Weather apps, smart thermostats, air quality monitors and radon tests all report numbers, but each uses its own unit and scale. Knowing what a reading of 1,013 hPa, 65 per cent or 150 Bq/m³ actually means helps you decide whether to open a window, turn the heating up or arrange a test.

The guides here focus on measurements you can take or check at home. For converting the units themselves, the temperature converter and the pressure converter cover Celsius, Fahrenheit, hectopascals, millibars and more, and the units of measurement guide explains where each unit comes from.

Which units are used for weather and home measurements?

Most UK weather and home readings use metric or SI units, with a few traditional scales alongside. The table lists the measurements you are most likely to meet and the tool that takes each one.

What is measured	Unit or scale	Tool	Where you see it
Temperature	Degrees Celsius (°C)	Thermometer	Forecasts, thermostats
Rainfall	Millimetres (mm)	Rain gauge	Forecasts, climate records
Wind speed	Miles per hour (mph), knots, km/h	Anemometer	Forecasts, shipping and aviation reports
Air pressure	Hectopascals (hPa), equal to millibars (mb)	Barometer	Weather maps, barometers
Relative humidity	Per cent (%)	Hygrometer	Home monitors, dehumidifiers
Outdoor air quality	Daily Air Quality Index, 1 to 10	Monitoring stations	UK-AIR forecasts
Sunburn risk	UV index, 1 to 11+	Forecast models and sensors	Weather forecasts
Radon	Becquerels per cubic metre (Bq/m³)	Passive radon detectors	Radon test results
Carbon monoxide	Parts per million (ppm)	CO alarm	Alarm displays

What temperature should your home be?

The NHS advises heating the rooms you use regularly, such as the living room and bedroom, to at least 18°C if you can. This matters most for older people and anyone with a long-term health condition. 18°C is 64.4°F; the Celsius to Fahrenheit converter handles other values.

Where you put a thermometer affects what it reads. Place it on an inside wall at about chest height, away from radiators, windows, direct sunlight and electrical equipment, and give it a few minutes to settle. Smart thermostats in hallways often read differently from the rooms you actually sit in.

How is rainfall measured?

Rainfall is measured as the depth of water that would collect on flat ground, in millimetres. One millimetre of rain falling on one square metre is exactly one litre of water, which makes rainfall easy to turn into volumes. For example, 10 mm of rain on a 20 m² roof is about 200 litres, before losses to splashing and evaporation.

A home rain gauge should stand in an open spot, away from walls, fences and trees, with its rim level and above the height of rain splashing off the ground. Read it at the same time each day and empty it after reading. The garden measuring guides cover gutters and other outdoor measurements.

How are wind speed and air pressure measured?

UK public forecasts usually give wind speed in miles per hour, while shipping and aviation use knots; air pressure is given in hectopascals, which are identical to millibars. One knot is one nautical mile per hour, about 1.15 mph. Wind is also described on the Beaufort scale, from Force 0 (calm) to Force 12 (hurricane force), which is based on observed effects such as moving branches or waves.

Average air pressure at sea level is about 1,013 hPa. Readings well above that usually come with settled weather, and falling pressure often signals wind and rain on the way. A home barometer must be adjusted for your height above sea level before its readings can be compared with a weather map.

How do you measure humidity at home?

Use a digital hygrometer, which shows relative humidity as a percentage of the moisture the air could hold at that temperature. Readings that stay high, especially in bedrooms and bathrooms, point to a risk of condensation and mould, while very dry air can irritate eyes and skin. The guide to measuring humidity at home explains where to place a hygrometer and how to act on the readings.

What is the Daily Air Quality Index?

The Daily Air Quality Index (DAQI) is the UK’s 1 to 10 scale for outdoor air pollution, split into four bands: Low (1 to 3), Moderate (4 to 6), High (7 to 9) and Very high (10). It combines measurements of five pollutants: ozone, nitrogen dioxide, sulphur dioxide and two sizes of particulate matter, PM2.5 and PM10, which are measured in micrograms per cubic metre (µg/m³).

