Unit System:

Process Inputs

🏢 Application

Psychrometric Properties

Atm. Pressure--
Dry Bulb Temp--
Wet Bulb Temp--
Dew Point--
Relative Humidity--
Humidity Ratio W--
Specific Volume--
Air Density--
Saturation Pressure--
Vapour Pressure--
Specific Enthalpy (h)--
COMFORT / APPLICATION STATUS
Formulae (corrected): p = 101.325×(1−0.0065z/288.15)^5.255 | pₛ = ASHRAE Wexler-Hyland (2009) | W = 0.621945×pᵥ/(p−pᵥ) | h = 1.006T + W×(2501+1.86T) | v = 0.287058×(T+273.15)×(1+1.6078W)/p | ρ = (1+W)/v | Twb: ASHRAE thermodynamic definition (Stull 2011 init) | Tdp: ARM coefficients (Alduchov & Eskridge 1996)

🔄 HVAC Air Process Calculator

Calculate cooling, heating, humidification & dehumidification loads for a given air flow.

State 1 — Inlet Air

State 2 — Outlet Air

🔧 Duct & Fan Calculator

Air Properties

Duct Geometry

Duct Results

📊 Psychrometric Chart — Current State Point

State point auto-updates from Tab 1 inputs. Comfort zone shown in green.

Educational Reference

📚 Psychrometrics — Complete Guide

A comprehensive reference for students and engineers covering all key terms, how to read the psychrometric chart, and how to use every feature of this calculator.

🔬 What is Psychrometrics?
Psychrometrics (also called hygrometry) is the branch of engineering science that studies the physical and thermodynamic properties of gas–vapour mixtures, most commonly moist air — a mixture of dry air and water vapour.

Understanding moist air is fundamental to HVAC engineering (Heating, Ventilation & Air Conditioning), building science, meteorology, industrial drying, food processing, pharmaceutical manufacturing, and any field where controlling temperature and humidity is critical to human comfort, product quality, or equipment reliability.

Why it matters

Human comfort, building energy use, equipment performance, and even structural integrity all depend on correctly characterising the moisture content of air. A psychrometric calculator transforms two known properties of an air sample into a complete set of thermodynamic state properties — eliminating tedious manual calculations while ensuring accuracy based on ASHRAE standards.

