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Humidifier Sizing Calculator

Cold outdoor air holds very little water. Once it leaks in and is heated, it dries the house. This calculator estimates how much water a humidifier must add to make up the difference.

Method reviewed October 26, 2026. Version 1.0.0. How it works

This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.

Your inputs

Unit system

Natural air leakage plus any mechanical ventilation. A blower-door test converted to natural conditions gives the leakage part.

EPA recommends keeping indoor humidity below 60 %, ideally 30–50 %. In cold weather, lower targets reduce window condensation.

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

Result

Water to add per day
6.71gal
Water to add per hour
2.332lb/h

About 6.7 gallons of water a day to hold 40 % RH at 70 °F.

  • Moisture from people, cooking and showers offsets part of this; the result is the outdoor-air load only.
Water to add per day
53.7pints
Outdoor air used
100CFM
Outdoor humidity ratio
10.5gr/lb
Target indoor humidity ratio
43.5gr/lb
Show the working
  1. Humidity ratios (ASHRAE psychrometric equations)
    outdoor 10.5 gr/lb, indoor target 43.5 gr/lb
  2. Dry-air flow = CFM × 60 ÷ outdoor specific volume
    100 × 60 ÷ 12.122 ft³/lb = 495 lb/h
  3. Water to add = dry-air flow × (W target − W outdoor)
    495 × 0.004712 = 2.332 lb/h
  4. Gallons per day = lb/h × 24 ÷ 8.34 lb per gallon
    6.71 gal/day

How the estimate works

Every pound of outdoor air that enters the house carries a small amount of moisture. To bring it up to your target indoor humidity, the humidifier must add the difference in humidity ratio between indoor and outdoor air:

  • Water per hour = dry-air flow (lb/h) × (indoor humidity ratio − outdoor humidity ratio)
  • Dry-air flow = outdoor airflow in CFM × 60 ÷ specific volume of the outdoor air

The humidity ratios and specific volume come from the ASHRAE psychrometric equations. The result is converted to gallons per day for comparison with humidifier ratings.

Getting the airflow

The airflow is the outdoor air entering the home: natural leakage plus any mechanical ventilation. If you have a blower-door result, convert it to natural air changes per hour first. You can enter the airflow directly or as air changes per hour with the house volume (CFM = ACH × volume ÷ 60).

Leaky houses need far more humidification than tight ones. Sealing air leaks usually does more for winter dryness than a bigger humidifier.

Choosing a target

EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. In very cold weather, a lower target protects windows and walls from condensation: if your windows sweat, the target is too high for that weather. The Indoor Humidity Check tool shows the highest humidity your coldest surface can tolerate.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Outdoor air entering the home
Airflow in CFM
Outdoor air (leakage + ventilation)
100 CFM
Outdoor temperature
20 °F
Outdoor relative humidity
70 %
Indoor temperature
70 °F
Target indoor relative humidity
40 %
Site elevation
0 ft

Result

Water to add per day
6.71 gal
Water to add per hour
2.332 lb/h
Water to add per day
53.7 pints
Outdoor air used
100 CFM
Outdoor humidity ratio
10.5 gr/lb
Target indoor humidity ratio
43.5 gr/lb

About 6.7 gallons of water a day to hold 40 % RH at 70 °F.

Assumptions

  • Steady state: the humidifier replaces the moisture difference between the outdoor air that enters and the indoor air it displaces.
  • Dry-air mass flow is based on the outdoor air’s specific volume; 1 US gallon of water ≈ 8.34 lb.

Limitations

  • Air leakage varies with wind and temperature; use a design-day leakage estimate.
  • Ignores moisture from occupants and activities and moisture stored in furnishings, so it slightly overstates the need.

Frequently asked questions

Why is the result higher than my humidifier rating?

Humidifier ratings are measured at specific conditions. Very cold, dry outdoor air and a leaky house can need more water than a small unit delivers. Reduce air leakage or lower the target humidity.

Does cooking and showering count?

They add moisture that offsets part of the load. This estimate ignores them, so it slightly overstates what the humidifier must supply.

How much does one gallon weigh?

About 8.34 pounds, which is what the tool uses to convert pounds of water to gallons.

Sources

  1. PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
  2. A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency. If possible, keep indoor humidity below 60 % (ideally between 30 and 50 %) relative humidity; condensation can be a sign of high humidity; insulate cold surfaces such as cold-water pipes.
  3. Air Change Rates in Rooms — The Engineering ToolBox. ACH = CFM × 60 / room volume.
  4. NIST Chemistry WebBook, SRD 69 — Thermophysical Properties of Fluid Systems (water) — National Institute of Standards and Technology. Saturated-liquid water density and viscosity, e.g. 62.364 lb/ft³ and 1.1211 cP at 60 °F; 61.377 lb/ft³ and 0.46602 cP at 140 °F.

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