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Flame & Flagstone

The tool

Patio heater BTU calculator

Put your dimensions in and see every multiplication. Then read why the published rules disagree with each other, and which one this calculator uses.

By Scooter M.

Published September 1, 2026

Last reviewed September 1, 2026

A notebook and measuring tools laid out on a wooden surface

Multiply your patio area in square feet by 20 BTU for an open patio in still air. Multiply by about 0.65 if there is a roof, or 0.5 for a roof plus screens. Multiply by up to 1.4 if the spot is exposed. A 12 x 14 ft open patio in average conditions comes to about 3,360 BTU/h.

Work out your patio

How much heater does this patio need?

What is overhead?
How exposed is it?

The answer

3,360 BTU/h

for 168 sq ft. That is 985 watts if you go electric, which one standard outlet can deliver.

The working

  1. 12 ft x 14 ft = 168 sq ft
  2. 168 sq ft x 20 BTU/sq ft = 3,360 BTU/h for an open patio in still air
  3. x 1 for “Open sky, no roof” = 3,360 BTU/h
  4. x 1 for average — some exposure = 3,360 BTU/h

A single heater covers a circle, not a rectangle. Your 168 sq ft would need a 7.3 ft radius from one central unit — if the patio is long and thin, two smaller heaters beat one big one every time.

Propane, per hour

$0.10

at $2.674/gal

Electric, per hour

$0.18

at 18.34 cents/kWh

20 lb cylinder lasts

128.3 h

at this burn rate

This is a sizing model with its assumptions on the page, not a measurement. Where the factors come from, and where published sources disagree with each other, is set out in full below.

Why the published rules disagree

Search for how many BTU a patio needs and you meet several confident, incompatible answers. That is not because anyone is being dishonest. It is because outdoor heating genuinely resists a clean rule, and different sources have simplified in different directions.

Here is what is actually in circulation, and what each one is really claiming.

The rule in circulationWhat it assumesWhere it breaks
20 BTU per square footAn open patio, still air, mild cold. The most widely repeated figure and the baseline this calculator uses.Says nothing about a roof or wind, which are the two things that change the answer most.
A published coverage radius (for example 9 ft)That the unit is central and unobstructed, and that feeling warmth counts as coverage.A radius is a circle: 9 ft is 254 sq ft. Most people read it as a diameter and halve their expectations by mistake.
Watts per square foot, borrowed from indoor heatingA closed room with a heat-loss calculation and a boundary.A patio has open sky on one side. Indoor load figures do not transfer and produce numbers far too high.
Retailer calculators ending at a product pageThat the answer is whichever of their units is nearest above the number.Not wrong so much as narrow. The output is a catalog item rather than a figure.

Every one of these is a simplification of the same underlying physics. None is a measurement, including ours.

What this calculator does, and what it is

It starts from the 20 BTU per square foot baseline, which is the most widely published figure in the category and the one closest to a consensus. Then it applies two corrections that the baseline leaves out and that change the answer more than model choice does.

  1. Area. Length times width of the part you actually use. Not the whole slab — the seating area.
  2. Baseline. Area x 20 BTU/sq ft. This is the open-patio, still-air number.
  3. Enclosure. x 1 for open sky, x 0.65 for a roof with open sides, x 0.5 for a roof plus screens or partial walls. A roof stops warmed air escaping upward, which is the largest single loss on an open patio.
  4. Exposure. x 0.9 for sheltered, x 1 for average, x 1.4 for genuinely exposed. Moving air strips warmed air away faster than a heater replaces it.

These factors are ours, and they are a model

The 20 BTU per square foot baseline is widely published. The enclosure and exposure multipliers are our own, chosen to reflect the physics in a way a reader can argue with, and they are not measurements. We publish them as numbers on the page rather than burying them in a black box precisely so you can disagree with a specific step. If you think 1.4 is too high for your windy corner, use 1.25 and the arithmetic still works.

