Free calculator

Humidifier sizing calculator

A humidifier isn't filling the room once — it's replacing the moisture that every air exchange carries away. The output you need scales with room volume, the RH gap, and how leaky the space is.

Room & setpoint

LengthftWidthftHeightft

Air temperature°C77 °F = 25 °C. Warm air holds more water, so the same RH gap costs more at higher temps.

Current RH%Target RH%

Air exchanges per hourACHSealed room ≈ 0.5–1, tent with passive intakes ≈ 2–5, active exhaust far higher.

Result

Room volume 800 ft³ (22.7 m³)

Output to hold 65% RH

3.1L/day

= 0.8 gal/day

One-time charge to raise the air

130mL

Tiny compared to the ongoing load — leakage is the real bill.

Shop for a unit rated ~20% above this (3.8 L/day) so it can cycle rather than run flat out — and expect transpiring plants to shrink the load as the canopy fills in.

Assumes make-up air enters at room temperature and your current RH. In winter, infiltrating air is colder and much drier once heated, so real demand runs higher than this estimate — size up if you heat.

Chasing an RH number? What actually matters is VPD — calculate it here →