Dew Point Calculator

The dew point is the temperature to which a parcel of air must be cooled, at constant pressure, for it to become saturated, that is, for water vapour to begin condensing as liquid dew. Because the cooling happens at the same total pressure, the dew point is the single humidity quantity that depends only on how much water the air actually contains. That makes it a far better descriptor of mugginess, condensation risk, and fog likelihood than relative humidity, which conflates temperature and moisture. This calculator takes air temperature and relative humidity and returns the dew point, an absolute humidity reading in grams per cubic metre, a plain-English comfort band, and a frost-point note whenever the air temperature drops below 0 °C. The Magnus-Tetens equation with the Sonntag / World Meteorological Organization coefficient set is used, and the inputs and outputs can be toggled between Celsius and Fahrenheit.

How to Use the Dew Point Calculator

1. Pick Celsius or Fahrenheit with the toggle near the top of the form. Both the temperature you type and the dew point you read out follow this choice, no separate unit box for the output.
2. Type the air temperature. Sensible defaults (20 °C and 50 %RH) are pre-loaded so the result box is populated on first paint.
3. Type the relative humidity as a percentage between 0 and 100. Values outside that range are ignored so the form never displays NaN.
4. Read the dew point, the absolute humidity in g/m³, the comfort band, and (when applicable) the frost-point note below the result box.
5. For common scenarios, comfortable indoor, muggy summer day, hot desert, cold winter heating, cold outdoor winter, click a preset button to load both fields at once.

If you change units while a value is loaded, the temperature is converted but the relative humidity is unchanged (relative humidity is unitless, so nothing needs to be done to it).

The Formula: Magnus-Tetens with Sonntag / WMO Coefficients

The Magnus-Tetens equation expresses the saturation vapour pressure over liquid water as a smooth function of temperature and folds in relative humidity through a logarithm. With the Sonntag / WMO coefficient set, valid from roughly −45 °C to +60 °C over water, the intermediate quantity γ and the dew point Td are given by:

• γ = (a · T) / (b + T) + ln(RH / 100)
• Td = (b · γ) / (a − γ)

with a = 17.62 and b = 243.12 °C. The air temperature T is in degrees Celsius. At 100 %RH the logarithm term vanishes, the equation collapses to Td = T, and the dew point equals the air temperature exactly, a useful sanity check that the implementation is wired correctly.

The absolute humidity in g/m³ is reported using a separate but consistent Magnus form:

• AH = 6.112 · exp((17.67 · T) / (T + 243.5)) · RH · 2.1674 / (273.15 + T)

where the factor 6.112 hPa is the saturation vapour pressure at 0 °C in the standard Magnus form, and 2.1674 is the molar-mass-to-temperature conversion that turns vapour pressure into grams of water per cubic metre of air. This expression is the most widely cited closed-form approximation for water-vapour density at temperatures above freezing; below freezing it remains continuous, although the underlying physical regime shifts to deposition onto ice rather than condensation onto liquid.

Frost-point regime. When the air temperature is below 0 °C the same Magnus equation still gives a single numerical answer, but the phase transition is no longer condensation from vapour to liquid. Water vapour instead deposits directly onto cold surfaces as ice (frost), and the surface temperature at which this happens is the frost point. Because the saturation vapour pressure over ice is slightly lower than over liquid water at the same temperature, the true frost point is marginally colder than the Magnus value, a correction of typically 0.5 to 1.5 °C in the range −5 °C to −15 °C. For most engineering uses the Magnus value is close enough; the calculator surfaces a frost-point note rather than applying a separate ice-coefficient set, to keep the displayed number consistent with the displayed formula.

Worked Examples

The five examples below cover the most common humidity scenarios. Each is computed directly with the formula above and matches the calculator's output to two decimal places.

Example 1, A 20 °C room at 50 %RH. A typical comfortable indoor reading.

γ = (17.62 · 20) / (243.12 + 20) + ln(50 / 100) = 352.4 / 263.12 + ln(0.5) ≈ 1.3391 + (−0.6931) ≈ 0.6460. Td = (243.12 · 0.6460) / (17.62 − 0.6460) ≈ 157.04 / 16.97 ≈ 9.26 °C. Absolute humidity: 6.112 · exp((17.67 · 20) / 263.5) · 0.5 · 2.1674 / 293.15 ≈ 8.64 g/m³.

Example 2, A 30 °C day at 80 %RH (muggy summer).