The table, if you would rather not use the tool

Patio areaOpen sky, no roofRoof overhead, open sidesRoof plus screens or partial walls
80 sq ft1,600 BTU/h1,040 BTU/h800 BTU/h
120 sq ft2,400 BTU/h1,560 BTU/h1,200 BTU/h
160 sq ft3,200 BTU/h2,080 BTU/h1,600 BTU/h
200 sq ft4,000 BTU/h2,600 BTU/h2,000 BTU/h
250 sq ft5,000 BTU/h3,250 BTU/h2,500 BTU/h
300 sq ft6,000 BTU/h3,900 BTU/h3,000 BTU/h
400 sq ft8,000 BTU/h5,200 BTU/h4,000 BTU/h

Area x 20 BTU per square foot for an open patio, then multiplied by 0.65 for a roof and 0.5 for a roof plus screens, and by 1 for average exposure. The reconciliation of where each factor comes from is on this page, not hidden.

The thing the number does not tell you

A BTU figure sizes the heat. It does not tell you the shape of the heat, and shape is where a lot of disappointment comes from.

One heater covers a circle. If the calculator says 4,000 BTU/h for a 200 sq ft patio and that patio is 8 ft by 25 ft, a single central heater with a 250 sq ft circle still leaves both ends cold, because the circle does not fit the rectangle. Two smaller units beat one larger one on any long, thin space, and no BTU total will tell you that.

The coverage area page works through the geometry, including how to check whether a manufacturer's radius claim is plausible for the output it comes with.

Worked example

A 12 x 16 ft covered porch on a sheltered side of the house. Area is 192 sq ft. Baseline is 192 x 20 = 3,840 BTU/h. Roof with open sides: x 0.65 = 2,496. Sheltered: x 0.9 = 2,246 BTU/h.

That is 2,246 BTU/h, which is well under what a single 1,500 W electric heater produces (5,118 BTU/h). So for that porch, one mounted electric infrared unit is comfortably sufficient, costs about 28 cents an hour, and a 48,000 BTU propane tower would be more than twenty times the required output — while also not fitting under the ceiling. The covered patio page covers the clearance side.

Then check what it costs

The prices every cost figure on this site uses

Your rates are not these rates. Every table on this site is the arithmetic, not the verdict — put your own number in and the answer moves.

Sizing and cost are the same calculation from opposite ends: the size tells you the output, and output times hours times your energy price tells you the bill. The cost page has the full tables, and the cheapest to run picks are the products that come out best once both are done.

Common questions

How many BTUs do I need for my patio heater?

Area in square feet times 20 for an open patio in still air. Multiply by 0.65 for a roof with open sides, or 0.5 for a roof plus screens. Multiply by up to 1.4 if the spot is genuinely exposed. A 200 sq ft open patio in average conditions works out at about 4,000 BTU/h.

Is 48,000 BTU too much for a small patio?

It is more than you need, and the cost of that is fuel rather than harm. 48,000 BTU covers about 2,400 sq ft on the standard rule. On a 150 sq ft patio you would run it low, which works fine — but you are also emptying a cylinder faster than a smaller unit would.

How many BTU do I need for a covered patio?

About a third less than the same patio would need in the open. A roof stops warmed air escaping upward, which is the largest loss outdoors. Our model uses a 0.65 multiplier, so a 200 sq ft covered patio needs about 2,600 BTU/h rather than 4,000.

Do I need two patio heaters?

If the space is long and thin, probably, regardless of the BTU total. One heater covers a circle, and a circle does not fit a rectangle. Two smaller units placed along the length beat one large one in the middle every time on that shape.

How many BTU is 1,500 watts?

5,118 BTU per hour. The conversion is 3,412 BTU per kWh, so 1.5 kW x 3,412 = 5,118. That is the ceiling on any plug-in electric heater in the U.S., because a 15-amp circuit derated for continuous load allows 1,440 W.

Sources

Every figure on this page either comes from one of these sources or is computed on the page from a figure that does. If you think one of them is wrong, tell us — we publish corrections.

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