Governing standards

This calculator is based on ASHRAE Fundamentals 2009, the authoritative standard for moist-air thermodynamics used worldwide by mechanical engineers and HVAC professionals.
🧩 Core Psychrometric Terms & Definitions
Dry Bulb Temperature
Tdb
Unit: °C (SI) · °F (Imperial)
The actual air temperature measured by a standard thermometer shielded from radiation and moisture. It is the most commonly referenced temperature in HVAC and the horizontal axis of the psychrometric chart. It does not account for humidity.
💡 Example: "The outdoor temperature is 35 °C" — this is the dry bulb temperature.
Wet Bulb Temperature
Twb
Unit: °C · °F
The temperature measured by a thermometer whose bulb is wrapped in a wet wick and exposed to airflow. Evaporation cools the wick. The wet bulb temperature is always ≤ dry bulb temperature. The difference (dry bulb − wet bulb) is the wet bulb depression and directly indicates humidity — a larger depression means drier air.
Stull (2011): Twb ≈ T·atan[0.151977√(RH+8.313659)] + atan(T+RH) − atan(RH−1.676331) + 0.00391838·RH^1.5·atan(0.023101·RH) − 4.686035
💡 Wet bulb = dry bulb only when RH = 100% (fully saturated air).
Dew Point Temperature
Tdp
Unit: °C · °F
The temperature at which air must be cooled (at constant pressure and humidity ratio) before water vapour begins to condense into liquid droplets. Below the dew point, condensation occurs on surfaces — the origin of morning dew, window condensation, and pipe sweating. Dew point is a direct measure of absolute moisture content.
ARM (Alduchov & Eskridge 1996): Tdp = 243.04·[ln(RH/100) + 17.625·T/(243.04+T)] / [17.625 − ln(RH/100) − 17.625·T/(243.04+T)]
💡 If Tdp = 20 °C, any surface cooler than 20 °C will collect condensation.
Relative Humidity
RH
Unit: % (dimensionless ratio × 100)
The ratio of the actual partial pressure of water vapour in the air to the saturation pressure of water vapour at the same temperature. It expresses how close the air is to being fully saturated. RH = 100% means the air is holding the maximum possible moisture — any further addition causes condensation.
RH = (pv / ps) × 100 %
💡 RH = 50 % at 25 °C means the air contains half the moisture it could hold at that temperature.
Humidity Ratio
W
Unit: kg/kg (SI) · lb/lb (Imperial)
Also called the specific humidity or moisture content. It is the mass of water vapour present per unit mass of dry air. Unlike relative humidity, it does not depend on temperature — it is an absolute measure of moisture content and does not change during sensible (temperature-only) heating or cooling.
W = 0.621945 × pv / (p − pv)   [ASHRAE 2009]
💡 W = 0.010 kg/kg means 10 g of water vapour per 1 kg of dry air. Typical indoor air: 0.007–0.012 kg/kg.
Specific Enthalpy
h
Unit: kJ/kg dry air · BTU/lb
The total heat content of moist air per unit mass of dry air, including both sensible heat (due to temperature) and latent heat (due to moisture content). Enthalpy is the key quantity for calculating HVAC loads — the energy required to change air from one state to another is simply the enthalpy difference times the mass flow rate.
h = 1.006·T + W·(2501 + 1.86·T)   kJ/kg dry air
💡 Cooling 5000 m³/h of air from h₁=85 kJ/kg to h₂=42 kJ/kg requires roughly 72 kW of cooling capacity.
Specific Volume
v
Unit: m³/kg dry air · ft³/lb
The volume occupied by one kilogram of dry air (plus its associated moisture). It is the inverse of density. Specific volume increases with temperature and decreases with pressure — hot, high-altitude air occupies more space per unit mass. Essential for converting between volumetric flow rates (m³/h) and mass flow rates (kg/s).
v = 0.287058 × (T + 273.15) × (1 + 1.6078·W) / p   m³/kg
💡 At sea level, 25 °C, 50% RH: v ≈ 0.858 m³/kg. At 40 °C: v ≈ 0.896 m³/kg.
Air Density
ρ
Unit: kg/m³ · lb/ft³
The mass of moist air per unit volume (= 1/v × (1+W)). Density decreases with rising temperature and increasing altitude. Critical for fan and duct sizing — fans are constant-volume machines, so reduced density (higher altitude or temperature) means less mass flow and less cooling capacity for the same fan speed.
ρ = (1 + W) / v   kg/m³   [ASHRAE]
💡 Standard sea-level air ≈ 1.204 kg/m³ at 20 °C. At 3000 m altitude it drops to ≈ 0.909 kg/m³.
Saturation Pressure
ps
Unit: kPa · psia
The maximum partial pressure water vapour can exert at a given temperature. It depends only on temperature and increases rapidly with it (approximately doubling every 10–11 °C near room temperature). When actual vapour pressure equals saturation pressure, the air is at 100% RH. Calculated using the ASHRAE Wexler-Hyland equation for maximum accuracy.
