How to Calculate BTU and Tonnage
Sensible, latent, and total heat explained the way it actually works in the field, plus the simple math to convert any BTU/hr number into tons of cooling.
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In This Guide
What a BTU and a Ton Actually Mean
A BTU (British Thermal Unit) is the amount of heat it takes to raise one pound of water by one degree Fahrenheit. On its own that number is almost useless in the field, so we talk about BTU per hour (BTU/hr), the rate at which a system moves heat. Every capacity you read off a nameplate or a load calc is really a BTU/hr figure.
A ton of cooling is just a bigger, rounder unit for that same rate. The term is a holdover from the ice trade: melting one ton of ice over 24 hours absorbs roughly 288,000 BTU, which works out to 12,000 BTU/hr. That single conversion is the backbone of every sizing conversation:
The one number to memorize
A 3-ton system is rated for 36,000 BTU/hr; a 5-ton for 60,000 BTU/hr.
Sensible, Latent, and Total Heat
An air conditioner does two jobs at once, and a good tech keeps them mentally separate:
- Sensible heat is the heat you can feel and measure with a thermometer. It changes the dry-bulb temperature of the air. Dropping supply air from 75°F to 55°F is sensible work.
- Latent heat is the heat tied up in moisture. Condensing water vapor out of the air on the coil removes latent heat without changing the dry-bulb temperature. It is what fills your condensate pan.
- Total heat is simply the two added together. It is the true capacity the coil is delivering, and it is what you compare against the equipment's rating.
Why it matters: A coil can post a healthy 20°F temperature split and still fall short on total capacity if it is not pulling enough moisture, and vice versa in a humid climate. Measuring only the temperature drop tells you the sensible story, not the whole story.
The Three Airside Formulas
These are the workhorse equations for checking capacity from airflow measurements. Each uses a constant baked out of air properties at standard conditions (0.075 lb/ft³ density, 60 min/hr).
Sensible
ΔT = dry-bulb temp split (°F) across the coil
Latent
ΔW = humidity ratio change in grains/lb
Total
Δh = enthalpy change (BTU/lb) from a psychrometric chart
Where the constants come from
- 1.08 = 0.075 lb/ft³ × 60 min/hr × 0.24 BTU/lb/°F (specific heat of air)
- 0.68 = 0.075 × 60 × 1076 BTU/lb ÷ 7000 grains/lb (latent heat of vaporization, converted for grains)
- 4.5 = 0.075 lb/ft³ × 60 min/hr (mass flow of standard air per CFM)
You will almost always reach for the sensible formula first, because a thermometer is faster than a psychrometric lookup. Use the total-heat formula (4.5 × CFM × Δh) when you need the real cooling capacity in a humid space, or when a customer is complaining about humidity even though temperatures look fine.
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Worked Example: A 3-Ton Split System
Problem:
You measure 1,200 CFM across the evaporator on a 3-ton system. Return air is 75°F dry-bulb, supply is 55°F dry-bulb. Using a psychrometer, the return grabs 65 grains/lb of moisture and the supply reads 50 grains/lb. Is the coil delivering its rated capacity?
Step 1: Find the sensible split and moisture change
ΔT = 75 − 55 = 20°F | ΔW = 65 − 50 = 15 grains/lb
Step 2: Sensible heat
Qs = 1.08 × 1200 × 20 = 25,920 BTU/hr
Step 3: Latent heat
Ql = 0.68 × 1200 × 15 = 12,240 BTU/hr
Step 4: Total heat
Qt = 25,920 + 12,240 = 38,160 BTU/hr
Step 5: Convert to tons
38,160 ÷ 12,000 = 3.18 tons
Result: Coil is performing
At roughly 3.18 tons of total capacity against a 3-ton (36,000 BTU/hr) rating, the coil is right where it should be. Rated capacities are measured at 80°F/67°F return conditions, so seeing a little over nameplate at higher indoor load is normal. Airflow also checks out: 1,200 CFM ÷ 3 tons = exactly the 400 CFM-per-ton target.
If total capacity had come in around 27,000 BTU/hr (2.25 tons) on that same 3-ton unit, you would start hunting for a low charge, a dirty coil, or restricted airflow before blaming the compressor.
Converting BTU/hr to Tonnage
Going either direction is one operation. Divide BTU/hr by 12,000 to get tons; multiply tons by 12,000 to get BTU/hr. This table covers the residential and light-commercial equipment you see every day.
| Tonnage | Nominal BTU/hr | Target Airflow (400 CFM/ton) |
|---|---|---|
| 1.5 Ton | 18,000 | 600 CFM |
| 2 Ton | 24,000 | 800 CFM |
| 2.5 Ton | 30,000 | 1,000 CFM |
| 3 Ton | 36,000 | 1,200 CFM |
| 3.5 Ton | 42,000 | 1,400 CFM |
| 4 Ton | 48,000 | 1,600 CFM |
| 5 Ton | 60,000 | 2,000 CFM |
Reading the model number
Most manufacturers encode tonnage in the model number in thousands of BTU/hr divided into the digits. A model ending in 036 is 36,000 BTU/hr (3 tons); 048 is 4 tons; 060 is 5 tons. When in doubt, the RLA and the coil size confirm it, but the model number is your fastest read on the truck.
Sensible Heat Ratio & What It Tells You
The sensible heat ratio (SHR) is the fraction of total capacity spent on temperature versus moisture:
From our example: 25,920 ÷ 38,160 = 0.68, so about 68% of the work was sensible, 32% latent.
A typical residential AC runs an SHR around 0.75 (75% sensible, 25% latent). Knowing the ratio helps you match equipment to climate:
- Hot and humid climates want a lower SHR (more latent capacity) so the system wrings out moisture. Oversized equipment short-cycles and leaves the space cold and clammy.
- Hot and dry climates want a higher SHR because there is little moisture to remove and almost all the work is dropping temperature.
The oversizing trap
Bumping a customer up a half-ton "to be safe" is the most common sizing error in residential HVAC. An oversized system satisfies the thermostat on sensible load before it runs long enough to pull latent load, so humidity stays high. Size to a Manual J load calc, not to a hunch, and treat the BTU-per-square-foot rules of thumb (roughly 18–25 BTU/sq ft in moderate climates) as a sanity check only.
Field Tips & Common Mistakes
- Get real CFM first. Every airside BTU number is only as good as your airflow figure. Measure it with a flow hood, TrueFlow grid, or static-pressure method, don't assume the fan is moving rated air.
- Use total heat for capacity, sensible for a quick check. Judging a coil on temperature split alone will fool you in humid weather where a big chunk of the work is latent.
- Grains, not percent RH, for latent. The 0.68 constant needs the humidity ratio in grains per pound, which you read off a psychrometric chart or a digital psychrometer, not relative humidity.
- Standard conditions assumed. The 1.08, 0.68, and 4.5 constants assume sea-level standard air. At high altitude the lower air density means you are moving less mass per CFM, so capacities drop; correct the constants for elevation on mountain jobs.
- Confirm airflow against 400 CFM/ton. If measured CFM is far off 400 per ton, fix airflow before you trust any capacity calculation, and before you touch the charge.
Quick recap
Sensible = 1.08 × CFM × ΔT. Latent = 0.68 × CFM × ΔW (grains). Total = sensible + latent, or 4.5 × CFM × Δh. Tons = total BTU/hr ÷ 12,000. Nail those five relationships and you can verify any air conditioner's output from the field.
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