Free dew point calculator · No sign-up · Runs entirely in your browser
Calculate dew point, absolute humidity and frost point from air temperature and relative humidity — and find the surface temperature at which condensation will form.
| Metric | Value |
|---|
| Dew point | Comfort level |
|---|---|
| Below 10°C | Very dry — comfortable (arid conditions) |
| 10–15°C | Comfortable — typical of Australian spring/autumn |
| 15–18°C | Slightly humid — comfortable |
| 18–21°C | Humid — somewhat uncomfortable |
| 21–24°C | Very humid — oppressive (tropical Australia) |
| Above 24°C | Extremely humid — dangerous (Darwin wet season) |
Air holds water vapour, and warm air can hold more of it than cold air. The dew point is the temperature you would have to cool that air to before it could no longer hold what it is carrying — at which point the excess condenses out as dew, fog, or the film on the inside of a window.
Because it is defined by the moisture actually present, dew point is a direct measure of how much water is in the air. That makes it more useful than relative humidity for almost every practical question.
This is the single most useful thing on the page. Relative humidity tells you how close air is to saturation at its current temperature — so the same percentage means completely different things at different temperatures.
| Air temperature | Relative humidity | Dew point | Water actually in the air |
|---|---|---|---|
| 5°C | 60% | −2.1°C | 4.08 g/m³ |
| 15°C | 60% | 7.3°C | 7.69 g/m³ |
| 25°C | 60% | 16.7°C | 13.81 g/m³ |
| 35°C | 60% | 26.1°C | 23.76 g/m³ |
| Dew point | Relative humidity | |
|---|---|---|
| Measures | Actual moisture content | How close to saturated, at current temperature |
| Changes when air is heated or cooled | No — stays constant | Yes — falls as air warms, rises as it cools |
| Comparable between places | Yes | Not really |
| Predicts condensation | Directly | Only with the temperature as well |
Dew point is derived from the Magnus equation, using the Alduchov–Eskridge coefficients that are standard in modern meteorology.
| Term | What it does |
|---|---|
| ln(RH ÷ 100) | Converts humidity into the fraction of saturation vapour pressure |
| (a × T) ÷ (b + T) | Saturation vapour pressure at the current temperature |
| a = 17.625, b = 243.04 | Empirical constants fitted for saturation over liquid water |
| Result Td | The temperature at which that vapour pressure becomes saturation |
A Sydney spring afternoon: 22°C with 60% relative humidity.
| Measure | Value |
|---|---|
| Air temperature | 22.0°C |
| Relative humidity | 60% |
| Dew point | 13.9°C |
| Spread (T − Td) | 8.1°C |
| Absolute humidity | 11.64 g/m³ |
| Comfort | Comfortable |
A dew point below 15°C is the range most people describe as pleasant. The 8.1°C spread means the air is well short of saturation, so fog is unlikely and washing will dry.
Cairns in the wet season: 32°C with 80% relative humidity.
| Measure | Value |
|---|---|
| Air temperature | 32.0°C |
| Relative humidity | 80% |
| Dew point | 28.1°C |
| Spread (T − Td) | 3.9°C |
| Absolute humidity | 27.04 g/m³ |
| Comfort | Extremely humid |
Each band is also named in full in the tables, so the colours are a convenience rather than the only way to read them.
| Dew point | How it feels | Typical Australian setting |
|---|---|---|
| Below 10°C | Very dry — may feel harsh on skin and airways | Inland winter, alpine areas, heated indoor air |
| 10–15°C | Comfortable | Most of southern Australia in spring and autumn |
| 15–18°C | Slightly humid but pleasant for most | Coastal summer mornings |
| 18–21°C | Noticeably humid — sticky | Sydney and Brisbane summer |
| 21–24°C | Very humid — oppressive | Brisbane and coastal Queensland in summer |
| Above 24°C | Extremely humid — sweat cannot evaporate effectively | Darwin and Cairns wet season |
This scale describes moisture, not heat. A 22°C dew point feels oppressive whether the air is 26°C or 34°C, because the limiting factor is your ability to shed heat by sweating rather than the temperature itself.
