Relative Humidity & Humidification Calculator

Estimate how relative humidity changes when outside air is heated or cooled, and the water addition or removal needed to reach an indoor humidity target.

This is an ideal-gas, constant-pressure HVAC screening calculation, not a sealed rigid-room model or a complete humidifier/dehumidifier design. It covers −20 to 50 °C and 60 to 110 kPa absolute pressure. RH is relative to liquid water, including supercooled water below 0 °C. Results below 0 °C do not establish freedom from frost.

Air conditions

Outside air temperature (°C)−20 to 50

Outside relative humidity (%)0 to 100

Absolute air pressure (kPa)60 to 110; 101.325 kPa is the sea-level reference, not the pressure at every site.

Indoor air temperature (°C)−20 to 50

Target indoor relative humidity (%)0 to 100; this is a calculation input, not a recommended humidity limit.

Room and ventilation

Room volume input

Room volume (m³)Greater than zero, up to 1,000,000,000

Volume unit converter. The room volume for the initial inventory is evaluated at the indoor temperature, before moisture adjustment.

Total supply air changes per hour (h⁻¹)0 to 100; includes recirculated air

Recirculated fraction of total supply airflow (%)0 to 100; the remainder is outside air

The ventilation airflow is room volume × total air changes per hour, referenced to indoor target temperature, humidity and pressure. Only the outside-air fraction creates the calculated ventilation moisture load. This is not an outside-air ACH input; if your ACH already counts outside air only, set recirculation to zero. Room inventory and continuous ventilation loads are separate results.

Calculated results

Outside humidity ratio, g water/kg dry air
Outside absolute humidity, g water/m³ outside air
Indoor RH after temperature change only, %
Indoor absolute humidity before moisture adjustment, g water/m³
Target humidity ratio, g water/kg dry air
Required moisture treatment
Required change in humidity ratio, g water/kg dry air
Room volume used, m³
Indoor moist-air density before moisture adjustment, kg/m³
Initial dry-air mass in the room, kg
One-time water addition or removal, kg (approximately L)
Outside-air fraction of the total airflow, %
Outside-air flow at indoor target conditions, m³/h
Ventilation water addition or removal, kg/h (approximately L/h)

Understanding relative humidity and water demand

Relative humidity compares the water-vapor partial pressure with the saturation vapor pressure at the same temperature. It is not the amount of water per kilogram of air. Humidity ratio is water mass per kilogram of dry air; absolute humidity is water mass per cubic metre of moist air. These units must not be interchanged.

During sensible heating at constant pressure, with no moisture exchange, humidity ratio remains constant and relative humidity falls. Absolute humidity in g/m³ also changes because the air volume changes. Cooling increases relative humidity until saturation is reached. Cooling below the dew point causes condensation; this calculator stops instead of reporting an impossible RH above 100%. A coil or condensation model is needed in that case. Frost can occur below 0 °C even when RH relative to liquid water is less than 100%.

Calculation basis

With pressure in Pa and temperature in °C: p_w = RH/100 × p_ws(T), and W = 0.621945 p_w/(p − p_w). Buck’s updated liquid-water equation is p_ws = 611.21 exp[(18.678 − T/234.5) T/(257.14 + T)]. No moist-air enhancement factor is applied.

The indoor RH before moisture adjustment is 100 p_w,out/p_ws(T_in). Initial dry-air mass is V(p − p_w,out)/[287.042(T_in + 273.15)]. Multiplying this mass by the target-minus-initial humidity ratio gives the one-time inventory change for a fixed dry-air mass. The sign distinguishes humidification from dehumidification; displayed quantities are magnitudes.

For ventilation, dry-air mass flow uses the entered volumetric flow and the indoor target vapor pressure. Water rate is dry-air mass flow × the outside-air fraction × |W_target − W_outside|. Recirculated air is assumed to return at the target condition. No rate can be determined from room volume and recirculation percentage alone.

Example and limitations

At 101.325 kPa, outside air at −10 °C and 30% RH contains approximately 0.53 g water/kg dry air. Heating it to 20 °C without adding water gives approximately 3.68% RH. Reaching 40% RH requires approximately 5.27 g water/kg dry air. For the 1,000 m³ example, the one-time inventory adjustment is about 6.34 kg; at one total air change per hour and 60% recirculation, the ventilation load is about 2.51 kg/h.

These are ideal moisture balances. Occupants, products, infiltration outside the stated airflow, open doors, moisture exchange with building materials, humidifier losses and thermal loads are not included. Kilograms of water are approximately equal to litres of liquid water for these screening results; evaporation energy and equipment capacity are not calculated. A truly sealed, fixed-volume room requires a different pressure and mass balance.

Actual m³/h and normal m³/h are different flow bases. Do not use this HVAC calculation for a hot saturated exhaust or an Nm³/h water-emission calculation without defining normal temperature, normal pressure and whether the flow is on a dry- or wet-gas basis.

Selecting a humidity target

Relative humidity can be measured outdoors with suitable calibrated instruments; measurement uncertainty depends on the sensor, installation and conditions. Cold air can have high RH while containing little water vapor. Temperature alone does not determine RH.

Indoor humidity targets depend on building conditions and use. For homes, the US EPA advises keeping RH below 60%, ideally 30–50%, to help control mold. This is residential moisture-control guidance, not a universal industrial or clinical specification. Persistent surface condensation warrants attention even if the room’s RH appears acceptable.

Application	What should determine the target?
General occupied buildings	Comfort, condensation risk, outdoor conditions and applicable ventilation requirements.
Materials, food and manufacturing	Product moisture stability, process requirements, storage conditions and supplier specifications.
Museums, archives and libraries	Collection-specific conservation requirements and control of damaging fluctuations.
Healthcare and controlled environments	Applicable facility requirements, room classification and specialist design criteria.
Electrostatic-sensitive processes	The complete electrostatic-discharge control program; humidity alone does not establish protection.

This calculator does not determine disease risk or prescribe humidity ranges for asthma, allergies, hospitals or individual products.

References and related tools

NCAR Earth Observing Laboratory: water-vapor pressure equations; PsychroLib: ASHRAE psychrometric relationships; US EPA: residential moisture and mold guidance.

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Lenntech BV cannot be held responsible for errors in the calculation, the program itself or the explanation. For questions or remarks please contact us.