Why line drying time is worth estimating
Line drying is the oldest laundry technology and, in many ways, still the best: it costs nothing in energy, it is gentle on fabric, sunlight naturally freshens and lightly disinfects, and it leaves clothes smelling like outdoors rather than like a heated appliance. Its one disadvantage is uncertainty. A load hung out on a bright, breezy morning may be dry by lunch, while the same load hung out on a humid, still afternoon may still be damp at dusk, and a load left out overnight may need another day entirely. That uncertainty causes real problems: washing started too late in the day, a school uniform still damp in the morning, or a load brought in half-dry and finished in a tumble dryer anyway, losing the energy saving that motivated line drying in the first place.
Estimating drying time before hanging out turns that uncertainty into a plan. If the estimate says three hours, a load hung at nine in the morning will be dry by noon, and the line will be free for a second load. If it says eight hours, the load should be hung as early as possible, or split, or started in the dryer and finished on the line, rather than discovered damp at bedtime. The estimate does not need to be exact to the minute; it needs to be good enough to choose a start time, to decide whether a second load is feasible, and to avoid the damp-at-dusk surprise. The calculator on this page gives that estimate from the three factors that matter most: how much laundry there is, how humid the air is, and how warm it is.
How the calculator works
Enter the number of garments, the relative humidity as a percentage, and the temperature in degrees Celsius. The calculator starts from a base drying load that grows with garment count, scales it up as humidity rises, because humid air absorbs moisture more slowly, and scales it down as temperature rises, because warm air holds more moisture and evaporates water faster. Ten garments at fifty percent humidity and twenty-five degrees therefore take about three hours, or one hundred and eighty minutes. One garment on a damp, cool day, ninety percent humidity and fifteen degrees, still takes about three point six hours, because humidity dominates when the air is nearly saturated. Twenty garments at sixty percent humidity and thirty degrees take about five hours, or three hundred minutes, showing how a larger load and moderate humidity can outweigh a warm day.
The formula is deliberately simple and should be read as an estimate, not a weather forecast. It does not include wind, which is the fourth major factor, because wind speed is harder for most people to know than temperature and humidity, both of which are shown on every phone weather app. Wind helps enormously: a steady breeze can halve drying time compared with still air at the same temperature and humidity, by constantly replacing the moist air next to the fabric with drier air. Sunlight helps too, both by warming the fabric directly and by warming the air around it, which is why a line in full sun dries faster than a line in shade at the same air temperature. Treat the calculator's figure as the still-air, shaded baseline, and expect faster drying on a breezy, sunny day, slower drying on a still, overcast one.
Humidity: the factor most people underestimate
Humidity is the single most powerful factor in drying, and the least intuitive, because it is invisible. Relative humidity is the percentage of moisture the air already holds compared with the maximum it can hold at that temperature, and evaporation slows as that percentage approaches one hundred, because the air has little capacity left to absorb more water. At fifty percent humidity, air can still absorb a great deal, and drying proceeds steadily. At eighty or ninety percent, the air is nearly saturated, evaporation crawls, and clothes can feel damp for hours even in warm weather, because the bottleneck is not heat but capacity. That is why a hot, humid day can dry laundry more slowly than a cooler, drier day, a result that surprises people who equate heat with drying.
Temperature interacts with humidity in a helpful way: warm air can hold more moisture than cool air, so the same absolute amount of water vapour represents a lower relative humidity at a higher temperature. Warming the air, whether by sunshine or by a heated room, therefore increases its drying capacity even if no moisture is removed. This is also why indoor drying in a heated room works, and why a dehumidifier helps indoor drying so much: it actively removes moisture from the air, lowering relative humidity and restoring the air's capacity to absorb water from fabric. A load dried indoors in a cold, damp, unventilated room, by contrast, may take a day or more, and risks mustiness, because the air saturates quickly and stays saturated.
Load size, fabric, and spacing
Load size matters in two ways: more garments hold more total water, and more garments crowded on a line shade and shelter each other from air movement. A line packed edge to edge dries much more slowly than the same garments spaced with gaps between them, because air must circulate around each item to carry moisture away. The calculator's load factor captures the total water effect, but spacing is a technique the user controls: leave a hand's width between garments where possible, hang larger items on the outer lines where wind reaches them first, and avoid overlapping items, which creates damp pockets that dry last. A second line, or a second load hung after the first is dry, often dries faster in total than one overloaded line, because circulation matters more than line length.
