What size pool heater do I need?

Work out both numbers and buy the bigger one. To heat from cold, the published Canadian method is gallons times 8.33 pounds times the rise in degrees Fahrenheit, divided by the hours you will wait. To hold a temperature, it is 10 BTU an hour per square foot of water surface per degree the water sits above the air. Pool Supplies Canada, whose method this is, also says to overestimate by about 20 percent for evaporation and weather. A 50,000 litre pool with 392 square feet of surface, lifted 13 degrees Celsius in a day against 18 degree air, lands near 300,000 BTU per hour.

Why is a pool heater sized in gallons when I measure in litres?

Because pool heaters are American products rated in BTU per hour, and the sizing method was written to match. There is nothing wrong with the method, it just arrives in the wrong units for a Canadian backyard. This page does the conversion once, in the open: your litres become US gallons at 3.785412 litres each per NIST, and a Celsius temperature DIFFERENCE becomes Fahrenheit by multiplying by 1.8, with no 32 degree offset because a gap between two temperatures does not carry one.

Does the size of the pool or the surface area matter more?

Both, for different jobs. Heating a cold pool is about volume: you are warming every litre, so a deep pool takes longer than a shallow one of the same shape. Holding a temperature is about surface, because that is where the heat leaves. That is why the two halves of the calculation use different measurements, and why a wide shallow pool can be harder to keep warm than a small deep one even though it takes less energy to bring up in the first place.

Is a gas heater or a heat pump better in Ontario?

A gas heater delivers its rated output whatever the weather; a heat pump moves heat out of the air, so its output falls as the air cools and most units want air above about 10 degrees Celsius to work well. In an Ontario May and September, which is exactly when you want the heater, that difference is the whole argument. Running costs point the other way: Apex Electric Mechanical puts a heat pump at $80 to $200 a month against $250 to $600 for natural gas.

Will a solar cover really change the size I need?

It changes the holding number more than any heater upgrade will, because most of what a pool loses overnight is evaporation off the surface rather than conduction through the shell. Pool Supplies Canada makes the point in the same guide: a solar cover reduces the time and energy the heater needs. It is the cheapest thing on the equipment pad and the only one that works while switched off.

What does a pool heater cost installed in the GTA?

Apex Electric Mechanical puts a natural gas heater at $3,500 to $9,500 all in for Toronto and the GTA and an electric heat pump at $5,000 to $11,000, while Swimpool 360 quotes $2,500 to $5,500 gas and $4,500 to $8,000 heat pump and PF Pools $3,000 to $5,500 gas. On top of the unit sit a gas line run at $15 to $50 a foot, electrical work at $500 to $2,500, a permit at $150 to $500 and TSSA or ESA inspection fees.

What size heater does this pool need?

Last updated:August 28, 2026By The Pool Trade Desk

Two questions, and the heater has to answer the harder one: how fast it can bring the pool up, and whether it can hold the temperature once it is there. Enter litres and degrees Celsius; the conversion to the gallons and Fahrenheit heaters are rated in is shown in the result.

How big does the heater have to be?

Water in the pool (litres)

Water surface area (sq ft)

Holding a temperature depends on surface, not volume.

Water temperature now (degrees C)

Temperature you want (degrees C)

Typical air temperature (degrees C)

A GTA May evening, not a July afternoon.

Hours you will wait for it (hours)

Halve this and the heater has to double.

Heater to look for

129,000BTU per hour, or about 38 kW

Your pool, in the units a heater is rated in3.785412 litres per US gallon (NIST): 13,200 US gal
Energy to lift the whole pool 13.0 degrees CThat is 23.4 degrees F. Gallons x 8.33 lb x the rise: 2,575,000 BTU
Rate to do it in 24 hoursThe heat-from-cold half of the method: 107,000 BTU/hr
Rate to HOLD it, 18.0 degrees F above the air10 BTU/hr per sq ft per degree F, per Pool Supplies Canada: 71,000 BTU/hr
The harder of the two, plus the published 20 percent marginFor evaporation and weather, which an open Ontario backyard has plenty of: 129,000 BTU/hr

What common sizes would do on this pool

150,000 BTU/hr heaterComfortably above what this pool needs: 17.2 hrs to heat
250,000 BTU/hr heaterComfortably above what this pool needs: 10.3 hrs to heat
400,000 BTU/hr heaterComfortably above what this pool needs: 6.4 hrs to heat

What that costs installed in the GTA

Natural gas heater, supplied and installedApex Electric Mechanical; Swimpool 360; PF Pools: $2,500 to $9,500
Electric heat pump, supplied and installedApex Electric Mechanical; Swimpool 360; PF Pools: $3,000 to $11,000

Get this size quoted by GTA heating companies

A heat pump is rated at a warm-air test condition and loses output as the air cools, so the number on the box is not what you get on a 10 degree May morning. A gas heater does not care about the air temperature and will hit its rating whenever you ask. That is the real difference between the two, and it matters more in Ontario than the efficiency figures do. A solar cover changes the holding number more than any heater upgrade does, because it stops the evaporation that is doing most of the losing.

