TryBuildCalcWhole House Humidifier Size Calculator (Gallons Per Day Sizing)
Size your whole-house humidifier instantly.
🕒 Last updated: September 12, 2026
Inputs
ℹ️Floor area of the space to be conditioned.
ℹ️A taller ceiling means more air volume to condition.
ℹ️How much outdoor air infiltrates — a tighter home loses less humidity and needs less capacity.
ℹ️Evaporative units top out around 21 gal/day; a steam unit (a different technology) can cover up to ~34.6 gal/day for larger/leakier homes.
Recommended Humidifier Size
12 Gal/Day
Next standard retail size at or above the 5.1 gal/day raw requirement
Home
Area: 1,200 sq ft
Ceiling height: 8 ft
Volume: 9,600 cu ft
Construction & Equipment
Tightness: Tight (new/well-sealed, ~0.5 air changes/hr)
K factor: 17.5
Humidifier type: Evaporative (bypass/fan-powered)
Occupant credit: No
Load
Raw requirement: 5.1 gal/day
Assumptions Used
Per HARDI Tech Tip #6 (derived from ARI Standard 630-74), humidification load (gal/day) = (house volume in cu ft x K) / 33,000, where K is a moisture-infiltration factor tied to construction tightness: 17.5 for tight (~0.5 air changes/hour), 27 for average (~1 ACH), or 38 for loose/pre-1930 construction (~2 ACH) — a tighter home loses less humidity to infiltration and needs proportionally less humidifier capacity. An optional, source-sanctioned occupant moisture credit (-2 gal/day) can be applied for "free" internal moisture from cooking, bathing, and laundry. The raw requirement is rounded UP to a standard tier for your selected humidifier type — Evaporative (bypass/fan-powered): 12, 17, 18, or 21 gal/day, the dominant whole-house brand's real evaporative product sizes; or Steam: up to 34.6 gal/day (a single-tier, higher-capacity technology using an electrode to boil water directly rather than evaporating it off a pad in the airstream — a genuinely different, typically pricier product, not just a bigger version of the same one). Never rounded down, so the displayed size never understates the raw requirement, and the SAME rounded-up number is used both for display and for choosing the tier, so the two can never look inconsistent with each other. This is a planning estimate — actual duct static pressure, water pressure/hardness, and drain configuration all affect a real installation; confirm your chosen unit's actual rated performance with a licensed HVAC contractor.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Humidifier / Dehumidifier Size Calculator.
Whole-house humidifier sizing
Per HARDI Tech Tip #6 (ARI Standard 630-74), humidification load = (house volume x K) / 33,000, where K reflects how many air changes per hour your home's construction allows.
This page defaults to a smaller, tightly-constructed 1,200 sq ft home with an evaporative unit — edit the inputs above to match your actual home.
Humidifier / Dehumidifier Sizing Formula: How Is It Determined?
Humidifier mode uses a volume-based load formula from an ARI/HARDI industry standard. Dehumidifier mode follows ENERGY STAR's own current dehumidifier buying guide, turned into a continuous area/dampness formula with adjustment factors.
Humidifier Mode
Volume (cu ft) = Area (sq ft) x Ceiling Height (ft)
Raw GPD = (Volume x K) / 33,000, minus 2 if occupant credit applied (floored at 0)
K = 17.5 (tight) / 27 (average) / 38 (loose)
Per HARDI Tech Tip #6 (derived from ARI Standard 630-74), K is a moisture-infiltration factor tied to how many air changes per hour a home's construction lets in. The raw requirement is rounded UP to a standard tier for your selected Humidifier Type — Evaporative: 12, 17, 18, or 21 gal/day; Steam: up to 34.6 gal/day (Aprilaire's Model 800, the common whole-house steam tier) — since evaporative and steam units are different technologies with very different real capacity ranges.
Dehumidifier Mode
Base Pints = ENERGY STAR anchor points for the dampness level, interpolated/extended by area
Height Multiplier = 1 + 0.125 x max(0, Ceiling Height - 8)
Raw Pints = (Base Pints x Height Multiplier) + Condition Add-ons
ENERGY STAR's own current buying guide gives a low/high pints-per-day figure for each dampness level at 300 sq ft and 2,000 sq ft (its own bracket boundary) — this calculator treats those as two points on a line and extends the same rate beyond 2,000 sq ft, rather than treating any area past a fixed cutoff as an unchanged, stale number. A taller ceiling scales the load up proportionally; below-grade space, missing vapor barriers, higher occupancy, nearby laundry, and a nearby pool/hot tub each add a flat amount (a separate, commonly-cited convention, not from ENERGY STAR's own table). The combined figure is rounded UP to a standard retail tier (22, 30, 35, 50, or 70 pints/day).
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Area / ceiling height | 1,200 sq ft / 8 ft |
| Construction tightness | Tight (new/well-sealed, ~0.5 air changes/hr) |
| Humidifier type | Evaporative (bypass/fan-powered) |
Sizing Steps
| Calculation | Result |
|---|---|
| Volume = 1,200 x 8 | 9,600 cu ft |
| Raw GPD = (9,600 x 17.5) / 33,000 | 5.1 gal/day |
| Nominal size, rounded up | 12 gal/day |
Therefore, this home needs a humidifier rated at approximately 12 gal/day. Get your specific unit's manufacturer performance data from a licensed HVAC contractor before purchasing.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.