By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed October 20, 2026
Home › Tools › Dew Point Calculator
This dew point calculator works five ways: dew point from air temperature and relative humidity, relative humidity from temperature and dew point, a dew point converted from one pressure to another, and dew point to parts per million by volume (ppmv) and back. Every mode runs on the saturation vapor pressure curve and its inverse, Td = Tn / (m / log10(Pw / A) − 1).
Always write the pressure next to a dew point. Air dried to −40 °C at atmospheric pressure condenses at about −20 °C once it is compressed to 7 bar(g), about 102 psig. A dew point quoted without its pressure basis can therefore be wrong by a full ISO 8573-1 class. Meters, sampling and the full class table are covered in our guide on how to measure dew point.
Calculator
Dew point, RH, pressure dew point and ppmv
Results update as you type. Pressures are gauge, referenced to 1013.25 hPa, in the unit chosen next to the first pressure box. Formula range: dew point over water −20 to +100 °C, frost point over ice −70 to 0 °C. Ideal gas, no enhancement factor; water vapor only, not the hydrocarbon dew point of natural gas.
How it works
Every mode first turns its inputs into one number, the partial pressure of water vapor, Pw. Relative humidity is Pw divided by the saturation pressure at the air temperature. The dew point is the temperature at which Pw would be the saturation pressure. And ppmv is Pw divided by the total absolute pressure, times one million.
Pws = A × 10m × T / (T + Tn) Td = Tn / (m / log10(Pw / A) − 1)
Here Pws is the saturation vapor pressure at temperature T, Pw is the actual water vapor partial pressure and Td is the dew point; A, m and Tn are fitted constants. The constants come from Vaisala’s published humidity conversion formulas.
Over water from −20 to +50 °C they are A = 6.116441 hPa, m = 7.591386 and Tn = 240.7263 °C, with a maximum error of 0.083 percent in vapor pressure. A second water set takes over from +50 to +100 °C. Over ice from −70 to 0 °C they are A = 6.114742 hPa, m = 9.778707 and Tn = 273.1466 °C.
Quick check: Vaisala’s own worked example, 40 °C at 50 percent RH, gives a 27.6 °C dew point. Set the unit to °C, enter those two numbers in the first mode and the calculator returns 27.6 °C. Many other web calculators use the Magnus constants 17.62 and 243.12 °C, or 17.625 and 243.04 °C. For air at 0 to 40 °C the three sets agree within 0.07 °C across the formula range, so the formula set is a negligible part of the error budget.
Pressure dew point
As long as nothing condenses, compression leaves the amount of water in a gas unchanged. It raises the water vapor partial pressure in proportion to the absolute pressure, and a higher Pw means a higher dew point. The conversion is two steps: Pw2 = Pw1 × P2 / P1 with both pressures absolute, then solve the dew point equation at Pw2. The ppmv value stays the same, because it is a mole fraction.
ISO 8573-1:2010 grades water vapor in compressed air, classes 1 to 6, by pressure dew point (PDP). Class 4 is +3 °C or lower, Class 2 is −40 °C or lower and Class 1 is −70 °C or lower. The pressure mode reports the class at the higher of the two pressures you enter, which is the line condition the standard grades.
Worked examples
Compressor intake. The compressor room is at 30 °C (86 °F) and 65 percent RH, and the system runs at 7 bar(g).
- Saturation pressure at 30 °C = 42.43 hPa
- Pw = 42.43 × 0.65 = 27.58 hPa, so the atmospheric dew point = 22.7 °C
- Pressure ratio = 8,013.25 / 1,013.25 = 7.908
- Pw at 7 bar(g) = 27.58 × 7.908 = 218.1 hPa, so the pressure dew point = 61.9 °C
Any surface in that line cooler than about 62 °C will collect water. That is why condensate drains from the aftercooler and the receiver before the air reaches the dryer.
Reading taken after expansion. A transmitter at a desiccant dryer outlet reads a frost point of −45 °C, but its sample line vents the air to atmosphere before the sensor. The line runs at 7 bar(g) and the specification is ISO 8573-1 Class 2.
