Last Updated on October 24, 2026 by Umar Farooq
How to calibrate a hygrometer: seal it in a bag with a cap of damp table salt for about 12 hours, then compare what it shows with the 75% relative humidity the salt holds the air at, and correct for the difference. It costs nothing, and it is the only honest way to find out whether the number you have been trusting is any good.
I would do this before buying a dehumidifier, not after. A hygrometer that reads ten points low will happily tell you a damp room is fine.
Why damp salt makes a reliable 75% reference
A saturated salt solution, meaning salt with just enough water that some crystals stay undissolved, holds the air above it at a fixed humidity. For sodium chloride (ordinary table salt) the figure is about 75%. One table of reference values, drawn from a NIST compilation, lists 75.7% at 20°C (68°F) and 75.3% at 25°C (77°F). A manufacturer’s instruction sheet gives 75.47% at 20°C for its 75% reference bottle.
So the value barely moves across normal room temperatures, and that is why salt is the standard home check. The UK’s National Physical Laboratory lists salt solutions as a field calibration method for hygrometers. If you want the wider picture of how instruments sense humidity at all, how humidity is measured covers it.
How to calibrate a hygrometer in five steps
Nothing here is dangerous, but keep the salt away from the sensor itself. That is my own precaution, not a manufacturer rule: salt dust on a delicate sensor is a poor trade.
- Make the slurry. Fill a bottle cap about three-quarters with table salt and add a few drops of water, ideally distilled, until it looks like wet sand. Some crystals must stay dry and undissolved. If it turns to brine, add more salt.
- Bag it. Put the cap and the hygrometer in a zip bag, display facing up, with the cap beside the unit rather than touching it. Seal it with the air still in.
- Keep the temperature steady. Somewhere around 20°C (68°F), out of sun and away from radiators.
- Wait. Sources I read give a range: at least five or six hours as a minimum, 12 hours for the manufacturer’s own procedure. I would leave it overnight and not open the bag.
- Read it through the bag. Note the number. If the unit shows 75%, it is spot on at that point. If not, write down the difference.
For a more careful result, one retailer guide suggests repeating the test and averaging the two readings. With readings of 70% and 72% it arrives at 71%, an offset of 4 points. I think that is worth the extra night if the hygrometer matters to you.
Do the offset sum, then adjust the unit or keep a sticky note
The offset is the salt reference minus what your hygrometer showed. Read low, and you add. Read high, and you subtract. The gap is in percentage points of humidity, not a percentage of the reading.
| Hygrometer showed | Offset | Add or subtract |
|---|---|---|
| 70% | +5 | Add 5 to its readings |
| 73% | +2 | Add 2 |
| 75% | 0 | Nothing |
| 78% | -3 | Subtract 3 |
That table is my own arithmetic, not a standard. What to do with it depends on the unit:
- Digital with a calibration mode: the manual says how to enter it. Set the display to 75%, then take it out of the bag.
- Dial with a screw on the back: turn it with a small screwdriver until the needle sits on 75%.
- Neither: keep the offset on a sticky note on the back and apply it in your head. Not elegant, entirely effective.
One honest limit. The salt test checks a single point. A unit that is right at 75% can still be off at 40%, and the NPL advises calibrating for the range and conditions of use. Our homes live between 30% and 60%, a long way from 75%. So treat a corrected reading as better, not as certified.
How far off is a cheap hygrometer allowed to be?
Further than the box says. The NPL states there is no “typical uncertainty” for humidity measurement, but very broadly good conditions might reach about plus or minus 2 to 3 %rh, and difficult ones plus or minus 5 to 10 %rh or worse, with drift as sensors age. I have not measured a batch of consumer units, so read that as the optimistic end for a cheap one.
The manufacturer sheet above sets a tighter pass mark for its professional probe: within plus or minus 2% of the chart value. That gives you a sensible yardstick. If a £10 unit lands within a few points of 75%, it is doing well. Ten points out is a gadget, not an instrument. What should you do with the corrected number? Compare it with the humidity range a home should sit in.
When to bin it instead of calibrating it
Calibrating cannot rescue everything. The manufacturer sheet is blunt about its own probes: outside the tolerance, replace it, because adjustments cannot be made. A cheap hygrometer has the same problem with a screw added.
My rules, which are judgement rather than a published standard:
- It reads more than about 5 points out and has no way to adjust.
- It is still wrong after adjusting, or the reading drifts between tests.
- Two hygrometers in the same room disagree by 10 or more points, and the salt test does not settle which is wrong.
- The sensor has been wet, or has been near a kitchen or shower for years.
Replacement is cheaper than the wrong decision: a dehumidifier bought on a bad reading is a far dearer mistake, and what size dehumidifier you need starts with a number you can trust.
Salt-test it once, then again before the damp season
NPL says relative humidity instruments might be recalibrated every 6 to 12 months, or more often if harshly treated. A home hygrometer does not need laboratory treatment, but a yearly overnight salt test is a fair habit, and a good moment is the start of autumn. Knowing how to calibrate a hygrometer turns it from a decoration into a tool: you will know its offset, you will know when to stop trusting it, and you will have spent a spoonful of salt finding out.
FAQ
How do you calibrate a hygrometer at home?
Seal it in a zip bag with a bottle cap of damp table salt, leave it for at least six hours (overnight is better), and read it through the bag. The salt holds the air at about 75% relative humidity, so the difference between 75% and your reading is its error.
How long does the salt test take?
Sources vary. One guide says at least five hours, others say six or six to eight as a minimum, and the manufacturer procedure I opened says about 12. Overnight covers all of them.
Is the salt test accurate?
It is accurate at one point, about 75% RH, and only as good as your seal and temperature. It will not tell you how the unit reads at 40%, which is where most homes sit, so use it to catch a bad hygrometer rather than to certify a good one.
Can you calibrate a hygrometer without salt?
You can check it against a second hygrometer, which the NPL describes as a useful check though not a calibration. Put both in the same spot for a few hours. If they disagree by a lot, you know one is wrong but not which.
How often should you recalibrate a hygrometer?
The NPL suggests every 6 to 12 months for relative humidity instruments. For a home unit, once a year is reasonable, sooner if it has been wet or the readings look odd.
