There is no honest BTU-per-square-foot answer for an outdoor patio, and every page that gives you one made it up. BTU per hour describes how fast an appliance burns fuel, not how much warmth reaches a person. Outdoors there is no envelope holding heat in, so the number cannot be converted into a heated area the way an indoor furnace rating can. Size by seats and distance instead: count the chairs you want warm, put the emitter close enough to reach them, and confirm the clearances in the manual fit that spot.
That is the whole answer. The rest of this explains why the arithmetic on page one falls apart, and what to do instead.
Before any of the sizing: this is a combustion appliance
A propane or natural gas patio heater burns fuel and produces carbon monoxide, which is colorless, odorless and invisible. Every one of these appliances is labeled for outdoor use only, and the label is not boilerplate.
The US Consumer Product Safety Commission advises against using portable propane space heaters indoors or in any confined space unless they are specifically designed for indoor use, and it warns that gas-fired radiant heaters run in small, poorly ventilated enclosed areas such as a tent or trailer present a carbon monoxide poisoning hazard that can cause death. WorkSafeBC's risk advisory on carbon monoxide in outdoor dining spaces, published July 2021, is blunter still for anyone tempted to wrap a patio in plastic for the season: "The best way to reduce the risk of exposure to CO when heating enclosed outdoor spaces is to use electric heaters, rather than natural gas or propane." It also notes that most manufacturers state gas and propane heaters should never be used in an enclosed area.
This belongs at the top of a sizing article for one specific reason. When a heater does not warm the space, people enclose the space. They add side panels to the gazebo, zip the canopy shut, run it in the open garage. That is the exact sequence the two authorities above are describing, and it starts with someone expecting a BTU number to heat a patio that it was never going to heat. Getting the expectation right is a safety matter, not just a shopping one.
Electric infrared heaters have no combustion and no combustion gases, which is why WorkSafeBC names them as the safer option for anything approaching enclosure. They have their own limits, covered below.
What "46,000 BTU" is actually measuring
A British thermal unit is a unit of energy. A rating on a gas appliance is a rate, properly written as BTU per hour, and on a burner it describes the fuel energy consumed at full output. It tells you how thirsty the appliance is. It does not tell you how much of that energy arrives at your shoulders.
Between the burner and you sit several losses that the rating never accounts for: heat going up as hot gas, heat radiating in directions where nobody is sitting, and heat carried away by moving air. Outdoors, all three are large and none is fixed.
Two appliances with the same number can feel completely different, because the emitter shape, the reflector and the mounting height decide where the energy goes. We have taken the same line on other headline specs on this site, including what a leaf mulcher's reduction ratio really means once the leaves are wet: the number is real, and it is measuring something narrower than the buyer assumes.
Why an outdoor rating cannot become a heated area
Indoor heating equipment gets sized by a published method. ANSI/ACCA 2 Manual J, the residential load calculation standard, is built around a building envelope: insulation, windows, doors, infiltration, orientation, climate data. It works because a house is a box that loses heat at a calculable rate, so you can size a system to replace those losses and hold a temperature.
A patio has no box. Take the walls, roof and insulation out of that calculation and there is nothing left to solve. There is no volume of air to hold at a temperature, because the air you warm leaves.
Three things follow, and they are the reason the per-square-foot rules cannot be rescued:
- You are not heating air, you are heating people. Outdoor heaters work mostly by radiant heat, which travels in straight lines from the emitter to whatever it strikes. You feel it on the side facing the heater. Step behind a chair back and it is largely gone.
- Distance is punishing. Radiant intensity from a small source falls off sharply with distance. As a rough physical rule, double your distance from the emitter and roughly a quarter of the intensity reaches you. Doubling the BTU rating does not undo that, because the falloff is geometry.
- Wind removes the rest. Any convected warmth is carried off by air movement, and moving air also strips heat from exposed skin faster, which is the effect the National Weather Service wind chill index describes. A breeze that would be pleasant in July can erase a heater in October.
