SizingKitOne quantity, six ways of writing it
Humidity calculator
Free, no signup: enter the air temperature and any single moisture figure you have — relative humidity, dew point, absolute humidity in grams per cubic meter, specific humidity or mixing ratio — and the other five come back together with the surface temperature at which water starts forming and the warmer one at which a surface crosses ASHRAE Standard 160’s 80% mold criterion. The printed dew point table below covers 50 to 95 °F against 20 to 90% relative humidity in 80 cells and is correct at any elevation.
- 100% free
- No signup
- 6 moisture units
- 80-cell dew point table
- Frost point below 32 °F
Give the air temperature and any one moisture figure
The other five come back with it. A moisture reading on its own says nothing — 10 g/m³ is dry air in a kitchen and saturated air in a cold room — so the temperature is not optional.
Default 0 ft — Sea level, where one standard atmosphere is 14.696 psia. Only two rows of the answer move with it. Relative humidity, dew point, vapor pressure and absolute humidity are set by the vapor pressure and the air temperature alone; the mixing ratio and the specific humidity are ratios against a mass of air, and that mass falls with the barometer. At 5,000 ft the same 50% at 75 °F is 78 gr/lb rather than 65.
RELATIVE HUMIDITY
—
of what this air could hold at its own temperature
DEW POINT
—
—
| Written as | Imperial | Metric |
|---|---|---|
| Vapor pressureThe physical quantity all five of the others encode. | — | — |
| Saturation pressure at this temperatureIts denominator: relative humidity is the row above divided by this one. | — | — |
| Absolute humidityWater per unit volume. Independent of the barometer. | — | — |
| Specific humidity qWater per unit mass of the mixture. Moves with the barometer. | — | — |
| Mixing ratio WWater per unit mass of the dry air alone. Always the larger of the two. | — | — |
| Gap between the last twoSmall enough to ignore indoors, not in a greenhouse. | — | |
Dew point by air temperature and relative humidity
80 cells, °F above °C, generated from the saturation curve rather than transcribed. Print it: it is right at any elevation, because a dew point is fixed by the vapor pressure the air carries and the barometer never enters the arithmetic.
| Air °F | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|
| 50 | 13-11 | 21-6 | 27-3 | 320 | 373 | 415 | 447 | 478 |
| 55 | 17-9 | 25-4 | 31-0 | 373 | 415 | 457 | 499 | 5211 |
| 60 | 20-6 | 29-2 | 362 | 415 | 468 | 5010 | 5412 | 5714 |
| 65 | 24-4 | 330 | 405 | 468 | 5110 | 5513 | 5915 | 6217 |
| 70 | 28-2 | 373 | 457 | 5110 | 5513 | 6015 | 6418 | 6719 |
| 75 | 31-0 | 425 | 499 | 5513 | 6016 | 6518 | 6820 | 7222 |
| 80 | 352 | 468 | 5412 | 6015 | 6518 | 6921 | 7323 | 7725 |
| 85 | 394 | 5010 | 5814 | 6418 | 7021 | 7423 | 7826 | 8228 |
| 90 | 446 | 5412 | 6217 | 6920 | 7423 | 7926 | 8328 | 8730 |
| 95 | 489 | 5915 | 6719 | 7323 | 7926 | 8429 | 8831 | 9233 |
ASHRAE Handbook—Fundamentals Ch. 1, Eqs. (5) and (6) — the Hyland and Wexler (1983) formulation for the saturation pressure of water vapor over ice and over liquid water, in IP units. The mixing ratio and the specific humidity above also use the molar mass ratio of water to dry air, 0.621945, from the same chapter; absolute humidity is the ideal gas law on the specific gas constant of water vapor, 85.78 ft·lbf/(lb·°R). Nothing on this page is a fitted curve, and nothing is read off a table.
How to turn one humidity reading into the other five
Two fields decide the answer. The third only matters for two of the six rows, and the page says which.
Type the air temperature first
Not the surface temperature and not the outdoor temperature — the temperature of the air the moisture reading came from. A moisture figure on its own is meaningless: 10 g/m³ is dry air in a kitchen and nearly saturated air in a walk-in cooler.
Pick the form your figure is already in
The select names all five and says where each usually comes from. If your instrument reports grams per kilogram, check its manual for whether that is per kilogram of moist air or of dry air — those are specific humidity and mixing ratio respectively, and the page reports both so you can see which one matches your reading.
Read the surface block, not just the humidity
Condensation is a property of a surface, not of a room. The two temperatures in the surfaces panel are the one below which water appears on glass, pipe or drywall, and the higher one below which that surface is wet enough for mold before any water is visible.
