Laundry Drying Time Estimator

How this laundry drying time estimate works

This calculator estimates how long it will take a load of laundry to dry when it is hung on a line, rack, or balcony. It uses the difference between the wet and dry weight of your clothes, the surrounding weather, and the airflow around the fabric to approximate how quickly water can evaporate. The result is an indicative drying time, not an exact prediction, but it should put you in the right ballpark for everyday planning.

The model is built around three main ideas:

• How much water has to leave the fabric (wet weight minus dry weight).
• How fast evaporation can happen, which depends on temperature, humidity, and airflow.
• Whether the fabric is heated by the sun, which speeds things up.

In practice, this means a small, light load on a warm, breezy, sunny balcony will dry much faster than a heavy load of towels on an indoor rack in cool, humid air. The calculator converts your entries into a simple evaporation rate and divides the total water to remove by that rate to estimate the total drying time.

Key quantities and formulas

The first step is to determine how much water your laundry is holding. If W_wet is the weight of the load right after the spin cycle, and W_dry is the weight of the same clothes when they are completely dry, then the mass of water to remove is:

Here, W is in kilograms (kg). For many everyday uses, you may not know the true dry weight. A common shortcut is to assume that after a good spin, the wet weight is roughly 1.8 to 2.2 times the dry weight for cotton garments, so you can estimate one from the other if needed.

The drying time is then approximated using a simplified evaporation model. We estimate the effective surface area of the fabric exposed to air as proportional to the dry weight, and we assume an evaporation rate that increases with airflow and decreases with humidity. A compact way to express the relationship is:

where:

• t is drying time in hours.
• W is the mass of water in the load (kg).
• A is the effective fabric area (m²), approximated from dry weight.
• RH is the relative humidity as a fraction (e.g., 0.50 for 50%).
• v is air speed over the clothes (m/s).
• S is a dimensionless sun exposure factor (about 1.0 in shade and higher in direct sun).
• k is a combined coefficient that captures details like fabric structure and how efficiently air can remove moisture.

The constant 0.1 m/s in the airflow term represents natural convection, which provides a small amount of air movement around the fabric even when the air feels still. Higher temperatures increase the saturation vapour pressure at the fabric surface, which effectively increases the k · A term in the model. In the background, the calculator adjusts its coefficient based on your temperature and sun exposure inputs to reflect that warm, sunlit fabric dries faster than cool, shaded fabric at the same humidity.

Interpreting your drying time result

The output is an approximate number of hours your laundry may take to feel dry to the touch under the conditions you entered. It is not exact, because real-world drying is affected by many small details, but you can use it as a planning guide.

Some practical tips when reading the result:

• Think in ranges, not single numbers. If the estimate is 3.8 hours, treat it as roughly 3–5 hours, depending on how evenly the clothes are spread out and whether the weather shifts.
• Consider the thickest item in the load. Jeans, hoodies, and towels dry more slowly than T-shirts. If the calculator suggests 4 hours for the load, the heaviest pieces may still feel slightly damp at that time.
• Watch for changing conditions. If the sun goes behind clouds, the wind drops, or evening humidity rises, real drying time can stretch beyond the original estimate.
• Indoor vs outdoor drying. Indoors with low airflow and higher humidity, the same load can take several times longer to dry than outdoors on a line, even at a similar temperature.

Use the estimate as a way to answer questions like “Will this load be dry before I leave for work?” or “Do I have enough time to dry these clothes on a rack before evening?” rather than as a guarantee.

Worked example: a typical mixed load on a balcony

To see how the pieces fit together, consider a simple scenario. Suppose you have a mixed load of shirts and light trousers that comes out of the washing machine weighing 4.0 kg right after the spin cycle. When fully dry, the same clothes weigh 2.5 kg.

We will assume:

• Wet weight: 4.0 kg
• Dry weight: 2.5 kg
• Ambient temperature: 24 °C
• Relative humidity: 55%
• Air speed: 0.4 m/s (a gentle breeze on a balcony)
• Sun exposure: mostly direct sun for several hours

The mass of water to remove is:

For light to medium-weight cotton garments hung with good spacing, a rough effective surface area might be on the order of 1 m² per kg of dry clothes. With 2.5 kg of dry fabric, that suggests an effective area of about 2.5 m². The model then uses your temperature, humidity, airflow, and sun exposure to compute an evaporation rate. At 24 °C, 55% relative humidity, light breeze, and direct sun, that rate might correspond to a drying time in the range of about 2.5–4 hours.

