What is AC tonnage?
A ton measures cooling capacity, not weight. Here is how tons relate to BTUs and why the right number depends on your house.
AC tonnage is a measure of cooling capacity, not how much the unit weighs. One ton equals 12,000 BTU per hour of heat removed from your home. A 3 ton air conditioner removes 36,000 BTU per hour.
The word comes from the old days of ice. One ton of cooling is roughly the heat needed to melt a ton of ice over 24 hours. You will never need to think about ice again, but the unit stuck.
How tons and BTUs relate
Residential central systems usually come in half-ton steps. The table shows the conversion.
| Tons | BTU per hour |
|---|---|
| 1.5 | 18,000 |
| 2 | 24,000 |
| 3 | 36,000 |
| 4 | 48,000 |
| 5 | 60,000 |
Mini-splits and window units usually carry a BTU label. Central systems carry a rating in tons. Converting between them is simple: divide BTU by 12,000.
More tons does not mean better cooling
This is the commonest mistake. A bigger unit cools the air quickly, then shuts off before it has removed much moisture. You get a house that feels cool and sticky, plus frequent starts and stops that wear parts.
Too small is a problem too. An undersized unit runs most of a hot afternoon and still falls behind.
What decides the right tonnage
Your house decides it, not a chart. These factors move the number:
- Climate and the hottest days the system must handle
- Attic and wall insulation
- Window area and direction
- Air leaks and the condition of the ducts
- Ceiling height, shade and number of occupants
Two homes with the same floor area can need different tonnage.
Ask for a load calculation
The standard method is a Manual J load calculation from ACCA. It adds up heat gain room by room. If a contractor picks a size by looking at the old unit or by square footage alone, ask them to show their numbers.
Where to find your current tonnage
The nameplate on the outdoor unit usually lists the capacity in BTU. Many model numbers also carry a two digit code divisible by 6, such as 36 for 3 tons, though the pattern varies by brand. Do not assume the existing size was correct. It may have been wrong from day one.
Our advice
Do not buy tonnage off a chart and do not copy the old size. Get at least two quotes, ask how each contractor chose the number and favor the one who shows a load calculation. Sizing and installation are work for a licensed professional.
Frequently asked questions
What does 1 ton of AC mean?
One ton equals 12,000 BTU per hour of cooling capacity. It describes how much heat the unit can remove, not its weight.
How many BTU is a 3 ton air conditioner?
A 3 ton unit is rated at 36,000 BTU per hour. Multiply tons by 12,000 to convert.
Is a higher tonnage AC better?
No. An oversized unit short cycles and leaves the house cool but humid. The right size comes from a load calculation of your own home.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Energy Savings at Home: Heating & Cooling
- US EPA. Section 608 of the Clean Air Act