Fabric type changes drying time as much as load size, though the calculator treats garments as average. Thin cotton shirts and synthetic sportswear dry quickly, often in one to two hours in good conditions, because they hold little water and present a large surface area. Thick cotton towels, jeans, and bedding hold far more water per item and dry slowly, sometimes taking a full day, because water must migrate from the inside of thick fabric to the surface before it can evaporate. A load of towels should therefore be treated as a larger load than the same count of shirts, and hung earliest, in the sunniest position, with the most spacing. Bedding, hung over two lines or folded over a line so air reaches both sides, dries faster than bedding bunched on one line.
Wind, sun, and line position
Wind deserves its reputation as the line dryer's best friend. A gentle breeze of even a few kilometres per hour continuously replaces moist air with drier air, and a stronger breeze also flutters fabric, exposing new surfaces and speeding evaporation further. A line sheltered by a wall or a fence, in a wind shadow, can dry noticeably more slowly than an exposed line a few metres away, even in the same garden. Sunlight adds direct radiant heating, warming the fabric above air temperature, which increases evaporation from the fabric surface itself, and ultraviolet light also has a mild disinfecting and whitening effect on white cotton, which is why whites dried in sun look brighter than whites dried indoors. The combination of sun and wind is ideal, and a load hung in sun and wind on a dry day is the fastest line drying there is.
Line position and orientation are practical choices that follow from this. A line running across the prevailing wind, rather than along it, exposes more fabric to moving air. A line in full sun for the middle of the day captures the warmest, driest hours, while a line that only catches morning sun may dry more slowly in the afternoon shade. Indoors, the equivalent choices are a rack near a window with airflow, or near a heat source with ventilation, never in a closed, cold room. A fan directed at an indoor rack mimics outdoor wind surprisingly well, and is far cheaper in energy than a tumble dryer, while a dehumidifier in the same room addresses the humidity bottleneck directly.
Planning the wash day
The practical use of an estimate is planning. A load estimated at three hours, hung at eight in the morning, will be dry by eleven, leaving the afternoon free and the line free for another load. A load estimated at six hours should be hung by nine to be dry by three, before the late-afternoon humidity rise and temperature fall that slow drying sharply. A load estimated at eight hours or more is a candidate for splitting, for starting earlier, or for a hybrid approach: a short tumble-dryer cycle to remove the bulk of the water, followed by line finishing, which still saves most of the energy and gives most of the fabric benefit. Evening hanging is usually a poor choice outdoors, because temperature falls and humidity rises after sunset, often re-dampening clothes that were nearly dry.
Seasonal planning follows the same logic. Summer, with long days, warm air, and low humidity, is the easiest line-drying season, and multiple loads per day are feasible. Winter, with short days, cool air, and high humidity, may allow only one load, hung early, and indoor drying with ventilation or dehumidification may be faster and more reliable than outdoor drying. Spring and autumn sit between, with good drying on dry, breezy days and poor drying on damp, still ones, which is why checking the day's humidity and temperature, and running the estimate, is more useful than following a seasonal rule. The calculator's inputs are exactly the numbers a weather app provides, so the estimate can be part of the morning routine, like checking for rain.
Indoor drying without damp problems
Indoor drying is often necessary, and it needs care to avoid the damp and mould problems that give it a bad reputation. A load of wet laundry releases roughly two litres of water into the room air as it dries, and if that moisture has nowhere to go, it condenses on cold walls and windows and feeds mould. Ventilation, opening a window slightly even in winter, or a dehumidifier, or both, prevents this, by giving the moisture an exit. Drying in a bathroom with an extractor fan running, or in a utility room with a window ajar, is far better than drying in a bedroom with the door closed. The estimate still applies indoors, but with the room's temperature and humidity, which a cheap hygrometer can measure, and with an adjustment for the fan or dehumidifier's help.
Common mistakes in line drying are mostly about timing and crowding. Hanging a load too late in the day, packing a line edge to edge, hanging towels and bedding last instead of first, and leaving clothes out overnight in dew, all slow drying or re-dampen nearly dry clothes. Bringing clothes in slightly damp and folding them away causes mustiness; bringing them in when fully dry, and folding promptly, preserves the fresh smell that is line drying's reward. A load that is not quite dry at dusk is better brought in and finished indoors with airflow than left out overnight, because overnight dew will undo hours of drying.
- Hang early, ideally before 9 a.m., for same-day drying.
- Space garments with gaps; never pack a line edge to edge.
- Hang towels and bedding first, in sun and wind.
- Check humidity, not just temperature, before hanging out.
- Bring in before dusk; finish damp loads indoors with airflow.
Enter your load, humidity, and temperature above, and plan today's wash around the estimate it gives you, rather than discovering at dusk that it needed longer.