FreeHVACCalculators

Cooling Capacity Calculator

Measure what an air conditioner is actually delivering, from the airflow and the dry-bulb and wet-bulb temperatures across the coil.

Method reviewed October 22, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

Result

Total capacity
34,879BTU/h
Total capacity
2.91tons

The coil is delivering about 34,879 BTU/h (2.91 tons) at these conditions.

Sensible capacity
25,920BTU/h
Latent capacity
8,959BTU/h
Sensible heat ratio
0.74
Enthalpy drop
6.46BTU/lb
Return-air enthalpy
28.43BTU/lb
Supply-air enthalpy
21.97BTU/lb
Return-air relative humidity
52%
Show the working
  1. Return-air enthalpy (from dry-bulb and wet-bulb)
    h = 0.240·t + W(1061 + 0.444·t) = 28.43 BTU/lb
  2. Supply-air enthalpy
    21.97 BTU/lb
  3. Total = 4.5 × density ratio × CFM × Δh
    4.5 × 1 × 1,200 × 6.459 = 34,879 BTU/h
  4. Sensible = 1.08 × density ratio × CFM × ΔT
    1.08 × 1 × 1,200 × 20 = 25,920 BTU/h
  5. Latent = total − sensible
    34,879 − 25,920 = 8,959 BTU/h

Total, sensible and latent

Total capacity comes from the drop in moist-air enthalpy across the coil: Q = 4.5 × CFM × Δh. Enthalpy is calculated from each dry-bulb and wet-bulb pair with the ASHRAE psychrometric equations, so it includes the moisture the coil condensed out.

Sensible capacity uses Q = 1.08 × CFM × ΔT. The difference between total and sensible is latent capacity — the moisture removal. Both constants are scaled for elevation.

Getting readings you can trust

  • Measure return air close to the unit and supply air downstream of the coil, where it is well mixed.
  • Use a properly ventilated wet-bulb or a calibrated RH probe; a 1 °F wet-bulb error moves total capacity by roughly 10 %.
  • Airflow is the other half of the equation: a 15 % airflow error is a 15 % capacity error.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Airflow across the coil
1,200 CFM
Return (entering) dry-bulb
75 °F
Return (entering) wet-bulb
63 °F
Supply (leaving) dry-bulb
55 °F
Supply (leaving) wet-bulb
53 °F
Site elevation
0 ft

Result

Total capacity
34,879 BTU/h
Total capacity
2.91 tons
Sensible capacity
25,920 BTU/h
Latent capacity
8,959 BTU/h
Sensible heat ratio
0.74
Enthalpy drop
6.46 BTU/lb

The coil is delivering about 34,879 BTU/h (2.91 tons) at these conditions.

Assumptions

  • 4.5 = 60 min/h × 0.075 lb/ft³ and 1.08 = 4.5 × 0.24 BTU/lb·°F for standard air; both are scaled by the air-density ratio at the site elevation.
  • Moist-air enthalpy is computed from dry-bulb and wet-bulb using ASHRAE psychrometric equations at the standard-atmosphere pressure for the elevation.

Limitations

  • Accuracy depends on airflow measurement and on taking temperatures in well-mixed air. A 1 °F wet-bulb error changes total capacity by roughly 10 %.
  • Measures the air-side capacity at the moment of measurement; it is not the rated capacity at AHRI conditions.

Frequently asked questions

Why is measured capacity lower than the rated tonnage?

Ratings are at standard test conditions. Lower indoor humidity, a hotter outdoor temperature, low airflow, a dirty coil or incorrect charge all reduce delivered capacity. Compare against the manufacturer’s expanded performance tables at the measured conditions, not the nameplate.

Can I use dry-bulb only?

Only for sensible capacity. Without wet-bulb (or humidity) readings the moisture removal is invisible, and total capacity can be badly underestimated in humid weather.

Sources

  1. Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
  2. PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
  3. Water vapor pressure formulations — UCAR / NCAR Earth Observing Laboratory. Hyland–Wexler saturation vapour pressure coefficients over water and ice.

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