Dew point is an absolute measure of moisture; relative humidity is not
Relative humidity tells you how close air is to saturation at its current temperature. Cool the same air without adding or removing water and its relative humidity climbs, even though the amount of water in it has not changed. That makes RH a poor way to compare two places, and a poor way to reason about condensation.
Dew point does not have that problem. It is the temperature at which the air's existing water vapour would saturate it, so it moves only when the actual water content moves. Air at 75 °F and 50% RH has a dew point of 55.1 °F. Warm that same air to 90 °F and the RH falls to about 30% while the dew point stays at 55.1 °F, because no water was added or removed. Two rooms with the same dew point contain the same amount of water per pound of air, whatever their temperatures.
This is why dew point is the number that predicts condensation. Any surface colder than the dew point of the air touching it will collect water, and no surface warmer than it will, regardless of what the relative humidity reads. A duct at 52 °F in a crawlspace at 78 °F and 70% RH will sweat because that air's dew point is 67.5 °F; the same duct in a conditioned room at 75 °F and 45% RH will stay dry because that air's dew point is 52.4 °F, though barely.
The third quantity in the family is the humidity ratio, quoted in the trade as grains of water per pound of dry air. That is the actual mass of water carried, and it is what the latent side of a load calculation is built from: the 0.68 × CFM × Δgrains term in the Manual J load calculator. Dew point and humidity ratio move together, since both depend only on the water present.
The Magnus-Tetens relation and why it takes the shape it does
Everything follows from the saturation vapour pressure curve, the pressure water vapour exerts when air is fully saturated at a given temperature. That curve is close to exponential, and the Magnus form fits it as
Pws = 6.112 × exp(17.62 T ÷ (243.12 + T)) in hPa, with T in °C.
The coefficients 17.62 and 243.12 are the values the World Meteorological Organization recommends for saturation over liquid water, and the 6.112 hPa is the saturation pressure at 0 °C, the triple point.
Relative humidity is defined as the actual vapour pressure divided by the saturation pressure at the dry bulb, so the actual vapour pressure is Pw = Pws(T) × RH/100. The dew point is then the temperature at which the saturation curve equals that actual pressure, so inverting the Magnus equation gives the dew point directly. Doing the algebra produces the two-line form the calculator uses: compute γ = ln(RH/100) + 17.62T/(243.12+T), then Td = 243.12γ ÷ (17.62 − γ).
Two properties are worth noticing. At RH = 100%, ln(1) = 0, so γ reduces to the temperature term and the formula returns exactly the dry bulb, as it must. And the dew point does not depend on barometric pressure at all: pressure enters only when converting vapour pressure into a humidity ratio, which is why the pressure input on this page changes the grains per pound and leaves the dew point alone.
Accuracy is better than about 0.1 °C over the range from −40 to +50 °C, which is far tighter than any field hygrometer. Below freezing the correct curve is saturation over ice rather than water, and the quantity is properly called the frost point.
Worked example: 75 F at 50% RH, and a duct at 52 F
A conditioned space reads 75 °F and 50% relative humidity, station pressure 29.921 inHg. A supply duct running through the space has a surface temperature of 52 °F.
- Convert to Celsius. (75 − 32) ÷ 1.8 = 23.889 °C.
- Temperature term. 17.62 × 23.889 ÷ (243.12 + 23.889) = 420.92 ÷ 267.01 = 1.57644.
- Saturation pressure. 6.112 × e1.57644 = 6.112 × 4.8374 = 29.566 hPa.
- Actual vapour pressure. 29.566 × 0.50 = 14.783 hPa.
- Gamma. ln(0.50) + 1.57644 = −0.69315 + 1.57644 = 0.88329.
- Dew point in Celsius. 243.12 × 0.88329 ÷ (17.62 − 0.88329) = 214.76 ÷ 16.7367 = 12.832 °C.
- Back to Fahrenheit. 12.832 × 1.8 + 32 = 55.10 °F.
- Humidity ratio. Total pressure is 29.921 × 33.8639 = 1,013.2 hPa. W = 0.621945 × 14.783 ÷ (1,013.2 − 14.783) = 9.194 ÷ 998.4 = 0.009209 lb/lb, which is 64.5 grains per pound.
- Condensation check. 52 − 55.10 = −3.1 °F. The duct is below the dew point and will sweat.
Two ways out. Lower the indoor dew point to below 52 °F, which means getting the room to about 44% RH at 75 °F, or insulate the duct so its outer surface sits above 55.1 °F. Insulation is the usual answer, and the required thickness follows from the same series-resistance arithmetic in the R-value to U-value converter: the outer surface temperature is the room temperature minus the fraction of the total resistance that lies outside the insulation, times the temperature difference across the whole assembly.
What dew point tells you about a building
In a cooled space, indoor dew point is the single best indicator of whether the equipment is doing its latent job. A conditioned room holding 75 °F should normally sit at a dew point somewhere in the low to mid 50s Fahrenheit, which corresponds to roughly 45 to 55% relative humidity. If the room reads 75 °F but the dew point is above 60 °F, the coil is not removing moisture: the usual causes are equipment oversized for the load so cycles are too short, airflow too high so the coil never gets cold enough, or an outdoor air path bringing moisture in faster than the coil removes it. Check the airflow side with the CFM per ton calculator, since airflow above about 450 CFM per ton pushes the coil surface up toward the entering dew point and moisture removal falls away.
