By Mario Vaghei, General Manager, Aire One Peel Heating & Cooling · Published November 9, 2026 · Facts verified November 5, 2026
Short answer
A BTU (British Thermal Unit) is the amount of energy needed to raise one pound of water by one degree Fahrenheit. On heating and cooling equipment the number is per hour: an air conditioner's BTU rating is how much heat it can remove in an hour, and a furnace's is how much heat it can make. One ton of cooling equals 12,000 BTU per hour. A bigger number is not better. The right size comes from a load calculation for your house, not from the square footage alone.
On this page
What a BTU measures · Tons and BTU: the cooling side · Input and output: the furnace side · How the right size is chosen · Why bigger is not better · What this means when you get a quote · Frequently asked questions
What a BTU measures
Carrier defines a BTU as "the amount of energy required to raise one pound of water by one degree Fahrenheit." Heating and cooling equipment is rated in BTU per hour, written BTU/h or BTUh.
- Air conditioners and heat pumps: the rating measures "the amount of heat an AC unit can remove per hour."
- Furnaces: BTU/h is "the number of BTUs a furnace can generate per hour."
Tons and BTU: the cooling side
Cooling equipment is often sold in tons. Carrier's conversion is simple: "One ton of cooling capacity is equivalent to 12,000 BTUs" per hour.
| Size in tons | Cooling capacity |
|---|---|
| 1.5 tons | 18,000 BTU/h |
| 2 tons | 24,000 BTU/h |
| 2.5 tons | 30,000 BTU/h |
| 3 tons | 36,000 BTU/h |
| 3.5 tons | 42,000 BTU/h |
| 4 tons | 48,000 BTU/h |
| 5 tons | 60,000 BTU/h |
Table: arithmetic from Carrier's conversion of 12,000 BTU/h per ton ("What is BTU?", read November 5, 2026).
Input and output: the furnace side
A furnace has two BTU numbers. The input is the fuel it burns. The output is the heat that reaches your home. The link between them is AFUE, which Carrier defines as "the percentage of fuel converted into usable heat for your home."
| Furnace input | AFUE | Heat delivered |
|---|---|---|
| 40,000 BTU/h | 95% | about 38,000 BTU/h |
| 60,000 BTU/h | 80% | about 48,000 BTU/h |
| 60,000 BTU/h | 96% | about 57,600 BTU/h |
Table: the first row is Carrier's own example ("What is a 40000 BTU Furnace?"); the other rows are the same arithmetic. Read November 5, 2026.
So two furnaces with the same input can deliver different amounts of heat. When you compare quotes, compare output and AFUE, not only the input number on the label.
How the right size is chosen
Carrier is direct about this: "square footage alone should not be used to determine the correct furnace size." The size should come from a load calculation that considers climate, insulation, windows, ductwork and the design of the home. In the United States that calculation is ACCA Manual J. In Canada the residential method for the same job is CSA F280.
Rules of thumb exist, and they are only that:
- Carrier's cooling baseline of 20 BTU per square foot is, in its own words, "a standard rough estimate". It is a United States figure.
- Carrier's furnace reference table gives a 60,000 BTU/h furnace an approximate range of 1,200 to 2,000 square feet, "depending on factors such as climate and insulation", and calls such ranges "general reference ranges, not equipment-sizing recommendations."
Our furnace size guide for Mississauga homes goes through the factors one by one.
Why bigger is not better
- Too big: Carrier says "Oversized furnaces heat spaces too quickly, causing them to cycle on and off frequently", which it links to uneven temperatures, reduced efficiency and more wear. That on-and-off pattern is furnace short cycling.
- Too small: "Undersized units run continuously without adequately heating the home."
What this means when you get a quote
Ask each contractor what the size is based on. A quote that names a load calculation is doing the job properly; one that only matches the label on your old unit may be repeating an old mistake. Our guides to what a new furnace costs in Mississauga and air conditioner installation cost in Mississauga show how size affects price. Call 905-564-8545 for a free in-home estimate.
Frequently asked questions
What does BTU stand for?
BTU stands for British Thermal Unit. Carrier defines it as the amount of energy required to raise one pound of water by one degree Fahrenheit.
How many BTU is one ton of air conditioning?
One ton of cooling capacity equals 12,000 BTU per hour, so a 2-ton air conditioner is 24,000 BTU/h and a 3-ton unit is 36,000 BTU/h.
Is a higher BTU furnace better?
No. An oversized furnace heats the space too quickly and cycles on and off frequently, which Carrier links to uneven temperatures, reduced efficiency and more wear. The right size matches the home's heat loss.
What is the difference between input and output BTU on a furnace?
Input is the fuel the furnace burns and output is the heat delivered to the home. AFUE is the percentage converted: Carrier's example is a 40,000 BTU furnace at 95% AFUE delivering about 38,000 BTU of heat.
How many BTU do I need for my house?
There is no reliable answer from square footage alone. Carrier says the size should come from a load calculation that considers climate, insulation, windows, ductwork and home design. In Canada the residential method is CSA F280.
Sources and references
- Carrier, "What is BTU?", read November 5, 2026.
- Carrier, "What is a 40000 BTU Furnace?", read November 5, 2026.
- Carrier, "Calculating Furnace Size", read November 5, 2026.