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How the calculator works

Outdoor heat is always an estimate, because there are no walls to hold it. The calculator uses the rule most heater makers publish:

Volume times temperature rise. It takes the area where people sit, counts the first 7 feet above the floor, and multiplies by the number of degrees you want to add.
Extra for open air. An open patio loses heat faster than a covered one, so the calculator adds 25 percent, and 50 percent for a windy spot.

Measure the seating area only. Nobody needs the far corner of the patio warm.

What the numbers mean in heaters

Standing propane heater: 40,000 to 48,000 BTU. It warms a circle of about 10 to 12 feet across in still air.
Propane fire table: about 50,000 BTU, but much of it goes straight up. It warms the people sitting round it.
Tabletop propane heater: about 10,000 BTU, for two people at one small table.
Electric infrared heater, 1,500 W: about 5,100 BTU. It heats people and surfaces directly instead of the air, so it works well under a roof where warm air would otherwise escape.

Fuel-burning heaters are for open air only. Under a roof, in a screened porch or on a covered balcony, use electric.
Many apartment buildings ban propane on balconies. Read the lease first.
Keep 3 feet clear around and above any heater.

Heat is one of twelve ways to keep using the patio when it gets cold. The rest are in cozy patio ideas for fall and winter.

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