Laundry Drying Time Estimator

How to use this laundry drying time estimator

Work down the form in order. Every field has a physical meaning, and the two water-content routes give the same answer when your scale and your machine agree.

1. Dry laundry mass. Enter the mass of the same garments when fully dry, in kilograms. If you have never weighed them, weigh the load once when it comes off the line and reuse that figure. A typical adult mixed wash is 4 kg to 6 kg dry.
2. Fabric mix. Choose the group that dominates the load. This sets three things at once: how much water the fabric holds after spinning, how much surface area a kilogram of it exposes to the air, and how deeply water hides inside the fibre once the surface is dry.
3. Water content from. Pick Measured wet and dry weights if you weighed the load straight out of the drum, or Estimated from final spin speed if you only know the machine setting. The unused field is ignored, so you never have to invent a number.
4. Wet weight or spin speed. Wet weight is taken immediately after the spin, before anything drips away. Spin speed is the final spin in revolutions per minute, typically 400 rpm to 1600 rpm on a domestic front loader.
5. Air temperature, relative humidity and air speed. Use the conditions where the laundry actually hangs, not the forecast for the region. Indoor air speed with the door shut is about 0.1 m/s to 0.2 m/s; a fan pointed across a rack gives 1 m/s to 2 m/s; a breezy garden line sees 2 m/s to 4 m/s.
6. Sun exposure. Shade covers indoor racks and north-facing lines. Bright overcast is roughly 350 W/m² on the cloth; clear summer midday sun is roughly 800 W/m².
7. Press Estimate drying time. The result panel gives the total hours, the split between the constant-rate and falling-rate stages, the peak evaporation rate, the fabric surface temperature, the ambient dew point, and the tumble-dryer energy the load avoids. The chart underneath sweeps relative humidity so you can see how close you are to the cliff, and the table shows the same sweep in numbers.

Change one input at a time and re-run to see which lever matters most for your situation. On a mild, humid day the spin speed usually wins; on a cold, damp day nothing outdoors wins and the honest answer is a rack indoors with a fan.

Drying-time formula: psychrometrics, spin speed and mass transfer

The calculation runs in five steps. Every constant below is used by the JavaScript on this page, so the numbers in the worked example can be reproduced by hand.

Step 1 — water left after the spin

If you measured the load, the water mass is simply the difference between wet and dry weights:

Formula: W = m_wet − m_dry, X_0 = W / m_dry

Here X₀ is the initial moisture ratio, the mass of water carried per kilogram of bone-dry cloth. It is the same quantity the EU spin-drying efficiency classes are defined on: class A is below 45 %, class C spans 54 % to 63 %, and class G is 90 % or more.

If you only know the spin speed, the estimator predicts the residual moisture content from the centrifugal field G generated in the drum. With drum radius r = 0.24 m and rotation rate N in rpm:

Formula: G = (((2πN)/60)^2 r) / g, X_0 = k_f ⋅ 2.836 ⋅ G^−0.277

The exponent and coefficient are fitted so that the curve passes through the well-documented anchors of about 80 % residual moisture at 600 rpm and about 50 % at 1400 rpm for cotton, which places 1000 rpm at 60 % and 1600 rpm at 46 %. The fabric factor k_f is 1.00 for a mixed cotton wash, 1.22 for terry towelling, 1.10 for denim, and 0.45 for synthetic base layers, which shed water far more readily.

Step 2 — how much vapour the air can still take

Saturation vapour pressure follows the Magnus form recommended by Alduchov and Eskridge and used throughout applied psychrometrics, with T in degrees Celsius and the result in pascals:

Formula: p_sat(T) = 610.94 ⋅ e^(17.625T)/(T+243.04)

Vapour density comes from the ideal gas law with the molar mass of water M = 0.018015 kg/mol and the universal gas constant R = 8.314 J/(mol·K):

Formula: ρ_v = (p ⋅ M) / (R ⋅(T + 273.15))

The ambient vapour density is ρ_a = RH · ρ_v(p_sat(T_air)). The dew point reported in the result is the temperature at which that same vapour density would saturate.

