Dew Point Calculator

What the dew point tells you

The dew point answers one question: how cool does the air have to get before water starts coming out of it. The National Weather Service defines it as the temperature the air must be cooled to, at constant pressure, to reach 100 percent relative humidity. Cool the air past that point and the vapor turns into fog, dew on the grass, or water running down a window pane.

Relative humidity answers a different question, and it is easy to misread. A 30 F morning with a 30 F dew point sits at 100 percent humidity and still feels dry, while an 80 F afternoon with a 60 F dew point sits at 50 percent and feels sticky. The La Crosse office of the NWS uses that exact pair to show why the percentage misleads: warm air holds more water at the same percentage, so the number moves when the thermometer moves. The dew point tracks the amount of water vapor itself, so it holds steady through the afternoon.

Three rules cover most cases. The dew point never sits above the air temperature. It equals the air temperature when the air is saturated. And the gap between the two, called the spread on this page, says how close the air is to condensing: a spread of 2 F means dew or fog by morning, while a spread of 30 F means the air is dry enough to pull moisture out of a wet towel. The weather service glossary adds that when the dew point comes within 5 degrees of the air temperature, fog becomes a possibility.

The Magnus formula, step by step

The calculator uses the Magnus form with the Alduchov and Eskridge coefficients, 17.625 and 243.04 C. Those two numbers were fitted together over the range from -40 C to 50 C to keep the worst error small, and mixing one coefficient from a different pair breaks the fit. Temperature enters the formula in Celsius, so a Fahrenheit reading is converted first.

Step one builds an intermediate value, gamma, from humidity and temperature: the natural log of RH divided by 100, plus 17.625 times T divided by 243.04 plus T. Step two inverts the relation: dew point equals 243.04 times gamma, divided by 17.625 minus gamma. The logarithm explains two behaviors. At 100 percent humidity the log term is zero, so the dew point comes out equal to the air temperature. At low humidity the log term turns negative, and the dew point falls well below the air temperature.

Take 80 F at 60 percent. In Celsius that is 26.67 C. The log term is ln(0.60), or -0.5108. The second term is 17.625 x 26.67 / 269.71 = 1.7426. Add the two and gamma is 1.2318. Dew point is 243.04 x 1.2318 / 16.393 = 18.26 C. Back in Fahrenheit that is 64.87 F, and the calculator reports 64.9 F. The spread is 80 - 64.9 = 15.1 F.

Run the same relation backwards to get humidity from a dew point. The vapor pressure ratio is exponential, so RH equals 100 times exp(17.625 Td / (243.04 + Td) minus 17.625 T / (243.04 + T)). The optional dew point field in the calculator does that conversion, which helps when a weather app gives you a dew point and you want to know what it implies for a room.

Worked example: a muggy afternoon and a dry room

A July afternoon at 95 F and 40 percent humidity sounds mild if you read only the percentage. The dew point comes out at 66.9 F, which is heavy air, and the spread is 28.1 F. The percentage is low only because 95 F air can hold a lot of water before it saturates.

A spring day at 70 F and 35 percent gives a dew point of 41.1 F and a spread of 28.9 F. The spread is nearly the same as the July case, but the actual moisture is far lower: 41 F air is dry enough that a wet towel dries overnight, while 67 F dew point air leaves the bathroom mirror fogged.

The third example shows the same lesson from the other side. At 85 F and 75 percent, the dew point is 76.2 F, with a spread of only 8.8 F. Air that warm and that wet sits near the top of what the Gulf Coast produces in summer, and a surface chilled by an air conditioner will collect water within minutes.

The 95 F row and the 70 F row carry almost the same spread, 28.1 F against 28.9 F, yet one afternoon feels thick and the other feels crisp. Spread tells you how far the air is from condensing. Dew point tells you how much water is in it. For comfort, the dew point is the number that matters.

Reading the comfort bands

The National Weather Service publishes three summer comfort bands based on dew point. At 55 F or below the air feels dry and comfortable. Between 55 and 65 F it starts to feel sticky, with muggy evenings. At 65 F and above there is a lot of moisture in the air, and it feels oppressive. The bands describe outdoor summer comfort, so a 55 F dew point on a January morning is just a dry cold day, not a comfort score.

