Furnace size calculator (BTU)
Enter your heated floor area, the age of the house and your region to get a rule-of-thumb heat loss and the common furnace sizes that cover it. Use it to sense-check a quote; a CSA F280 room-by-room load calculation is the proper sizing method in Canada.
Estimate your furnace size
Estimates update as you change the inputs. Nothing is sent anywhere.
Estimated heat loss on the coldest days
Rule of thumb for your inputs
Ticks mark the output of common furnace sizes, labelled by input.
Output to aim for
Nearest common size
About 76,800 BTU/h output at 96%
A 60,000 BTU/h input furnace (about 57,600 output) falls just short of this range. A load calculation decides between the two.
- Above grade1,800 sq ft × 34 to 40
- Finished basement
- Colder region+10%
A CSA F280 room-by-room load calculation is the proper sizing method in Canada.
Get the real number for your home: a free, no-obligation quote.
Rule of thumb only: 25 to 45 BTU/h per sq ft depending on age and insulation, calibrated for southern and eastern Ontario. Homes in Northern Ontario, and any home with major additions or renovations, need a measured load calculation.
On this page
Input BTU and output BTU are different numbers
Furnace size is quoted as input: the gas it burns per hour, in BTU/h. What heats your home is output, input multiplied by efficiency. A 96% furnace turns 60,000 BTU/h of input into about 57,600 BTU/h of heat; a 1990s mid-efficiency furnace at about 80% delivers only 48,000 from the same input. So copying the number on your old furnace's label is a mistake: the same input in a new high-efficiency furnace is already a step up in heat.
| Input | Output at about 96% | Output at about 80% (older mid-efficiency) |
|---|---|---|
| 40,000 BTU/h | 38,400 | 32,000 |
| 60,000 BTU/h | 57,600 | 48,000 |
| 80,000 BTU/h | 76,800 | 64,000 |
| 100,000 BTU/h | 96,000 | 80,000 |
| 120,000 BTU/h | 115,200 | 96,000 |
Why a bigger furnace is not better
An oversized furnace reaches the thermostat setting quickly, shuts off and restarts a few minutes later. That short cycling brings temperature swings, noisy blasts of hot air, more starts on the igniter and inducer, and less time at steady, efficient output. Ducts sized for a smaller furnace can struggle with the extra airflow: whistling registers and shutdowns on the high-limit switch are the signs.
Many older Ontario homes have furnaces bigger than they need. The first one was often sized generously, the house has since gained insulation, better windows and air sealing, and each replacement copied the old label. Two-stage and modulating furnaces soften the problem because they spend most of the winter on low, but they are no reason to skip proper sizing. Settle the size first, then check typical Ontario furnace prices for the type you want.
Central air changes the answer
The furnace blower also moves air for the air conditioner, roughly 400 cubic feet per minute per ton of cooling. A 3-ton AC needs about 1,200 CFM, which can rule out the smallest furnaces even when the heat loss is low. The AC size calculator covers the cooling side.
How this calculator works it out
Heat loss per square foot
Age and insulation pick a band inside the 25 to 45 BTU/h per sq ft rule of thumb (table below).
Climate and basement
Central and eastern Ontario adds about 10%. A finished, heated basement counts at about half the above-grade rate, because the soil around it stays far warmer than winter air.
Output and size
The middle of the range is the output to aim for, because the rule of thumb is already generous. The tool picks the smallest common input size whose output, at about 96%, covers it, and names the next size up only when that pick falls short of the top of the range.
| Home age and insulation | BTU/h per sq ft | For 1,800 sq ft |
|---|---|---|
| Newer and airtight | 25 to 30 | 45,000 to 54,000 |
| 1990s to 2010s | 29 to 35 | 52,000 to 63,000 |
| 1960s to 1980s | 34 to 40 | 61,000 to 72,000 |
| Before 1960 or drafty | 39 to 45 | 70,000 to 81,000 |
Worked example
A 2,000 sq ft two-storey house in London, built in 1995, with an unfinished basement. The 1990s to 2010s band gives 29 to 35 BTU/h per sq ft, so the heat loss estimate is 58,000 to 70,000 BTU/h. Southwestern Ontario needs no climate adjustment.
The output to aim for is about 64,000 BTU/h, the middle of that range, and the smallest common furnace that covers it is 80,000 BTU/h input (about 76,800 output). Furnaces come in steps of about 20,000 BTU/h, so the pick can land well above the middle. A 60,000 input furnace (about 57,600 output) falls just short. That is the decision a room-by-room load calculation should settle: if the house measures below about 57,600 BTU/h, the smaller furnace fits better and usually costs less. The furnace cost calculator prices either size.
What a proper load calculation looks at
A CSA F280 calculation works room by room instead of using one number for the house. Ontario's building code points to it for sizing heating in new houses, and it is good practice for replacements.
- Each room separately, so every room gets the right airflow
- Insulation in the walls, attic and basement, and the window area and type in each room
- Air leakage, ideally from a blower door test if the home has had an energy assessment
- The winter design temperature for your town, several degrees colder in Ottawa or Barrie than in Windsor
- Ducts that run through unheated spaces such as an attic, garage or crawlspace
Ask for the heat-loss figure in writing with each quote, and if two installers recommend different sizes, ask each to show the number behind it. The furnace price guide and furnace buying guide cover what else to compare.
Get a quote sized for your home
Ask for a free in-home estimate from a participating local heating and cooling company, and ask them to show the heat-loss number behind the size they recommend.
Free, no obligation. About a minute to request.
Questions people ask
What size furnace do I need for a 2,000 sq ft house in Ontario?
By the 25 to 45 BTU/h per sq ft rule of thumb, a 2,000 sq ft home loses about 50,000 to 90,000 BTU/h on the coldest days, depending on age and insulation. That points to furnaces between 60,000 and 100,000 BTU/h input. A tight, well-insulated home often measures lower, which is why a load calculation matters before you choose.
What is the difference between furnace input and output BTU?
Input is the gas the furnace burns per hour; output is the heat it delivers to your ducts. Output equals input times efficiency. A 96% furnace rated at 80,000 BTU/h input delivers about 76,800 BTU/h. An older 80% furnace with a 100,000 input delivers about 80,000, nearly the same heat as the smaller new furnace.
Can I size a furnace from the old one?
Only as a starting point. Many older furnaces were oversized to begin with, and the house may have had insulation, window or air sealing upgrades since. A new high-efficiency furnace also delivers more heat per BTU of input than an old mid-efficiency one. Ask the installer for a heat-loss calculation rather than a nameplate match.