How many gallons per day (GPD) of humidifier output does your home or room need? Size whole-house, portable, and furnace-mounted humidifiers from square footage, ceiling height, and home tightness — using AprilAire's official sizing method.
Whole-house humidifier sizer
Formula (AprilAire official): GPD = sq ft × (ceiling ft / 8) / k, where k = 300 tight, 150 average, 100 loose.
sq ft
ft
Health note: keep indoor RH between 30% and 50% (EPA). Above 50% sustained encourages mold; if you see window condensation, lower the humidistat setpoint.
Portable / room humidifier sizer
Based on an industry room-sizing table (secondary source, spot-checked against a second guide).
sq ft
gal
Reverse: GPD → max coverage
You know the unit's GPD — how many square feet can it cover? (8-ft ceilings.)
GPD
AprilAire model picker
Coverage per AprilAire Form 222; 720A rated up to 6,250 sq ft (tightly constructed). Between sizes → next larger (5% fit margin).
sq ft
AprilAire model ladder (GPD + coverage, 8-ft ceilings)
Model
GPD
Tight
Average
Loose
500A
12
3,000
1,500
750
600A
17
4,000
2,000
1,000
700A
18
4,200
2,100
1,050
720A
21
6,250
~3,150*
~2,100*
*Estimated from 21 GPD (manufacturer publishes tight-construction coverage only).
Climate / season helper — humidifier or dehumidifier?
Health note: the EPA target is 30-50% indoor RH. Humidifiers add moisture (winter/dry); dehumidifiers remove it (damp/basement). RH above 50% for long periods risks mold.
Operating cost: bypass vs fan-powered vs steam
Water cost covers evaporated water only; bypass units flush extra water to drain. Electricity: bypass 0 W (uses furnace blower), fan-powered ~96 W, steam ~1,500 W typical.
Source: AprilAire "Humidifier Regional Applications" official sizing guidelines (GPD per AHRI Guideline F). Formula: GPD = sq ft × (ceiling/8) / k, k = 300/150/100. Some older AprilAire documents show higher values (e.g. 8.5 GPD tight at 2,000 sq ft); this tool uses the current official sizing chart.
Not what you're looking for?
Dehumidifier size calculator — the opposite job: removes moisture (pints/day) for damp basements and humid climates.
For a 2,000 sq ft home of average tightness with 8-ft ceilings, the AprilAire sizing formula gives 2,000 / 150 = 13.3 gallons per day (the official AprilAire chart rounds this cell to 13.4 GPD). On the AprilAire model ladder, the 700A (18 GPD, covers up to 2,100 sq ft average) is the minimum fit, with the 720A (21 GPD) as the headroom option. If your home is loose (drafty), the same house needs 20.0 GPD; if it is tight, only 6.7 GPD.
Yes — humidifier sizing follows air volume, not just floor area. A 2,000 sq ft home with 10-ft ceilings has the same volume as a 2,500 sq ft home with 8-ft ceilings, so at average tightness it needs 2,000 x (10/8) / 150 = 16.7 GPD instead of 13.3 GPD — a 25% increase. This calculator multiplies your square footage by ceiling/8 automatically.
A huge one. For a 2,000 sq ft home: tight construction (0.5 air changes per hour) needs 6.7 GPD, average (1 ACH) needs 13.3 GPD, and loose (2 ACH) needs 20.0 GPD — a 3x spread. Air sealing and insulation shrink the humidifier you need, which is why the calculator asks about your home's tightness.
Industry sizing tables (secondary source, spot-checked) put a 250-500 sq ft room at 1.5-2.0 gallons per day of output. Do not confuse daily output (GPD) with tank capacity: the tank size only sets how often you refill, not how much moisture the unit delivers. A 1-gallon tank on a 2.0 GPD unit runs about 12 hours before refilling.
They do opposite jobs. A humidifier ADDS moisture — for winter or dry climates when indoor RH falls below 30%. A dehumidifier REMOVES moisture — for basements, humid summers, or any space where RH stays above 50% and mold becomes a risk. Check your current RH: below 30% points to a humidifier, above 50% points to a dehumidifier. Try our dehumidifier size calculator if you need to remove moisture: /tools/dehumidifier-size-calculator/
The EPA recommends keeping indoor humidity between 30% and 50%, so 40% is fine — it is inside both the EPA range and the 35-45% band AprilAire calls optimal. Indoor airPLUS notes that ducted systems automatically limit humidity to 40%. Watch your windows: if condensation forms on cold glass, lower the humidistat setpoint, because persistent condensation invites mold.
Multiply gallons per day by 3.78541. The AprilAire chart's 13.4 GPD cell for a 2,000 sq ft average home equals 13.4 x 3.78541 = 50.72 liters per day. The calculator has an imperial/metric toggle that shows liters per day and square meters alongside GPD and square feet.
Yes, with a furnace-mounted unit. The AprilAire 720A delivers 21 GPD, which at tight construction covers 21 x 300 = 6,300 sq ft — consistent with the manufacturer's up-to-6,250 sq ft claim. Portable units are a different class: a large console portable tops out around 12 GPD for up to 3,000 sq ft, and tabletop units only handle single rooms.
How the math works
Whole-house sizing uses AprilAire's official method: GPD = square feet × (ceiling height / 8) / k, with k = 300 for tight, 150 for average, and 100 for loose construction. The divisor k encodes the air-change rate (0.5 / 1 / 2 ACH): leakier homes lose moisture faster and need proportionally more output — loose needs exactly 3× the tight value. Evaporative ratings assume the blower runs and plenum conditions per AHRI Guideline F; a family of four adds roughly 2 GPD through daily activities. Portable room sizing follows an industry table (secondary source) from 1.5 GPD for 500 sq ft up to 12 GPD for 3,000 sq ft.
Disclaimer: This is an educational sizing estimator, not a substitute for a licensed HVAC professional. Keep indoor relative humidity between 30% and 50% (EPA); sustained humidity above 50% encourages mold growth, and condensation on windows means the humidistat setpoint is too high. Oversized humidifiers can cause condensation damage — when between sizes, step up one model but do not jump two.
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