The bands map onto real decisions. Below 55 F, open windows cool a house without bringing in moisture. Around 60 F, a fan still works but the air feels clammy. Above 65 F, an air conditioner does double duty: it lowers the temperature and pulls water out of the air as the coil drops below the dew point. That is why a window unit can leave a room at 78 F and still feel better than 78 F outside, and why the condensate drain runs hardest on the most humid days.

Indoor targets differ slightly from outdoor comfort. The EPA mold guide recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, and treating condensation on windows, walls or pipes as a signal to act. At 70 F indoors, that range corresponds to a dew point between 37.1 F and 50.5 F, which is the number to compare against the temperature of a cold surface.

Condensation on windows, pipes and walls

Condensation needs one condition: a surface colder than the dew point. Indoor air at 70 F and 40 percent has a dew point of 44.6 F, so a single-pane window with a 35 F pane fogs while a double-pane unit with a 55 F inner pane stays clear. Raise the indoor humidity to 50 percent and the dew point climbs to 50.5 F, and the double-pane window starts to sweat too. The table below turns common indoor conditions into the surface temperature where fogging begins.

The same arithmetic explains a sweating cold water pipe in summer. A basement at 75 F and 60 percent humidity has a dew point near 60 F, so a pipe carrying 50 F water collects droplets all day and drips onto the floor. Insulating the pipe raises its surface temperature above the dew point and stops the drip without touching the humidity.

Windows are the cheapest dew point sensor in a house. A little condensation in the bottom corner on the coldest night of the year is normal. Water running down the glass every morning in January points to indoor humidity that is too high for the outside temperature, and the fix is ventilation or a lower humidifier setting, not a better window. The EPA guide puts the same point plainly: dry wet surfaces within 24 to 48 hours, because mold needs water and will use whatever the window provides.

Below freezing: frost point, not dew point

When the dew point falls below 32 F, vapor can deposit straight onto a surface as ice crystals without becoming liquid first. The temperature at which that happens is the frost point, and it sits slightly above the water-based dew point because ice has a lower saturation vapor pressure than supercooled water at the same temperature.

The ice-surface fit uses its own coefficients, 22.587 and 273.86, in the same Magnus shape. At 20 F and 80 percent humidity the water fit gives a dew point of 14.8 F and the ice fit gives a frost point of 15.4 F. The half degree matters in practice: a windshield at 15 F is cold enough for frost even though it sits 0.2 F above the reported dew point of 14.8 F. The difference grows as the air gets drier, which is why cold clear mornings often produce a frosted windshield with a low reported humidity.

Two caveats come with the frost branch. The ice fit is calibrated over a narrower temperature range than the water fit, and below about -40 F the gap between the two saturation curves grows fast enough that both readings become rough guides. And frost on a surface is a statement about that surface, not about the air: a car roof can frost while the windshield stays clear if the two surfaces shed heat at different rates.

Where the formula stops working

The Magnus fit assumes ordinary surface pressure. Dew point depends on pressure as well as moisture, so the same amount of water vapor reads differently in Denver than in New York. At 5,000 feet the air is thinner, and a calculation built for sea level drifts by a few degrees. For a bedroom or a backyard that error does not matter. For an altitude chamber or a weather balloon, it does.

The formula also assumes the humidity reading is honest. Most home hygrometers are accurate to a few percent relative humidity, and that error carries straight into the dew point. A sensor reading 60 percent when the air is at 55 percent puts the dew point about 2 F too high, which can turn a clear window into an expected fog. Cheap sensors drift as they age, and a bathroom sensor that has seen years of steam is not a reliable reference.

Finally, the result is a point value for well mixed air. Air sitting against a cold window forms its own thin layer at a different temperature, and air in direct sun on a still day can run well above the shaded reading that a weather station reports. Use the number as a guide to the bulk air, then check the surface you care about: the window, the pipe, the basement wall. If the surface reads colder than the dew point, water is already on its way.