γ = (17.62 · 30) / (243.12 + 30) + ln(0.8) = 528.6 / 273.12 + (−0.2231) ≈ 1.9352 − 0.2231 ≈ 1.7121. Td = (243.12 · 1.7121) / (17.62 − 1.7121) ≈ 416.24 / 15.91 ≈ 26.17 °C. Absolute humidity: ≈ 24.28 g/m³. Comfort band: "very humid", most people will feel sticky and want air conditioning.

Example 3, A 25 °C room at 100 %RH (saturation).

γ = (17.62 · 25) / (243.12 + 25) + ln(1.0) = 440.5 / 268.12 + 0 ≈ 1.6428. Td = (243.12 · 1.6428) / (17.62 − 1.6428) ≈ 399.40 / 15.98 ≈ 25.00 °C.

The dew point equals the air temperature, exactly as expected at saturation. Absolute humidity: ≈ 23.03 g/m³. This is the maximum water the air can hold at 25 °C.

Example 4, A 0 °C morning at 60 %RH (cold, damp).

γ = (17.62 · 0) / (243.12 + 0) + ln(0.6) = 0 + (−0.5108) ≈ −0.5108. Td = (243.12 · −0.5108) / (17.62 − (−0.5108)) ≈ −124.21 / 18.13 ≈ −6.85 °C. Absolute humidity: ≈ 2.91 g/m³. Comfort band: "bone dry". In the field the temperature would have to drop to about −6.85 °C for dew to start forming on grass and car roofs, a useful frost-forecasting number.

Example 5, A 35 °C desert afternoon at 20 %RH (hot but dry).

γ = (17.62 · 35) / (243.12 + 35) + ln(0.2) = 616.7 / 278.12 + (−1.6094) ≈ 2.2173 − 1.6094 ≈ 0.6079. Td = (243.12 · 0.6079) / (17.62 − 0.6079) ≈ 147.79 / 17.01 ≈ 8.69 °C. Absolute humidity: ≈ 7.92 g/m³. Comfort band: "dry". The dry air lets the body cool by evaporation, so the apparent temperature is much lower than the 35 °C reading even though the heat index is high.

Example 6, Fahrenheit round-trip. 68 °F = 20 °C, 50 %RH. From Example 1, Td = 9.26 °C = 48.66 °F. The unit toggle preserves the underlying humidity calculation and only re-expresses the temperature at the input/output boundary.

Why Dew Point Beats Relative Humidity for Comfort

Relative humidity is a ratio, the partial pressure of water vapour divided by the saturation vapour pressure at the current temperature. Because the saturation pressure rises steeply with temperature, the same relative humidity can represent very different absolute moisture contents. A 5 °C morning at 90 %RH contains roughly 6.5 g/m³ of water, while a 30 °C afternoon at 30 %RH contains around 9.5 g/m³, the second is half-as-saturated in relative terms but holds about 50 % more water. The dew point removes this ambiguity: it is the temperature at which the air becomes saturated, and it rises monotonically with the actual water content of the air.

Weather presenters, HVAC engineers, and aviation forecasters all use dew point as the primary humidity metric. The general public-comfort bands based on dew point are roughly:

• Td ≤ 0 °C: bone dry, static likely, mucous-membrane irritation.
• 0 °C < Td ≤ 10 °C: dry, comfortable for cool environments, low condensation risk.
• 10 °C < Td ≤ 16 °C: comfortable, the default indoor target for most homes.
• 16 °C < Td ≤ 19 °C: humid, beginning to feel muggy, condensation on cool glass possible.
• 19 °C < Td ≤ 24 °C: very humid, oppressive for many, dehumidification recommended indoors.
• Td > 24 °C: oppressive, tropical / sauna-like, severe condensation risk on any cold surface.

The calculator assigns one of these bands automatically and prints a one-line interpretation underneath the result.

Condensation on Windows, Walls, and Other Building Elements

In a building envelope, condensation forms when a surface temperature falls below the dew point of the indoor air. Single-pane windows in winter are the classic failure mode: on a −5 °C night with 21 °C indoor air at 40 %RH, the indoor dew point is roughly 7 °C. A single-pane glass centre can easily cool below 7 °C, and water condenses on the inside surface. Double-glazing, low-e coatings, and warm-edge spacers all push the inner-glass temperature up to keep it above the dew point.

Exterior walls suffer the same physics but with longer time constants. Moisture that condenses inside the wall cavity (often on the back of the interior vapour barrier or on the sheathing) accumulates over weeks, encouraging mould and rot. The dew-point calculator is used at the design stage to choose insulation levels and vapour-control layers that keep every surface in the wall above the indoor dew point under design outdoor conditions. Bathroom and kitchen exhaust fans are sized to keep the indoor dew point low enough that no wall surface falls below it during typical use.