Step 3 — the fabric surface temperature

Wet cloth is not at air temperature. Evaporation removes latent heat, so the surface settles where absorbed sunlight balances the sensible heat gained from the air plus the latent heat carried away. Using the Chilton–Colburn analogy to link the heat-transfer coefficient to the mass-transfer coefficient (ρc_pLe^2/3 ≈ 1086 J/(m³·K) for air and water vapour), the balance solved by bisection on this page is:

Formula: (α ⋅ I ⋅ f) / h_m = 1086 ⋅(T_s − T_air) + h_fg ⋅(ρ_s(T_s) − ρ_a)

with solar absorptance α = 0.55 for mixed-colour laundry, irradiance I from the sun-exposure choice, a projection factor f that converts the load's 0.25 m² of sun-facing area per kilogram into a share of its total exposed area, and the latent heat of vaporisation h_fg = 2.454 MJ/kg at 20 °C. Set I to zero and this equation returns the wet-bulb temperature: in shade at 20 °C and 60 % relative humidity, wet laundry sits near 14.8 °C.

Step 4 — the evaporation rate

Mass transfer from the wetted surface uses the standard convective form, with the exposed area A taken as the dry mass multiplied by a fabric-specific specific area (2.2 m²/kg for a mixed cotton wash, 1.5 for towelling, 1.2 for denim, 3.0 for thin synthetics):

Formula: m ˙ = h_m ⋅ A ⋅(ρ_s − ρ_a), h_m = 0.0025 + 0.0056 ⋅ v^0.8

The 0.0025 m/s floor is natural convection, so still air never gives a zero rate, and the 0.8 exponent on air speed v is the turbulent flat-plate dependence that the heat- and mass-transfer analogy predicts for flow along a hanging sheet.

Step 5 — hours, in two stages

While the whole surface is still wet the rate is constant, so the first stage takes the water above the critical moisture ratio X_c and divides it by the rate. Below X_c the wet patches shrink and the rate falls in proportion to the remaining free water, which gives the classical logarithmic falling-rate term down to a target ratio X_t:

Formula: t = (m_dry(X_0 − X_c)) / (m ˙) + (m_dry(X_c − X_e)) / (m ˙) ⋅ ln (X_c − X_e) / (X_t − X_e)

The equilibrium moisture ratio X_e is the regain the fabric settles at in that air, from the Henderson sorption isotherm fitted to published cotton regain data: 7.5 % at 65 % relative humidity, 10.5 % at 80 %, and 17 % at 95 %. The load is called dry when it reaches X_t = X_e + 0.03, which is the point at which cotton feels dry to the hand. Because X_e climbs steeply near saturation, the model correctly refuses to dry anything in air that is already at 100 % relative humidity.

Worked example: 5 kg cotton wash on a shaded, breezy line

These are the values the form loads with, so you can press Estimate drying time and compare every figure directly.

• Dry laundry mass: 5.0 kg
• Fabric mix: mixed cotton wash
• Water content from: measured wet and dry weights
• Wet weight after spin: 8.0 kg
• Air temperature: 20 °C
• Relative humidity: 60 %
• Air speed over the clothes: 1.0 m/s
• Sun exposure: shade or indoors

The load holds 8.0 − 5.0 = 3.0 kg of water, an initial moisture ratio of 60 %, which is EU spin-drying efficiency class C. Exposed area is 5.0 kg × 2.2 m²/kg = 11.0 m². Air speed gives a mass-transfer coefficient of 0.0025 + 0.0056 × 1.0^0.8 = 0.0081 m/s.

Formula: m ˙ = 0.0081 ⋅ 11.0 ⋅(0.012648 − 0.010349) = 2.048 × 10^−4 kg/s

Saturation at 20 °C is 2333 Pa by the Magnus form above, so the air at 60 % holds a vapour density of 0.010349 kg/m³. Solving the surface balance with no sun puts the cloth at 14.8 °C, where saturated vapour density is 0.012648 kg/m³. The deficit is therefore 0.0023 kg/m³ and the rate works out at 2.048 × 10⁻⁴ kg/s, which is 737 grams of water per hour.

The constant-rate stage runs from 60 % down to the critical ratio of 15 %, removing 5.0 × 0.45 = 2.25 kg and taking 3.05 hours. The falling-rate stage runs from 15 % down to the target of 9.8 %, with an equilibrium regain of 6.8 % in this air, and adds 0.56 hours. The calculator therefore reports 3.6 hours, or 3 h 36 min, together with a note that a tumble dryer would have spent about 2.9 kWh (vented or condenser) or 1.7 kWh (heat pump) removing the same 3.0 kg of water.

