Humidity Load Calculator (Grains)
Turn an airflow and the grains-of-moisture difference between outdoor and indoor air into a latent cooling load and daily water removal.
How it works
Latent BTU/hr = 0.68 × CFM × Δgrains · Water (lb/hr) = 4.5 × CFM × Δgrains ÷ 7,000 · Pints/day = lb/hr × 24 ÷ 1.043
Moisture in air is measured in grains per pound of dry air (7,000 grains = 1 pound of water). The latent load — the energy needed to condense that water out — is 0.68 times the airflow times the grain difference between the humid outdoor air and your drier indoor target. Converting the same grain difference to a mass flow gives the pints of water per day your AC or dehumidifier must actually remove, which is why humid climates feel so much harder to cool than dry ones at the same temperature.
Worked example
Pulling in 100 CFM of 120-grain outdoor air toward a 65-grain indoor target is a 55-grain difference: latent load = 0.68 × 100 × 55 = 3,740 BTU/hr. As water, 4.5 × 100 × 55 ÷ 7,000 = 3.54 lb/hr, or about 81 pints per day.
Frequently asked questions
What is a “grain” of moisture?
A grain is a tiny unit of weight — 7,000 grains make one pound. In HVAC it counts the water vapor carried in each pound of dry air, so “65 grains per pound” means that much water is suspended in the air.
What is latent vs sensible load?
Sensible load is the heat that changes air temperature; latent load is the energy to remove moisture without changing temperature. Total cooling load is the sum, and in humid climates the latent share is large — which is why an oversized AC that cools fast but runs briefly leaves rooms cold and damp.
Where do the constants 0.68 and 4.5 come from?
They bundle standard air density and the latent heat of water at typical conditions: 0.68 converts CFM and grains directly to BTU/hr, while 4.5 (60 min × 0.075 lb/ft³) converts CFM to pounds of air per hour before the grains-to-pounds step.
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This calculator is for educational purposes. Double-check important results before acting on them.