How to Use and Calibrate a Hygrometer (Salt Test Included)

Quick answer: A hygrometer measures relative humidity, which is how much moisture is in the air compared with the most it could hold at that temperature. To use one, put it where you want to know the humidity (a room, a humidor or inside a storage jar), close everything up and give it time to settle before you read it. To check it's accurate, seal it in a bag with a damp pinch of table salt for at least 6 hours. Salt and water hold 75%, so whatever the gap is, that's your hygrometer's error.

Most people buy a hygrometer, drop it in a jar and then believe whatever it says for the next three years. And that works fine, right up until the screen and the jar stop agreeing with each other. It says 62% and the herbs crumble when you touch them. Or it says 55% and there's a faintly damp smell when you open the lid.

Nine times out of ten the hygrometer isn't actually broken. It got read too soon, or it's sitting in a bad spot, or it has drifted a few points over the months and nobody noticed. We sell these little things, so we get asked about this a lot, and we'll be honest: the older version of this page didn't help much. It talked mostly about homes, gave a humidity range we couldn't source, and suggested a damp-cloth test that doesn't really prove anything. So we've rewritten it. Below is what a hygrometer measures, how to use and read one (including inside a jar, which almost nobody covers), how to test it with salt, and what to do once you know the number is off.

What Is a Hygrometer, and What Is It Used For?

The National Weather Service keeps its definition short. A hygrometer is "an instrument which measures the humidity of the air" (NWS glossary). The figure you see on the screen is relative humidity, usually written RH.

The "relative" bit catches people out. The NWS explains that "relative humidity is a function of both moisture content and temperature" (NWS glossary). In plain terms, if you warm the same air up, the RH goes down, even though not a drop of water has gone anywhere. Cool it down again and the RH climbs back. So when a jar reads a few points different in the evening than it did at lunchtime, it may just be the room getting colder.

As for what they're used for, it's a longer list than you'd guess. People keep them in living rooms and bedrooms to stay comfortable and catch damp before it turns into mold. Cigar smokers have them in humidors, and wine people have them in cellars, because cigars and corks both suffer when the air is wrong. If you play guitar or violin, there's a fair chance one lives in the case, since dry winter air can crack a wooden instrument. They turn up in reptile tanks, grow tents and greenhouses too. And then there are storage jars for dried herbs, spices and loose tea, which is the use we care about most here.

How Does a Hygrometer Work?

Pretty much every hygrometer you'll find for sale is one of three types.

The digital ones, including ours, use a tiny electronic sensor. On most, that's a capacitive sensor, a thin layer of polymer or metal oxide that soaks up water vapor and changes its electrical properties as it does. The little circuit turns that change into a percentage. They're cheap, small and quick to react. Wikipedia's hygrometer page puts a calibrated capacitive sensor at around ±2% RH between 5% and 95%, and adds that "uncalibrated, this is two to three times worse." They're also "subject to contamination, drift and aging effects," which is the whole reason it's worth testing one now and then.

Dial hygrometers are the analog kind you see in old humidors. Inside there's a coil of metal and paper, or sometimes a strand of hair, that changes shape as it takes on moisture and nudges a needle. They look the part, but the cheap ones aren't great. The same Wikipedia page says the metal-paper coil type can be off by "10% or more."

Then there's the psychrometer, which uses two thermometers, one of them with a wet wick. Water evaporating from the wick cools it down, and the difference between the two readings gives you the humidity from a chart. They're accurate, but they belong in a weather station, not a jar.

For anything small and sealed, we'd go digital every time. A dial won't fit in most jars, and a 10-point error is no use when the range you're trying to hit is only 10 points wide in the first place.

How to Use a Hygrometer

In a room

Put it somewhere that isn't going to skew the reading, so not in direct sun, and not next to a heating vent, radiator, window, kettle or bathroom door. Somewhere around chest height on an inside wall gives a fair picture of the room. If you've just moved it, give it at least half an hour before you believe what it says. And don't judge a room on one reading. Check it a few times across the day, or across a week, and you'll get a much better idea of what's going on.

Inside a storage jar

This is where a small digital hygrometer really earns its place, and it works a bit differently.

