How long does washing take to dry outside?

A cotton t-shirt in good conditions: a couple of hours. Jeans: potentially all day. A thick towel on a damp, still afternoon: possibly still damp at dusk. These are honest answers to a question that has no single correct one. Weather decides whether it's worth hanging anything out. Fabric has a big say on how long it takes. What follows is an explanation of both sides of that equation, so you can make better calls with the information you have.

What you control

Fabric

A laboratory study by Laing et al. (2007) tested 17 different fabrics under controlled conditions, using precision balances to measure the exact point at which each became dry.¹ The findings are the most reliable set of comparative drying figures available in textile science.

Pure polyester dried roughly 25 minutes faster than pure cotton across samples of identical size and weave. The 65/35 polyester-cotton blend common in school uniforms and work shirts landed almost exactly between the two: about 12 minutes faster than pure cotton, 13 minutes slower than pure polyester. The difference is explained by fibre structure. Synthetic fibres are hydrophobic: water sits on the surface rather than penetrating the fibre, so there is less work to do.² Cotton is hydrophilic and absorbs water into the fibre structure itself. More water in means more time to shift it.

Wool is more complicated. The study found that thickness matters more than fibre type. A thin woven wool can dry faster than a heavy synthetic. A dense Merino knit was among the slowest fabrics tested, holding significantly more moisture between its fibres than the weight alone would suggest. Wool that feels dry to the touch may not be. It can hold up to 30% of its own weight in moisture without feeling wet at the surface.

The Laing findings, modelled onto real garment sizes by Dodd et al., produce these benchmarks under fair weather conditions: low wind, 15 to 20°C (59-68°F), no rain blah blah blah...

Thickness and layers

A single layer of jersey dries faster than a doubled-over hem. A jeans pocket sitting flat against the outer denim is four layers thick: the outer fabric, the pocket lining, the pocket bag, and the rear outer denim, and that's assuming it isn't pressing directly on to a rear pocket. Turning jeans inside out pulls the pocket bags to the surface and reduces the thickest point from four layers to two.

The stubborn last 1%

The Laing study found that the definition of "dry" changes everything. For a heavy wool knit, reaching the final 0.5% residual moisture from 2% took 48 minutes longer than getting from fully wet to mostly dry. The evaporation rate slows sharply as the accessible surface moisture is exhausted, leaving only water trapped deep inside the fibre structure.

This explains something many people have experienced without a name for it: a jumper that feels dry to the touch but feels cold and slightly clammy when you put it on. That last 1% of moisture is still there. It just takes as long to remove as the first 20%.

Colour

In direct sun, dark fabric absorbs more solar radiation and reaches a higher surface temperature, which accelerates evaporation. In shade, this reverses: dark fabric radiates heat away faster, so it actually runs cooler than a lighter equivalent in the same conditions.³ The colour effect only exists where there is direct solar radiation to absorb. On a cloudy day, it makes no meaningful difference.

Spin speed

Every revolution of the drum removes water before the clothes ever reach the line. Higher spin speeds measurably reduce residual moisture content. The difference between 800 RPM and 1,400 RPM affects total drying time outdoors in a way you'll notice. Use the highest spin speed the fabric's care label permits. It's the one variable you control entirely before the weather becomes relevant.

Giving the load an extra spin is an entire article by itself...

How you hang things

Items bunched together share humid air. Spaced items dry independently. An unfolded towel has twice the exposed surface area of one draped over a line. A doubled-over towel isn't drying both sides at the same rate. The inner surfaces sit in their own microclimate, and they'll still be damp when the outer surface feels dry.

What you can't control, but can read

Wind

The most important variable, and the most underrated. When fabric loses moisture, a thin layer of saturated air forms immediately against the surface. We call it "the soggy halo". That layer acts as a lid: until it's cleared away, the fabric underneath cannot dry, regardless of temperature.⁴ A gentle breeze doesn't just feel helpful. It mechanically removes the barrier that stops evaporation. On a still day, even warm air does very little.

