Latent Heat Calculator
Latent heat is the energy associated with moisture addition or removal from air — it changes humidity without changing temperature. The constant 0.68 derives from: 0.075 lb/ft³...
Formula
Source: Engineering Toolbox, ASHRAE Fundamentals | Last reviewed: June 8, 2026
Examples
0 BTU/hr
= 2720 BTU/hr
- cfm = 400
- delta_W = 10
400 CFM removing 10 grains/lb moisture = 2,720 BTU/hr latent
0 BTU/hr
= 24480 BTU/hr
- cfm = 1200
- delta_W = 30
1,200 CFM at 30 grains/lb moisture removal
0 BTU/hr
= 1700 BTU/hr
- cfm = 500
- delta_W = 5
500 CFM light dehumidification = 1,700 BTU/hr latent
Where is this used?
Swimming pool HVAC: high latent loads from evaporation require dedicated dehumidification.
Humidity control in museums and data centers: maintaining stable relative humidity.
Fresh air pretreatment: outdoor air in humid climates carries significant latent load.
Frequently Asked Questions
What is a typical humidity ratio for HVAC calculations?
At standard conditions (75°F, 50% RH), the humidity ratio is about 65 grains/lb. After cooling to 55°F saturated (typical supply air), it drops to ~64 grains/lb. The ΔW is small for recirculated air but can be 80-120 grains/lb for outdoor air on a humid summer day.
Why does latent heat matter for cooling?
In humid climates, latent cooling can be 30-50% of total cooling load. An undersized or poorly designed cooling coil may satisfy temperature but leave the space clammy and uncomfortable due to inadequate moisture removal. This is why coil selection must consider both sensible and latent capacity.
How do I convert relative humidity to humidity ratio?
This requires psychrometric calculations involving temperature, barometric pressure, and saturation pressure. Use a psychrometric chart or calculator. As a rough guide, at 75°F: 50% RH ≈ 65 grains/lb, 60% RH ≈ 78 grains/lb, 40% RH ≈ 52 grains/lb.
Reviewed for accuracy
· Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.