ASHRAE 2009 Wexler-Hyland Eq. 6 (liquid, 0–200 °C): ln(ps) = C₈/T + C₉ + C₁₀T + C₁₁T² + C₁₂T³ + C₁₃·ln(T)
Vapour Pressure
pv
Unit: kPa · psia
The partial pressure exerted by water vapour in the air mixture. It is the product of saturation pressure and relative humidity: pv = RH × ps. Vapour pressure drives moisture movement — vapour moves from high to low partial pressure, causing diffusion through building materials and driving evaporation and condensation processes.
pv = RH × ps(T)
Atmospheric Pressure
p
Unit: kPa · psia
The total pressure of the air–vapour mixture. At sea level it is 101.325 kPa (standard atmosphere). It decreases with altitude, which affects all psychrometric properties. The calculator automatically adjusts pressure based on altitude entered, or you can override it manually for non-standard conditions such as pressurised rooms or wind tunnel testing.
p = 101.325 × (1 − 0.0065·z / 288.15)^5.255   kPa
💡 At 1500 m altitude (e.g. Mexico City), p ≈ 84.6 kPa — this lowers boiling points and changes HVAC load calculations significantly.
Sensible Heat
Qs
Unit: kW · BTU/hr
Heat that changes air temperature without affecting moisture content (humidity ratio W remains constant). On the psychrometric chart, a purely sensible process moves horizontally — temperature changes, W does not. Sensible heat ratio (SHR) expresses what fraction of total heat transfer is sensible.
Qs = ṁ × 1.006 × ΔT   kW (where ṁ is mass flow in kg/s)
Latent Heat
QL
Unit: kW · BTU/hr
Heat associated with a change in moisture content (phase change of water) at constant temperature. Adding moisture (humidification) requires latent heat; removing it (dehumidification through condensation) releases latent heat. Latent loads are often dominant in tropical and humid climates, sometimes exceeding sensible loads.
QL = ṁ × 2501 × ΔW   kW (approximately)
💡 Latent heat of vaporisation of water ≈ 2501 kJ/kg at 0 °C, declining to ≈ 2442 kJ/kg at 25 °C.
Sensible Heat Ratio
SHR
Unit: dimensionless (0 – 1)
The ratio of sensible heat gain to total heat gain (sensible + latent). An SHR of 1.0 means all heat transfer is sensible (no moisture change). An SHR of 0.7 is typical for occupied buildings in temperate climates. Lower SHR values indicate humid environments requiring significant dehumidification.
SHR = Qs / (Qs + QL)
📊 How to Read the Psychrometric Chart
ℹ️
The psychrometric chart is a graphical representation of the thermodynamic properties of moist air. Every point on the chart represents a unique state point — a complete description of the air condition. Learning to read it is one of the most valuable skills for any HVAC engineer.
📐 Chart Lines & Regions — What Each Line Means
Saturation Curve (100% RH)
The bold curved boundary on the left. Air states ON this curve are fully saturated. States to the LEFT are physically impossible (supersaturated fog). This is the most important line on the chart.
Constant RH Lines
Curved lines arcing upward from left to right (10%, 20%…90%). Moving horizontally to the right at constant W increases temperature and reduces RH. Moving vertically up at constant T increases W and RH.
Dry Bulb Temperature Lines
Vertical lines running from bottom to top. The x-axis is dry bulb temperature. Moving right = warmer air. These are the easiest lines to identify on the chart.
Constant Humidity Ratio (W) Lines
Horizontal lines. The y-axis (right side) shows humidity ratio W in g/kg or kg/kg. A horizontal process = sensible heating or cooling — W does not change.
Wet Bulb / Enthalpy Lines
Diagonal lines sloping down to the right. Lines of constant wet bulb temperature and approximately constant enthalpy both run in this direction, allowing you to read enthalpy by following the line to the saturation curve.
Comfort Zone (Green Region)
The shaded polygon representing the ASHRAE 55 human comfort zone (approx. 20–26 °C, 30–60% RH). State points inside this zone require no active conditioning for human comfort in light clothing.
Current State Point (Purple Dot)
The plotted point from Tab 1 inputs. Its position on the chart tells you everything about the air condition at a glance — you can visually see how far it is from saturation, from the comfort zone, and what processes would be needed to reach a target state.
Constant Specific Volume Lines
Slightly diagonal lines. Specific volume increases toward the upper right (hot, moist air). Used when converting between volumetric and mass flow rates.

