GPD vs mL/hr: Converting Humidifier Output Specs
Humidifier output is published in either gallons per day (GPD) or milliliters per hour (mL/hr), with no consistent rule for which brands use which. The conversion is fixed — 1 gallon = 3,785.41 mL — so you can always put two models on the same footing before comparing them.
The one thing to know: Manufacturers split roughly evenly between rating output in gallons per day (GPD) and milliliters per hour (mL/hr), and a straight unit conversion (1 gallon = 3,785.41 mL) lets you compare any two models on the same footing regardless of which unit a given brand chose to publish. When a spec sheet gives only a sq ft claim and no GPD or mL/hr figure at all — which happens more often than you'd expect, including on some tank-size-forward budget models — that is itself useful information: it means the coverage claim can't be independently checked against a verified output rate.
The conversion formulas
mL/hr to GPD: (mL/hr × 24) ÷ 3,785.41 = GPD
GPD to mL/hr: (GPD × 3,785.41) ÷ 24 = mL/hr
Worked examples from real models
| Model | Published spec | Converted |
|---|---|---|
| Levoit LV600S | 500 mL/hr (mfr.) | ≈3.17 GPD (converted) |
| Honeywell HCM350 | up to 2 GPD (mfr.) | ≈315.5 mL/hr (converted) |
| Pure Enrichment MistAire | 150 mL/hr (mfr.) | ≈0.95 GPD (converted) |
| AIRCARE Alliance H12600 | 12 GPD (mfr.) | ≈1,892.7 mL/hr (converted) |
Full table with tank capacity and derived room volume in the Humidifier Output Index.
This page is a unit-conversion reference, not a health or safety determination.
Frequently Asked Questions
Multiply mL/hr by 24 to get mL/day, then divide by 3,785.41 (mL per gallon). For example, 500 mL/hr × 24 = 12,000 mL/day ÷ 3,785.41 = about 3.17 GPD.
Multiply GPD by 3,785.41 to get mL/day, then divide by 24. For example, 2 GPD × 3,785.41 = 7,570.8 mL/day ÷ 24 = about 315.5 mL/hr.
There's no industry-wide standard — it appears to be a brand or regional convention rather than a technical difference. Evaporative/whole-house brands (Honeywell, AIRCARE) tend to use GPD; ultrasonic tabletop brands (Levoit, Pure Enrichment) tend to use mL/hr.
Not automatically — always convert to the same unit before comparing. A 500 mL/hr unit (about 3.17 GPD) outputs more than a unit rated at 2 GPD, but a 150 mL/hr unit (about 0.95 GPD) outputs less. Never compare raw mL/hr numbers to raw GPD numbers without converting.