The short answer
Seal the hygrometer in a bag with a capful of damp salt for 8 to 12 hours; a saturated salt solution holds the air at about 75 percent relative humidity, so the difference from 75 is your correction.
How accurate are they
Makers usually state accuracy of ±2 to ±5 percent relative humidity, often best in the middle of the range. Readings drift over time, especially after exposure to very humid air.
The salt test
- Put a few tablespoons of table salt in a bottle cap and add a few drops of water to make a damp paste (not a solution).
- Place the cap and the hygrometer in a sealed zip-top bag.
- Leave it at a steady room temperature for 8 to 12 hours.
- Read it: it should show 75 percent. If it shows 71, add 4 to future readings.
Placement
Keep it at breathing height, away from humidifiers, kitchens, bathrooms, exterior walls and direct sun, and give it an hour after moving it.
Products mentioned
Questions readers ask
Can I recalibrate a digital hygrometer?
Some have a calibration setting; otherwise note the offset and adjust readings in your head.
Why do two hygrometers show different readings?
Each has its own error; the salt test tells you which to trust.