Why are dehumidifiers rated at 80F and 60 percent RH?

AHAM tests every unit at the same 80F/60% RH conditions so capacities can be compared fairly. A unit will remove far fewer pints per day in a cool 65F basement, which is why a margin is essential.

What grains-per-pound difference should I enter?

It is the humidity ratio gap between the humid outdoor air leaking in and your indoor target. In a humid summer climate the outdoor air may be 30 to 50 grains per pound above a 50 percent RH indoor target; mild climates are lower.

How is the infiltration moisture load calculated?

The tool converts your air-change rate to CFM, then applies the standard psychrometric relation that one CFM of air carrying a given grains-per-pound difference adds a known mass of water per hour, scaled to pints per day.

Why add a 20 percent margin?

Real basements run cooler than the 80F test rig, infiltration spikes on humid days, and units lose capacity as filters load. A 20 percent margin keeps the unit able to hold setpoint under worst-case conditions without short cycling at maximum output.

Should I count the slab if it has a vapor barrier?

If a continuous, sealed vapor barrier is present, set the damp-slab option to no. Bare or cracked slabs and open crawlspaces can add several pints per day per thousand square feet from vapor diffusion, so leave it on when in doubt.

Whole-House Dehumidifier Sizing Calculator

Picking a dehumidifier by floor area alone ignores where the moisture actually comes from. This calculator builds the latent load from its real sources — people, cooking, showers, the humid air leaking in, and vapor rising off a damp slab — and then recommends a standard AHAM-rated unit with enough headroom to hold setpoint on the worst days.

How it works

Each source is estimated in pints per day and summed:

occupants    = persons × 3 pints/day
cooking      ≈ 5 pints/day
bathing      = showers × 1 pint
infiltration = 4.5 × CFM × (grains/7000) lb/hr → pints/day
slab/crawl   ≈ 4 pints/day per 1000 ft² if damp
recommended  = (sum of all loads) × 1.2, rounded up to an AHAM size

The infiltration term uses the standard psychrometric mass-flow relation, where 4.5 times CFM gives the air mass flow in pounds per hour and the grains difference (divided by 7000 grains per pound) gives the moisture each pound of air carries.

Example and tips

A tight 1,500 ft² basement with three occupants, 0.35 ACH of humid infiltration, a 30-grain outdoor-indoor difference, four showers a day, and a damp slab works out to roughly 30 to 35 pints per day of moisture. With the 20 percent margin the tool recommends a 50 pint AHAM unit. Place the dehumidifier where air circulates freely, drain it to a condensate pump or floor drain so it runs continuously, and remember that sealing the slab and tightening infiltration shrinks the load far more cheaply than oversizing the equipment.