Each band comes with health advice, especially for adults and children with lung or heart problems. Forecasts and current readings for your area are published on Defra’s UK-AIR website, and many weather apps now show the same index.

How do you test your home for radon?

Radon is measured in becquerels per cubic metre (Bq/m³) using small passive detectors left in the home, usually for three months. The UK Health Security Agency (UKHSA) advises reducing radon where the annual average in a home is above the action level of 200 Bq/m³, and sets a target level of 100 Bq/m³ as the ideal outcome of any remedial work.

Radon is a natural radioactive gas from the ground, and levels vary greatly between areas and even between neighbouring houses. UKHSA’s UKradon website has maps showing affected areas and sells validated test kits. A short test with a cheap electronic monitor can give a useful indication, but decisions about remedial work should be based on a proper long-term test.

How do you monitor carbon monoxide at home?

Fit a carbon monoxide (CO) alarm made to BS EN 50291 in every room with a fuel-burning appliance, such as a boiler, open fire or wood burner. Carbon monoxide has no smell or colour, so an alarm is the only practical way to detect it. In England, private and social landlords must fit a CO alarm in any room used as living accommodation that has a fixed combustion appliance, other than a gas cooker.

If a CO alarm sounds, open doors and windows, turn off appliances if you can, leave the property and call the National Gas Emergency Service on 0800 111 999. Seek medical help if anyone has headaches, dizziness or nausea. Have gas appliances checked every year by a Gas Safe registered engineer.

What mistakes should you avoid with home environment readings?

• Putting sensors in the wrong place. A thermometer in sunlight or a hygrometer next to the kettle gives misleading readings.
• Acting on a single reading. Temperature, humidity and especially radon vary through the day and the year, so look at averages.
• Mixing pressure units. hPa and mb are the same, but inches of mercury (inHg) on older barometers are not.
• Relying on cheap monitors for safety. Low-cost air quality and radon monitors are useful indicators, not substitutes for proper tests or certified alarms.

Weather and home health guides in this section

• How to measure humidity in your home: hygrometers, placement and what the readings mean.
• Temperature converter: Celsius, Fahrenheit and kelvin.
• Fahrenheit to Celsius converter: for older recipes, thermometers and US forecasts.
• Pressure converter: hectopascals, millibars, pascals, bar and psi.
• Speed converter: miles per hour, knots, km/h and metres per second.

Latest weather and home health guides

Related sections

For measuring rooms, windows and radiators, see the home and DIY measuring guides. The measuring tools section covers how to choose and check instruments, and the units of measurement guide explains SI units such as the pascal and the becquerel.

Frequently asked questions

What temperature should I keep my home at in the UK?

The NHS advises heating the rooms you use regularly, such as the living room and bedroom, to at least 18°C if you can, especially for older people and anyone with a health condition.

Is a millibar the same as a hectopascal?

Yes. One millibar equals one hectopascal, so a pressure of 1,013 mb is the same as 1,013 hPa.

What is a high radon level in a home?

UKHSA advises reducing radon where the annual average is above the action level of 200 Bq/m³. The target level for remedial work is 100 Bq/m³.

What do the Daily Air Quality Index numbers mean?

The index runs from 1 to 10. Readings of 1 to 3 are Low, 4 to 6 Moderate, 7 to 9 High and 10 Very high, and each band has its own health advice.

How much water is 1 mm of rain?

One millimetre of rain on one square metre is one litre of water. 10 mm of rain on a 20 m² roof gives about 200 litres.

Sources

• Keep warm, keep well, NHS
• What is the Action Level?, UKradon, UK Health Security Agency
• Daily Air Quality Index, Defra UK-AIR
• What does this forecast mean?, Met Office
• Smoke and carbon monoxide alarms: explanatory booklet for landlords, GOV.UK