Dew Point Calculator

Q: What is the difference between dew point and relative humidity, and which one matters more?

Relative humidity is the ratio of current water vapor to the maximum amount the air could hold at the current temperature, expressed as a percentage. It changes constantly as temperature changes — a 95% RH morning at 60 °F can become 40% RH by afternoon at 85 °F without any moisture entering or leaving the air. Dew point is the temperature at which the current moisture content would saturate the air; it stays nearly constant through the day unless moisture is added or removed. For comfort, health, and condensation prediction, dew point is the more meaningful number. A dew point of 65 °F always feels noticeably humid regardless of whether the air temperature is 75 °F or 95 °F. Meteorologists and HVAC engineers prefer dew point for this stability, while the general public is more familiar with relative humidity from weather reports.

Q: How is dew point used to assess outdoor comfort and heat stress?

Dew point is one of the best single-value comfort and heat-stress indicators. Below 50 °F, the air feels dry and comfortable for most people. From 50–60 °F, comfort is unaffected by humidity. From 60–65 °F, people start noticing moisture but conditions are tolerable. From 65–70 °F, the air feels muggy and exercise becomes more taxing. From 70–75 °F, conditions are uncomfortable for sustained outdoor activity, and from 75 °F upward they become oppressive and physiologically dangerous because evaporative cooling — the body's main heat-removal mechanism — becomes ineffective. Sustained dew points above 80 °F are rare but lethal; the famous July 1995 Chicago heat wave saw dew points near 80 °F and over 700 deaths. For outdoor athletic events, governing bodies increasingly use dew point alongside WBGT for activity restrictions.

Q: When does condensation form on windows, pipes, or AC ducts, and how do I prevent it?

Condensation forms when a surface temperature drops to or below the surrounding air's dew point. In a home, this happens on cold-water pipes in humid basements, on AC ducts running through humid attics, on single-pane windows during winter when indoor humid air contacts the cold glass, and on the underside of insulated cathedral ceilings if water vapor reaches the cold roof deck. Prevention has two paths: lower the indoor dew point (by dehumidifying, ventilating, or air conditioning) or raise the surface temperature (by insulating the cold surface). For windows, dual-pane or triple-pane glazing raises the inner surface temperature above typical indoor dew points. For pipes, foam sleeves work. For roof decks, vapor barriers plus ventilation prevent moist indoor air from reaching cold surfaces. In commercial HVAC, supply-air dew point design is a primary spec to avoid duct condensation.

Q: What are common mistakes when interpreting dew point?

The most frequent mistake is conflating dew point with relative humidity — a 40% RH at 95 °F has a dew point near 67 °F (humid!), while 80% RH at 50 °F has a dew point near 44 °F (dry). People look at the high RH number and feel the cool air is humid; the low dew point tells them otherwise. Another error is using dew-point formulas across freezing — below 32 °F, frost-point physics differs slightly and most simple formulas slightly mis-estimate. Plugging Fahrenheit temperatures straight into the Celsius Magnus formula is another classic error — it can overstate the dew point by more than 10 °F; always convert to Celsius first, as this calculator does. People also confuse dew point with wet-bulb temperature; they are related but distinct (wet bulb accounts for the cooling from evaporation). Finally, ignoring that wind and sun exposure modify perceived comfort despite a fixed dew point can lead to underestimating heat stress.

Q: When should I NOT use this calculator?

For NIST-traceable scientific accuracy use the Arden Buck or Hyland-Wexler formulations; the Magnus form used here is accurate to a few tenths of a degree across normal weather ranges. Do not apply it for below-freezing dew points without acknowledging the frost-point distinction; for ice/water phase calculations, use a frost-point formula. It is not the right tool for high-altitude or low-pressure conditions (above ~6,000 ft); the relationship between RH and dew point shifts with pressure. For HVAC engineering specs and certification, use psychrometric chart software (e.g., ASHRAE Handbook tables) rather than a single-formula estimate. Near saturation (RH above about 95%) small errors in the humidity reading dominate the result, so use a calibrated hygrometer. Finally, this calculator measures the relationship between T and RH; it does not measure either input — you still need a hygrometer or a reliable weather station to obtain T and RH accurately.

Estimate the temperature at which air becomes saturated and dew or frost begins to form, using an approximation of the Magnus formula. Useful for assessing humidity comfort, condensation risk, and HVAC sizing.

Last updated: September 2026

Air Temperature (°F)

Relative Humidity (%)

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Formula below · 3 sources (weather.gov, journals.ametsoc.org, ashrae.org) · Updated Sep 2026

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About this calculator

Dew point is the temperature to which a parcel of air must be cooled at constant pressure for water vapor to begin condensing into liquid water. Unlike relative humidity (which shifts with temperature), dew point is a direct measure of absolute moisture content, making it the better single-number indicator of how humid the air actually is. The calculator uses the Magnus formula with the constants used by the US National Weather Service (Bolton 1980, a = 17.67, b = 243.5 °C). The air temperature is first converted to Celsius, T_C = (T − 32) × 5/9; then γ = ln(RH/100) + a·T_C/(b + T_C) and DP_C = b·γ/(a − γ); the result is converted back to Fahrenheit, DP = DP_C × 9/5 + 32. It is accurate to about ±0.4 °F over normal weather ranges. Variables and edge cases: dew point can never exceed air temperature (if RH = 100% then DP = T); RH must be above 0%. Below freezing (T ≤ 32 °F), the relevant quantity is technically the frost point, which is slightly higher than the liquid-water dew point because of differences in vapor pressure over ice vs. water. Dew points above ~75 °F are physiologically dangerous because evaporative sweat cooling becomes inefficient. Comfort guide: <55 °F dry, 55–60 °F comfortable, 60–65 °F sticky, 65–70 °F uncomfortable, ≥70 °F oppressive.