Area-based sizing works when a machine physically covers ground. That is why what size lawn sweeper you need is a question with a real answer and this one is not. A sweeper's width times your yard gives passes. A heater's BTU times your patio gives nothing.
The rules of thumb on page one, and what they do to one small patio
Search this question and you get confident arithmetic from home improvement retailers, appliance stores, propane suppliers and patio heater sellers. The page-one set we read on 2026-09-02 included Lowe's, BBQGuys, Ferrellgas, Abt and Woodland Direct. Not one of them cites a standards body, a test method, or any source at all for its multiplier.
Here is what those published rules make of the same twelve by twelve foot patio, 144 square feet. We are printing this to show the spread, not as guidance.
| The rule as published on page one | What it makes of a 12 by 12 patio |
|---|---|
| 20 BTU per hour per square foot | About 2,900 BTU per hour |
| 30 to 40 BTU per hour per square foot for most North American climates | About 4,300 to 5,800 |
| 50 to 60 BTU per hour per square foot in cold climates | About 7,200 to 8,600 |
| One heater per 300 to 350 square feet of uncovered patio | One full-size unit, commonly sold at around 40,000 to 50,000 |
The first row and the last row appear in the same body of advice, sometimes on the same page, and they are roughly sixteenfold apart. A set of rules that disagrees with itself by that margin is not a measurement. It is a habit.
There is a second formula in circulation: cubic feet times desired temperature rise. That one is an indoor load formula. It requires an enclosed volume and a temperature you intend to hold, and a patio supplies neither.
We are not going to print a BTU-per-square-foot figure, including a cautious one, because we cannot name a body that published one or a test that produced one. On safety-adjacent equipment, a number we cannot stand behind is worse than no number.
What nobody publishes, and we did look
There is a real standard for these appliances. CSA/ANSI Z83.26 and CSA 2.37, "Gas-fired outdoor infrared patio heaters", is published by CSA Group as a National Standard of Canada and approved by ANSI as an American National Standard. The current third edition was published in 2020, superseding the 2014 and 2007 editions, and it covers newly produced gas-fired outdoor infrared patio heaters for natural gas or propane. It is a standard for construction, safe operation and acceptable performance, and it is the mark worth having on an appliance you are going to run near people.
What we could not find, searching on 2026-09-02, is any published test method or rating body that certifies how many square feet an outdoor heater warms. The coverage claims live on packaging and retail listings, they are stated in incompatible units from one brand to the next, sometimes a radius in feet, sometimes an area, sometimes just the BTU number, and none of the ones we read pointed to a procedure. If a coverage standard exists, it is not visible from the buying journey.
That absence is the actual finding. The reason every page gives you a different multiplier is that there is nothing underneath any of them.
So how many BTU of patio heater do you need, in practice
Work from the seat, outward. This is the same order that makes an aerator rental or purchase decision tractable: start from what you actually do, not from the spec sheet.
1. Count the seats you want warm, and measure how far the emitter can be from them. Three or four chairs pulled into a group are a solvable problem. A whole open patio is not, at any BTU rating.
2. Decide the mounting. Freestanding units put the emitter high, which spreads a weak cone over a wider circle. Tabletop units put a small emitter close, which is more effective per unit of fuel and reaches almost nobody else. Wall and ceiling mounted electric infrared aims a fixed cone at a fixed area, and manufacturers publish a mounting height range for their models. Getting that height wrong wastes more output than choosing the wrong BTU number would.
3. Pick the fuel for how you will use it. Propane is portable and refillable and the tank runs down on cold nights. Natural gas is permanent and needs a gas line. Electric infrared has no combustion, no tank and no fumes, and needs a suitable circuit.
4. Read the manual before you buy, not after. Manuals are downloadable from the manufacturer while you can still choose a different model. Look for the minimum clearance to combustibles, which is stated separately for the sides and for the top and is not the same figure from one model to the next, whether a tip-over shutoff is fitted, and any statement about use under an overhead structure. If those clearances do not fit the spot you had in mind, that is the buying decision, not the BTU rating.