Air temperature down the side, relative humidity across the top. Shading follows the comfort scale above.
| Air temp | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|---|
| 10°C | -6.8 | -3.0 | 0.0 | 2.6 | 4.8 | 6.7 | 8.4 | 10.0 |
| 15°C | -2.4 | 1.5 | 4.7 | 7.3 | 9.6 | 11.6 | 13.4 | 15.0 |
| 20°C | 1.9 | 6.0 | 9.3 | 12.0 | 14.4 | 16.4 | 18.3 | 20.0 |
| 25°C | 6.2 | 10.5 | 13.9 | 16.7 | 19.1 | 21.3 | 23.2 | 25.0 |
| 30°C | 10.5 | 14.9 | 18.4 | 21.4 | 23.9 | 26.2 | 28.2 | 30.0 |
| 35°C | 14.8 | 19.4 | 23.0 | 26.1 | 28.7 | 31.0 | 33.1 | 35.0 |
| 40°C | 19.1 | 23.8 | 27.6 | 30.8 | 33.5 | 35.9 | 38.0 | 40.0 |
Note the diagonal: at 100% humidity the dew point equals the air temperature in every row. That is what saturation means.
This is where dew point stops being meteorology and starts being a household problem. Condensation forms on any surface that is at or below the dew point of the surrounding air. Nothing else is required.
| Indoor conditions | Dew point | Condensation appears on any surface below |
|---|---|---|
| 20°C, 50% RH | 9.3°C | 9.3°C |
| 20°C, 65% RH | 13.2°C | 13.2°C |
| 20°C, 80% RH | 16.4°C | 16.4°C |
| 18°C, 70% RH | 12.4°C | 12.4°C |
The same physics explains cold spots on external walls, condensation inside wardrobes on south-facing walls, and water pooling on bathroom ceilings. In each case a surface is sitting below the dew point of the air touching it.
Mould needs moisture, and persistent condensation supplies it. Australian health authorities identify indoor damp and mould as a health concern, particularly for people with asthma, allergies or respiratory conditions.
Indoor relative humidity in roughly the 30–60% range is a widely used comfort and moisture-control target, though the right figure for your home depends on your climate, construction and heating. More useful than a per-room percentage is knowing where the moisture comes from and where it shows up — because they are rarely the same room.
| Room | Role | What helps |
|---|---|---|
| Bathroom | Major moisture source — a hot shower releases a large volume of vapour quickly | Run the exhaust fan during and for 15–20 minutes after, door closed. Squeegee the screen |
| Kitchen | Major source — boiling, steaming and dishwashers | Range hood on while cooking; lids on pots |
| Laundry | Major source — a single load dried indoors releases litres of water | Dry outdoors when possible; vent condenser and vented dryers outside |
| Bedrooms | Moderate source overnight, and often the coldest rooms | Crack a window or vent briefly each morning; avoid drying washing here |
| Living areas | Where moisture accumulates and where unflued gas heaters add more | Ventilate daily, even briefly, in winter |
| Wardrobes and corners | Where the problem appears — still air against cold external walls | Leave gaps behind furniture; keep wardrobe doors ajar occasionally |
The gap between air temperature and dew point — the spread — is one of the most informative numbers in a weather observation.
| Spread (T − Td) | What it indicates |
|---|---|
| 0°C | Saturated air — fog, mist or active condensation |
| Under 2.5°C | Fog likely if the air cools further overnight |
| 2.5–5°C | Humid, low cloud base, high chance of dew by morning |
| Above 10°C | Dry air, high cloud base, rapid evaporation |
Overnight, the temperature often falls toward the dew point and stops there, because further cooling releases latent heat as moisture condenses. This is why the dew point is a good guide to the overnight minimum, and why humid nights stay warmer.
What is dew point?
