Geometry

Patio heater coverage area

Manufacturers publish a radius. Buyers read a diameter. The gap between those two is a factor of four in area, and it explains a lot of disappointment.

Coverage claims in this category are almost always quoted as a radius, and are almost always read as a width. The arithmetic makes that mistake expensive.

Area of a circle is pi times radius squared. At a 9-foot radius that is 254 square feet, and the circle is 18 feet across. Read the same 9 feet as a diameter and you get a 4.5-foot radius and 64 square feet. Same number, four times the area, and no way to tell from the number alone which one the marketing meant.

The last column of that table is the useful one, and it is a test anyone can run in ten seconds. Take the claimed radius, work out the area, and multiply by 20 BTU per square foot. If the result is far more than the unit's rated output, the claim is describing where you can detect warmth, not where the patio is warm.

Worked through on a real example: the Amazon Basics tower publishes a 9-foot radius and 46,000 BTU/h. Nine feet is 254 sq ft, which at 20 BTU/sq ft needs 5,090 BTU/h. The unit produces 46,000. The claim is therefore conservative by a wide margin, which is unusual enough in this category to be worth pointing out. The full write-up is here.

Run the same check on a 1,500 W electric heater claiming to cover a large open patio and it fails immediately: 5,118 BTU/h supports about 256 sq ft on the same rule, and that is on an open patio in still air with the beam pointed at you.

This is the practical part. Heaters cover circles; patios are rectangles; and the mismatch is why a correctly sized heater can still leave people cold.

Take a 10 x 20 ft patio, 200 sq ft. On the sizing rule that is 4,000 BTU/h, which any tower exceeds. Put one tower in the middle with a 254 sq ft circle and the numbers look comfortable. But a 254 sq ft circle is 18 feet across, and the patio is 20 feet long — so the two ends are outside it, and the circle also spills sideways onto the lawn, where it does nothing.

Two smaller units, one at each end, cover the rectangle far better with the same total output. That is a placement decision no BTU total will surface, and it is the most common reason a heater underdelivers on a long, narrow space.

A heater's coverage circle is measured at ground or seating level, and it depends on how high the source is. A tall tower at seven feet spreads its output over a wider, thinner circle; a wall-mounted infrared unit at eight feet, aimed down, produces a tighter, more intense cone.

Practically: mount radiant heaters at seven to nine feet and aim them at the chairs, not at the middle of the space. And accept that a tower's coverage falls off with distance rather than stopping at an edge — the published radius is where the effect becomes negligible, not a boundary.

Everything above assumes an open patio. Put a roof over the same heater and the effective coverage grows substantially, because the warmed air that would have risen away is now trapped and circulating.

That is the same effect the enclosure multiplier in the BTU calculator accounts for, seen from the other direction. It is also why a modest electric unit performs far above its published output under a pergola, and why the covered patio page recommends much smaller heaters than an open patio of the same size.