5. Expect a heated seat, not a heated patio. More units placed near people beat one large unit placed for symmetry, and a windbreak that is not an enclosure does more than extra output does. The wind is the variable that decides your evening.
How little heat they really give
This is the part the category does not say out loud. A patio heater extends the usable season at a specific seat by a modest margin. It does not turn a cold patio into a warm room, and no BTU number available to a homeowner will.
If you are honest about what the appliance does, it is a good purchase: the difference between going inside at eight and staying out until ten. If you bought it expecting the patio to feel like the living room, you will be disappointed at any price, and you will be tempted to enclose the space, which is where this stops being a comfort question. We take the same view of lawn aerator shoes and of the hopper capacity printed on a lawn sweeper: the honest answer is about what the thing does at the low end of its claims, not the high end.
Where this article stops
A permanently installed natural gas heater involves gas fitting, which is licensed work in every jurisdiction we are aware of, and it is a licensed gas fitter's job, not a weekend one. Do not connect, extend or alter a gas line, a regulator or an orifice.
A hardwired electric infrared heater is a licensed electrician's job, and outdoor circuits carry their own requirements for weatherproof enclosures and ground fault protection. A plug-in unit is homeowner territory only if the circuit is rated for it and the cord is the one the manufacturer supplied.
Any statement about clearance, overhead structures or ventilation for your appliance comes from your manual and your local authority. Not from us, and not from a retailer's buying guide.
Frequently asked questions
How many BTU patio heater do I need? There is no defensible BTU-per-square-foot answer for an outdoor space, because a patio has no envelope to hold heat and no standards body publishes a coverage rating. Size by seats and distance: count the chairs you want warm, place the emitter close enough to reach them, and check that the manual's clearances fit that position.
Does a higher BTU rating mean a warmer patio? Not reliably. BTU per hour describes fuel energy consumed, not warmth delivered. Emitter shape, reflector, mounting height, your distance from it and the wind all decide what reaches you, and radiant intensity from a small source falls off sharply with distance, so doubling the rating does not double the comfort at your chair.
Why does every site give a different BTU per square foot number? Because none of them is measuring anything. The page-one rules we read on 2026-09-02 ranged from 20 to 60 BTU per hour per square foot, alongside a separate rule of one heater per 300 to 350 square feet of uncovered patio, which is roughly sixteen times more heat for the same space. No source names a test method or an authority.
Is a patio heater safe under a gazebo or a canopy? Gas and propane heaters are labeled for outdoor use only, and CPSC warns that gas-fired radiant heaters in small, poorly ventilated enclosed areas present a carbon monoxide hazard that can cause death. WorkSafeBC advises electric heaters rather than gas or propane wherever an outdoor space is being enclosed. Your specific appliance's manual states whether any overhead use is permitted, and if it does not name your situation, the answer is no.
Is electric infrared or propane better for a small patio? For a small, fixed seating group close to a wall or ceiling mount, electric infrared aims its output where people sit, has no combustion gases and needs no tank. Propane suits a group that moves around a yard with no power nearby. Running cost is a separate question that depends on local fuel and electricity rates.
Yard Insight has not tested a patio heater, and this article makes no claim otherwise. No BTU-per-square-foot figure, coverage area or clearance distance is recommended anywhere above, deliberately. CPSC and WorkSafeBC are named as the authorities for the carbon monoxide guidance quoted, and CSA/ANSI Z83.26 with CSA 2.37 as the construction and safe-operation standard for gas-fired outdoor infrared patio heaters; the page-one sizing rules quoted in the table are attributed to the retailer and supplier pages they came from and are not endorsed. Sources read 2026-09-02. If anyone develops headache, dizziness, nausea or confusion around a fuel-burning heater, get into fresh air and seek medical advice.
