run times are at full output; lower settings last longer. Figures use 21,600 BTU per pound of propane and are rounded — see our BTU sizing & running-cost guide for the full method.
How the calculator works
Sizing a propane heater comes down to two numbers: how much heat the space loses, and how much heat the unit puts out. Heat output is measured in BTU per hour. The bigger and less insulated the space, the more BTU you need.
We start from the same rule of thumb most manufacturers publish:
- Well-insulated indoor room: ~20–30 BTU per square foot
- Average or older indoor room / basement: ~30–35 BTU per square foot
- Uninsulated garage, workshop or job site: ~40–45 BTU per square foot
- Tent or ice shanty: a small radiant heater is usually plenty — but ventilation and an oxygen-depletion sensor matter more than raw BTU
- Outdoor patio: square footage doesn't apply the same way; a standing patio heater warms a radius around it rather than a sealed room
For run time we use propane's energy content — about 21,600 BTU per pound. Multiply a tank's weight by 21,600 and divide by the heater's BTU output. A 20 lb tank (≈ 432,000 BTU) on a 9,000 BTU heater lasts roughly 48 hours; the same tank on a 45,000 BTU forced-air heater lasts under 10.
Match your result to a heater
Once you know the range, these guides cover the pick for each use case:
- Best indoor propane heaters — indoor-safe radiant units with an oxygen-depletion sensor
- Best propane heaters for a garage or workshop
- Best forced-air (torpedo) heaters and tank-top heaters for big, drafty spaces
- Best propane heaters for camping and ice fishing
- Best patio propane heaters for outdoor areas
A note on indoor safety
Only run a heater indoors if it is specifically rated indoor-safe with an oxygen-depletion sensor (ODS). Keep a window cracked for ventilation and use a battery-powered carbon-monoxide detector. Outdoor, patio and forced-air heaters are not safe in enclosed spaces. See our indoor safety & carbon monoxide guide.
Frequently asked
How many BTUs do I need per square foot?
For a well-insulated indoor room, plan on roughly 20–30 BTU per square foot. Older or drafty rooms need about 35+ BTU/ft², and uninsulated garages, workshops and job sites often need 40+ BTU/ft². Outdoor spaces don’t follow the formula — coverage radius matters more than area.
How long does a propane heater run on a 20 lb tank?
Multiply the tank weight by 21,600 (BTU per pound of propane) and divide by the heater’s BTU output. A 20 lb tank holds about 432,000 BTU, so a 9,000 BTU heater runs roughly 48 hours; a 45,000 BTU tank-top heater lasts under 10 hours. Lower heat settings stretch that further.
How long does a 1 lb propane cylinder last?
A 1 lb cylinder holds about 21,600 BTU. On a small 9,000 BTU indoor heater that is roughly 2–3 hours at full output, and longer on a low setting.
What size propane heater do I need for a 2-car garage?
A typical 2-car garage is about 400–600 sq ft and, being uninsulated, needs roughly 40+ BTU/ft² — so about 18,000–30,000+ BTU. Most people use a tank-top or forced-air (torpedo) heater there, not an indoor-safe radiant unit.