Step-by-step: Locating a State Point

  1. Start with dry bulb temperature. Find your temperature on the horizontal x-axis and draw a vertical line upward.
  2. Find your second property. If you know RH, find the appropriate curved RH line. If you know humidity ratio W, find the horizontal line on the right-hand y-axis scale. If you know dew point, find that temperature on the saturation curve, then draw a horizontal line across.
  3. Mark the intersection. The crossing point of your two property lines is the air's state point. This single dot encodes all other properties.
  4. Read wet bulb temperature. Follow the diagonal (constant enthalpy/wet bulb) line up-left from the state point to where it meets the saturation curve — read the temperature there.
  5. Read dew point. From the state point, draw a horizontal line left until it hits the saturation curve — read the temperature there.
  6. Read enthalpy. Follow the diagonal enthalpy line to the scale on the upper-left boundary to read enthalpy in kJ/kg.
🔄 Air-Conditioning Processes on the Chart

Each HVAC process traces a specific path on the psychrometric chart. Understanding these paths allows engineers to design systems and calculate energy loads.

❄️
Sensible Cooling
Temperature decreases, humidity ratio W stays constant. The state point moves horizontally to the LEFT. This is what a simple chilled-water coil does when the coil surface temperature is above the dew point. No moisture is removed — only temperature drops.
Path: Horizontal ← Left
🌡️
Sensible Heating
Temperature increases, W stays constant. The state point moves horizontally to the RIGHT. A heating coil or furnace performs this process. As temperature rises, relative humidity drops — this is why indoor air becomes dry in winter when cold outdoor air is heated.
Path: Horizontal → Right
💧
Cooling & Dehumidification
The most common air-conditioning process. A cooling coil whose surface is below the air's dew point both cools and removes moisture. The state point moves down and to the left — both T and W decrease. The process ends at the coil's apparatus dew point on the saturation curve, then mixes with bypassed air.
Path: Down ↙ Left (toward saturation curve)
💦
Humidification
Moisture is added to the air. Steam humidification (isothermal) moves the state point nearly vertically upward — W increases, T barely changes. Evaporative (adiabatic) humidification moves the point diagonally along a constant wet-bulb line toward saturation — cooling occurs as moisture is added.
Path: ↑ Up (steam) or ↗ Diagonal (evaporative)
🌀
Mixing of Two Air Streams
When two air streams are mixed (e.g., return air + fresh outside air), the resulting mixed state point lies on the straight line connecting the two original state points on the chart. Its exact position depends on the mass-flow ratio — a weighted average of the two states.
Path: Straight line between two state points
🏜️
Desiccant Dehumidification
A desiccant wheel adsorbs moisture from the air, removing W while simultaneously releasing the heat of adsorption — so the air becomes drier but hotter. The state point moves up and to the right (W decreases, T increases). Regeneration reverses this on the exhaust side.
Path: ↗ Up-Right (drier, hotter)
🛠️ How to Use This Calculator — Tab by Tab
✅
Quick start: All tabs update in real time as you type. Switch between SI (Metric) and Imperial (US) using the toggle bar at the top of the page. Results always show both unit systems where helpful.

🌡️ Tab 1 — State Point Calculator

This is the core calculator. Enter any two known air properties and the tool calculates all remaining psychrometric properties instantly.
  1. Set Altitude or Pressure. Enter your site altitude in metres (or feet in Imperial mode). The atmospheric pressure auto-calculates using the standard atmosphere equation. Or manually override pressure for special conditions.
  2. Enter Dry Bulb Temperature. Type the air temperature in the Dry Bulb field.
  3. Choose Input Mode. Select your second known property from the dropdown: Relative Humidity (most common), Wet Bulb Temperature, Dew Point, or Humidity Ratio.
  4. Enter the second value. Type the value for the mode you selected.
  5. Read all outputs. The right panel instantly shows Wet Bulb, Dew Point, RH, Humidity Ratio, Specific Volume, Air Density, Saturation Pressure, Vapour Pressure, and Specific Enthalpy.
  6. Check Application Status. Select your application type (Human Comfort, Industrial, Pharma, Server Room, or Food Processing) to see whether the current air state is within acceptable limits for that application, with specific warnings for temperature, humidity, mould risk, and heat stress.
  7. Export PDF. Click "Generate PDF Report" for a formatted engineering report including all values, formulae, and references.

🔄 Tab 2 — HVAC Process Calculator

Calculate the energy loads for air-conditioning processes between two air states.
  1. Enter State 1 (Inlet Air). Input the dry bulb temperature and relative humidity of the incoming air — typically outdoor air or return air.
  2. Enter State 2 (Outlet Air). Input the desired supply air conditions after the HVAC process.
  3. Enter Air Flow Rate. The volume flow in m³/h (or cfm in Imperial) determines the magnitude of the calculated loads.
  4. Read the results. The calculator outputs: sensible heat load (kW), latent heat load (kW), total heat load (kW), mass flow rate (kg/s), and water added or removed (kg/h). These values feed directly into equipment selection and coil sizing.

📐 Tab 3 — Duct & Fan Calculator

Size ducts and check velocity limits for a given airflow, using the actual density of moist air at your conditions.
  1. Enter air conditions. Temperature, RH, and altitude — used to calculate actual air density.
  2. Select duct shape. Rectangular (enter width and height) or circular (enter diameter).
  3. Enter flow rate and duct length. Volume flow in m³/h and duct run length in metres.
  4. Enter roughness. Galvanised steel = 0.15 mm (default), concrete = 1.5 mm, flexible duct = 0.9 mm.
  5. Read results. Air velocity, cross-sectional area, hydraulic diameter, Reynolds number, friction factor, and pressure drop are all calculated. Velocity is flagged if outside recommended ASHRAE limits (supply air 3–8 m/s, return 2–5 m/s).