1. Put it inside, with the contents. You want it reading the air in the jar, not the room. A mini hygrometer can sit on top of whatever's in there or slide down the side of the glass.
2. Close the lid and wait. In a sealed jar, the air and the contents trade moisture until they match. METER Group, who make lab equipment for exactly this kind of measurement, put it like this: "the relative humidity of the air in the headspace will equilibrate with the water activity of the sample" (METER Group). That doesn't happen instantly, and every time you open the lid some room air gets in and throws it off again. We'd give it an hour or two at least, and longer if you've only just filled the jar.
3. Read it through the glass if you can. If you have to open up to see the screen, read it straight away, because it'll start drifting toward the room's humidity within a few minutes.
4. Use one per jar. A jar of herbs and a jar of tea can sit at quite different levels. Ours come six to a pack in red, green and purple, partly so you can color-code jars and tell which reading is which from across the shelf.

How to Read a Hygrometer (and What the Numbers Mean)

A digital hygrometer usually shows you two numbers. The big one with the % sign is relative humidity, and the smaller one is temperature (ours reads in °F).

Whether a number is good or bad depends entirely on where the hygrometer is sitting. A jar of dried herbs wants much damper air than your living room does, which surprises people the first time they see it.

Where	Too dry	Where you want it	Too damp
A room in your home	Below 30%	30% to 50%	Above 60%
A storage jar of dried herbs	Below 55%	55% to 65%, aiming for 62%	Above 65%

The room figures come from the EPA, which says to keep indoor humidity "below 60 percent (ideally between 30 and 50 percent)" to keep mold away (EPA). That replaces the "25% to 55%" the old version of this page used, which we couldn't trace back to anything. The jar figures come from ASTM D8197, an industry standard for dried flower, which sets the safe range at a water activity between 0.55 and 0.65 (ASTM D8197). In a sealed jar that works out at roughly 55% to 65% RH. Drier than that and things go brittle and lose their smell. Damper, and you're giving mold a chance. If you want the longer version of why the middle of that range matters, our guide on whether flower goes bad goes into it.

So 63% in your living room is on the damp side, but 63% in a jar of herbs is fine, which is why it helps to know which row of the table you're actually looking at.

How Accurate Is a Hygrometer?

Less accurate than the screen makes it look, honestly. A display showing 62% isn't promising you the air is exactly 62%. Every hygrometer comes with a rated accuracy, and the small digital ones are usually somewhere around ±2% to ±5% RH. Ours are rated to within 5% RH, which is normal for something this size and this price.

In a room, 5 points either way hardly matters. A jar's a different story, because the safe range there only runs from 55% to 65%. If your hygrometer says 62% and could be wrong by 5 in either direction, the real number might be anywhere from 57% to 67%. That covers the entire safe range, with a bit of mold territory on the end.

None of that makes cheap hygrometers a bad buy, though. It just means it's worth testing yours once, so you know which way it leans and by how much. A hygrometer that reads 3 points low every single time is still really useful, as long as you know it reads 3 low.

How to Calibrate a Hygrometer: The Salt Test

The salt test works because a saturated mix of salt and water holds the air above it at one fixed humidity. The US National Bureau of Standards (now NIST) measured it back in 1977. Saturated table salt holds 75.3% RH at 77°F (25°C) and 75.5% at 68°F (20°C) (Greenspan, NIST 1977), and the paper actually lists hygrometer calibration as one of the things these fixed points are good for. That also answers a question we get a lot, which is whether the temperature matters. It barely does, since any normal room gives you 75%, give or take a fraction. What does matter is that the temperature stays steady while you wait.

You'll need some table salt, a little water, a bottle cap or tiny dish, and a zip bag or airtight container the hygrometer fits in.

1. Make the salt damp, not dissolved. Put about a teaspoon of salt in the cap and add water a few drops at a time until it looks like wet sand. If it all dissolves into a puddle, add more salt. There has to be some undissolved salt left for this to work.
2. Seal it in with the hygrometer. Cap and hygrometer go in the bag side by side, with the salt kept off the sensor. Leave a bit of air in there and seal it.
3. Leave it for at least 6 hours. Overnight is better. Put it somewhere the temperature won't swing around, so not on a windowsill or next to a radiator. Read it too early and you'll catch a number that's still climbing, which gives you the wrong offset.
4. Read it without opening the bag. Room air shifts the reading within minutes, so look through the plastic.
5. Work out the gap from 75. If it says 75, lovely, it's spot on. If it says 72, it's reading 3 low. If it says 79, it's 4 high.

If you've got several hygrometers, test them all in the same bag at the same time. It saves salt, and you'll spot straight away if one of them disagrees with the rest.

Adjust it, or remember the offset

Some hygrometers have a calibration setting, or a little screw on the back if it's a dial, and you can set it to 75% while it's still sealed in the bag. If yours doesn't have one, that's fine. We'd just write the offset on a scrap of tape and stick it on the back, then add or subtract every time you read it. A hygrometer that's 3 low and shows 59% in your jar is really telling you about 62%.

