The most common patio heater mistake isn't buying the wrong fuel — it's buying the wrong size. An undersized heater run flat-out still leaves guests cold, and an oversized one wastes money every hour it runs. Sizing comes down to one calculation plus three adjustments, and you can do it in five minutes with a tape measure. Here's the whole method.
The Base Calculation: BTUs per Square Foot
Measure the area you actually need to heat — not your whole patio, just the zone where people sit or stand. For open outdoor space, the working rule is roughly 30–40 BTU per square foot in mild conditions, rising to 50–60 BTU per square foot for cold or exposed sites. So a 200 sq ft dining zone needs around 6,000–8,000 BTU on a mild evening, but 10,000–12,000 BTU to stay comfortable on a cold, breezy one. Size for the conditions you'll host in, not the best-case night.
Quick Reference: Coverage by Heater Output
- ~11,000 BTU (typical tabletop propane): up to ~50 sq ft — a bistro table, two or three seats
- ~20,000 BTU (2,000–6,000W electric, mounted): ~60–110 sq ft per unit — a covered seating zone or dining nook
- 34,000–40,000 BTU (standard freestanding propane): ~150–200 sq ft — the classic 8–10 ft radius circle around the post
- 50,000–75,000 BTU (high-output gas, commercial-grade): ~200–300+ sq ft per unit — large patios, restaurant terraces, event spaces
The Three Adjustments That Change Your Number
Wind exposure: open, breezy sites can lose a third or more of effective coverage — round up a size, or add wind screening. Ceiling height: mounted radiant heaters have a sweet spot around 8–9 ft; every foot above that spreads the same heat over more area, so high pergolas need higher output or angled mounting. Coverage shape: a freestanding mushroom heater throws a circle; wall- and ceiling-mounted units throw a directional rectangle. Long, narrow spaces are almost always better served by two smaller directional units than one big central one — heat where the people are, not the geometric center.
Sizing Larger Spaces: Zones, Not One Big Unit
Past ~250 sq ft, stop shopping for a bigger heater and start planning zones. Divide the space into seating areas, size each zone with the math above, and place a unit per zone. This is exactly how restaurant terraces do it — consistent warmth at every table rather than a hot spot by the heater and cold corners everywhere else. For high-traffic or hospitality settings, purpose-built units run longer duty cycles and mount out of the way: browse our commercial patio heater collection. For homes, flexible placement wins: see the freestanding patio heater collection to position heat exactly where your seating lives.
Worked Example: A 12 × 16 ft Patio
That's 192 sq ft. Sheltered and mild climate: 192 × 35 ≈ 6,700 BTU — one mounted electric unit or a tabletop propane heater per seating cluster covers it. Exposed site, cool shoulder-season evenings: 192 × 55 ≈ 10,600 BTU — one standard 34,000 BTU freestanding unit handles it with headroom (rated coverage matters more than raw BTU here), or two directional mounted units at either end for even warmth. Still deciding between fuels? Start with our gas vs. electric vs. infrared buying guide, then come back to the math.
Every patio heater at Wood Fire Home ships free within the US. Bromic models carry a 1-year parts warranty — with a 7-year element warranty on Tungsten Electric heaters — and all other brands are backed by their manufacturer's warranty; check the specific model's terms on its product page.
By Matthew Murphy