How to use

Example 1 — typical summer day. T = 80 °F, RH = 60%. Step 1: T_C = (80 − 32) × 5/9 = 26.67 °C. Step 2: γ = ln(0.60) + 17.67 × 26.67 / (243.5 + 26.67) = −0.5108 + 1.7441 = 1.2333. Step 3: DP_C = 243.5 × 1.2333 / (17.67 − 1.2333) = 300.3 / 16.437 = 18.27 °C. Step 4: DP = 18.27 × 9/5 + 32 = 64.9 °F — noticeably humid but not oppressive. Example 2 — comfortable dry day. T = 75 °F, RH = 40%. T_C = 23.89 °C; γ = ln(0.40) + 17.67 × 23.89 / 267.39 = −0.9163 + 1.5787 = 0.6624; DP_C = 243.5 × 0.6624 / 17.0076 = 9.48 °C; DP = 49.1 °F — dry and comfortable.

Frequently asked questions

What is the difference between dew point and relative humidity, and which one matters more?

Relative humidity is the ratio of current water vapor to the maximum amount the air could hold at the current temperature, expressed as a percentage. It changes constantly as temperature changes — a 95% RH morning at 60 °F can become 40% RH by afternoon at 85 °F without any moisture entering or leaving the air. Dew point is the temperature at which the current moisture content would saturate the air; it stays nearly constant through the day unless moisture is added or removed. For comfort, health, and condensation prediction, dew point is the more meaningful number. A dew point of 65 °F always feels noticeably humid regardless of whether the air temperature is 75 °F or 95 °F. Meteorologists and HVAC engineers prefer dew point for this stability, while the general public is more familiar with relative humidity from weather reports.

How is dew point used to assess outdoor comfort and heat stress?

Dew point is one of the best single-value comfort and heat-stress indicators. Below 50 °F, the air feels dry and comfortable for most people. From 50–60 °F, comfort is unaffected by humidity. From 60–65 °F, people start noticing moisture but conditions are tolerable. From 65–70 °F, the air feels muggy and exercise becomes more taxing. From 70–75 °F, conditions are uncomfortable for sustained outdoor activity, and from 75 °F upward they become oppressive and physiologically dangerous because evaporative cooling — the body's main heat-removal mechanism — becomes ineffective. Sustained dew points above 80 °F are rare but lethal; the famous July 1995 Chicago heat wave saw dew points near 80 °F and over 700 deaths. For outdoor athletic events, governing bodies increasingly use dew point alongside WBGT for activity restrictions.

When does condensation form on windows, pipes, or AC ducts, and how do I prevent it?

Condensation forms when a surface temperature drops to or below the surrounding air's dew point. In a home, this happens on cold-water pipes in humid basements, on AC ducts running through humid attics, on single-pane windows during winter when indoor humid air contacts the cold glass, and on the underside of insulated cathedral ceilings if water vapor reaches the cold roof deck. Prevention has two paths: lower the indoor dew point (by dehumidifying, ventilating, or air conditioning) or raise the surface temperature (by insulating the cold surface). For windows, dual-pane or triple-pane glazing raises the inner surface temperature above typical indoor dew points. For pipes, foam sleeves work. For roof decks, vapor barriers plus ventilation prevent moist indoor air from reaching cold surfaces. In commercial HVAC, supply-air dew point design is a primary spec to avoid duct condensation.

What are common mistakes when interpreting dew point?

The most frequent mistake is conflating dew point with relative humidity — a 40% RH at 95 °F has a dew point near 67 °F (humid!), while 80% RH at 50 °F has a dew point near 44 °F (dry). People look at the high RH number and feel the cool air is humid; the low dew point tells them otherwise. Another error is using dew-point formulas across freezing — below 32 °F, frost-point physics differs slightly and most simple formulas slightly mis-estimate. Plugging Fahrenheit temperatures straight into the Celsius Magnus formula is another classic error — it can overstate the dew point by more than 10 °F; always convert to Celsius first, as this calculator does. People also confuse dew point with wet-bulb temperature; they are related but distinct (wet bulb accounts for the cooling from evaporation). Finally, ignoring that wind and sun exposure modify perceived comfort despite a fixed dew point can lead to underestimating heat stress.

When should I NOT use this calculator?

For NIST-traceable scientific accuracy use the Arden Buck or Hyland-Wexler formulations; the Magnus form used here is accurate to a few tenths of a degree across normal weather ranges. Do not apply it for below-freezing dew points without acknowledging the frost-point distinction; for ice/water phase calculations, use a frost-point formula. It is not the right tool for high-altitude or low-pressure conditions (above ~6,000 ft); the relationship between RH and dew point shifts with pressure. For HVAC engineering specs and certification, use psychrometric chart software (e.g., ASHRAE Handbook tables) rather than a single-formula estimate. Near saturation (RH above about 95%) small errors in the humidity reading dominate the result, so use a calibrated hygrometer. Finally, this calculator measures the relationship between T and RH; it does not measure either input — you still need a hygrometer or a reliable weather station to obtain T and RH accurately.