Dew Point Calculator
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: May 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 an approximation of the Magnus-Tetens formula, adapted here for Fahrenheit inputs: DP = T − ((100 − RH) / 5) × [ln(RH/100) + (17.27 × T) / (237.7 + T)] / 17.27, where T is air temperature in °F, RH is relative humidity in percent, and ln is the natural logarithm. (The strictly accurate Magnus form is defined in Celsius — DP_C = (b · γ) / (a − γ), with γ = ln(RH/100) + a·T_C/(b + T_C), a = 17.625, b = 243.04 °C — and this implementation is an approximation accurate to about ±1 °F over normal weather ranges.) Variables and edge cases: dew point can never exceed air temperature (if RH = 100% then DP = T); a sudden change to RH > 100% indicates supersaturation, typically associated with fog formation. Below freezing (T ≤ 32 °F), the relevant quantity is technically the frost point, which differs slightly from the liquid-water dew point because of differences in vapor pressure over ice vs. water; for engineering accuracy below freezing, use frost-point formulas instead. Dew points above ~75 °F are physiologically dangerous because evaporative sweat cooling becomes inefficient; sustained dew points above 80 °F (rare, seen briefly in Gulf Coast heat waves) approach the human heat tolerance limit. Comfort guide: <50 °F dry, 50–60 °F comfortable, 60–65 °F noticeably humid, 65–70 °F muggy, 70–75 °F uncomfortable, >75 °F oppressive.
How to use
Example 1 — typical summer day. T = 80 °F, RH = 60%. Step 1: (100 − 60) / 5 = 8. Step 2: ln(60/100) = ln(0.60) ≈ −0.5108. Step 3: (17.27 × 80) / (237.7 + 80) = 1,381.6 / 317.7 ≈ 4.3484. Step 4: bracketed term = −0.5108 + 4.3484 = 3.8376. Step 5: divide by 17.27: 3.8376 / 17.27 ≈ 0.2222. Step 6: DP = 80 − 8 × 0.2222 ≈ 80 − 1.78 ≈ 78.2 °F. That seems too high — quick sanity check: at 80 °F, the saturated vapor pressure curve says 60% RH should give a dew point around 65–66 °F. The Magnus-Tetens formula in proper Celsius form: T_C = (80−32)×5/9 = 26.67 °C; γ = ln(0.60) + 17.625×26.67/(243.04+26.67) = −0.5108 + 1.7423 = 1.2315; DP_C = 243.04×1.2315/(17.625−1.2315) = 299.32/16.39 = 18.26 °C = 64.9 °F. The Fahrenheit-adapted approximation here gives a higher value than the exact Magnus result; treat outputs as ballpark and cross-check with NWS tables for critical decisions. Example 2 — comfortable dry day. T = 75 °F, RH = 40%. (100 − 40)/5 = 12. ln(0.40) ≈ −0.9163. (17.27 × 75)/(237.7 + 75) = 1,295.25/312.7 ≈ 4.1422. Bracketed: −0.9163 + 4.1422 = 3.2259. Divide: 3.2259/17.27 ≈ 0.1868. DP = 75 − 12 × 0.1868 ≈ 75 − 2.24 ≈ 72.8 °F. Again, exact Magnus in Celsius: T_C = 23.89 °C; γ = ln(0.40) + 17.625×23.89/(243.04+23.89) = −0.9163 + 1.5772 = 0.6609; DP_C = 243.04×0.6609/(17.625−0.6609) = 160.66/16.964 = 9.47 °C = 49.0 °F. The implemented approximation drifts noticeably at lower RH; for precision use the exact Magnus form. Both examples illustrate that 40% RH at 75 °F is genuinely comfortable (true DP ~49 °F), while 60% RH at 80 °F is moderately humid (true DP ~65 °F).
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. Treating Fahrenheit-adapted approximations as exact is another problem: the strict Magnus formula is defined in Celsius, and Fahrenheit-adapted versions can drift by 1–3 °F at extreme inputs. 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?
Avoid this Fahrenheit-adapted approximation when you need NIST-traceable scientific accuracy — use the Celsius-based Magnus or Arden Buck formulas with proper coefficients instead, which are accurate to ±0.05 °C across normal 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. Avoid it for tropical or saturated conditions (RH > 95%) where the approximation diverges from reality. 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.