A digital hygrometer with a capacitive sensor, checked once against a saturated salt solution, is the best combination of cost and accuracy for a house. Analog dial hygrometers run 10–20 points off and rarely hold calibration; smart, connected models add logging but the sensor inside is the same capacitive part. Your phone has no humidity sensor in it — a weather app is showing you the airport reading, not your room.
What actually measures humidity, and what does not
A hygrometer works by putting a material in the airstream whose physical property changes with moisture, then reading that change electrically or mechanically. A capacitive sensor uses a polymer film that absorbs water vapor and changes its electrical capacitance as it does; a resistive sensor reads a similar change in electrical resistance. Both are cheap to manufacture, which is why they show up in everything from a $10 dial gauge to a $200 smart display.
The mechanical alternative — the coiled-hair or synthetic-fiber dial hygrometer sold in most hardware stores — works on an older principle: the fiber lengthens and shortens with humidity, and a lever translates that into a needle position. It needs no battery, which is its only real advantage.
Three kinds of hygrometer, compared
| Type | Typical accuracy | Holds calibration? | Cost |
|---|---|---|---|
| Analog dial (hair/fiber) | ±10–20 points, worse with age | No — drifts continuously | $8–$15 |
| Digital, capacitive sensor | ±2–5 points fresh; check yearly | Reasonably well for 1–2 years | $10–$25 |
| Smart / Wi-Fi or Bluetooth | ±2–5 points — same sensor as above | Same as digital; app adds logging, not accuracy | $20–$60 |
The jump in accuracy that matters is analog to digital. Digital to smart buys history and remote alerts, not a more accurate reading — the sensor element inside a $60 Wi-Fi model and a $12 basic digital model is frequently the same part from the same handful of manufacturers.
Placement still decides more than the sensor does
Buying the most accurate meter available does not help if it sits somewhere unrepresentative — three feet from a heating vent, on a sunny windowsill, or in the one room in the house that runs driest. Where to put a hygrometer and when to read it covers placement and timing in full; the short version is chest height, away from vents and glass, and read first thing in the morning before any window opens.
A perfectly calibrated meter in the wrong spot loses to a five-point-off meter in the right one — placement moves the reading further than sensor quality does.
Calibrate it before you trust it: the salt test
Make a salt slurry
Half-fill a bottle cap or small dish with ordinary table salt (sodium chloride) and add water a few drops at a time until it is damp throughout but not dissolved — a wet paste, not a puddle.
Note the meter's current reading
Write down what your hygrometer shows right now, before you touch the calibration step, so you have a before-and-after to compare.
Seal both in a bag together
Put the salt dish and the hygrometer's sensing face side by side in a zip-top bag, press out the air, and seal it. They must share the same sealed air space — do not let the meter touch the salt or the water.
Leave it for 8 hours at a stable temperature
A saturated salt solution holds the air directly above it at 75% relative humidity at room temperature, reliably enough that this is the standard bench check for consumer hygrometers. A spot away from a vent or window keeps the temperature — and so the 75% figure — steady.
Read it without opening the bag
Most digital meters show through the plastic clearly enough to read in place. Opening the bag lets outside air in and undoes hours of equilibrium in seconds.
Record the offset
Subtract your meter's reading from 75. A meter reading 68% is 7 points low; one reading 81% is 6 points high. Add or subtract that offset from every reading the meter gives you from here on, or use the unit's calibration screw or button if it has one.
What to do with the offset
An offset under about 3 points is close enough to ignore for household decisions — running a dehumidifier or humidifier off the 30–50% band does not need lab precision. An offset past 5 points is worth correcting every time you read the meter, and past 10 points the meter is worth replacing rather than mentally adjusting forever; that much drift is rarely stable and tends to keep moving.
Re-run the test once a year for a digital meter and roughly every six months for an analog dial, since a fiber hygrometer drifts continuously rather than settling at one offset the way a capacitive sensor tends to.
The phone-sensor myth
No mainstream smartphone sold today carries a built-in relative-humidity sensor. A handful of Android models included one around 2014–2016 for fitness features and it was quietly dropped — component cost and case-sealing requirements made it not worth keeping. Every "humidity" reading a weather app shows you on a current phone is pulled from the nearest official weather station, often an airport miles away, not measured by the device in your hand.
