How to Calibrate a Hygrometer: Step-by-Step Guide

Even the best hygrometer will drift over time. Capacitive sensors age, mechanical hair elements shift, and the accumulated exposure to varying conditions can push a reading a few percent away from the true value. In a home setting this might not matter much. In a greenhouse, a cigar humidor, or a pharmaceutical storage room, a reading that is 5% too low or too high can mean the difference between conditions that are fine and conditions that are causing damage.

Calibrating a hygrometer brings it back to accuracy. It is a simple process — the most common method requires nothing more than a zip-lock bag, some table salt, and a few hours — and it is one of the most valuable pieces of maintenance you can perform on any humidity-measuring instrument.

This guide covers two calibration methods: the salt test for consumer and semi-professional instruments, and reference sensor calibration for more precise applications. It also explains what to do with the result and how often to repeat the process.

Why hygrometers need calibration

All hygrometers use a sensor that responds to water molecules in the air. Over time, the sensor's response changes slightly — typically drifting toward lower or higher readings depending on the conditions it has been exposed to. High humidity environments tend to push capacitive sensors higher over time; very dry environments can shift them lower. Temperature extremes, dust, and chemical exposure can also affect sensor response.

For most consumer digital hygrometers, drift of 2–5% over two to three years is typical. Analog hair hygrometers may drift faster, particularly if they have been stored in very dry or very humid conditions. A single calibration check per year is sufficient for most applications; more critical environments benefit from more frequent checks.

Method 1: The salt test (most common)

The salt test exploits a property of saturated salt solutions. When table salt (sodium chloride) is dissolved in water to the point of saturation — when no more salt can dissolve — the solution maintains a relative humidity of exactly 75% in the air immediately above it, regardless of the ambient temperature. This gives a known reference point against which to check and calibrate a hygrometer.

What you need

• Table salt (not iodised)

• A small bottle cap or container

• A zip-lock bag large enough to contain both the container and the hygrometer

• Water

• 6–8 hours of undisturbed time

Step-by-step instructions

• Fill the small container with table salt to about half full.

• Add a small amount of water — just enough to dampen the salt thoroughly without making it liquid. The salt should look wet but not be swimming in water. If the mix is too wet, add more salt; if too dry, add a few more drops of water.

• Place the container of damp salt and the hygrometer inside the zip-lock bag. Make sure neither is touching the other directly.

• Seal the bag, leaving as little air inside as possible.

• Leave in a stable temperature environment — away from sunlight, radiators, and draughts — for at least 6 hours, and preferably overnight.

• After the equilibration period, without opening the bag, read the hygrometer display.

• At equilibrium, the reading should be 75% RH. Note the difference between what the hygrometer shows and 75%.

What to do with the result

If your hygrometer has a calibration offset adjustment — a small button or menu setting that lets you add or subtract a fixed value to the reading — set it to the difference you measured. If the reading showed 72%, set an offset of +3%. If it showed 78%, set an offset of -3%.

If your hygrometer does not have a calibration offset adjustment, note the offset and apply it mentally to future readings. A hygrometer consistently reading 3% low should have 3% added to every reading you take. This is less convenient but perfectly valid for many applications.

Method 2: Reference sensor calibration

For more accurate calibration — or when you need to check accuracy across multiple humidity levels rather than just at 75% — a humidity calibrator or reference sensor is the more rigorous approach. A portable humidity generator creates a stable, known humidity environment at a selectable setpoint, allowing you to check your instrument at several points across its operating range and identify whether it is biased consistently or whether the error changes at different humidity levels.

This method is used for professional instruments, for calibrating multiple sensors to a common standard, and for any application where accuracy across the full humidity range matters rather than just at a single reference point. Browse our humidity calibrators and generators for options.

Calibration methods compared

How often should you calibrate?

For home and general use: once a year is sufficient for most consumer digital hygrometers. The salt test takes less than ten minutes to set up and requires no special equipment.

For cigar humidors, greenhouses, and instrument storage: calibrate every six months, or whenever conditions in the monitored space change significantly — a new humidifier, a change of season, or a long period of the hygrometer being stored rather than in use.