- Pw at a −45 °C frost point = 0.07205 hPa
- Pw at 7 bar(g) = 0.07205 × 7.908 = 0.570 hPa
- Pressure frost point = −26.0 °C, which is Class 3, not Class 2
The vented reading looked five degrees better than the −40 °C limit. At line pressure the air misses it by 14 degrees. To meet Class 2 at 7 bar(g), the vented reading has to be −56.9 °C or lower.
ppmv specification. A gas specification allows 16 ppmv of water at 7 bar(g). In the dew point from ppmv mode, 16 ppmv at 7 bar(g) returns a frost point of −40.0 °C. The same 16 ppmv at 0 bar(g) returns −57.0 °C, because the partial pressure is lower by the same 7.908 ratio.
Application note
Spot checks on refrigerated dryers. Take a dryer that delivers exactly the Class 4 limit, +3 °C pressure dew point at 7 bar(g). Vent that air to atmosphere and a handheld dew point meter reads a frost point of about −21 °C. The low number does not make the dryer better than specified, so convert it back to line pressure before you judge it.
Frost point below zero
Below 0 °C the same air has two possible numbers. Ice has a lower saturation pressure than supercooled water at the same temperature: about 2.60 hPa against 2.87 hPa at −10 °C. A frost point of −10 °C therefore equals a dew point over water of −11.2 °C. Vaisala’s conversion notes point out that below 0 °C ice normally forms rather than supercooled water, so the calculator defaults to frost point.
Relative humidity is a different case. By a long-standing World Meteorological Organization convention, RH is calculated against saturation over water even below freezing, and the calculator follows it. If your instrument reports sub-zero values over water, switch the selector so your entries and results match it.
Accuracy and limits
Input error outweighs formula error by more than an order of magnitude. Vaisala’s 0.083 percent vapor pressure error is about 0.01 °C of dew point near room temperature. At 25 °C and 60 percent RH, a 2 %RH error moves the computed dew point by about 0.5 °C. At 5 %RH the same 2 %RH error spans a frost point of −20.9 to −11.8 °C, a 9-degree band.
That is why dry gas is measured with a dew point sensor rather than calculated from RH. For that job our online dew point meter reads −100 to +80 °C Td with ±2 °C accuracy in air or nitrogen.
The tool covers water vapor only. It also rests on two assumptions. The pressure conversion assumes ideal gas and ignores the enhancement factor, the small extra water vapor that real air holds at saturation under pressure. Vaisala notes the effect is negligible at or below atmospheric pressure but must be considered well above it, so treat high-pressure results as close estimates.
Gauge pressures are referenced to 1013.25 hPa. At altitude, enter your absolute pressure minus 1013.25 hPa in the selected unit, which gives a negative number; the absolute vs gauge pressure calculator does that step.
Related guides and tools
For measurement methods, sampling rules and the full ISO 8573-1 water table, read how to measure dew point. If your pressure is in a unit the selector does not list, convert it first with the pressure unit converter.
For continuous monitoring on a network, the 608 series dew point transmitter reports dew point over RS485 Modbus from −60 to +60 °C. On a compressed air audit, pair the dew point check with a compressed air flow meter so moisture and air consumption are measured in the same survey.
FAQ
How do I calculate the dew point?
Find the water vapor pressure first: multiply the saturation pressure at the air temperature by RH/100. Then solve the saturation equation backward for the temperature at which that pressure is saturated. At 30 °C and 65 percent RH, Pw is 27.58 hPa and the dew point is 22.7 °C.
How to convert pressure dew point to atmospheric dew point?
Find the vapor pressure at the pressure dew point, multiply it by atmospheric pressure over line pressure (both absolute), and solve for the new dew point. A −40 °C pressure dew point at 7 bar(g) becomes a frost point of −56.9 °C at atmosphere.
How to calculate dew point in compressed air?
Measure at line pressure if you can, because ISO 8573-1 grades pressure dew point. If the sensor sits after an expansion, convert its reading with the line pressure. A 30 °C, 65 percent RH intake compressed to 7 bar(g) has a 61.9 °C pressure dew point before any drying.
Request a quote
Once you know the dew point your process needs at its real pressure, send us the gas, line pressure, target dew point and output signal, and we will match a transmitter or handheld meter to it. Tell us the application and we configure one unit, not a shelf part. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.