How close is close enough?

Anything within the hygrometer's rated accuracy means it's working the way it's supposed to. For ours, that's anywhere from 70% to 80% on the salt test. We'd still note the offset either way, though, because "in spec" isn't the same as exact, and in a jar it's the exact number you're after. If it's further out than its rating, or gives you a different answer every time you test it, it's time for a new one.

Why we dropped the damp-cloth test

The old version of this guide told you to wrap your hygrometer in a damp cloth and look for about 95%. Other sites say to use a wet paper towel and look for 100%. We'd skip both. A wet cloth doesn't hold any known, fixed humidity the way saturated salt does, so there's nothing reliable to compare your reading against. Salt gives you a proper reference point, and it's right in the range you actually use.

How Often Should You Calibrate a Hygrometer?

Test a new one as soon as it arrives, so you know its offset from day one. After that, once a year is a sensible habit. It's also worth re-testing any time the reading stops matching how the contents feel, or after the hygrometer has been dropped, got wet or spent time somewhere very hot or damp. Strong solvents, glues and adhesive tapes are worth keeping away from it too, since contamination is one of the things that pushes these sensors out over time.

What to Do When the Reading Is Off

Once you trust the hygrometer, the number in the jar tells you what to do next.

If it's sitting below about 55%, whatever's inside will feel crisp and probably crumble. Dried herbs can usually be brought back, though. Our guide on how to rehydrate dry flower walks through it, and with a two-way pack it takes a day or two.

Above about 65% is the one to take seriously. Open the jar and have a proper look and smell. Anything fuzzy or musty means mold, and that gets thrown out, not saved. If it all looks fine, leave the lid off for a while so it can air out before you seal it up again.

And if the reading keeps swinging up and down, the jar is usually the problem rather than the hygrometer. Either the lid isn't sealing properly, the jar's being opened a lot, or it's somewhere that warms up and cools down through the day. Moving it somewhere cool and dark usually helps a bit.

It's worth remembering that a hygrometer only tells you what's going on, and it can't fix anything by itself. To hold a jar steady you need something that adds moisture when it's dry and takes it back when it's damp, which is what our 2-Way 62% Humidity Packs do. A properly sealed jar from our stash jars stops moisture moving in and out in the first place. Put a hygrometer in the same jar and you can actually watch the pack doing its job.

Our Mini Digital Hygrometers

We make ours for exactly this kind of job. The ONGROK Digital Hygrometers are small enough to go inside a jar, a bag or a case. They read humidity from 10% to 99% RH, accurate to within 5%, and temperature from -58°F to 158°F, accurate to within 2°F. You get six in a pack in red, green and purple, with the LR44 batteries already fitted, for $29.99. Our advice is the same as above: salt-test them when they arrive, put a bit of tape with the offset on the back of each one, and then you'll know exactly where every jar stands.

Hygrometer FAQ

How do I know if my hygrometer is accurate?

You don't, until you test it. Seal it in a bag with a bottle cap of damp salt overnight and read it through the plastic. It should say 75%, and however far off it is, that's its error.

How long does a hygrometer take to give an accurate reading?

Longer than most people wait. In a room, give it half an hour after moving it. In a jar, an hour or two, and more if you just filled it, since the contents have to catch up with the air.

Where's the best place to put a hygrometer in a house?

Pretty much any inside wall at chest height, as long as it's away from sunny windows, vents, the kitchen and the bathroom. Anywhere that heats up, cools down or gets steamy will give you readings that say more about that spot than about the room.

Is 70% humidity in a house bad?

It's high, yes. The EPA suggests staying below 60% indoors, ideally 30 to 50%, because damp air is what mold needs. A dehumidifier, or just better airflow in that room, usually brings it down.

Do I need a separate hygrometer for every jar?

Ideally, yes. Two jars on the same shelf can sit several points apart depending on how full they are, what's in them and how often they get opened. One hygrometer moved between jars spends most of its time settling.

Can you check humidity without a hygrometer?

Only roughly. Condensation on the inside of windows and a musty smell mean damp air, and static shocks mean dry air. In a jar, herbs that snap are too dry and herbs that feel spongy are too damp. For an actual number, you need the hygrometer.

What's the difference between a hygrometer and a hydrometer?

They do completely different jobs, even though the names are only one letter apart. A hygrometer reads moisture in the air, while a hydrometer measures the density of a liquid, which is what brewers and people checking car batteries use. For a room, a humidor or a jar, it's a hygrometer you want.