Dew point is the temperature to which air must be cooled for it to become saturated with water vapour. At that point moisture begins condensing out as dew, fog or condensation on surfaces. Because it depends only on the water actually present, it is a direct measure of atmospheric moisture.
How is dew point calculated?
Using the Magnus equation. Calculate γ = ln(RH ÷ 100) + (17.625 × T) ÷ (243.04 + T), then dew point = (243.04 × γ) ÷ (17.625 − γ), with T in °C and RH as a percentage. At 22°C and 60% humidity this gives a dew point of 13.9°C.
What is the difference between dew point and relative humidity?
Dew point measures how much water is actually in the air, and does not change when the air is heated or cooled. Relative humidity measures how close the air is to saturation at its current temperature, so it changes constantly as temperature changes. Two places both reporting 60% humidity can hold vastly different amounts of moisture.
What is a comfortable dew point?
Below 10°C feels dry, 10–15°C is generally comfortable, 15–18°C is slightly humid but pleasant for most people, 18–21°C feels noticeably sticky, and above 21°C is oppressive. Above 24°C, sweat evaporates so poorly that the body struggles to cool itself regardless of the air temperature.
Does a higher dew point mean more humidity?
It means more moisture in absolute terms, which is usually what people mean. It does not necessarily mean higher relative humidity — air at 30°C with a 20°C dew point is around 55% relative humidity, while air at 22°C with the same 20°C dew point is close to 89%. Same moisture, different percentage.
Why do my windows fog up in winter?
Because the glass is colder than the dew point of the room air. A living room at 20°C and 65% humidity has a dew point of 13.2°C, so any surface below that collects moisture — and single glazing on a cold night sits well under it. Ventilating to lower the dew point, or insulating to raise the glass temperature, both stop it. Heating alone does not, because it adds no path for the moisture to leave.
How does dew point affect mould?
Mould needs sustained moisture, and condensation supplies it. Whenever a wall, window or ceiling surface sits at or below the dew point of the surrounding air, water collects there. Reducing indoor moisture through ventilation, or raising cold surface temperatures through insulation, removes the water supply. Persistent damp usually indicates a building fault that needs professional assessment.
Can dew point predict fog?
It is one of the better indicators. When the air temperature and dew point converge, the air is saturated and fog forms. A spread under about 2.5°C in the evening, with clear skies and light wind, is a strong signal for overnight fog — because the temperature will often fall to meet the dew point and stop there.
What is the temperature-dew point spread?
The spread is simply air temperature minus dew point. A large spread means dry air, high cloud bases and rapid evaporation. A small spread means the air is close to saturation, with low cloud, likely fog and slow drying. Pilots also use it to estimate cloud base: roughly the spread in °C divided by 2.5, times 1,000 feet.
What is a dangerous dew point?
Dew points above 24°C are physiologically demanding because sweat evaporates poorly, so the body's main cooling mechanism becomes ineffective. Combined with high air temperature this produces serious heat stress. Northern Australia regularly reaches this range during the wet season. The heat index combines both figures into a single risk estimate.
How is dew point used in HVAC?
Air conditioners dehumidify by passing air over a coil colder than its dew point, condensing moisture out — that is the water draining from a split system. Dew point also determines whether ductwork or chilled pipes will sweat, and dehumidifier capacity is specified in litres of water removed rather than humidity percentage, because absolute moisture is what is being extracted.
Is dew point the same as frost point?
No. Below 0°C, water vapour can deposit directly as ice, and saturation vapour pressure over ice is lower than over supercooled water. The frost point is therefore higher than the dew point — at −15°C air and 60% humidity, the dew point is −21.0°C while the frost point is −18.9°C. The gap widens as conditions get colder.
Dew point answers questions that relative humidity cannot: whether your windows will fog, whether fog will form overnight, whether the air will actually feel oppressive, and whether a surface is at risk of condensation. It is a single figure that stays put while everything around it changes.
For the heat side of the picture, our heat index calculator combines this moisture with air temperature into a feels-like figure, and the wind chill calculator covers the cold end of the year.