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Humidifier Capacity Calculator

Work out humidifier capacity instantly with clear inputs, formula shown and shareable results.

About Humidifier Capacity Calculator

A humidifier has to replace the moisture that leaks away with infiltrating dry winter air. Converting volume and air change rate to CFM, then multiplying by air density and the grains of moisture that must be added per pound of air, gives the water the unit must evaporate each hour.

Formulas

Moisture load

lb/h = CFM x 60 x 0.075 x Δgrains / 7000; gal/day = lb/h x 24 / 8.34

Latent load

BTU/h = 0.68 x CFM x Δgrains

Worked example

With the default values below, Humidifier Capacity Calculator returns Humidifier capacity required of 5.55. Change any input above and every figure here is recalculated in your browser.

Inputs used

Space volumeminimum 1 ft³
12,000 ft³
Infiltration rateminimum 0.05 ACH
0.5 ACH
Moisture to addminimum 1 grains/lb
30 grains/lb

Results

Humidifier capacity required
5.55 gal/day
Moisture addition rate
1.93 lb/h
Infiltration airflow
100 CFM
Latent heat load
2,040 BTU/h

Step by step

  1. 1Values used
  2. 2Moisture load
  3. 3Latent load
  4. 4Humidifier capacity required= 5.55 gal/day
  5. 5Moisture addition rate= 1.93 lb/h
  6. 6Infiltration airflow= 100 CFM
  7. 7Latent heat load= 2,040 BTU/h

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Humidifier Capacity figure worked out?

lb/h = CFM x 60 x 0.075 x Δgrains / 7000; gal/day = lb/h x 24 / 8.34. A humidifier has to replace the moisture that leaks away with infiltrating dry winter air.

Why does a tight house need less capacity?

Almost all winter humidity loss is infiltration. Halving the air change rate halves the humidifier duty, which is why air sealing usually beats a bigger humidifier.

What values do I need to enter?

This calculator takes 3 inputs: Space volume, Infiltration rate, Moisture to add. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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