Indoor swimming pools and cold-storage facilities push the dew-point requirements to extremes, pool halls are kept at 28 to 30 °C with 50 to 60 %RH, which is a dew point of roughly 18 °C; cold-storage rooms must keep their interior dew point below the coldest surface to prevent frost heave and ice formation.

HVAC, Dehumidification, and Greenhouse Control

Air-conditioning systems are typically rated by their sensible heat ratio (the fraction of total cooling that goes into dry-bulb temperature reduction versus latent heat removal from water vapour). For occupied comfort, an HVAC system must hold the indoor dew point in the 10 to 13 °C band, which usually corresponds to 30 to 55 %RH at typical indoor temperatures. When the outdoor air is humid (dew point above 18 °C) the system must run more latent cooling, which is why coastal or tropical climates often need dedicated dehumidification stages even when the air temperature is moderate.

Greenhouses have the opposite requirement: high humidity promotes disease pressure (botrytis, downy mildew) but very low humidity stresses the plants and wastes water. Commercial greenhouse set-points target leaf wetness control by holding the dew point within roughly 0.5 to 1 °C of the leaf temperature at night and 2 to 3 °C below the leaf temperature during the day. Sensors that report dew point directly, rather than computing it from relative humidity, are now standard in commercial installations.

Industrial dryers, compressed-air systems, and lithium-battery dry rooms all use dew point as the primary control variable. Compressed-air quality classes (ISO 8573-1) are written in dew point at line pressure. A −40 °C pressure dew point corresponds to very dry air; a class 1 air system specifies −70 °C pressure dew point for the driest applications. The same Magnus form is used as the underlying reference, with adjustments for line pressure.

Aviation: Temperature-Dew-Point Spread and Fog Risk

In aviation, the difference between air temperature and dew point is called the spread (T − Td). A small spread means the air is close to saturation; a spread of zero means fog, mist, or low cloud is imminent. Standard aviation forecasts include the spread in METAR and TAF reports, and pilots use it to estimate the likelihood of instrument conditions, carburettor icing in piston engines, and airframe icing in clouds.

Empirical rules used by forecasters and pilots include:

• Spread ≤ 1 °C: fog very likely, possibly thick. IFR conditions probable.
• 1 °C < spread ≤ 3 °C: mist or shallow fog likely overnight. Marginal VFR.
• 3 °C < spread ≤ 5 °C: low-cloud formation possible if the air cools further. VFR likely.
• Spread > 5 °C: low cloud unlikely, no fog. Clear VFR.

The dew point itself travels with the air mass: a tropical maritime air mass with a dew point of 22 °C will feel humid at any temperature, while a continental polar air mass with a dew point of −10 °C will feel bone dry. The spread can therefore change rapidly with time of day (as the temperature rises or falls) while the dew point changes only when the air mass itself changes. Forecasters prefer to think in terms of dew point; the spread is a derived quantity.

The calculator displays the spread as a worked value alongside the dew point so aviation, marine, and HVAC users can read both numbers at once.

Common Mistakes

Treating 100 %RH as the only time condensation happens. Condensation happens whenever a surface temperature falls below the dew point, not just at saturation. A 22 °C room at 40 %RH has a dew point around 7 °C, and any window, water pipe, or exterior wall colder than 7 °C will condensate, even though the relative humidity is "only" 40 %.

Using relative humidity to compare days at different temperatures. A 50 %RH morning at 5 °C is not the same humidity as a 50 %RH afternoon at 30 °C; the second holds roughly four times as much water. Use the dew point to compare moisture between days.

Applying the Magnus formula below its validity range. The Sonntag / WMO coefficient set is valid from roughly −45 °C to +60 °C over water. Outside that range, for example, in cryogenic applications or above-100 °C industrial processes, the formula drifts from the true saturation curve and a different correlation (or a tabulated vapour-pressure table) should be used.

Confusing frost point with dew point below freezing. Below 0 °C, water vapour deposits onto cold surfaces as ice rather than condensing as liquid dew. The Magnus equation still gives a single numerical answer, but the surface condensate is frost, and the underlying physical regime is different. The calculator surfaces this as an explicit note.

Forgetting that relative humidity sensors drift. Capacitive polymer RH sensors, the most common inexpensive type, typically drift by 1 to 3 %RH per year in field conditions and should be recalibrated annually. A 3 %RH error at 25 °C / 50 %RH shifts the dew-point reading by roughly 1 °C, small in absolute terms but enough to mis-classify the comfort band.

