Indoor Humidity Levels and the Best Hygrometers to Measure Them

A hygrometer is "an instrument which measures the humidity of the air," in the National Weather Service's definition, and what home models show is relative humidity: how much moisture is in the air compared with the most it could hold at that temperature. EPA's target for a home is below 60 percent, ideally between 30 and 50 percent. To measure it, the TempPro TP50 is the simple pick, and the Aranet4 HOME is the one to get if you want history on your phone.

Readings depend on where you put the hygrometer and how accurate its sensor is, and every maker states that accuracy differently. Specs and guidance checked November 1, 2026.

• 30 to 50%EPA's ideal range for indoor relative humidity
• 60%The level EPA says to stay below
• 75%Relative humidity over saturated table salt at room temperature (NIST)

What the number means

The National Weather Service defines relative humidity as "a dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated." It adds the part that trips people up: because the amount of moisture air can hold depends on temperature, "relative humidity is a function of both moisture content and temperature." The same air reads differently beside a warm radiator and beside a cold window, so where you put a hygrometer matters as much as which one you buy.

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01Relative humidity+RH: moisture relative to saturation

Relative humidity is the amount of atmospheric moisture present relative to the amount present at saturation, expressed as a percent.

Sources: 1: The National Weather Service defines relative humidity as a percentage ratio of actual moisture to the amount present at saturation.

02Moisture and temperature+Both affect the RH reading

Relative humidity depends on both moisture content and temperature.

Sources: 2: Relative humidity depends on both moisture content and temperature.

03Place at breathing height+Airthings View Plus example

Airthings recommends placing View Plus at breathing height.

Sources: 3: Airthings recommends placing View Plus at breathing height and at least three feet from vents, fans, windows or doors.

04Leave 3 feet clear+From vents, fans, windows or doors

Place View Plus at least 3 feet from vents, fans, windows or doors. The distance is Airthings’ instruction for this monitor.

Sources: 3: Airthings recommends placing View Plus at breathing height and at least three feet from vents, fans, windows or doors.

Airthings View Plus placement example; schematic locations. Follow the placement instructions for your own monitor.

Source notes3 references+

1. 1

   The National Weather Service defines relative humidity as a percentage ratio of actual moisture to the amount present at saturation.

   forecast.weather.gov ↗ — source for The National Weather Service defines relative humidity as a percentage ratio of actual moisture to the amount present at saturation.

   A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.

2. 2

   Relative humidity depends on both moisture content and temperature.

   forecast.weather.gov ↗ — source for Relative humidity depends on both moisture content and temperature.

   Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature.

3. 3

   Airthings recommends placing View Plus at breathing height and at least three feet from vents, fans, windows or doors.

   www.airthings.com ↗ — source for Airthings recommends placing View Plus at breathing height and at least three feet from vents, fans, windows or doors.

   At breathing height, at least 3 feet from vents, fans, windows, or doors.

EPA's guidance gives the targets. Its moisture guide says to keep indoor humidity "below 60 percent (ideally between 30 and 50 percent) relative humidity," and its humidifier fact sheet says "Do not humidify to indoor relative humidity levels exceeding 50 percent." EPA also gives the visual check: "If water condenses on windows, walls, or pictures, relocate the humidifier, lower its humidistat setting, or reduce its use."

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01Measure the room+Use a hygrometer

A hygrometer measures humidity levels. Also check surfaces for condensation.

• Check surfaces → Condensation on surfaces?

Sources: 4: EPA says hygrometers can measure humidity levels., 3: If water condenses on windows, walls or pictures, EPA advises relocation, a lower humidistat setting or reduced humidifier use.

02Condensation on surfaces?+Windows, walls or pictures

If water condenses on these surfaces, EPA advises adjusting the humidifier’s placement, setting or use.

• Yes → Adjust humidifier use
• No → Aim for EPA’s ideal

Sources: 3: If water condenses on windows, walls or pictures, EPA advises relocation, a lower humidistat setting or reduced humidifier use.

03Adjust humidifier use+Relocate it, lower setting or reduce use

Relocate the humidifier, lower its humidistat setting or reduce its use when water condenses on windows, walls or pictures.

Outcome

Sources: 3: If water condenses on windows, walls or pictures, EPA advises relocation, a lower humidistat setting or reduced humidifier use.

04Aim for EPA’s ideal+30 to 50 percent RH

EPA’s ideal indoor relative-humidity range is between 30 and 50 percent.

• Humidifier on → Do not exceed 50
• No humidifier → Keep below 60 percent

Sources: 1: EPA recommends indoor relative humidity below sixty percent, ideally between thirty and fifty percent.

05Do not exceed 50+percent RH while humidifying

Do not humidify to indoor relative-humidity levels above 50 percent.

• Indoor limit → Keep below 60 percent

Sources: 2: EPA says not to humidify above fifty percent indoor relative humidity.

06Keep below 60 percent+General indoor guidance

EPA advises keeping indoor humidity below 60 percent if possible. This is an upper limit; the ideal band remains 30 to 50 percent.

