Why do clothes dry faster on some days?

Clothes dry by evaporation, and the speed depends on how much more water the air can take. Warm air holds far more water vapour than cold air, so a warm, dry day dries laundry fast, while at 100% humidity nothing dries at all. Wind carries off the damp air next to the fabric, and sun warms the clothes themselves.

Tool description

This calculator estimates how long a load of laundry takes to dry. Enter the Temperature in °C or °F (from −30 to 60 °C) and the Humidity, then pick the Air movement, the Drying spot (indoors, outdoors in shade or outdoors in sun) and the Clothes (light, medium or heavy). Hung up at starts on the current time.

You get the Drying time, rounded to 5 minutes, the clock time the load is Dry by, and a Drying conditions rating from how long a medium load would take: Excellent up to 2 hours, Good up to 5, Fair up to 12, Poor beyond. Over 48 hours shows as "Over 2 days". It all runs in your browser.

How it works

The base times are for indoors in still air at 20 °C and 50% humidity: 4 hours for light clothes, 8 for medium and 16 for heavy. The tool scales them by the vapour pressure deficit, how much more water the air can take, using the Tetens formula with T in °C:

deficit = 0.6108 × e^(17.27 × T / (T + 237.3)) × (1 − humidity / 100)
drying time = base time × (deficit at 20 °C and 50% / deficit) ÷ spot ÷ air

The spot factor is 1 indoors, 1.3 in shade and 2 in sun. The air factor is 1 for still air, 1.7 for a light breeze or fan and 2.5 for strong wind.

Examples

Weather Clothes Drying time Conditions
20 °C, 50%, indoors, still air Medium 8 h Fair
28 °C, 40%, in sun, light breeze Medium 1 h 15 min Excellent
22 °C, 60%, indoors with a fan Light 2 h 35 min Fair
10 °C, 80%, in shade, light breeze Heavy 34 h 30 min Poor
5 °C, 90%, indoors, still air Heavy Over 2 days Poor

Features

  • Works in Celsius or Fahrenheit
  • Shows the time your laundry will be dry, not only how long it takes
  • Rates the drying weather from Excellent to Poor