Relative Humidity Calculator

An expert tool to calculate relative humidity based on temperature and dew point.

Dew point cannot be higher than air temperature.

Visual representation of Relative Humidity.

What is the Formula Used to Calculate Relative Humidity?

Relative humidity is a crucial metric in meteorology and environmental science, representing the amount of moisture in the air as a percentage of the maximum amount the air could hold at its current temperature. It is not a measure of the absolute amount of water vapor but a ratio. A high relative humidity means the air is close to saturation, which can lead to condensation, dew, or fog. This concept is vital for predicting weather, assessing human comfort, and for many industrial processes. For a deeper dive, check out our guide on understanding atmospheric moisture.

The Relative Humidity Formula and Explanation

The core formula used to calculate relative humidity is a simple ratio. It compares the actual amount of water vapor in the air to the maximum possible amount at that temperature.

RH (%) = (E / E_s) × 100

To use this formula, we first need to find the values for E and E_s, which are typically derived from temperature and dew point using a more complex empirical formula, such as the Magnus-Tetens approximation:

Vapor Pressure (E or E_s) = 6.112 × exp((17.67 × T) / (T + 243.5))

Here, ‘T’ is the temperature in Celsius. You use the dew point temperature to find the Actual Vapor Pressure (E) and the air temperature to find the Saturation Vapor Pressure (E_s). Our dew point calculator can help with related conversions.

Description of variables in the relative humidity formula.

Variable	Meaning	Unit	Typical Range
RH	Relative Humidity	Percentage (%)	0 – 100%
E	Actual Vapor Pressure	hectopascals (hPa)	0 – 70 hPa
Es	Saturation Vapor Pressure	hectopascals (hPa)	1 – 110 hPa
T	Temperature (Air or Dew Point)	°C, °F, K	-50 to 50 °C

Practical Examples

Example 1: A Comfortable Day

Let’s imagine a pleasant spring day where the conditions are:

• Inputs: Air Temperature = 20°C, Dew Point = 10°C
• First, we calculate E_s at 20°C, which is approximately 23.37 hPa.
• Next, we calculate E at 10°C, which is approximately 12.27 hPa.
• Result: RH = (12.27 / 23.37) × 100 ≈ 52.5%. This is a comfortable level of humidity.

Example 2: A Humid Summer Afternoon

Now consider a hot and sticky summer day:

• Inputs: Air Temperature = 32°C (89.6°F), Dew Point = 24°C (75.2°F)
• E_s at 32°C is approximately 47.54 hPa.
• E at 24°C is approximately 29.83 hPa.
• Result: RH = (29.83 / 47.54) × 100 ≈ 62.7%. While not extremely high, combined with the heat, it would feel quite muggy. For more on this, see our heat index formula tool.

How to Use This Relative Humidity Calculator

Our tool makes finding the formula-based relative humidity simple. Just follow these steps:

1. Enter Air Temperature: Input the current air temperature into the first field.
2. Enter Dew Point: Input the current dew point temperature. The dew point must be less than or equal to the air temperature.
3. Select Units: Choose whether your input temperatures are in Celsius or Fahrenheit from the dropdown menu.
4. Read the Result: The calculator instantly updates, showing the final relative humidity percentage, along with the intermediate vapor pressure values used in the calculation.

Key Factors That Affect Relative Humidity

Relative humidity is a dynamic value influenced by several environmental factors.

• Temperature: This is the most significant factor. If the amount of water vapor stays constant, a decrease in temperature will cause relative humidity to increase, and an increase in temperature will cause it to decrease.
• Water Vapor Content: Adding moisture to the air (e.g., through evaporation from bodies of water) will increase relative humidity if the temperature is stable.
• Atmospheric Pressure: Changes in pressure can affect the air’s volume and temperature, indirectly influencing RH.
• Wind and Air Movement: Wind can transport moist or dry air from one region to another, rapidly changing local humidity.
• Proximity to Water: Coastal areas typically have higher humidity than inland deserts because of the vast source of evaporating water.
• Vegetation: Plants release water vapor through a process called transpiration, which can significantly increase local humidity in densely forested areas. Understanding this is key to using tools like an air density calculator correctly.

Frequently Asked Questions (FAQ)

Q1: What does 100% relative humidity mean?
A: It means the air is completely saturated with water vapor. It cannot hold any more moisture at its current temperature. If the temperature drops even slightly, the water vapor will condense into liquid water, forming dew or fog.

Q2: Can relative humidity be over 100%?
In nature, this is extremely rare and is called supersaturation. It requires a very pure air mass free of condensation nuclei (like dust or pollen). For all practical purposes, 100% is the maximum.

Q3: Is high relative humidity always uncomfortable?
Not necessarily. In cold weather, higher humidity can make the air feel warmer and more comfortable. In hot weather, however, high humidity prevents sweat from evaporating, which is why it feels “muggy” and uncomfortable.

Q4: Why does it feel “drier” indoors during winter?
When you heat cold outside air, you increase its temperature drastically. While the absolute amount of moisture in the air stays the same, the temperature increase significantly raises the air’s capacity to hold water, thus the relative humidity plummets, often to uncomfortable lows.

Q5: What is the difference between relative humidity and dew point?
Relative humidity is a ratio that depends on the current temperature. Dew point is an absolute measure of the moisture in the air; it’s the temperature to which the air would need to cool to reach 100% relative humidity. A higher dew point always means more moisture in the air, regardless of the current temperature.

Q6: How do you use the formula for Fahrenheit?
The scientific formula relies on Celsius. To use it with Fahrenheit, you must first convert your Fahrenheit temperatures to Celsius using the formula: C = (F – 32) × 5/9. Our calculator does this automatically for you.

Q7: What is a good indoor relative humidity level?
For comfort and health, most experts recommend keeping indoor relative humidity between 30% and 50%.

Q8: Where can I find more advanced humidity data?
For complex engineering and scientific work, professionals often use a psychrometric chart online, which plots the relationships between all major air properties.