Two calculations, and why they use different measurements

Method

Heating a cold pool is a volume problem. Every litre has to be warmed, so the energy is the whole mass of water times the rise: gallons, times 8.33 pounds a gallon, times the rise in degrees Fahrenheit. Divide that by the number of hours you are prepared to wait and you have a rate in BTU per hour. Halve the hours and the heater has to double, which is the single most useful thing to understand before reading a quote.

Holding a temperature is a surface problem. Heat leaves a pool mostly by evaporating off the top, so the published rule scales with square feet rather than litres: 10 BTU an hour per square foot per degree the water sits above the air. A wide shallow pool can be harder to keep warm than a small deep one that took longer to heat in the first place. The tool works out both numbers and recommends the larger, plus the 20 percent the method itself asks for.

One conversion is worth spelling out, because getting it wrong is a classic and invisible error. A temperature DIFFERENCE in Celsius becomes Fahrenheit by multiplying by 1.8 and nothing else. The 32 in the everyday formula converts a point on the scale, not a gap between two points, and carrying it into a sizing calculation would add 32 degrees of heating to every pool. The tool multiplies by 1.8 and shows both figures so you can see it did.

What the arithmetic cannot see is Ontario. A heat pump is rated at a warm test condition and its real output falls as the air cools, which matters precisely in May and September when you want the heater most. A gas heater hits its rating whenever it is asked. And a solar cover changes the holding number more than stepping up a heater size does, because it stops the evaporation that is doing most of the losing.

Frequently asked questions

What size pool heater do I need?: Work out both numbers and buy the bigger one. To heat from cold, the published Canadian method is gallons times 8.33 pounds times the rise in degrees Fahrenheit, divided by the hours you will wait. To hold a temperature, it is 10 BTU an hour per square foot of water surface per degree the water sits above the air. Pool Supplies Canada, whose method this is, also says to overestimate by about 20 percent for evaporation and weather. A 50,000 litre pool with 392 square feet of surface, lifted 13 degrees Celsius in a day against 18 degree air, lands near 300,000 BTU per hour.
Why is a pool heater sized in gallons when I measure in litres?: Because pool heaters are American products rated in BTU per hour, and the sizing method was written to match. There is nothing wrong with the method, it just arrives in the wrong units for a Canadian backyard. This page does the conversion once, in the open: your litres become US gallons at 3.785412 litres each per NIST, and a Celsius temperature DIFFERENCE becomes Fahrenheit by multiplying by 1.8, with no 32 degree offset because a gap between two temperatures does not carry one.
Does the size of the pool or the surface area matter more?: Both, for different jobs. Heating a cold pool is about volume: you are warming every litre, so a deep pool takes longer than a shallow one of the same shape. Holding a temperature is about surface, because that is where the heat leaves. That is why the two halves of the calculation use different measurements, and why a wide shallow pool can be harder to keep warm than a small deep one even though it takes less energy to bring up in the first place.
Is a gas heater or a heat pump better in Ontario?: A gas heater delivers its rated output whatever the weather; a heat pump moves heat out of the air, so its output falls as the air cools and most units want air above about 10 degrees Celsius to work well. In an Ontario May and September, which is exactly when you want the heater, that difference is the whole argument. Running costs point the other way: Apex Electric Mechanical puts a heat pump at $80 to $200 a month against $250 to $600 for natural gas.
Will a solar cover really change the size I need?: It changes the holding number more than any heater upgrade will, because most of what a pool loses overnight is evaporation off the surface rather than conduction through the shell. Pool Supplies Canada makes the point in the same guide: a solar cover reduces the time and energy the heater needs. It is the cheapest thing on the equipment pad and the only one that works while switched off.
What does a pool heater cost installed in the GTA?: Apex Electric Mechanical puts a natural gas heater at $3,500 to $9,500 all in for Toronto and the GTA and an electric heat pump at $5,000 to $11,000, while Swimpool 360 quotes $2,500 to $5,500 gas and $4,500 to $8,000 heat pump and PF Pools $3,000 to $5,500 gas. On top of the unit sit a gas line run at $15 to $50 a foot, electrical work at $500 to $2,500, a permit at $150 to $500 and TSSA or ESA inspection fees.

Where these figures come from

Provenance

The sizing method is Pool Supplies Canada's, used unchanged. The unit conversions come from NIST and the Canada Energy Regulator, and the installed prices from three GTA heating companies.

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