Frequently Asked Questions

What exactly is the dew point?

The dew point is the temperature to which a parcel of air must be cooled, at constant pressure, for it to become saturated with respect to liquid water, that is, for water vapour to begin condensing as liquid dew. Because cooling at constant pressure does not change the absolute water content, the dew point depends only on how much water the air holds, which is why it is a much better humidity descriptor than relative humidity.

Why do weather presenters and HVAC engineers prefer dew point to relative humidity?

Relative humidity is a ratio that depends on temperature, so the same RH can mean very different absolute moisture contents at different temperatures. Dew point depends only on the absolute water content and is therefore directly comparable across days, climates, and seasons. A high dew point always means the air is genuinely humid; a high relative humidity may simply mean the air is cool.

Which coefficient set does this calculator use?

The calculator uses the Magnus-Tetens form with the Sonntag / WMO coefficient set over liquid water: a = 17.62 and b = 243.12 °C. This set is valid from roughly −45 °C to +60 °C and is the set recommended by the World Meteorological Organization for routine meteorological use.

What is the frost point, and how does it differ from the dew point?

The frost point is the temperature at which water vapour deposits directly as ice on a cold surface. Below 0 °C the dew point and frost point diverge slightly because the saturation vapour pressure over ice is lower than over liquid water. The calculator surfaces a frost-point note rather than a separate numerical value, because the difference is small (typically under 1.5 °C in the −5 to −15 °C range) and the Magnus equation gives a single continuous answer across the freezing boundary.

What does absolute humidity tell me that relative humidity does not?

Absolute humidity is the mass of water per unit volume of air, expressed here in grams per cubic metre. It does not depend on temperature, so a 5 °C morning and a 30 °C afternoon with the same absolute humidity contain the same total water, even though their relative humidities will be very different. It is the humidity metric used in HVAC sizing, industrial drying, and compressed-air systems.

What comfort band should I aim for indoors?

For most homes and offices the comfortable indoor band is a dew point between 10 °C and 16 °C, which corresponds to roughly 30 to 55 %RH at 21 °C. Below 10 °C the air feels dry; above 16 °C it begins to feel muggy and condensation on cool glass becomes likely.

can the Dew Point Calculator, Td, Frost Point & Absolute Humidity be used for professional or commercial purposes?

yes, the Dew Point Calculator, Td, Frost Point & Absolute Humidity provides mathematically correct results that are suitable for professional, commercial, and educational use. For the Dew Point Calculator, Td, Frost Point & Absolute Humidity, For the Dew Point Calculator, Td, Frost Point & Absolute Humidity, For high-stakes applications (medical, legal, financial), verify results with a domain expert. For the Dew Point Calculator, Td, Frost Point & Absolute Humidity, the Dew Point Calculator, Td, Frost Point & Absolute Humidity formulas used are well-established and validated against reference standards.

For the Dew Point Calculator, Td, Frost Point & Absolute Humidity, How often are the underlying formulas updated?

For the Dew Point Calculator, Td, Frost Point & Absolute Humidity, the Dew Point Calculator, Td, Frost Point & Absolute Humidity formulas are based on established scientific, mathematical, or industry-standard references and rarely require updates. when standards change, the Dew Point Calculator, Td, Frost Point & Absolute Humidity is updated to reflect the current authoritative source. For the Dew Point Calculator, Td, Frost Point & Absolute Humidity, For the Dew Point Calculator, Td, Frost Point & Absolute Humidity, Each calculator's references section lists the specific sources used.

References

• WMO-No. 8, Guide to Instruments and Methods of Observation. World Meteorological Organization. The authoritative reference for meteorological measurement practice, including the Magnus-Tetens form with the Sonntag coefficient set used for routine dew-point calculation over water.
• Sonntag D. (1990), Important new values of the physical constants of 1986, vapour pressure formulations based on the ITS-90, and psychrometer formulae. Zeitschrift für Meteorologie 40 (5), pp. 340 to 344. The source of the a = 17.62, b = 243.12 °C coefficient set used here.
• NIST Special Publication 811, Guide for the Use of the International System of Units (SI). For the Dew Point Calculator, Td, Frost Point & Absolute Humidity, National Institute of Standards and Technology. The SI-units reference for temperature (Celsius, Kelvin, Fahrenheit) and the conventions used for absolute humidity in grams per cubic metre.
• ISO 8573-1, Compressed air, Contaminants and purity classes. The international standard that defines compressed-air purity by pressure dew point, for which the Magnus-Tetens equation is the underlying reference calculation.