Indoor Climate · Calculator
Dew Point Calculator: Dew Point, Wet Bulb and Relative Humidity
Dew point is the temperature at which air becomes saturated and moisture starts to condense. At 75 °F and 50% relative humidity it is about 55 °F, which most people find pleasant, and the wet-bulb temperature is about 62.7 °F. Working backwards, 75 °F air with a 55 °F dew point is at about 50% RH. Any surface colder than the dew point, such as a window, duct or cold pipe, will collect condensation.
Dew point calculator
The formula behind the calculator
The calculator uses the Magnus approximation, a well-tested fit to the saturation vapor pressure of water that is accurate to within a few tenths of a degree across normal indoor and outdoor conditions. Temperature is converted to Celsius first.
T(°C) = (T(°F) − 32) × 5 ÷ 9
γ = ln(RH ÷ 100) + b × T ÷ (c + T)
Dew point (°C) = c × γ ÷ (b − γ)
with b = 17.62 and c = 243.12 °C
The result is converted back to Fahrenheit (× 9 ÷ 5 + 32). Relative humidity must be above zero, since the logarithm of zero is undefined.
Reverse mode. Solving the same equation for humidity gives RH from the air temperature T and dew point Td, both in °C, with the same constants:
RH (%) = 100 × exp( b × Td ÷ (c + Td) − b × T ÷ (c + T) )
The dew point cannot be higher than the air temperature; if you enter one that is, the calculator says so instead of returning more than 100%.
Wet bulb. The wet-bulb figure uses Roland Stull's 2011 empirical fit, with T in °C and RH in %:
Tw = T × atan(0.151977 × (RH + 8.313659)^0.5) + atan(T + RH) − atan(RH − 1.676331) + 0.00391838 × RH^1.5 × atan(0.023101 × RH) − 4.686035
It assumes sea-level pressure and is intended for roughly 5–99% RH and −20 to 50 °C (−4 to 122 °F), where it is typically within about 1 °C (2 °F) of a full psychrometric calculation. Treat it as approximate, and at high altitude or for charging decisions use a psychrometric chart or a proper wet-bulb reading.
Worked example
Indoor air at 75 °F and 50% RH:
- T = (75 − 32) × 5 ÷ 9 = 23.89 °C
- γ = ln(0.50) + 17.62 × 23.89 ÷ (243.12 + 23.89) = −0.693 + 1.576 = 0.883
- Dew point = 243.12 × 0.883 ÷ (17.62 − 0.883) = 12.8 °C
- 12.8 °C × 9 ÷ 5 + 32 = 55.1 °F
- Wet bulb (Stull): 20.53 + 1.557 − 1.550 + 1.188 − 4.686 = 17.04 °C, or about 62.7 °F
Reverse mode, 75 °F air with a 55 °F (12.78 °C) dew point:
- b × Td ÷ (c + Td) = 17.62 × 12.78 ÷ 255.90 = 0.880
- b × T ÷ (c + T) = 17.62 × 23.89 ÷ 267.01 = 1.576
- RH = 100 × exp(0.880 − 1.576) = 100 × 0.498 = 49.8%
Those match the calculator's default results. Any surface in that room below about 55 °F, such as an uninsulated cold-water pipe or a supply duct carrying 55 °F air, will start to sweat.
Dew point versus relative humidity
Relative humidity tells you how full the air is compared with what it could hold at its current temperature. Because warm air holds more water, the same RH means very different amounts of moisture at different temperatures. Dew point is an absolute measure: it only changes when water is added or removed.
| Air temperature | RH | Dew point (approx.) |
|---|---|---|
| 70 °F | 30% | 37 °F |
| 70 °F | 50% | 51 °F |
| 75 °F | 50% | 55 °F |
| 75 °F | 60% | 60 °F |
| 80 °F | 70% | 69 °F |
| 90 °F | 50% | 69 °F |
That is why a summer afternoon at 90 °F and 50% RH feels sticky while a 70 °F room at 50% feels fine: the dew point is nearly 20 °F higher.
Comfort ratings by dew point
| Dew point | How it typically feels |
|---|---|
| Below 40 °F | Dry; common indoors in winter |
| 40–55 °F | Comfortable for most people |
| 55–60 °F | Pleasant, slightly humid |
| 60–65 °F | Sticky, especially when active |
| 65–70 °F | Muggy |
| Above 70 °F | Oppressive |
For indoor air, holding the dew point at or below about 55 to 60 °F in summer keeps rooms comfortable and helps an air conditioner avoid a clammy feel. See the indoor humidity guide for target RH by season.
Using dew point to prevent condensation
- Ducts and vents: supply air leaves the coil at roughly 55 °F. In an attic or crawlspace with a dew point above that, uninsulated ducts and grilles drip. See condensation on AC vents.
- Windows in winter: the inside glass surface can be far colder than the room. Lower the indoor dew point (by reducing RH) as outdoor temperatures fall.
- Basements: cool foundation walls can sit below the dew point of summer air, which is why basement dehumidifier settings matter.
- AC coils: an air conditioner only removes moisture because its coil is below the air's dew point. More on that in does AC remove humidity.
Accuracy and limits
The Magnus constants used here (17.62 and 243.12 °C) are intended for temperatures from roughly −45 to 60 °C (−49 to 140 °F) over liquid water. Below freezing, the frost point over ice is slightly higher than the dew point shown. The bigger source of error is usually the humidity sensor itself: cheap hygrometers can be off by 3 to 5 RH points, which shifts the dew point by about 1 to 2 °F.
Dew point in HVAC work
Technicians use dew point in several everyday checks:
- Duct insulation: the outer surface of duct insulation must stay above the surrounding air's dew point, which is why ducts in humid attics and crawlspaces need a continuous vapor jacket.
- Condensate expectations: the bigger the gap between room dew point and coil temperature, the more water an AC removes. An evaporator at 45 °F in a room with a 60 °F dew point drains far more than one in a room with a 48 °F dew point.
- Pipe sweating: cold-water lines in summer and refrigerant suction lines sweat when they are below the dew point, so they are insulated with closed-cell foam.
- Dehumidifier performance: a basement dehumidifier struggles when the dew point falls into the 40s °F, because its coil cannot get far enough below it without icing.
For comfort, the ideal AC temperature guide explains how a lower dew point lets you set the thermostat higher and still feel comfortable.
Frequently asked questions
What is a good indoor dew point?
In summer, about 50 to 55 °F feels comfortable and keeps mold risk low. In winter it naturally falls lower, often into the 30s °F, and should stay low enough to avoid window condensation.
Can dew point be higher than air temperature?
No. At 100% RH, dew point equals air temperature. Above that, water condenses out as fog or dew.
Is dew point the same as wet bulb temperature?
No. Wet bulb sits between the dew point and the dry bulb temperature, and only equals them both at 100% RH. At 75 °F and 50% RH the dew point is about 55 °F and the wet bulb about 62.7 °F; the calculator shows both, with wet bulb as a sea-level approximation. Wet bulb is used in AC charging charts; dew point is used for condensation and comfort.
How do I get relative humidity from the dew point?
Enter the air temperature and dew point in the "Reverse" fields. The calculator solves the Magnus formula backwards: RH = 100 × exp(b × Td ÷ (c + Td) − b × T ÷ (c + T)). For example, 75 °F air with a 55 °F dew point is at about 50% RH.
Last reviewed 2026-10-28