Dew Point Calculator
What this calculator does
This dew point calculator estimates the temperature at which the air would become saturated if cooled without changing its moisture content. Dew point is one of the clearest ways to describe how much water vapor is actually in the air.
Relative humidity depends on temperature, so it can be misleading when you compare a cool morning with a hot afternoon. Dew point removes much of that ambiguity by tying the moisture content to a saturation temperature. That makes it useful for weather interpretation, comfort checks, condensation risk, and humidity communication.
Inputs explained
- Air temperature: Enter the current dry-bulb air temperature in C or F according to the page controls.
- Relative humidity: Enter the current RH percentage from 0 to 100.
- Unit handling: The calculator converts as needed but solves the approximation in Celsius internally.
How it works / method
The page uses a Magnus-style approximation, which is a common practical way to estimate dew point from air temperature and relative humidity. The algorithm first computes an intermediate humidity term and then solves for the dew point temperature. The result is presented in both C and F along with a plain-language comfort label so the meaning is easier to understand.
Formula used
alpha = ln(RH / 100) + (aT / (b + T)); Td = (b alpha) / (a - alpha)
In this implementation, a = 17.27 and b = 237.3 with temperature handled in C. This is an approximation model, not a direct laboratory psychrometric measurement.
Practical note: Dew point estimates depend on the chosen approximation and the quality of the humidity measurement. Small sensor errors in RH can shift the calculated dew point by several degrees, especially near saturation.
Calculate the temperature at which dew forms.
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Comfort Level: --
Step-by-step example
Suppose the air temperature is 30 C and the relative humidity is 60 percent. Dew point helps translate that pair into a single moisture indicator.
- Enter 30 for air temperature.
- Enter 60 for relative humidity.
- The calculator estimates a dew point near 21 C.
- That is high enough to feel muggy for many people and indicates substantial moisture in the air.
- If a cooled surface drops below that dew point, condensation becomes possible.
Use cases
- Comparing humid summer days in a way that is less temperature-dependent than relative humidity alone.
- Checking whether indoor surfaces, ducts, or windows may approach condensation conditions.
- Understanding why fog, dew, or cloud formation is more likely when air temperature approaches dew point.
- Supporting greenhouse, storage, or HVAC interpretation where moisture content matters directly.
Assumptions and limitations
- This calculator uses a standard approximation and assumes air behaves in the normal practical range for this type of formula.
- If sensors are inaccurate or rounded aggressively, the dew point result will inherit that error.
- Very precise psychrometric work may require pressure-aware or instrument-based methods rather than a compact online estimate.
- Comfort labels are broad interpretations and should not be treated as a universal human response scale.
For humidity inversion problems, use the related relative humidity from dew point calculator. For evaporative cooling limits, compare the wet-bulb temperature page.
Frequently Asked Questions
The Magnus approximation does the job. Compute γ = ln(RH/100) + (17.625 × T)/(243.04 + T), then dew point Td = 243.04 × γ / (17.625 − γ), with T and Td in °C. So at 25°C and 60% RH, γ comes to about 1.96, giving Td ≈ 16.7°C. You need both ambient temperature and RH to begin — neither alone is enough. The formula is accurate to within about 0.4°C across normal weather conditions, which is fine for forecasting and HVAC work.
For most people in temperate climates, dew points below 16°C (60°F) feel pleasant. Between 16 and 18°C the air starts feeling sticky, 18–21°C is uncomfortable, and above 21°C (70°F) it gets oppressive. The reason it tracks comfort better than RH is that dew point is an absolute measure of moisture content. RH of 60% at 15°C feels dry, but 60% at 32°C feels muggy — the dew points are 7°C and 23°C respectively, and your body is responding to that.
Common bands used by weather services: below 13°C (55°F) feels dry, 13–16°C is comfortable, 16–18°C noticeably humid, 18–21°C uncomfortable, and above 21°C (70°F) outright oppressive. Tropical monsoon air often runs 24–26°C dew points. These bands are rule-of-thumb, not law — acclimatisation, age, and activity level shift the perceived line. A trained outdoor labourer in Chennai may shrug at 24°C; a tourist from a temperate country at the same dew point will struggle.
A 70°F (about 21°C) dew point means the air contains enough water vapour that it would saturate if cooled to that temperature. In practice, that level is muggy and uncomfortable. Sweat evaporates slowly, clothes stick, and the night does not bring much relief because the air cannot cool below the dew point. National Weather Service guidance treats 70°F as the entry to "oppressive." Outdoor sport at this dew point with high temperature is when heat-stroke calls start coming in.
No, and this confusion costs people money on humidifiers. Dew point is the temperature at which air becomes saturated — an absolute moisture indicator that does not change as the room warms or cools. Relative humidity is a percentage of the air's current moisture-holding capacity at the current temperature. Heat a room and RH drops even though no water has been removed; the dew point stays the same. For comfort and health work, dew point is the better single number to track.
The Magnus form is the workhorse: Td = (b × γ) / (a − γ), where γ = ln(RH/100) + a × T/(b + T), with a = 17.625 and b = 243.04°C. Inputs are temperature T in °C and relative humidity RH as a percent. Output Td is in °C, valid roughly from −40°C to +50°C. There are higher-order forms — Arden Buck, Sonntag — that are more accurate at extreme conditions, but for everyday weather this version stays well within ±0.5°C.
The "muggy" line is around 18°C (65°F) for most people. Below that, sweat evaporates fast enough that you stay comfortable; above it, evaporation slows and your body's cooling system starts working harder. By 21°C dew point, the air feels wet on your skin even in a breeze. Coastal cities in tropical zones — Mumbai in monsoon, Kolkata in July — sit at 24–26°C for weeks. Locals adapt; visitors do not. Heat-illness statistics climb wherever dew point stays above 24°C overnight.
You cannot do it from RH alone — you also need the air temperature. Once you have both, the Magnus formula does the conversion: Td = 243.04 × γ / (17.625 − γ), where γ = ln(RH/100) + 17.625T/(243.04+T). At 30°C and 50% RH, γ ≈ 1.16 and Td ≈ 18.4°C. Without temperature, RH is just a ratio with no absolute meaning. This is the single most common mistake I see — people treating RH like a moisture content number when it is not.
Sources & references
- NOAA National Weather Service - Dew point glossary
- NOAA National Weather Service - Relative humidity glossary
- NOAA National Weather Service - Dry bulb, wet bulb, and dew point definitions
- NOAA National Weather Service - Vapor pressure glossary
- American Meteorological Society - Stull wet-bulb approximation
- NIST Chemistry WebBook - Water data
- Schema.org - FAQPage and WebApplication vocabulary