One ton of air conditioning equals 12,000 BTU/hr of nominal cooling capacity.
So:
- 1.5 tons = 18,000 BTU/hr
- 2 tons = 24,000 BTU/hr
- 2.5 tons = 30,000 BTU/hr
- 3 tons = 36,000 BTU/hr
- 4 tons = 48,000 BTU/hr
- 5 tons = 60,000 BTU/hr
That conversion is simple.
HVAC sizing is not.
Why is cooling measured in tons?
The term comes from the amount of heat involved in melting a ton of ice over 24 hours.
In modern HVAC, "ton" is simply a capacity unit.
It does not mean the air conditioner weighs a ton.
Nominal tonnage is a product category
A 3-ton condenser is commonly associated with 36,000 BTU/hr.
But actual delivered capacity changes with:
- Outdoor temperature
- Indoor temperature
- Indoor humidity/wet-bulb
- Airflow
- Equipment pairing
- Altitude
That is why Manual S uses manufacturer performance data.
Manual J load is a building requirement
Suppose Manual J says:
- Total cooling load: 31,000 BTU/hr
That does not mean the building "is 2.58 tons" in the same sense as an equipment model.
The load is the building's required heat-removal rate at the design condition.
The equipment has to be selected to meet that load under its actual operating condition.
Do not divide by 12,000 and round up blindly
31,000 / 12,000 = 2.58 tons.
The wrong next step is:
"Round up to 3 tons. Done."
The correct next step is Manual S:
- Identify candidate equipment
- Check delivered total capacity
- Check sensible capacity
- Check latent performance
- Apply sizing limits
- Check airflow
A nominal 3-ton system may be the answer.
The division alone does not prove it.
Sensible capacity can differ from total capacity
A 36,000 BTU/hr total capacity does not mean all 36,000 BTU/hr is available for lowering temperature.
Part of cooling capacity may be latent moisture removal.
That is why Manual J reports:
- Sensible load
- Latent load
- Total load
Equipment has a sensible/latent split too.
Heat-pump tonnage is even more misleading in heating
A "3-ton heat pump" does not deliver 36,000 BTU/hr of heating at every outdoor temperature.
Heating output may change substantially as temperature falls.
Cold-climate heat-pump sizing therefore needs:
- Manual J heating load
- OEM low-ambient capacity
- Manual S
Do not convert furnace BTU to "heat-pump tons" directly.
Why homeowners hear conflicting tonnage recommendations
One contractor says 3 tons.
Another says 4 tons.
Possible reasons include:
- Different load assumptions
- Rule-of-thumb sizing
- Different equipment models
- Different design conditions
- One contractor is rounding up
- One performed Manual J and Manual S
Ask for the calculation and the equipment-selection basis.
Tonnage does not tell you airflow automatically
The familiar 400 CFM/ton rule is not universal.
Airflow depends on:
- Equipment
- Sensible/latent requirements
- Climate
- Manufacturer settings
See HVAC Airflow per Ton: Why 400 CFM Is Not Universal.
Replacement tonnage can change
If the old house had a 4-ton AC and the new load supports a smaller system, that does not mean the calculation is automatically wrong.
The old system may have been oversized.
Or the building may have changed through:
- Insulation
- Air sealing
- New windows
- Duct improvements
See Should a Replacement HVAC System Be the Same Size as the Old One?.
Bottom line
The conversion is simple:
1 ton = 12,000 BTU/hr
But HVAC design requires more than converting the Manual J total into nominal tons.
Manual J tells you the building load.
Manual S tells you which actual equipment can meet it.