Key takeaways
- We won't give you a tonnage from square footage, because the evidence doesn't support one.
- The 2021 IRC calls for equipment sizing from building loads, using Manual S and Manual J or other approved methods.
- One firm's 40 load calculations ranged from 624 to 3,325 square feet per ton.
No square-footage rule can tell you, including for a 2,000-square-foot house. So we don't offer a calculator that gives a whole-house tonnage. Here's why, and what to ask for instead.
What the code asks for
2021 IRC Section M1401.3 says equipment is to be sized using ACCA Manual S or other approved methods, based on loads calculated with ACCA Manual J or other approved methods. Your local code decides what applies.
How much real houses vary
ACCA's blog published 40 Manual J calculations from one firm, mostly in hot and mixed climates:
| Measure | Square feet per ton |
|---|---|
| The common rule of thumb | 400 to 600 |
| Average of the 40 | About 1,431 |
| Range of the 40 | 624 to 3,325 |
For a 2,000-square-foot house, that range is:
| Basis | Tons |
|---|---|
| Rule of thumb | 3.3 to 5 |
| The firm's range | 0.6 to 3.2 |
This is one firm's project set, not a national sample or sizing guidance. It just shows how little floor area predicts by itself. The same post warns that Manual J's own inputs can be inflated, too.
What to ask a contractor
Ask for the load calculation, and go through these inputs. Each one is a question to ask, not a right answer we can give you:
- Which outdoor and indoor design temperatures did you use?
- What window values and orientations did you use?
- What insulation and air leakage did you assume?
- Where are the ducts, and did you account for their losses?
- How many occupants did you assume?
- Did you calculate room by room, as well as the whole house?
- Which Manual S selection did you use to choose the equipment?
A quick check on a quote
Once you have a proposed size, you can do some simple math. It doesn't tell you the right size, but it gives you a question to ask.
Square feet per ton = conditioned floor area ÷ tons
For 2,000 square feet and 4 tons, that's 500 square feet per ton. If there's no load report behind that number, ask how they got it.
| Unit | Conversion |
|---|---|
| 1 ton of cooling | 12,000 BTU per hour |
| 1 kW | 3,412 BTU per hour |
Why not just keep the old size?
Your old system may not have been sized correctly, and your home may have changed since. New windows, insulation or air sealing can change your load.
Don't try these yourself
Only Section 608 certified technicians can buy refrigerant for stationary equipment, and EPA counts attaching gauges as technician work. Venting refrigerant is prohibited. Whether you need a permit or licensed contractor depends on your area, and 2021 IRC R105.1 covers replacing mechanical systems.
For a single room, use the room AC calculator.
Frequently asked questions
What size AC do I need for 2,000 square feet?
Floor area alone can't answer that. Published load calculations for real homes range widely, so ask for a load calculation for your home.
Is there a free Manual J calculator?
Load tools exist for contractors and others, and some may be free. We haven't reviewed them. This site doesn't offer one, because we'd need to carry out the method properly.
Is 400 to 600 square feet per ton wrong?
It's a rule of thumb rather than a calculation. In the one set of real calculations above, most homes came in well beyond it.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Residential Code, Chapter 14, Section M1401.3
- ACCA: Manual J load calculations vs. rules of thumb
- EPA Section 608: Refrigerant sales restriction, technician certification, venting prohibition