If you take the same load indoors, everything else being equal but with air speed of 0.1 m/s (stagnant indoor air) and shade instead of sun, the calculation slows down. The estimated drying time might increase to something like 6–8 hours or more, which is why indoor racks often leave clothes damp well into the evening.

Typical drying times in common conditions

The table below shows some illustrative scenarios to help you compare your own result with typical situations. These are rounded ranges, not precise predictions, and assume a reasonably spaced load of mostly cotton clothing.

Use your own estimated drying time alongside these examples to judge whether your setup is unusually slow or fast. For instance, if the calculator suggests 10–12 hours for an indoor rack on a cool, humid day, that aligns well with the “Cool, humid day outdoors” scenario being similarly slow.

Assumptions and limitations of the model

To keep the calculator simple and fast, several assumptions are built into the model. These make it useful for everyday planning, but they also limit how precisely it can match every real-world situation.

• Typical household fabrics. The model is tuned for common cotton and cotton-blend clothing. Very thick items (duvets, heavy wool blankets, dense bath mats) or technical sports fabrics may dry faster or slower than predicted.
• Evenly spaced laundry. It assumes clothes are hung so that air can reach most surfaces. Crowded racks, clothes layered on top of each other, or items bunched on hangers can dramatically slow drying.
• Steady weather conditions. The estimate treats temperature, humidity, and airflow as roughly constant over the drying period. In reality, these can change hourly, especially outdoors.
• Uniform air speed. The air speed input is a simple average. Gusty wind, oscillating fans, or opening and closing windows will create variations the model does not track explicitly.
• Approximate humidity. Many people do not have a hygrometer. If you guess humidity (for example, “summer, feels muggy” vs “winter, dry heated room”), the estimate will be correspondingly approximate.
• No mechanical drying. The tool is for air drying only. It does not model tumble dryers, heated airers, or dehumidifier-assisted drying, although you might loosely approximate dehumidifier use as “lower humidity and slightly higher airflow”.
• Clothes “feel” dry, not laboratory dry. The point at which clothes feel acceptably dry to wear is subjective. The calculator targets this everyday sense of dryness rather than a strict moisture content threshold.

Because of these assumptions, you should treat the output as a guide. If timing is critical (for example, you must pack clothes for travel in a few hours), build in a safety margin beyond the predicted time and, if possible, increase airflow or move the drying location to improve conditions.

About this calculator

The underlying model is inspired by standard evaporation theory, including concepts similar to Dalton’s law of evaporation, and by typical measurements of how long laundry takes to dry under different household conditions. To keep the tool accessible, the physical details are condensed into a single empirical coefficient that has been tuned to match realistic drying times for common fabrics and line or rack setups.

It is not a research-grade simulator and should not be used in place of professional engineering tools or controlled experiments. Instead, it is designed to answer practical questions like:

• “If I hang this load now, will it be dry before bedtime?”
• “Will an indoor rack be enough, or do I need to use a fan or dehumidifier?”
• “How much faster will clothes dry on a sunny balcony than in a shaded room?”

If you are experimenting with ways to speed up drying, you can use the calculator repeatedly to compare scenarios—changing one factor at a time, such as airflow or humidity—to see which adjustments have the biggest effect on estimated drying time.

Practical tips to improve drying time

While the calculator helps you estimate drying time, a few simple changes can often reduce that time significantly:

• Increase airflow. Use a fan, open windows, or choose a breezier location. Even modest air movement greatly increases evaporation.
• Lower indoor humidity. In damp climates or tightly sealed homes, a dehumidifier in the drying room can noticeably shorten drying times.
• Maximize surface area. Shake out garments, avoid folding items over the line more than necessary, and give each piece some space.
• Prioritize sun when possible. Direct sunlight warms fabric and helps disinfect it; even partial sun can make a clear difference compared with shade.
• Spin thoroughly. Using a higher spin speed (if your machine and fabrics allow) reduces the initial water content and shortens air-drying time.

You can reflect each of these changes directly in the calculator’s inputs—for example, by increasing the airflow value when you add a fan, or reducing the wet weight if you use a higher spin setting—then compare the new estimated drying time to your original scenario.