Start with the honest version: there is no square-footage formula for an outdoor heater, and anyone who gives you one is guessing with confidence. Indoor sizing rules work because a room has an envelope — walls, a ceiling, insulation — that holds warmed air in, so the volume you are heating is a real, bounded quantity. A patio has none of that. The air you warm rises and leaves. What actually reaches you is radiant heat traveling in a straight line from the element to your skin, exactly like sunlight.
That single fact changes the whole question. The variables that matter are how far you sit from the heater, whether anything is between you and it, and how windy it is— not how many square feet of paving you own. Radiant heat follows an inverse-square relationship: double your distance and you feel roughly a quarter of the warmth. Sit three feet from a modest heater and you are warm; stand twelve feet from a huge one and you are not.
So the sizing question becomes a much more useful one: who is sitting where? Answer that and the tier falls out immediately.
The four tiers, by who is sitting where
Every patio heater on the market lands in one of four bands. Find your situation and the band is the answer.
9,000 BTU and below — one or two people, close.A small radiant propane heater on a 1 lb cylinder. On an open patio this is nearly useless at any real distance; a couple of feet away, pointed at you, it is plenty. This is the tier for a workbench, a blind, an ice shanty or a porch step where you are the only person who needs warming. The Mr. Heater Portable Buddy publishes 9,000 BTU and is the default in this band, partly because it is one of the few small heaters that publishes its safety hardware — a tip-over switch and an oxygen depletion sensor.
10,000 to 11,000 BTU — one table. A tabletop unit. This looks absurdly small next to a standing heater, and it works anyway, because everyone at a table is within two or three feet of the middle of it. A tenth of the output at a quarter of the distance delivers more to your hands and chest than a big heater across the patio does. If your evenings happen at a table rather than in a circle of chairs, this is the efficient buy and a seven-foot mushroom heater is overkill you also have to store. Best tabletop patio heaters ranks that tier.
40,000 to 50,000 BTU — a seating circle. The standard freestanding mushroom or pyramid heater, and the right answer for most patios. The reflector at the top spreads the output downward and outward over a group rather than a point, which is what you want when four to six people are seated around it and some of them are standing up and moving. The Amazon Basics unit publishes 46,000 BTU and is the cheapest full-size heater worth buying; best propane patio heaters covers the tier properly.
50,000 to 80,000 BTU — a volume, not a patio. At this output you are no longer buying a patio heater, you are buying a space heater that happens to run on propane. The Dura Heat LPC80 publishes an adjustable 50,000 to 80,000 BTU range and is a convectionheater rather than a radiant one — it warms the air in a 360-degree column and lets it circulate, which only makes sense where there is a volume of air to warm. An open garage, a barn, a shop, a large tent. On an open patio it is wasteful, because the air you paid to heat blows away.
Radiant or convection — the distinction that decides the tier
This matters more than the BTU number and almost nobody mentions it.
Radiant heaters— every mushroom heater, pyramid heater, tabletop unit and electric infrared panel — warm objects in their line of sight. They do not meaningfully warm the air. That is a feature outdoors: there is no point heating air that is about to leave. It also means they are directional and distance-sensitive, and that a person behind a chair back or a table edge gets much less than the number suggests.
Convection heaters warm the air itself. Outdoors in the open that is money burned, which is why convection units are sold for garages, shops and tents rather than patios. In a part-enclosed space they are the better answer, because the volume stays put long enough to get warm.
So: open patio, radiant, size by distance. Enclosed or part-enclosed volume, convection, and now ventilation becomes the governing concern rather than output.
Propane, electric and the watt conversion
Comparing a propane heater to an electric one in BTU is straightforward, and the result surprises people. One watt is about 3.4 BTU per hour, so a 1,500 W electric heater is roughly 5,100 BTU— about half an 11,000 BTU tabletop propane unit, and roughly a ninth of a 46,000 BTU standing one.
That is a real gap and it is not the whole story. A 1,500 W plug-in heater is the practical ceiling of a standard household circuit, so if you want more electric output than that you are into 240 V hardwired territory. What electric buys instead is placement: no combustion means no carbon monoxide question, so an electric infrared unit can go where no propane heater should — a screened porch, a three-walled nook, a tight gazebo. Where that is your situation, size down and go electric rather than up and propane. Propane vs electric patio heater has the running-cost half of the argument.
One big heater or two small ones?
Almost always two small ones, and the inverse-square relationship is why.
Put one 80,000 BTU heater in the middle of a long patio and the people nearest it are uncomfortably hot while the ends of the table are cold — because the person at six feet is getting roughly a quarter of what the person at three feet gets. Put a 46,000 BTU heater at each end and both groups sit in the useful range. Same total fuel, dramatically better result.
The rule, then: count seating groups, not square feet. One heater per group of four to six seated people, positioned within a few feet of them rather than at the geometric center of the patio. If your patio has one table and one lounge corner, that is two heaters, and no single unit of any size will do the job as well.
What ruins a correctly sized heater
Three things, in order of how often they are the real problem:
- Wind.By a wide margin the biggest factor, and the one no amount of extra BTU fixes — a breeze strips warmed air away and carries radiant benefit with it by forcing you to sit further back from a flame that is being blown about. The answer is a windbreak and closer placement, not a bigger unit. A sheltered patio at 46,000 BTU beats an exposed one at 80,000.
- Distance. People put heaters at the edge of the patio, out of the way, and then conclude the heater is too small. Move it in. Three feet closer is worth more than ten thousand extra BTU.
- An empty base, or no shelter at all for the unit itself.Not a heat issue but the usual way heaters die: water in the burner or the ignition. Fill a water-fillable base, and cover the unit when it is cool — patio heater covers costs a fraction of the heater.
Where enclosure overrides size entirely
One case where you should stop thinking about BTU and think about ventilation instead.
Anything that burns propane, natural gas or kerosene produces carbon monoxide, and the quantity scales with the output — so the bigger the heater, the more combustion products in the same place. Outdoors in moving air that is a non-issue. On a screened porch, under a tight gazebo, in a garage with the door part open, or in a tent, it is a real one. The CPSC reports more than 200 people in the United States dying every year from accidental non-fire carbon monoxide poisoning associated with consumer products, and recommends CO alarms on every level of a home and outside sleeping areas.
In those spaces: choose an electric infrared heater, or if it has to be combustion, choose an appliance actually rated for it, ventilate deliberately and run a CO alarm. The longer version is in carbon monoxide and outdoor heaters and is a propane heater safe indoors. For covered patios specifically, best patio heaters for covered porches is picked on exactly this basis.
A four-step sizing routine
- Count the people and how they sit. One or two close up, a table, or a circle of chairs. That picks your tier before you look at a single product.
- Measure the distance from where the heater can realistically stand to the nearest and furthest person. If that spread is more than about six feet, you want two heaters rather than one bigger one.
- Decide how enclosed the space is. Open: propane, any tier. Part-enclosed or roofed: electric, and size down without apology.
- Check the fuel logistics. A 20 lb tank needs outdoor, upright storage and is not included with most heaters; 1 lb cylinders are convenient and expensive per hour; electric needs a socket you actually have outside. How long propane heaters last covers the run-time side.
When the answer is not a heater at all
If what you want is something to sit around rather than warmth delivered to your shoulders, a heater is the wrong purchase and no amount of sizing will fix it. A fire pit is a focal point that produces heat as a side effect; a patio heater is an appliance that produces heat and nothing else. Patio heater vs fire pit makes that comparison honestly, and if the budget is the constraint, fire pits under $100 gets you a flame for less than most of the heaters on this page.
