💧 Relative Humidity

Relative Humidity Calculator

Q: How do you calculate relative humidity from dry-bulb and dew point temperature?

Relative humidity equals the saturation vapour pressure at the dew point divided by the saturation vapour pressure at the dry-bulb temperature, multiplied by 100. Using the Magnus approximation: RH = 100 x exp(17.625 x Tdp / (243.04 + Tdp)) / exp(17.625 x Tdb / (243.04 + Tdb)), where temperatures are in Celsius. For example, a dry-bulb of 25°C and dew point of 15°C gives RH of about 53%. This is the most accurate field method because dew point is an absolute measure that does not change with temperature, unlike wet-bulb which depends on airflow and psychrometer design.

Q: What is the recommended indoor relative humidity range in Canada?

ASHRAE Standard 55 recommends indoor relative humidity between 30% and 60% for occupant thermal comfort. In Canadian winters, outdoor air is extremely dry and infiltration drives indoor RH below 30%, causing dry skin, static electricity, and respiratory discomfort. Most Canadian building codes and health guidelines recommend maintaining indoor RH between 35% and 50% in winter. Above 60% RH promotes mould growth, dust mites, and condensation on cold surfaces. In climates like Vancouver with mild humid winters, summer dehumidification is rarely needed but winter humidification typically is not either. In Toronto and prairie cities, winter humidification is standard practice in residential and commercial buildings.

Calculate relative humidity from dry-bulb and dew point, dry-bulb and wet-bulb, or humidity ratio. Visual comfort gauge and zone indicator included. Use with the dew point calculator and full psychrometric calculator.

⚙ Input Method

💧 Air Conditions

Units

Elevation i

Dry-Bulb Temperature *

°F

Dew Point Temperature *

°F

Wet-Bulb Temperature *

°F

Humidity Ratio *

gr/lb

💧 Relative Humidity Results

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Relative Humidity

0%30%60%100%

Property	Imperial	Metric

Export:

Relative humidity in Canadian buildings: comfort, health, and building envelope

Relative humidity is the ratio of the actual water vapour pressure in air to the maximum possible vapour pressure at the same temperature, expressed as a percentage. At 100% RH, air is saturated and condensation occurs. At 0% RH, air is completely dry. The comfort range for most people is 30-60% RH at typical indoor temperatures.

Canada's climate creates two distinct seasonal challenges. In winter, extremely cold and dry outdoor air infiltrates buildings. When -20°C outdoor air at 80% RH enters a building and warms to 21°C, its RH drops to about 12% — well below the comfort threshold. Humidification systems add moisture to bring RH up to 35-50%. In summer, humid outdoor air (particularly in Ontario and Quebec) must be dehumidified by cooling coils to keep indoor RH below 60%.

Why dew point is more useful than relative humidity for diagnosis

Relative humidity changes with temperature even when the actual moisture content stays constant. If you heat air from 10°C to 25°C without adding or removing moisture, RH drops from about 70% to about 30% — but the actual water vapour content is identical. This is why dew point is the better diagnostic tool: it stays constant when only temperature changes. Use the dew point calculator to find the dew point from your RH reading, then use that dew point to assess condensation risk on cold surfaces regardless of temperature changes in the space.

Measuring relative humidity accurately in the field

Capacitive RH sensors (most modern digital hygrometers) are accurate to ±2-3% when calibrated. Psychrometer methods using wet-bulb and dry-bulb temperatures are less accurate below 50% RH because the wet-bulb depression becomes small and measurement errors dominate. For building commissioning and coil performance verification, a calibrated digital sensor is preferred. Always allow the sensor to stabilize for at least 10 minutes in the measured space before recording a reading.

Frequently Asked Questions

How do you calculate relative humidity from dry-bulb and dew point temperature?+

RH equals saturation vapour pressure at the dew point divided by saturation vapour pressure at dry-bulb temperature, times 100. Using the Magnus formula: RH = 100 × exp(17.625 × Tdp / (243.04 + Tdp)) / exp(17.625 × Tdb / (243.04 + Tdb)) where temperatures are in °C. A dry-bulb of 25°C and dew point of 15°C gives about 53% RH. This is the most accurate field method since dew point is an absolute measure. Use the psychrometric calculator for all properties at once.

What is the recommended indoor relative humidity range in Canada?+

ASHRAE 55 recommends 30-60% RH for occupant comfort. Canadian winters drive indoor RH below 30% in most cities without humidification. Target 35-50% RH in winter for comfort and to prevent static electricity and dry respiratory passages. Above 60% RH promotes mould and condensation on cold surfaces. Use the condensation risk calculator to check if your indoor RH will cause condensation on windows or walls at your local outdoor design temperature.