How to Measure Humidity in a Room (and What the Reading Means)
You cannot judge damp by feel or by looking at the windows. A small device called a hygrometer tells you the relative humidity of a room in seconds, and it is the number that decides whether you need a dehumidifier at all. Here is how to measure it, where to put it and how to read the result.
The quick answer
Use a digital hygrometer (a humidity meter, often combined with a room thermometer). Stand it on a shelf or table in the room you are worried about, away from the kettle, bathroom door, radiator and window, wait 10-15 minutes and read the relative humidity percentage. For most UK homes the comfortable range is 40-60%. A basic hygrometer such as the TempPro TP49 costs a few pounds and runs on a button battery.
What the reading means
Relative humidity (RH) is how much moisture the air holds compared with the most it could hold at that temperature. The table shows what to do with the number. If your reading sits between 40% and 60% but you still see mould, the cause is usually a cold surface or a leak rather than room humidity, so check the wall as well as the number.
| Reading | What it means | What to do |
|---|---|---|
| Below 30% | Very dry air | Dry skin, static and a sore throat are likely; do not run a dehumidifier |
| 30-40% | Slightly dry | Fine in winter with the heating on; no action needed |
| 40-60% | Comfortable and healthy | This is the target range for most rooms |
| 60-70% | Damp | Condensation is likely; ventilate and consider a dehumidifier |
| Above 70% for days | Mould risk | Mould and dust mites thrive; a dehumidifier is worth running |
Where to put the hygrometer
Place it at roughly chest height in the middle of the room, away from windows, radiators, direct sunlight and the dehumidifier's own air outlet, because all of these skew the reading. Take readings in the room you are actually worried about, such as a bedroom with condensation or a bathroom, rather than a hallway. If you are testing a cold corner where mould appears, put a second one there and compare.
When to check
Humidity changes through the day. It rises after showers, cooking and drying washing, and it is usually highest overnight and first thing in the morning in a bedroom. Read it at the same times for a few days and note the highest figure. A single morning reading after a shower tells you little; a pattern of 65-70% every evening tells you a lot.
What to do if it is too high
First fix the source: ventilate after showers, use extractor fans, cover pans and avoid drying washing on radiators in closed rooms. If the reading still sits above 60%, a dehumidifier is the practical answer. Our guides to where to put a dehumidifier and how long a dehumidifier takes to work cover the next steps, and the best dehumidifier UK comparison lists the models worth considering. For visible black spots, see our dehumidifier for mould guide. If you already know you need one, these are the models we review most: the Meaco Arete One 20L for a family home, the Meaco Arete One 10L for a flat or bedroom, the Meaco DD8L Pro for cold rooms and laundry, and the Bosch Dry 4000 as a well-known alternative.
Is a dehumidifier's built-in humidity display enough?
Most dehumidifiers show a humidity reading, but it measures the air right beside the machine, which is often drier than the rest of the room because the unit is pulling moisture through it. A separate hygrometer on the far side of the room gives a more honest picture. It also lets you check that the dehumidifier is actually bringing the whole room down to your target, and that you are not running it for longer than you need to. Running cost matters too, so see how much a dehumidifier costs to run in our running cost guide.
Analogue or digital?
Digital models are the better choice: they are easier to read, usually include a thermometer and often store the highest and lowest readings. Analogue dial hygrometers are cheap but less accurate and drift over time. If you want to track readings from your phone and get alerts when humidity climbs, a Bluetooth model is available, but a simple digital one is enough for deciding whether you need a dehumidifier. For the bigger picture on choosing a unit, read our guide to buying a dehumidifier and our comparison of desiccant and compressor models.
Ready to compare models? See our best dehumidifier uk comparison →
Frequently asked questions
What is a good humidity level for a home in the UK?+
Aim for 40-60% relative humidity. Below 30% the air feels dry, and above 60-70% condensation, damp and mould become likely.
Can I measure humidity without a hygrometer?+
Not reliably. Condensation on windows or a musty smell suggest high humidity, but they do not give you a number. A digital hygrometer costs a few pounds and shows the exact reading.
How accurate are cheap hygrometers?+
Budget digital models are usually accurate to within a few percent, which is enough to tell whether a room is comfortable or damp. Two cheap units placed side by side will sometimes differ slightly, so look at the trend rather than a single figure.
Where is the best place to put a hygrometer?+
On a shelf or table in the middle of the room at about chest height, away from windows, radiators, sunlight and the dehumidifier's air outlet.
What humidity should I set my dehumidifier to?+
Most people set it to 50-55%. Going much lower makes the compressor work harder for little benefit and leaves the air drier than most people like.
Does a dehumidifier have a hygrometer built in?+
Most have a humidity sensor and display, but it reads the air beside the machine. A separate hygrometer elsewhere in the room gives a better picture of the whole space.