Grains of Moisture Calculator

Grains per pound: the practical unit for dehumidifier sizing and latent load analysis

One grain equals 1/7000 of a pound, a unit carried over from the troy grain used to measure gunpowder and medications and documented as a standard mass unit in NIST Special Publication 811. In North American HVAC, grains per pound (gr/lb) expresses the mass of water vapour per pound of dry air. It's numerically the same quantity as pounds per pound multiplied by 7000, or g/kg multiplied by 7. HVAC engineers in Canada often work with both gr/lb (for equipment specs from US manufacturers and legacy psychrometric charts) and g/kg (for metric calculations and energy modelling). This calculator outputs both and handles the conversion automatically.

The key advantage of grains over relative humidity is that grains measure actual moisture mass. A dehumidifier rated at 70 pints per day (about 33 litres per day) removes a fixed mass of water, roughly 72,000 grains per hour. To match that rate against a building load, calculate the grain difference between space conditions and the dehumidifier discharge conditions, multiply by the air circulation rate in lb/hr, and compare against the unit's rated capacity with the dehumidifier sizing calculator. Relative humidity gives you no such direct path to a mass removal rate. The moisture content calculator covers the same calculation in g/kg units for metric dehumidification sizing.

The 0.68 rule: latent load in BTU/hr from CFM and gr/lb

A widely used HVAC shortcut relates latent load directly to CFM and grain difference: Q_latent (BTU/hr) ≈ 0.68 × CFM × ΔW (gr/lb). This comes from the full formula Q = ṁ × ΔW/7000 × h_fg, where ṁ = CFM × 60 × 0.075 lb/ft³ (standard air density) and h_fg = 1076 BTU/lb of water. Substituting: Q = CFM × 60 × 0.075 × ΔW/7000 × 1076 = CFM × ΔW × 0.693. Rounding gives 0.68 for a slight conservative adjustment. This rule is accurate to within 3% at sea level and typical indoor temperatures. At higher elevations (Calgary at 1045 m, Edmonton at 671 m), air density is lower, so the true coefficient drops to about 0.60 in Calgary and 0.63 in Edmonton. This tool uses the exact formula with actual air density from the entered elevation, so the "grain difference" mode gives more accurate latent loads for higher Canadian sites than the 0.68 rule.

Sizing dehumidifiers: pints per day and what the numbers mean

Residential and commercial dehumidifiers are rated in pints per day (US pints) of water removed at a standard test condition. One US pint of water weighs about 1.04 lb, so 70 pints/day equals about 72.8 lb/day or 3.03 lb/hr of moisture removed. To verify whether a dehumidifier meets your load, convert its pint rating to grains per hour and compare to the space moisture load in grains per hour. For a pool room, gymnasium, or commercial laundry in a Canadian building, the load calculation needs to account for evaporation sources, ventilation air moisture content, and envelope infiltration gains, the last of which the infiltration calculator can quantify separately, all of which this tool can compute individually using the grain values. See the coil load calculator for the full psychrometric load including sensible cooling alongside the latent grain removal.