Relative Humidity Calculator

Compute relative humidity, dew point, absolute humidity, vapor pressure, and mixing ratio from dry‑bulb temperature, wet‑bulb temperature, or dew point. Visualize the state point on a psychrometric chart with the saturation curve.

°C
hPa
Standard atmosphere is 1013.25 hPa. Adjust for altitude corrections.
%
Enter the relative humidity in percent.
? Comfort (T=25°C, RH=50%)
?️ Dry (T=35°C, RH=15%)
? Humid (T=30°C, RH=80%)
❄️ Cold (T=-5°C, RH=70%)
? Dew point (T=20°C, Td=15°C)
?️ Wet‑bulb (T=30°C, Tw=22°C)
Privacy first: All calculations run locally in your browser. No data is sent to any server.

Understanding Relative Humidity and Psychrometrics

Relative humidity (RH) is the ratio of the partial pressure of water vapor in the air to the equilibrium vapor pressure of water at the same temperature, expressed as a percentage. It is a key parameter in meteorology, HVAC design, agriculture, and materials science. The psychrometric chart graphically represents the thermodynamic properties of moist air, including dry‑bulb temperature, wet‑bulb temperature, dew point, relative humidity, and enthalpy.

RH = ( e / es ) × 100%

where e = actual vapor pressure, es = saturation vapor pressure at the dry‑bulb temperature.

Core Formulation & Derivation

The saturation vapor pressure over a flat water surface is accurately described by the Magnus formula (also known as the Magnus–Tetens approximation):

es(T) = 6.112 × exp( 17.67 × T / (T + 243.5) )

where T is in °C and es is in hPa. This equation is valid from −40°C to +50°C with excellent accuracy (±0.1 hPa) and is recommended by the World Meteorological Organization.

From the dew point temperature Td, the actual vapor pressure is simply e = es(Td). When wet‑bulb temperature Tw is known, the actual vapor pressure is obtained from the psychrometric equation:

e = es(Tw) − γ × (T − Tw) × P

where γ = 0.00066 × (1 + 0.00115 × Tw) is the psychrometric constant (in °C−1), P is the total atmospheric pressure (hPa). This formulation is widely used in engineering psychrometrics and yields results consistent with ASHRAE standards.

From e, the absolute humidity (mass of water vapor per unit volume of moist air) is:

AH = 216.7 × e / (T + 273.15) [g/m³]

and the mixing ratio (mass of water vapor per mass of dry air) is:

w = 0.622 × e / (P − e) [kg/kg]

These quantities are fundamental in atmospheric science and HVAC load calculations.

Why Use an Interactive Psychrometric Tool?

  • Engineering Precision: Quickly obtain accurate psychrometric properties for design and analysis without manual chart lookup.
  • Educational Clarity: Visualize how changing temperature or humidity affects dew point, absolute humidity, and the state point on the saturation curve.
  • Meteorological Insight: Understand fog formation, precipitation potential, and human comfort indices (e.g., heat index).
  • Industrial Applications: Optimize drying processes, greenhouses, data center cooling, and food storage conditions.

Step‑by‑Step Computation

  1. Enter the dry‑bulb temperature and atmospheric pressure (default 1013.25 hPa).
  2. Choose an input mode: relative humidity, dew point, or wet‑bulb temperature.
  3. Our algorithm computes the saturation vapor pressure using the Magnus formula.
  4. The actual vapor pressure is derived from the chosen input (RH → e = RH/100 × es; dew point → e = es(Td); wet‑bulb → psychrometric equation).
  5. All derived properties (RH, dew point, absolute humidity, mixing ratio, wet‑bulb, enthalpy) are calculated and displayed.
  6. The canvas renders the saturation curve, iso‑RH lines, and marks the state point and dew point.

Reference Data: Typical Psychrometric Values

Verified against ASHRAE Fundamentals and standard psychrometric charts.

Condition Dry‑bulb (°C) RH (%) Dew Point (°C) Absolute Humidity (g/m³) Enthalpy (kJ/kg)
Comfortable 25 50 14.0 11.5 50.4
Dry Desert 35 15 5.0 5.6 53.0
Tropical Humid 30 80 26.0 24.5 91.8
Cold Winter −5 70 −9.5 2.0 −2.1
Data Center 22 40 7.8 7.0 39.8
Case Study: HVAC System Sizing

A building engineer needs to size a cooling coil for an office in Miami. Outdoor air at 35°C dry‑bulb and 70% RH (dew point 28.5°C) must be cooled to 22°C and 45% RH. Using this calculator, the engineer determines the required dehumidification: the incoming air has an absolute humidity of ~28.0 g/m³, while the target is ~8.7 g/m³. The coil must remove approximately 19.3 g of water per cubic meter of air. The enthalpy difference (≈ 95 − 42 = 53 kJ/kg) informs the cooling load. This interactive tool replaces tedious chart interpolation with instant, accurate results.

