Everyday
Whole House Humidifier Size Calculator
Enter your home's size, how tight the envelope is, and what heats it. The page works out the gallons per day a humidifier must deliver under AHRI Guideline F, then matches the load to a bypass, fan-powered, or steam unit.
How it works
Behind the numbers
Sizing follows AHRI Guideline F, the chart behind most furnace-mounted humidifier ratings. At 8-foot ceilings it calls for 4.3 gallons per day per 1,000 square feet in a tight home, 8.6 in an average one, and 12.7 in a leaky one, because the dry outdoor air leaking in is what you are really humidifying. The calculator scales that by ceiling height, so a 2,000 square foot average home with 9-foot ceilings lands near 19 gallons per day.
Gallons per day is only half the answer, because the three types cover different ranges. Bypass units run about 12–18 GPD and suit homes up to roughly 1,500 square feet; fan-powered units at 18–25 GPD hit their stride in the 1,500–2,500 range; steam units at 20–35+ GPD handle larger or leaky homes and keep output where evaporative types fade. On a heat pump, evaporative output drops with the cooler supply air, so the calculator sizes up a step and leans toward steam.
The rating on the box is a capacity, not a setting, and running too high ends with wet windows. The safe maximum falls as it gets colder: roughly 40–45% above 20°F outside, 35–40% from 0 to 20°F, and 30–35% below zero. The calculator shows the ceiling for the coldest temperature you design for. If the glass sweats anyway, dial the setting back before suspecting the unit.
FAQ
Questions, answered
What size whole house humidifier do I need?
Start with square footage and envelope tightness under AHRI Guideline F: at 8-foot ceilings, plan 4.3 gallons per day per 1,000 square feet for a tight home, 8.6 for average, 12.7 for leaky. A 2,000 square foot average home lands near 17 GPD. Taller ceilings raise the number, and on a heat pump, go one step up or pick steam, since evaporative output drops with cooler supply air.
Should I get a bypass, fan-powered, or steam humidifier?
Match the type to the load. Bypass units (about 12–18 GPD) fit homes up to roughly 1,500 square feet. Fan-powered (18–25 GPD) is the sweet spot for 1,500–2,500. Steam (20–35+ GPD) covers larger or leaky homes and holds output where evaporative types fade. Steam costs more up front — roughly $1,600–$2,800 installed versus $550–$950 for bypass — and needs a dedicated 240V circuit.
What should I set my humidifier to in winter?
Aim for 35–45% relative humidity in winter, inside the EPA's 30–50% healthy band, but lower the setting as it gets colder: about 40–45% when it is above 20°F outside, 35–40% from 0 to 20°F, and 30–35% below zero. Condensation or frost on the windows means the setting is too high — dial it back before blaming the unit.
Does a heat pump change what size humidifier I need?
Yes. Heat pumps deliver cooler supply air than furnaces, which weakens bypass and fan-powered humidifiers (both are evaporative). Homes on heat pumps usually size up a step, and steam is often the better pick there, since it boils water on its own instead of relying on hot duct air.
How much water does a whole house humidifier use?
About its rated gallons per day on heating days: a 17 GPD unit puts roughly 17 gallons into the air each day it runs. Bypass flow-through models send extra water down the drain while running, so they use more than the rating. Steam and fan-powered units use close to what they evaporate.