Outcome

Sources: 1: EPA recommends indoor relative humidity below sixty percent, ideally between thirty and fifty percent.

EPA ideal: 30 to 50 percent relative humidity. The general advice to keep below 60 percent does not extend the humidifying limit above 50 percent.

Source notes4 references+

1. 1

   EPA recommends indoor relative humidity below sixty percent, ideally between thirty and fifty percent.

   www.epa.gov ↗ — source for EPA recommends indoor relative humidity below sixty percent, ideally between thirty and fifty percent.

   If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity.

2. 2

   EPA says not to humidify above fifty percent indoor relative humidity.

   www.epa.gov ↗ — source for EPA says not to humidify above fifty percent indoor relative humidity.

   Do not humidify to indoor relative humidity levels exceeding 50 percent.

3. 3

   If water condenses on windows, walls or pictures, EPA advises relocation, a lower humidistat setting or reduced humidifier use.

   www.epa.gov ↗ — source for If water condenses on windows, walls or pictures, EPA advises relocation, a lower humidistat setting or reduced humidifier use.

   If water condenses on windows, walls, or pictures, relocate the humidifier, lower its humidistat setting, or reduce its use.

4. 4

   EPA says hygrometers can measure humidity levels.

   www.epa.gov ↗ — source for EPA says hygrometers can measure humidity levels.

   Hygrometers, available at local hardware stores, may be used to measure humidity levels.

Quick picks

Best basic hygrometer: TempPro TP50

The TP50 does one job: temperature and humidity on an LCD, refreshed every 10 seconds, with the day's highs and lows and a Dry, Comfortable or Wet indicator. TempPro lists its humidity tolerance as ±2% between 30% and 80% and ±3% outside that band, across a 10 to 99% range. It's calibratable, runs on one AAA battery, and can stand on a table, stick on with its magnetic back or hang. At 3 1/8 x 2 5/8 x 1 in, it also fits in a stocking.

Two things to know. TempPro's product title says "Large Backlit," but its own spec table says there's no backlight, so plan on reading it with the lights on. And its Dry, Comfortable and Wet icon uses TempPro's own bands, so read the number against EPA's 30 to 50% rather than trusting the face. ThermoPro's TP50 page now redirects to TempPro's, so listings may use either name. Registering it extends the warranty to 3 years.

Check price on Amazon

Best for history on your phone: Aranet4 HOME

The Aranet4 HOME is a CO2 monitor first, with a true NDIR sensor, but it's also a careful hygrometer. Aranet's datasheet lists relative humidity at ±3% accuracy over 0 to 85%, with a long-term drift of 0.5% a year, and temperature at ±0.3 °C. The e-ink screen shows CO2, temperature and humidity at a glance, and the Aranet4 app connects over Bluetooth to show history (Aranet's manual says the app stores up to 90 days, with CSV export; the monitor itself keeps 15 days at the default 5-minute interval, per the datasheet).

It runs on two AA batteries, and Aranet's datasheet estimates about 2.8 years at the default 5-minute interval with Bluetooth on. The trade-offs: its radio is Bluetooth (Aranet's datasheet lists no Wi-Fi), it measures CO2, temperature, humidity and pressure but not particles, and it isn't for damp rooms: its operating range stops at 85% relative humidity, non-condensing.

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Best whole-home dashboard: Airthings View Plus

The View Plus is the pick if humidity is one of several things you want to watch. It has seven sensors (radon, PM2.5, CO2, VOCs, temperature, humidity and pressure), and Airthings' product sheet lists humidity accuracy "Within 3%." Readings show on an e-ink screen and sync over Wi-Fi to the Airthings app, and it runs on six AA batteries or USB-C.

Airthings also publishes placement rules worth using for any monitor: put it at breathing height, at least 3 feet from vents, fans, windows and doors, and never outdoors or anywhere above 85% humidity. The trade-offs: it's large for a nightstand at 17 cm wide, and it doesn't support Apple HomeKit.

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Best for local, open-source monitoring: AirGradient ONE

The AirGradient ONE measures temperature and humidity with a Sensirion SHT40, which AirGradient lists at ±2% relative humidity and ±0.2 °C, alongside CO2, particles and VOC and NOx indexes. It has a small OLED display and an LED bar you can switch off, connects over Wi-Fi, and works locally with Home Assistant (version 2024.6 or later, with current firmware) and Homey, so the cloud is optional. AirGradient says it's fully open source.

The trade-offs: it runs on USB power, and the 5 V plug of more than 2 A it needs isn't in the box, and AirGradient's page describes its online dashboard both as "subscription free" and as "12 months data platform / dashboard and API access," so check the current terms.

See the AirGradient ONE at AirGradient

How to check a hygrometer's reading

You don't need a lab, only a known humidity. NIST's reference tables for saturated salt solutions give one you can make at home: air in a closed space over saturated sodium chloride (table salt) settles at 75.47% relative humidity at 20 °C and 75.29% at 25 °C.