For surfaces, the working rule is margin rather than sign. A surface exactly at the dew point is already condensing, and a surface a degree above it will condense as soon as anything cools it slightly or humidity rises slightly. Design to keep surfaces at least 5 °F above the design dew point, and remember that the coldest spot on an assembly governs, not the average: a window frame, a fastener through insulation or a duct hanger will be colder than the surface beside it.
In winter the same arithmetic runs the other way. Indoor air at 70 °F and 40% RH has a dew point of 44.6 °F, so any interior surface below that will collect water. A double-glazed window at U-0.30 with 0 °F outside runs an interior glass surface in the mid 50s, which is safe; a single-glazed window in the same conditions runs far colder and will stream. That is why winter humidification has to be limited by the coldest glass in the building rather than by comfort alone.
Dew point in Fahrenheit by dry bulb and relative humidity
| Dry bulb | 30% RH | 40% | 50% | 60% | 70% | 80% |
|---|---|---|---|---|---|---|
| 60 F | 28.4 | 35.6 | 41.3 | 46.1 | 50.2 | 53.8 |
| 70 F | 37.1 | 44.6 | 50.5 | 55.5 | 59.8 | 63.5 |
| 75 F | 41.5 | 49.0 | 55.1 | 60.2 | 64.5 | 68.4 |
| 80 F | 45.8 | 53.5 | 59.7 | 64.9 | 69.3 | 73.3 |
| 90 F | 54.4 | 62.4 | 68.9 | 74.2 | 78.9 | 83.0 |
Read across a row and the dew point rises with humidity at fixed temperature; read down a column and it rises with temperature at fixed RH, because warmer air at the same RH holds more water.
Key terms
- Dew point
- The temperature at which air becomes saturated with the water vapour it already contains. Independent of barometric pressure and unchanged by heating or cooling the air.
- Frost point
- The same idea below 0 °C, where vapour deposits directly as ice. Saturation pressure over ice is slightly lower than over supercooled water, so the frost point is a little higher than the water-based dew point.
- Humidity ratio
- Mass of water vapour per unit mass of dry air, quoted in the trade as grains per pound; 7,000 grains make one pound.
- Dew point depression
- Dry bulb minus dew point. Zero at saturation, and a direct measure of how far the air is from condensing.
- Vapour pressure
- The partial pressure water vapour contributes to the total atmospheric pressure. Relative humidity is this divided by its saturation value at the dry bulb.
Where dew point calculations go wrong
- Measuring temperature and humidity in different places. Both readings must come from the same air. A wall-mounted hygrometer and a probe in the supply duct describe two different states and combining them gives a meaningless dew point.
- Using room air conditions to judge a surface inside a wall or an attic. The air touching that surface is the air that condenses on it. A duct in a vented crawlspace sees crawlspace air, which can have a dew point 15 F above the conditioned space.
- Trusting a cheap hygrometer near saturation. Capacitive RH sensors commonly drift, and an error of 5 percentage points at 75 F moves the dew point by roughly 2 F. Where the answer matters, calibrate against a sling psychrometer or a chilled-mirror instrument.
- Assuming the coldest surface is the one you measured. Thermal bridges, fasteners, framing and window frames all run colder than the field of an assembly. Condensation appears at the coldest point first.
- Confusing dew point with wet bulb. They coincide only at saturation. Wet bulb is always between the dew point and the dry bulb, and it is the quantity coil performance data is indexed on.
- Applying the water-based formula below freezing. The calculation still returns a number, but the correct quantity there is the frost point over ice, which is slightly higher.
Dew point among the other psychrometric measures
Every psychrometric state of moist air is fixed by any two independent properties at a given pressure. Dry bulb plus relative humidity, dry bulb plus wet bulb, and dry bulb plus dew point are all equivalent descriptions, and the calculators on this site convert between them. If you have a sling psychrometer rather than a hygrometer, the relative humidity calculator takes a dry-bulb and wet-bulb pair and returns RH, humidity ratio and dew point together. If you need the wet bulb the manufacturer's coil tables are indexed on, the wet bulb temperature calculator works it out from dry bulb and RH.
The three temperatures always order themselves the same way: dew point ≤ wet bulb ≤ dry bulb, with equality only at saturation. Wet bulb sits between the other two because evaporating water into the air cools the thermometer while raising the air's humidity, so the process stops short of full saturation at the dew point.
For design work, dew point is what you carry between the outdoor design condition and the indoor target. The grains difference between them is the input to the latent load, and the calculation of how much moisture ventilation air brings in follows directly from the ventilation rate, which the air changes per hour calculator will give you. A house that ventilates at 60 cfm with a 40 grain difference is importing 0.68 × 60 × 40 = 1,632 BTU/h of latent load continuously, which is a meaningful share of a small system's moisture capacity.