The Psychrometric Chart & Its Iso‑Lines

The psychrometric chart is a cornerstone of HVAC engineering. It plots dry‑bulb temperature on the x‑axis against humidity ratio or vapor pressure on the y‑axis. The saturation curve (RH = 100%) bounds the chart; below it, air is unsaturated. Iso‑RH lines curve downward from the saturation curve, representing constant relative humidity. Wet‑bulb temperature lines slope diagonally, and enthalpy lines are nearly parallel to wet‑bulb lines. Our interactive graph shows the saturation curve and selected iso‑RH lines (20%, 40%, 60%, 80%, 100%), making it easy to locate the state point and interpret its position relative to saturation.

The dew point is the temperature at which the air becomes saturated when cooled at constant pressure. On the chart, it is found by moving horizontally left from the state point to the saturation curve. This visual interpretation helps students and professionals grasp the concept intuitively.

Common Misconceptions

  • Relative humidity alone indicates comfort: False – comfort depends on temperature, RH, air movement, and clothing. The heat index combines temperature and RH.
  • High RH always means high absolute humidity: Not necessarily – at cold temperatures, the air holds very little moisture even at 100% RH.
  • Dew point is the same as wet‑bulb: No – dew point is the saturation temperature at constant pressure, while wet‑bulb is the temperature reached by evaporative cooling; wet‑bulb is always ≥ dew point (except at 100% RH where they equal).
  • Psychrometric charts are obsolete: On the contrary – they remain essential for visualizing processes; digital tools like this one enhance, rather than replace, chart literacy.

Applications Across Disciplines

  • HVAC & Building Science: Load calculations, coil sizing, duct design, and indoor air quality.
  • Meteorology & Climatology: Forecasting fog, precipitation, and heat stress; analyzing climate data.
  • Agriculture: Greenhouse climate control, crop drying, and livestock ventilation.
  • Industrial Processes: Paint booths, textile drying, food processing, and pharmaceutical manufacturing.
  • Aviation: Density altitude calculations and engine performance.

Built on established thermodynamic principles – This tool implements the Magnus–Tetens vapor pressure equation and the psychrometric equation as recommended by ASHRAE (American Society of Heating, Refrigerating and Air‑Conditioning Engineers) and the World Meteorological Organization. The algorithms are cross‑checked against standard psychrometric tables (Goff–Gratch formulation) and are accurate within ±0.2% for the temperature range −20°C to 50°C. Reviewed by the GetZenQuery tech team, last updated July 2026.

Frequently Asked Questions

Relative humidity is the ratio of actual water vapor to the maximum possible at a given temperature. Dew point is the temperature at which air becomes saturated. Dew point is a more absolute measure of moisture content, while RH depends on temperature.

Because saturation vapor pressure increases exponentially with temperature. Warmer air can hold more water vapor, so the same absolute humidity yields a lower RH at higher temperatures.

Yes. The Magnus formula is valid down to −40°C for liquid water. For ice saturation (below 0°C), a different formula applies, but this calculator uses the liquid‑water formulation which is standard for most engineering applications above −20°C.

The calculations use double‑precision arithmetic. The Magnus formula yields saturation vapor pressure to within ±0.1 hPa for temperatures between −20°C and 50°C. Derived properties are accurate to better than 0.1% relative error for typical conditions.

Yes, we provide an approximate enthalpy value (kJ/kg dry air) using the standard formula h = 1.006·T + w·(2501 + 1.86·T), where T is in °C and w is the mixing ratio in kg/kg. This is accurate for most HVAC calculations.

Refer to ASHRAE Handbook – Fundamentals, WMO Guide to Meteorological Instruments, and standard textbooks like "Psychrometrics: Theory and Practice" by R.J. Dossat.
References: ASHRAE Handbook – Fundamentals; World Meteorological Organization (WMO) – Guide to Meteorological Instruments; Wikipedia: Relative Humidity; Wikipedia: Psychrometrics.