1. 1

   Make the reference

   Put a small dish of table salt in a container that seals, and add just enough water to make it damp; some salt should stay undissolved.

2. 2

   Seal it in

   Place the hygrometer beside the dish, not touching it, and close the lid. Keep it out of sun and away from heat.

3. 3

   Wait

   Leave it until the reading stops changing. The air inside has to settle before the number means anything.

4. 4

   Read the offset

   The difference from about 75% is the error at that level. Correct it if your hygrometer is calibratable, like the TP50, or note it.

One check isn't the whole story. TempPro's own tolerance shows that accuracy can change across the range (±2% between 30 and 80%, ±3% outside), so a hygrometer that reads true at 75% can still be a little off at 40%. NIST's tables list magnesium chloride at about 33% if you want a second point. Aranet's 0.5% a year drift figure is also a reason to repeat the check now and then.

Buying guide

Accuracy is a tolerance, not a promise. Compare each maker's stated figure: ±2% (TP50 within 30 to 80%, AirGradient), ±3% (Aranet) or within 3% (Airthings). Two hygrometers a couple of points apart can both be within spec.

Decide whether you need history. A high and low record (TP50) shows what happened overnight. An app with stored data (Aranet4, Airthings, AirGradient) shows patterns across days and rooms, which helps if you're running a humidifier or a dehumidifier.

Place it like it matters. Keep it away from heat, sun and the humidifier's own output, and follow Airthings' rule of breathing height and 3 feet from vents, windows and doors. Because relative humidity depends on temperature, a hygrometer on a windowsill isn't measuring the room.

Tap a label to explore. You can also hover or use your keyboard.

01Relative humidity+RH: moisture relative to saturation

Relative humidity is the amount of atmospheric moisture present relative to the amount present at saturation, expressed as a percent.

Sources: 1: The National Weather Service defines relative humidity as a percentage ratio of actual moisture to the amount present at saturation.

02Moisture and temperature+Both affect the RH reading

Relative humidity depends on both moisture content and temperature.

Sources: 2: Relative humidity depends on both moisture content and temperature.

03Place at breathing height+Airthings View Plus example

Airthings recommends placing View Plus at breathing height.

Sources: 3: Airthings recommends placing View Plus at breathing height and at least three feet from vents, fans, windows or doors.

04Leave 3 feet clear+From vents, fans, windows or doors

Place View Plus at least 3 feet from vents, fans, windows or doors. The distance is Airthings’ instruction for this monitor.

Sources: 3: Airthings recommends placing View Plus at breathing height and at least three feet from vents, fans, windows or doors.

Airthings View Plus placement example; schematic locations. Follow the placement instructions for your own monitor.

Source notes3 references+

1. 1

   The National Weather Service defines relative humidity as a percentage ratio of actual moisture to the amount present at saturation.

   forecast.weather.gov ↗ — source for The National Weather Service defines relative humidity as a percentage ratio of actual moisture to the amount present at saturation.

   A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.

2. 2

   Relative humidity depends on both moisture content and temperature.

   forecast.weather.gov ↗ — source for Relative humidity depends on both moisture content and temperature.

   Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature.

3. 3

   Airthings recommends placing View Plus at breathing height and at least three feet from vents, fans, windows or doors.

   www.airthings.com ↗ — source for Airthings recommends placing View Plus at breathing height and at least three feet from vents, fans, windows or doors.

   At breathing height, at least 3 feet from vents, fans, windows, or doors.

How we choose

We haven't lab-tested these hygrometers. Every pick comes from research:

• Stated accuracy. Humidity and temperature tolerances come from each maker's product page, datasheet or product sheet, quoted as the maker gives them.
• Data and power. Display, logging, app, connectivity and batteries from the same official pages and manuals.
• Reference values. Definitions come from the National Weather Service, targets from EPA, and the salt-check humidity from NIST's published tables.
• No health claims. We report what a hygrometer measures, not what a humidity level does for your body.

Our full method is on How we choose.

Verdict

Aim for EPA's 30 to 50% and measure before you adjust. The TempPro TP50 is the simple, calibratable hygrometer for one room, and the Aranet4 HOME adds phone history and an NDIR CO2 sensor on two AA batteries. For a whole-home dashboard, the Airthings View Plus adds radon and particles, and the AirGradient ONE suits people who want local, open-source data. Whichever you buy, a salt check against NIST's 75% tells you how far to trust it.

Sources

Specs and guidance checked November 1, 2026:

• National Weather Service glossary: hygrometer and relative humidity
• EPA: A brief guide to mold, moisture and your home and Use and care of home humidifiers
• NIST: Greenspan, Humidity Fixed Points of Binary Saturated Aqueous Solutions, Journal of Research of the National Bureau of Standards, 1977
• TempPro: TP50
• Aranet: Aranet4 HOME datasheet (PDF) and user manual (PDF)
• Airthings: View Plus, its specifications and product sheet (PDF)
• AirGradient: ONE indoor monitor and its spec sheet (PDF)