Relative vs Absolute Humidity: What Dehumidifiers Control

Relative vs absolute humidity: what each means

Relative humidity (RH) is the percentage of water vapor in the air compared with the maximum amount the air can hold at a given temperature. Warm air holds more moisture than cool air, so the same quantity of water vapor results in a higher RH when temperatures drop. This is why a basement can feel damp in summer even if the absolute amount of water in the air has not changed much.

Absolute humidity is the actual concentration of water vapor in the air, usually expressed as a mass per volume. It is a direct measurement of the moisture content, independent of temperature. While absolute humidity tells you how much water is present, it does not indicate how the air feels or how likely mold is to grow. For those purposes, relative humidity is the more useful number.

Why dehumidifiers use relative humidity

Dehumidifiers are designed to combat the effects of high relative humidity. The EPA's guidance on mold control states that the key to mold control is moisture control, and the CDC advises keeping indoor humidity as low as you can, no higher than 50% all day long. These targets are expressed as relative humidity because they directly reflect the conditions that lead to condensation, musty odors, and mold growth on surfaces.

Energy Star uses relative humidity to categorize indoor conditions. It defines a space as slightly to moderately damp when RH is 50-75%, very damp at 75-90%, and wet at 90-100%, and it bases recommended dehumidifier capacities on these categories. A dehumidifier's job is to pull a room from these high RH levels down to a target like 50% or lower. That is why the humidistat on the unit is calibrated in relative humidity and why you set it as a percentage.

For a detailed look at where to set your humidistat, see the recommended humidity settings guide.

How dehumidifier ratings tie to relative humidity

Dehumidifier capacity is rated in pints per 24 hours under test conditions established by the U.S. Department of Energy. The DOE test procedure for portable dehumidifiers specifies a temperature of 65°F, which is meant to represent a typical basement environment. Because cooler air holds less moisture, the amount of water a unit removes is measured at that specific condition. The Energy Star testing page explains that the new procedure uses this cooler temperature to reflect real-world basement use.

The capacity you need depends on the size of the space and the existing dampness, which Energy Star describes in relative humidity terms. A dehumidifier's rating tells you how many pints it can remove per day, and that rating is tied to the RH conditions in the test. When you set the humidistat, you are choosing a relative humidity target, and the unit runs until it reaches that level. To learn more about how capacity is measured and what the numbers mean, see the pint rating explained article.

The table below summarizes Energy Star's recommended capacity ranges for different conditions in a small to medium room (under 2,000 sq. ft.) and a large room (over 2,000 sq. ft.).

Practical advice for your home

To use a dehumidifier effectively, start by measuring the relative humidity in the space with a hygrometer. If the reading is above 50%, run the unit until it reaches your target. The recommended humidity settings guide offers more detail on where to set the dial for different rooms and seasons.

Because relative humidity depends on temperature, you may see different RH readings in the same room at different times of day. A cool basement may show 70% RH even though the absolute humidity is modest. That is exactly the situation a dehumidifier is meant to address. If you have a damp basement, a portable unit with a drain hose is often the simplest solution. For a summary of the best basement models, see our best basement dehumidifiers guide. If you are dealing with a smaller space, a compact unit may be enough; browse the under 50 pint options.

Why absolute humidity matters less

Absolute humidity still matters in some contexts, such as calculating how much water a dehumidifier must remove to lower RH in a given volume of air. But for day-to-day control, relative humidity is the metric you can act on. You cannot set a dehumidifier to a specific absolute humidity level, because that number changes with temperature. The humidistat is calibrated in RH, and the recommended targets from public health agencies are expressed as RH.

What to pick for your use

Questions

What is the difference between relative and absolute humidity?

Relative humidity is a percentage that compares the amount of water vapor in the air to the maximum the air can hold at that temperature. Absolute humidity is the actual mass of water vapor per volume of air, independent of temperature. Dehumidifiers use relative humidity for their controls and ratings.

Why do dehumidifier ratings mention relative humidity?

Dehumidifier capacity is rated in pints per day under test conditions set by the U.S. Department of Energy, and the purpose of the unit is to lower the relative humidity in a space. The Energy Star program describes dampness conditions in relative humidity ranges, and the humidistat on the unit is set as a percentage of RH.

How high should the relative humidity be in my home?

The CDC advises keeping indoor humidity as low as you can, no higher than 50% all day long. The EPA notes that controlling moisture is the key to mold control. For more specific guidance, see the recommended humidity settings article.

Will a dehumidifier work in a cold basement?

The DOE test procedure for portable dehumidifiers uses a temperature of 65°F, and Energy Star notes that below 65°F frost can form on the coils, which may affect performance. Some units have anti-frost sensors that manage defrost cycles. If your basement is cooler, look for a dehumidifier designed for low-temperature operation. See the low temperature performance article for details.