📊 Tab 4 — Psychrometric Chart

  1. The chart automatically plots the state point from Tab 1. Switch to Tab 4 after entering your conditions to see the visual representation.
  2. The purple dot is your current state point. The green shaded region is the ASHRAE 55 human comfort zone.
  3. RH curves (10% to 100%), constant W gridlines, and the saturation curve are all displayed for reference.
  4. The chart updates live whenever you change Tab 1 inputs and then return to the Chart tab.
🏢 Application Comfort & Condition Limits

Different applications require different temperature and humidity ranges. The calculator checks the current state point against these limits automatically.

ApplicationTemp Range (°C)RH Range (%)Key Reason
Human Comfort (ASHRAE 55)20 – 26 °C30 – 60%Thermal comfort, no dry skin, no mould risk
Industrial18 – 27 °C40 – 60%Worker comfort, prevent static electricity, material integrity
Pharma / Cleanroom20 – 25 °C45 – 55%Regulatory compliance, product stability, contamination control
Server Room (ASHRAE A2)18 – 27 °C40 – 55%Prevent condensation on electronics, avoid static discharge
Food Processing5 – 15 °C35 – 50%Inhibit microbial growth, extend shelf life, prevent condensation
⚠️
Mould risk occurs when surface temperatures approach the dew point (within ~3 °C) combined with RH > 70%. The calculator flags this condition automatically. In buildings, mould growth typically begins when surface RH exceeds 80% for sustained periods.
📐 Engineering Formula Reference
Core Psychrometric Equations (ASHRAE 2009)
Atmospheric Pressurep = 101.325 × (1 − 0.0065z / 288.15)^5.255 kPa
Saturation PressureASHRAE Wexler-Hyland Eq. 5 (ice) / Eq. 6 (liquid)
Humidity RatioW = 0.621945 × pv / (p − pv) kg/kg
Specific Enthalpyh = 1.006·T + W·(2501 + 1.86·T) kJ/kg
Specific Volumev = 0.287058·(T+273.15)·(1+1.6078W) / p m³/kg
Moist Air Densityρ = (1 + W) / v kg/m³
Dew PointARM (Alduchov & Eskridge 1996): a=17.625, b=243.04°C
Wet BulbStull (2011) initial + ASHRAE thermodynamic refinement
Sensible LoadQs = ṁ_dry × Cp_air × ΔT kW
Latent LoadQL = ṁ_dry × hfg × ΔW kW (hfg ≈ 2501 kJ/kg)
Darcy-Weisbach (Ducts)ΔP = f × (L/D_h) × (ρv²/2) Pa
Hydraulic DiameterD_h = 4A / P_wetted m
📖
Key References: ASHRAE Fundamentals Handbook 2009 · Alduchov & Eskridge (1996), J. Applied Meteorology · Stull (2011), J. Applied Meteorology & Climatology · ASHRAE Standard 55-2020 (Thermal Comfort) · ISO 7933 (Heat Stress)
💡 Practical Tips & Common Mistakes
🎯 Always specify altitude
At high altitude, lower atmospheric pressure means air holds less mass per unit volume. Cooling and heating loads in m³/h are reduced, but per-kg loads are the same. Always enter site altitude for accurate results — a difference of 1000 m changes density by ~10%.
🌡️ Wet bulb ≤ Dry bulb always
If your wet bulb reading exceeds dry bulb, your instrument is faulty or the wick is not wet. The wet bulb can only equal the dry bulb when RH = 100%. This is a quick sanity check for any psychrometric measurement.
💧 Dew point is absolute
Unlike relative humidity, dew point temperature does not change when air is heated. If outdoor air at Tdp = 10 °C is heated to 30 °C indoors, the dew point is still 10 °C — only RH drops (to ~28%). Use dew point for reliable moisture content comparisons.
⚡ Enthalpy drives equipment sizing
The cooling or heating capacity required is always: Q = ṁ × |h₁ − h₂| kW, where ṁ is dry air mass flow in kg/s. Never use volumetric flow directly — convert using specific volume first. This is the most common error in HVAC load calculations.
📏 Duct velocity rules of thumb
Low-velocity systems: supply 3–5 m/s, return 2–4 m/s. Medium-velocity: up to 8 m/s. High-velocity: 8–12 m/s (with sound attenuation). Velocity above 5 m/s in occupied spaces causes noticeable noise. The duct calculator flags out-of-range velocities automatically.
🏗️ Mould prevention
Keep surface temperatures at least 3 °C above the dew point of the room air. In a room at 22 °C / 60% RH (Tdp ≈ 14 °C), wall surfaces should stay above 17 °C. Insulate cold bridges, pipes, and corners. The calculator warns you when mould risk conditions exist.