Technical specifications
| Forms converted | Six: relative humidity, dew point, vapor pressure, absolute humidity, specific humidity and mixing ratio. Any one of them plus the air temperature fixes the other five exactly — there is no iteration and no lookup table involved. |
|---|---|
| Which ones carry the barometer | Two of six. Relative humidity, dew point, vapor pressure and absolute humidity are set by the vapor pressure and the temperature alone. Mixing ratio and specific humidity are ratios against a mass of air, so 70 °F at 50% is 54.5 gr/lb at sea level and 65.6 gr/lb at 5,000 ft. |
| Below freezing | The saturation curve inverted under 32 °F is the one over ice, ASHRAE Handbook—Fundamentals Ch. 1 Eq. (5), so what comes back is a frost point and the page relabels the row. Air at 20 °F and 60% relative humidity frosts a surface at 9.6 °F. |
| Printed table | 80 cells — 50 to 95 °F in 5 °F steps against 20 to 90% relative humidity in 10% steps, each cell carrying °F over °C. It spans 12.9 °F in the driest corner to 91.6 °F in the wettest. |
| Comfort ceiling | ASHRAE Standard 55 caps the comfort zone at 0.012 lb of water per lb of dry air, a 62.3 °F dew point. Expressed as relative humidity that is 76.6% at 70 °F, 64.7% at 75 °F and 54.8% at 80 °F — which is why a single percentage figure cannot be the whole rule. |
| Mold criterion | ASHRAE Standard 160 sets germination at 80% surface relative humidity on a 30-day running mean, evaluated between 41 and 104 °F surface temperature. Across the range of the table above, that threshold sits 5.2 to 6.5 °F warmer than the dew point. |
| Absolute humidity | Ideal gas law on the specific gas constant of water vapor, 85.78 ft·lbf/(lb·°R) from ASHRAE Ch. 1. Saturated air at 68 °F carries 17.29 g/m³, which is 7.55 gr/ft³ in the units a US instrument reports. |
| How the table is produced | All 80 cells are computed from the saturation equation when the page loads rather than shipped as a picture, which is why they can carry two temperature scales where a scanned chart carries one. |
Frequently asked questions
Specific humidity or mixing ratio — which one is my instrument reporting?
Check the denominator, because both are quoted in grams per kilogram and they are not the same number. Specific humidity divides the water by the whole mixture, mixing ratio divides it by the dry air alone, so the mixing ratio is always the larger of the two. The gap is 0.8% at 70 °F and 50% relative humidity, which nobody notices indoors, and 3.2% at 95 °F and 90%, which shows up in a greenhouse or a drying room.
Why does entering an elevation change some rows and not others?
Because only two of the six figures have air pressure in them. Relative humidity is a ratio of two vapor pressures, the dew point is the temperature at which one of them saturates, and absolute humidity is water per cubic meter — none of the three asks how much air shares that volume. The mixing ratio and the specific humidity are ratios against a mass of air, and that mass falls with the barometer, so those two rise as you climb even though nothing about the water changed.
It says frost point below freezing. Why not dew point?
Because below 32 °F water vapor deposits straight to ice without passing through liquid, and the saturation pressure over ice is lower than over supercooled water. ASHRAE gives two equations for exactly this reason and the page switches between them at the freezing point. It matters on a refrigeration coil: the coil begins to frost when its surface reaches this temperature, which is not 32 °F and is usually well below it.
My windows sweat at 45% relative humidity. Isn't that supposed to be safe?
45% is a property of the room air and condensation is a property of the glass, and the two only meet through the dew point. Air at 70 °F and 45% has a dew point near 48 °F, so any surface colder than that collects water no matter how comfortable the room reads — and single glazing on a 10 °F night runs far colder than 48 °F at its edges. The fix is either a warmer surface or less water in the air, and the page gives you the number that each has to beat.
What relative humidity actually stops mold?
There is no room humidity that stops it, because mold responds to the humidity at the surface it is growing on, not to the average of the room. ASHRAE Standard 160 puts the threshold at 80% surface relative humidity on a 30-day running mean while the surface sits between 41 and 104 °F, and this page converts that into the surface temperature your air puts it at. Keeping the room between 30 and 50% as the EPA advises lowers the whole curve, but a cold corner behind a wardrobe can still cross it.
Is 10 g/m³ of absolute humidity dry or humid?
Neither, until you say what temperature the air is. 10 g/m³ is about 40% relative humidity in a 77 °F room and slightly over 100% in a 50 °F cellar, where it would be fog. Absolute humidity is genuinely useful in a growing room or an archive because it does not move when the air is heated, and genuinely misleading anywhere it is quoted without the temperature beside it.
Can I get relative humidity from a dew point without knowing the pressure?
Yes, and that is one of the few shortcuts in this subject that is exactly right rather than approximately right. Relative humidity is the saturation pressure at the dew point divided by the saturation pressure at the air temperature, and both come from the temperature alone. The elevation field on this page changes nothing in that conversion; leave it wherever it is and the answer stands.
About the five names one measurement goes by
Every figure on this page is the partial pressure of water vapor in the air, divided by something different each time. Relative humidity divides it by the saturation pressure at the same temperature, which is why it moves when you heat air that has gained no water at all — the numerator held still and the denominator grew. Absolute humidity divides the water by the volume, specific humidity by the mass of the mixture, and the mixing ratio by the mass of the dry air alone. Four different denominators is why a humidity figure quoted without its form is worth about as much as a pressure quoted without gauge or absolute.
The practical half of this is condensation, and the mistake there is a category error rather than an arithmetic one. People try to prevent it by setting a room humidity, which cannot work, because water forms on a surface when that surface falls below the dew point of the air touching it — a number the room average does not contain. The useful discipline is to compare the coldest surface you can find with the dew point of the air around it, and the mold threshold sits several degrees warmer than that: a rim joist or a duct in an unconditioned attic can be growing something long before anything visibly drips. Winter is when this bites, and it is the same season a furnace size calculator has you drying the house out by heating it.
Reference tables and arithmetic are all this page is. Whether a wall assembly is going to accumulate moisture over a season is a hygrothermal analysis with a vapor retarder, an air barrier and a climate file in it, and that is work a building scientist does and signs. If you need the water in the air expressed as heat rather than as a moisture figure — the latent half of a cooling load, or the enthalpy change across a coil — the psychrometric calculator carries that, and the room load it feeds into is on the BTU calculator.
Where the numbers are worked out
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The dew point table is generated cell by cell in your own browser from the saturation equation, not fetched as an image, so the page loads and prints identically on a phone with no signal in a crawlspace.