| Parameter | Value |
|---|---|
| Dew point | — |
| Partial vapor pressure | — |
| Saturation vapor pressure | — |
| Absolute humidity | — |
| Wet-bulb temperature | — |
| Saturation deficit | — |
| Mixing ratio w | — |
| Specific humidity q | — |
This tool turns air temperature and relative humidity into the key micro-meteorological numbers you need: dew point, absolute humidity, mixing ratio and specific humidity. It also shows pressure-dependent values engineers care about. Read on for plain language, a US-friendly example and handy tips you can use on the job.
Table of Contents
What this calculator gives you
Use it to find the dew point for a given air state, compute how many grams of water are in each cubic meter, get the mixing ratio and the specific humidity. These last two depend on total air pressure so we let you enter pressure explicitly. The tool also tells you how close the air is to saturation and the vapor pressure deficit, both useful for HVAC, drying processes and agriculture.
Quick input notes
- Temperature — enter in degrees Celsius or provide Fahrenheit and the tool will convert.
- Relative humidity — percent between 0 and 100.
- Pressure — default 1013.25 hPa. You can enter in inches of mercury and we’ll convert to hPa for the pressure-dependent outputs.
Main formulas
Naturated vapor pressure by Magnus-Tetens:
$$e_s = 6.112 \cdot \exp\!\left(\frac{17.27\,T}{237.7 + T}\right)\quad[\text{hPa}]$$
Partial vapor pressure at given relative humidity:
$$e = \frac{\mathrm{RH}}{100}\; e_s\quad[\text{hPa}]$$
Dew point via helper α:
$$\alpha = \ln\!\left(\frac{\mathrm{RH}}{100}\right) + \frac{17.27\,T}{237.7 + T},$$
$$T_{\mathrm{dew}} = \frac{237.7\,\alpha}{17.27 – \alpha}\quad[^\circ\mathrm{C}]$$
Absolute humidity in grams per cubic meter:
$$\mathrm{AH} = 216.7 \cdot \frac{e}{T + 273.15}$$
Mixing ratio w and specific humidity q depend on total pressure P:
$$w\ [\mathrm{kg/kg}] = 0.622\cdot\frac{e}{P – e},$$
$$ w\ [\mathrm{g/kg}] = 1000\cdot w$$
$$q\ [\mathrm{kg/kg}] = \frac{w}{1 + w},$$
$$ q\ [\mathrm{g/kg}] = 1000\cdot q$$
Worked example
Inputs: 77 °F, 60% RH, pressure 29.92 inHg (standard sea-level).
Conversions used by the calculator:
- 77 °F → 25.0 °C
- 29.92 inHg → 1013.25 hPa
- Saturated vapor pressure at 25 °C:$$e_s = 6.112 \cdot \exp\!\left(\frac{17.27\cdot 25}{237.7 + 25}\right) =$$
$$ = 31.6\ \text{hPa}.$$ - Partial vapor pressure at RH = 60%:$$e = 0.60 \cdot e_s \approx 18.96\ \text{hPa}.$$
- Absolute humidity:$$\mathrm{AH} = 216.7 \cdot \frac{e}{25 + 273.15} =$$
$$ = 13.79\ \text{g/m}^3.$$ - Dew point:compute \(\alpha\) then$$T_{\mathrm{dew}} \approx 16.7^\circ\mathrm{C} \quad(\approx 62.1^\circ\mathrm{F}).$$
- Mixing ratio and specific humidity at P = 1013.25 hPa:$$w \approx 0.01187\ \mathrm{kg/kg} = 11.87\ \mathrm{g/kg},$$$$q \approx 0.01173\ \mathrm{kg/kg} = 11.73\ \mathrm{g/kg}.$$
Why these numbers matter — practical takeaways
Absolute humidity tells you how much water is physically present per cubic meter. That’s what drying processes, desiccant sizing and product storage care about. Dew point tells you the temperature at which surfaces will start forming liquid. Compare surface temperature to dew point to predict condensation risk.
📊 Mixing ratio and specific humidity are mass-based metrics used in psychrometric calculations, HVAC load estimates and when you need pressure-consistent values across altitude changes or sealed systems.
Quick tips for field and HVAC use
- When you enter pressure, use inHg if you’re in the US. We convert automatically. If you don’t know local pressure use 29.92 inHg as a standard baseline.
- For indoor comfort, aim for a dew point under about 60 °F to avoid clamminess and mold risk. Museums and storage often target lower dew points.
- Calibrate sensors regularly. A 2% error in RH at 80% RH causes a much larger error in absolute humidity than the same error at 30% RH.
- In drying work log both AH and dew point. AH shows drying capacity while dew point shows surface condensation risk.
Common sources of error
Uncalibrated RH probes, sensor radiation errors when measuring in direct sun, and using pressureless approximations for pressure-sensitive outputs. Rapid transient conditions also give misleading single-point readings. Average readings over several minutes for more stable results.
🌡 Use this calculator to turn raw temperature and RH into engineering-useful humidity metrics. Enter pressure when you need mixing ratio or specific humidity that’s correct for altitude or sealed systems. Check sensors and cross-validate with a handheld chilled-mirror or oven method for the most demanding applications.
Recommended books
- Fundamentals of Air Conditioning — practical HVAC reference
- Psychrometrics: Theory and Applications — hands-on psychrometrics
- ASHRAE Handbook — Fundamentals — authoritative HVAC and humidity data
David Parry — Senior Engineering Analyst
Specializing in electronics and physics-based simulations with 20+ years of engineering experience. David ensures the mathematical and physical accuracy of the tools at ProCalcLab.