Humidity

Once humidity climbs past 70%, the vapour pressure deficit collapses.⁵ That deficit, the gap between how much moisture the air currently holds and how much it could hold, is the driving force for evaporation. When the gap closes, there is nowhere for the water in your towel to go. The Royal Meteorological Society is direct about this: clothes dry faster on a cold, dry day than on a warm, humid one.⁶

Solar radiation

Solar radiation drives evaporation directly, not just by warming the air. An item in direct sun absorbs radiant energy regardless of the thermometer reading, which is why washing on a line can be significantly warmer than the ambient temperature suggests.⁷ This is also why clear, cold, sunny days can be better drying days than warm, overcast ones.

There's a secondary benefit worth knowing. UV radiation from direct sunlight has a measurable bactericidal effect on fabric. A study published in the Journal of the European Academy of Dermatology and Venereology found that socks exposed to sun for three days showed significantly higher rates of negative fungal cultures than those kept indoors.⁸ UV-C, the most germicidal wavelength, is largely blocked by the atmosphere, but UV-A and UV-B reach the surface and contribute. Line-drying in direct sun is also, quietly, disinfecting.

Temperature

Temperature matters, but it sits third in the hierarchy behind wind and humidity. Warmer air can hold more moisture, which widens the vapour pressure deficit and supports faster evaporation. Research following the Laing study found that each additional degree of ambient temperature may reduce drying time by around two minutes, all else being equal. But warm, still, humid air still dries slowly. Don't let a warm forecast override a high humidity reading.

Precipitation and air pressure

Rain resets progress. Even a brief shower can add more moisture than an hour of reasonable conditions can remove. Moderate rainfall is measured in whole millimetres per hour, while evaporation rates are typically a tenth of that.⁹ Air pressure is a useful proxy for stability: high pressure tends to mean settled, drier conditions. Low pressure tends to mean the opposite.

The line itself

Direction

An east-west line means garments hang facing north and south. In the northern hemisphere, the sun travels across the southern sky throughout the day, so the south-facing side of every garment receives direct solar radiation through peak hours. A north-south line puts one face in permanent shade for half the day.¹⁰ If you're choosing where to site a permanent line, orient it east-west.

If you are using a rotary airer, who knows where your clothes are pointing? They might be still or spinning like a fairground ride.

Covered drying

A pergola or porch overhang keeps rain off, which matters on a marginal day when showers are possible but conditions are otherwise reasonable. The trade-off is reduced direct solar radiation. On a clear, good drying day, the cover costs more than it gives. On an uncertain one, it can make the difference between getting a wash out and leaving it.

Sublimation. Wait, what?

Washing can dry in freezing temperatures. Ice transitions directly to water vapour without passing through liquid (a process called sublimation), provided the air is dry enough and there is some movement.¹¹ It is slow, and UK winters rarely combine the required conditions (well below freezing, low humidity, a breeze) in the way that makes it practical.

Why DryTime has no calculator

The science for a laundry drying time calculator exists. Laing et al. established precise drying times for 17 fabrics under controlled conditions. Research following that study produced a predictive model suggesting that each additional degree of ambient temperature may reduce drying time by around two minutes, all else being equal. The data is there.

The problem is the user-input side. Capturing fabric type, fabric weight, garment construction, starting moisture content, colour, spin speed, and how each item is hung requires an interface that becomes more complicated than the problem it's solving. Threads on laundry forums reference tools with exactly this outcome. "I can't make head or tail of it," wrote one reviewer, describing a drying time calculator. The number of inputs required to produce a better estimate has made the tool harder to use than the benefit it brings.

What actually works

Meteorologists modelling clothes drying often arrive at the Penman-Monteith equation, developed for agricultural irrigation, applied to vapour pressure deficit, net radiation, and wind speed.¹² DryTime uses a custom equation, with the same weather inputs and scientific trend directions, but hey, we're not drying horizontal fields covered in plants, are we? Either way, the working definition seems to be: a washing day is any day when a full load of heavy fabrics would be dry within four hours.

Four hours of good conditions. That is the working heuristic for the hardest item to dry. Lighter fabrics need less. Synthetics need considerably less. The conditions side of the equation is the part you can read, forecast, and act on. Fabric type, pocket orientation, and spin speed are fixed by the time the clothes reach the line.

What remains is the question of whether this morning or afternoon is worth it? Or even overnight? That's the one DryTime answers.

To the forecast

Hero image credit: Photo by Agê Barros on Unsplash

Last updated: April 2026 by the DryTime family in Somerset, UK.