Can I calibrate the humidity on my thermostat?

On several models, yes. Honeywell Home T9 and T10 Pro thermostats have an Indoor Humidity Display Offset of minus 12% to 12%, Sensi has a Humidity Offset of up to 25% either way, and ecobee has Humidity Correction on the Thresholds screen. The Google Nest help pages we checked do not describe one.

Does a wrong humidity reading affect how my system runs?

It can, if you use a humidity feature. ecobee says an inaccurate humidity reading means its air conditioner overcool feature, AC Overcool Max, may not work correctly. Sensi says its thermostat uses the offset number as the actual humidity, so an offset changes when dehumidification starts and stops.

Where do I see the humidity on a Nest thermostat?

Google says to press the thermostat ring to open the Quick View menu, where the current humidity is shown. The Nest app and Google Home app also show it when you tap your thermostat.

Can I turn the humidity reading off on an ecobee?

On the Smart Thermostat Premium, Enhanced and Lite, yes, through Humidity Monitoring on the Thresholds screen. ecobee says it cannot be disabled when a humidifier or dehumidifier is connected, and humidity features such as AC Overcool Max become unavailable while it is off.

Thermostat humidity reading seems wrong

Relative humidity changes with temperature and location, so two instruments in different spots rarely agree. Air from inside the wall behind the thermostat, nearby vents, windows or sunlight can skew it. If the gap stays the same, several models let you offset the humidity reading: up to 12% either way on a Honeywell Home T9 and 25% on Sensi.

Why do two humidity readings disagree?

Because humidity on a thermostat is relative humidity, and relative humidity depends on the air temperature at the sensor. A spot that is a little warmer reads lower humidity than a cooler spot in the same room, even though the amount of moisture in the air is the same. A hygrometer on a shelf and a thermostat on a wall are measuring two slightly different pockets of air.

Small gaps between two instruments are normal. A large gap, or one that swings through the day, usually has a cause you can find. It also matters more if your thermostat uses the air conditioner (AC) to dehumidify, as explained below.

What makes the thermostat reading drift?

ecobee's page on inaccurate humidity readings names two causes, and both apply to any brand.

Air from inside the wall. ecobee says the most common cause is a hole around the wiring behind the thermostat, which lets the sensors pick up air from inside the wall. That air is usually a different temperature and humidity from the room. Sealing it is installation work, so ask whoever installed the thermostat or a licensed HVAC technician.
Location. ecobee lists return vents, windows, doors and direct sunlight as placements that make readings inaccurate. Sensi makes a similar point about locations near windows or a bathroom.

Because the same things skew the temperature, a humidity problem often shows up alongside a temperature one. The causes of a wrong temperature are covered on thermostat shows the wrong room temperature.

Can you offset the humidity reading?

Several brands let you shift the displayed humidity. The ranges are quite different:

Humidity reading offsets by brand and modelUnits: Range in % relative humidity.
Thermostat	Setting name	Range(% relative humidity)	Where you change it
Honeywell Home T9	Indoor Humidity Display Offset (ISU 14020), in 1% steps	minus 12 to 12	Menu > Preferences > Indoor display offsets
Honeywell Home T10 and T10+ Pro	Indoor Display Offsets: Indoor Humidity (ISU 14020)	minus 12 to 12	Installer setup
Emerson Sensi	Humidity Offset	minus 25 to 25	Sensi app: settings gear > System settings
Emerson Sensi Lite	Humidity Offset, in 5% steps	minus 25 to 25	Sensi app
ecobee	Humidity Correction	Not stated	Thermostat only: Installation Settings > Thresholds
Google Nest	Not stated	Not stated	Not stated

From each manufacturer's guides and support pages, listed under Sources. Not stated means the pages we checked do not give it.

The wording behind these settings differs in a way that matters. Honeywell describes its T9 setting as a display offset: the thermostat displays up to 12% lower or higher than the actual measured humidity. Sensi says its thermostat "will use the new number as the actual humidity percentage", and that the offset applies to thermostats, not sensors. ecobee says its Humidity Correction adjusts the humidity sensor reading for greater accuracy.

When should you change the offset?

Only when the error is steady. ecobee's thresholds page says to adjust Humidity Correction only if your humidity is a consistent amount above or below the actual humidity, and to look for the cause if the difference varies. The same logic applies to every brand. Compare the thermostat with a hygrometer placed right next to it, at a few different times. If the gap is the same each time, an offset is reasonable. If it changes with the sun or when the system runs, the location or the wall cavity is the more likely cause, and an offset will be right at one hour and wrong at another.

Does the reading affect dehumidification?

Yes, on systems that act on humidity. ecobee warns that inaccurate humidity readings mean AC Overcool Max may not work correctly, since that feature keeps cooling until the humidity target is reached. Google Nest Cool to Dry turns the air conditioner on when humidity rises above 75% with cooling off. A reading that is too high can make either feature run more than you expect. How these settings behave is on overcool to dehumidify.

On the ecobee Smart Thermostat Premium, Enhanced and Lite, you can disable Humidity Monitoring entirely on the Thresholds screen, unless a humidifier or dehumidifier is connected. ecobee says AC Overcool Max, the eco+ temperature adjustment for humidity and high or low humidity alerts become unavailable while it is off. The neighboring Temperature Correction setting is explained on how to calibrate an ecobee.

Thermostat humidity does not match your hygrometer

Did you compare them side by side, at the same spot?

Yes
Is the gap about the same at different times of day?
- Yes
  A steady offset. Use the humidity offset for your model from the table above.
- No
  Something nearby changes the reading. Check for vents, windows, doors and sunlight, and ask the installer about air from the wall behind the thermostat.
No
Put the hygrometer next to the thermostat for an hour first. Readings in different rooms are expected to differ.

Frequently asked questions

Can I calibrate the humidity on my thermostat?: On several models, yes. Honeywell Home T9 and T10 Pro thermostats have an Indoor Humidity Display Offset of minus 12% to 12%, Sensi has a Humidity Offset of up to 25% either way, and ecobee has Humidity Correction on the Thresholds screen. The Google Nest help pages we checked do not describe one.
Does a wrong humidity reading affect how my system runs?: It can, if you use a humidity feature. ecobee says an inaccurate humidity reading means its air conditioner overcool feature, AC Overcool Max, may not work correctly. Sensi says its thermostat uses the offset number as the actual humidity, so an offset changes when dehumidification starts and stops.
Where do I see the humidity on a Nest thermostat?: Google says to press the thermostat ring to open the Quick View menu, where the current humidity is shown. The Nest app and Google Home app also show it when you tap your thermostat.
Can I turn the humidity reading off on an ecobee?: On the Smart Thermostat Premium, Enhanced and Lite, yes, through Humidity Monitoring on the Thresholds screen. ecobee says it cannot be disabled when a humidifier or dehumidifier is connected, and humidity features such as AC Overcool Max become unavailable while it is off.