HVAC
HVAC Tonnage Calculator
Convert between BTU and tons and sanity check a proposed capacity against the space.
Result
A rule-of-thumb estimate for early conversations. Equipment should be selected on a room-by-room load calculation (ACCA Manual J) and the ducts designed to match (Manual D). Oversized equipment short-cycles, dehumidifies poorly and wears out early, so guessing high is not the safe option it looks like.
Get a Free Business AuditWhat moves the number
- The load calculation the tonnage should come from, rather than floor area alone.
- Whether the equipment is single stage, two stage or variable capacity.
- Duct capacity, which caps the tonnage that can actually be delivered.
Where this one goes wrong
Replacing like for like on tonnage without asking whether the original was right. A system that has been oversized for twenty years gets replaced with another oversized system because the nameplate is treated as the specification.
Questions
About this calculator
How many BTU is a ton?
Twelve thousand BTU per hour. The unit comes from the cooling effect of melting a ton of ice over a day, which is why the number looks arbitrary.
Should I match the tonnage of the old unit?
Only after checking that the old one was correct. Replacing an oversized system with the same tonnage perpetuates the comfort problems the customer has been living with.
Does variable capacity change the sizing decision?
It makes the decision more forgiving, because the equipment can run at part load for longer. It does not remove the need for a load calculation, since the maximum still has to cover design conditions.
The formula
BTU = area x climate factor x envelope adjustment + occupant load; tons = BTU / 12,000
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How to use this calculator
- Conditioned area (sq ft) (sq ft). Enter your own figure; the page starts at 1,800 sq ft.
- Climate (BTU per sq ft). Enter your own figure; the page starts at Warm south.
- Insulation and windows. Enter your own figure; the page starts at Average.
- People in the home. Enter your own figure; the page starts at 3.
The result updates as you type, so you can try a figure, see what it does to cooling tonnage, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Conditioned area (sq ft) | 1,800 sq ft |
| Climate (BTU per sq ft) | Warm south |
| Insulation and windows | Average |
| People in the home | 3 |
What comes out
| Result | Value |
|---|---|
| Cooling tonnage main result | 3.4 |
| Nearest standard size | 3.5 |
| Cooling capacity | 40,200 |
| Area per ton | 537 |
About 3.4 tons of cooling, which rounds to a 3.5 ton unit at roughly 537 sq ft per ton.
How sensitive the answer is
Holding everything else steady and changing only conditioned area (sq ft), this is what happens to cooling tonnage. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Conditioned area (sq ft) (sq ft) | Cooling tonnage |
|---|---|
| 900 sq ft | 1.7 |
| 1,350 sq ft | 2.5 |
| 1,800 sq ft example above | 3.4 |
| 2,700 sq ft | 5.0 |
| 3,600 sq ft | 6.7 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Conditioned area (sq ft) (sq ft)
- Measured in sq ft, starting at 1,800 sq ft.
- Climate (BTU per sq ft)
- Choose from Cool north, Moderate, Warm south, Hot south, Hot and humid, Desert.
- Insulation and windows
- Choose from Older, draughty, Average, Tight, new build.
- People in the home
- A number, between 0 and 40, starting at 3.
What the results tell you
| Result | What it is |
|---|---|
| Cooling tonnage | The headline result. A decimal figure. |
| Nearest standard size | A decimal figure. |
| Cooling capacity | A number. |
| Area per ton | A number. |
Before you rely on this number
Sizing, capacity and load figures here are planning estimates. A real installation depends on the manufacturer's data for the specific equipment, the duct system it is going into, and a proper load calculation for the building. Use this to sanity-check a number or price a job, not to select equipment.
Keep going
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Estimate cooling capacity for a space as a cross-check on a load calculation, not as a replacement for one.
Open calculatorHVAC BTU Calculator
Get a planning figure for the BTU a space needs, as a sanity check against a proper load calculation rather than a substitute for one.
Open calculatorHVAC Load Calculator
Produce a planning load figure from the envelope and climate, as preparation for a proper Manual J rather than a substitute.
Open calculatorEstimating is the easy part
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