That distinction matters indoors specifically because outdoor and indoor humidity routinely disagree by 20–30 points. A basement, a closet or a bathroom can sit at a completely different reading than whatever the weather app displays, and no phone app closes that gap — it cannot measure a number it has no sensor for.
A few dedicated humidity-measuring accessories plug into a phone's port or connect over Bluetooth and do carry a real capacitive sensor; those work like any other digital hygrometer above. The app alone, with no attached hardware, never does.
Smart, connected hygrometers: worth the extra cost?
The sensor inside a smart hygrometer is not more accurate than a $12 digital unit — see the comparison table above. What the extra cost buys is logging (a graph of the last 30 or 90 days), remote alerts if a basement or crawl space crosses a threshold while you are away, and sometimes a second temperature or CO₂ sensor bundled in.
That is genuinely useful in a room you cannot check in person often — a vacation-home basement, a rental you do not visit weekly, a crawl space with a recently installed vapor barrier you want to confirm is holding. In a bedroom or living room you walk through daily, a basic digital unit checked with the salt test gives you the same number for a fraction of the price.
Two meters beat one accurate meter in the wrong room
Humidity is not a single number for a house — a living room can read a comfortable 45% while a north-facing bedroom or an unfinished basement sits 15–20 points higher. A second cheap, salt-tested hygrometer left in whichever room actually worries you tells you more than upgrading the one meter you already own.
Once you have a trustworthy reading, the percentage itself is not always the most useful form of it. The absolute humidity converter turns a relative-humidity reading into grains per pound or grams per cubic meter, which is the form that lets you compare a basement reading against outdoor air directly — relative humidity alone cannot do that because it moves with temperature even when no water has actually moved.
How much this actually changes what you do
Everything on this site — the 30–50% target band, the point at which a surface starts growing mold, when to size up a dehumidifier — assumes the number feeding the decision is roughly right. A meter running 10 points low can show a comfortable 42% in a room that is actually pushing 52%, which is enough to explain condensation or a musty smell that a homeowner cannot otherwise account for.
The salt test costs nothing and takes one evening. It is the cheapest diagnostic step on this entire site, and skipping it is the most common reason a reader follows every other piece of advice here and still cannot explain what they are seeing.
Common questions
What is the most accurate way to measure indoor humidity at home?
A digital hygrometer with a capacitive sensor, checked against a saturated salt solution so you know its exact offset. Fresh out of the box it typically reads within 2–5 points; the salt test tells you whether your specific unit is doing better or worse than that.
Are digital hygrometers more accurate than analog dial hygrometers?
Yes, usually by a wide margin. A hair- or fiber-based dial hygrometer commonly runs 10–20 points off and keeps drifting with age and humidity swings, while a capacitive digital sensor starts closer to accurate and holds that calibration for a year or two.
Can my smartphone measure indoor humidity?
No. Almost no current smartphone contains a humidity sensor — the "humidity" a weather app shows is pulled from the nearest official weather station, not measured by the phone. A dedicated hygrometer, or a plug-in sensor accessory, is required for an actual indoor reading.
How often should a hygrometer be recalibrated?
Roughly once a year for a digital unit and every six months for an analog dial, using the salt test. Retest sooner if a reading suddenly looks implausible against what you can feel or see in the room.
Is a smart, Wi-Fi-connected hygrometer worth the extra cost?
Only if you need the logging or remote alerts — the sensor itself is typically the same capacitive part used in a basic $12 digital meter, not a more accurate one. It earns its price in a space you cannot check in person often, like a vacation-home basement or a rental.
What offset is too large to just mentally correct for?
Past about 10 points off in the salt test, the meter is usually worth replacing rather than adjusting by hand every time — that much drift rarely stays fixed at one offset and tends to keep moving further off over time.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions
- ASHRAE Standard 41.6 — Standard Method for Measurement of Moist Air Properties — reference method for humidity measurement and instrument accuracy that the saturated-salt bench check approximates for consumer hygrometers