For professional and laboratory applications: follow the calibration interval specified in the instrument's documentation, typically every 12 months, with a calibration certificate issued at each service.

The two-salt test: checking accuracy at a second reference point

The standard salt test checks accuracy at a single reference point — 75% RH. For most consumer applications this is sufficient. If you want to understand how your hygrometer behaves at a different part of its range — particularly at the lower end, which matters more for dry environments like guitar storage rooms or arid climates — you can add a second check point using magnesium chloride.

Saturated magnesium chloride solution maintains 33% relative humidity at equilibrium at room temperature. The two-salt method uses separate sealed bags — one with sodium chloride (75% RH) and one with magnesium chloride (33% RH) — to check the hygrometer at two points across its operating range. If the error is similar at both points, the sensor has a consistent offset that can be dialled out. If the error differs significantly between the two points, the sensor's linearity is poor and simple offset adjustment will not fully correct it.

Magnesium chloride is available from laboratory supply companies and some hardware stores. It is sold as a road de-icing agent under various trade names — check the label to confirm it is pure magnesium chloride rather than a blend, since blended de-icers may not produce the correct reference humidity.

Frequently asked questions

What is the salt test for hygrometers?

The salt test is a method for checking and calibrating a hygrometer using a saturated solution of table salt (sodium chloride). A saturated salt solution maintains a relative humidity of exactly 75% in the air above it, providing a fixed reference point. By placing the hygrometer in a sealed bag with the salt solution for 6–8 hours, you can check whether its reading matches 75% and adjust the calibration offset accordingly.

What if my hygrometer reads 80% during the salt test?

A reading of 80% during the salt test means your hygrometer is reading 5% too high. If it has a calibration offset adjustment, set it to -5%. If it does not, subtract 5% from every reading you take going forward. For example, a reading of 65% on a hygrometer that runs 5% high would actually represent 60% true relative humidity.

Can I use iodised salt for the salt test?

It is better to use plain table salt without iodine for the salt test. The iodine additive can theoretically affect the saturation equilibrium slightly, though in practice the effect is small. If plain salt is not available, iodised salt will still give a useful result — it is considerably better than no calibration check at all.

What is a humidity calibrator?

A humidity calibrator is a device that creates a controlled, stable humidity environment at a known setpoint, used to check and calibrate humidity sensors. Simple versions are sealed chambers containing saturated salt solutions at specific concentrations. More sophisticated portable humidity generators allow you to set any desired humidity level and maintain it stably enough to calibrate sensors across their full operating range.

My hygrometer cannot be calibrated — should I replace it?

If your hygrometer has no calibration offset adjustment and you discover it is reading significantly off, you have two options: note the offset and apply it manually to every reading, or replace the unit with one that includes a calibration adjustment. For an offset of 3% or less, noting the difference and applying it mentally is entirely practical. For larger offsets or critical applications, replacement is the better choice. Most mid-range and above digital hygrometers include a calibration offset feature — check before purchasing if this matters to you.

Shop humidity calibration tools at Weather Scientific

For humidity calibrators, generators, and professional calibration equipment, browse our environmental meters collection. For a complete guide to choosing a hygrometer, see our complete hygrometer buyer's guide.

Bob Batemen

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Bob Batemen is a dedicated contributor to WeatherScientific.com, bringing a wealth of expertise in weather management and environmental science. Bob combines a deep understanding of environmental systems with practical experience in weather forecasting, climate patterns, and the implementation of sustainable weather-related solutions. Over the years, Bob has developed a keen interest in how climate change impacts global weather patterns, disaster risk management, and the mitigation of extreme weather events.

Bob's professional experience spans both private and public sectors, where they have contributed to the development of weather-sensitive infrastructure, environmental policy, and climate adaptation plans.

As a contributor to WeatherScientific.com, Bob shares insightful articles, guides, and analyses on emerging weather trends, cutting-edge weather technologies, and their environmental implications. Their passion for blending science with practical applications continues to shape their work, providing readers with valuable, informed perspectives on the ever-evolving world of weather and environmental management.