Now change one thing at a time. Selecting direct summer sun lifts the cloth to 16.8 °C and drops the estimate to 2.1 hours. Leaving it in shade but raising humidity to 80 % pushes it to 6.6 hours, and 90 % takes it to 12.5 hours. Switching the water source to a 1400 rpm spin instead of the measured weights starts the load at 2.50 kg of water rather than 3.00 kg and finishes in 2.9 hours — the same saving as an hour of sunshine, bought for a few watt-hours in the drum.

Reading the result and comparing typical drying situations

The headline number is the time to reach a hand-dry state, not the time until the last seam in the heaviest garment has given up its moisture. Treat it as a central estimate with roughly ±25 % of spread from garment spacing, thickness and the weather changing while the load hangs.

The rows below are produced by the same model, so you can enter any of them and reproduce the time exactly. They are useful as calibration points for judging your own conditions.

Two patterns are worth noticing. Airflow is nearly free and very effective indoors, where a fan across a rack cuts the time by more than half. And thick, absorbent fabrics are slow not because they dry differently but because they start with far more water per kilogram and expose far less area per kilogram: towelling holds 22 % more water than a mixed cotton wash at the same spin speed and presents only two thirds of the surface.

Limitations and assumptions behind this drying model

This is an engineering estimate built from textbook relations, not a computational fluid dynamics simulation of your garden. The following assumptions matter enough to state plainly.

• One lumped load, not individual garments. The whole wash is treated as a single wetted surface at one temperature. In reality thin shirts finish hours before the waistband of a pair of jeans, and the estimate tracks the average.
• Effective area is a fabric-specific constant per kilogram. Garments crowded on a rack, folded double over a line, or hung inside each other expose far less than the assumed area, and the true time can be 50 % longer.
• Conditions are held steady. The entered temperature, humidity, air speed and irradiance apply for the whole drying period. Evening humidity rises sharply outdoors, so an estimate that runs past dusk is optimistic.
• Solar input is a flat-plate approximation. Absorptance is fixed at 0.55 and the sun-facing area at 0.25 m² per kilogram of dry laundry. Dark fabrics absorb more, white sheets much less, and no allowance is made for the sun moving behind a wall.
• The spin correlation assumes a 0.24 m drum radius. Compact and top-loading machines differ, and real residual moisture also depends on load size and how well the drum balances. Measuring the wet weight once is always more accurate than the spin-speed route.
• The sorption isotherm is fitted to cotton. The Henderson fit matches published regain well above 60 % relative humidity and understates it by one or two percentage points below 40 %, where the effect on total time is small.
• Below freezing is flagged, not modelled. The equations still run, but frozen laundry sublimates rather than evaporates and passes through a stiff stage the model does not represent.
• Saturated air is capped. At relative humidity above 99.5 % the driving deficit is effectively zero. Rather than printing an infinite or absurd number, the calculator caps the answer at 120 hours and says the air cannot accept more moisture.
• Tumble-dryer figures are test-standard averages. The 0.98 kWh and 0.55 kWh per kilogram of water are derived from certified test loads, not from your specific appliance, cycle or ambient temperature.

For anything time-critical, add a margin. Spreading the load out, opening a window and pointing a fan across the rack are the three actions that map directly onto the terms this model actually uses.

Sources checked for this laundry drying calculator

Sources: the psychrometric relations, mass-transfer analogy, spin-efficiency classes and dryer energy figures used above were checked against the following authorities.

• ASHRAE Handbook — Fundamentals, chapters on psychrometrics and on mass transfer, for saturation vapour pressure, vapour density, wet-bulb behaviour and the Chilton–Colburn analogy between heat and mass transfer. ashrae.org — ASHRAE Handbook
• US Department of Energy, Uniform Test Method for Measuring the Energy Consumption of Clothes Dryers, 10 CFR Part 430 Subpart B Appendix D2, for the standard test load, the 57.5 % initial moisture content and the Combined Energy Factor definition. ecfr.gov — 10 CFR Part 430
• ENERGY STAR product criteria and certified-product data for clothes dryers, for the Combined Energy Factor levels used to derive kilowatt hours per kilogram of water removed. energystar.gov — clothes dryers
• EU energy labelling for household washing machines and washer-dryers, Regulation (EU) 2019/2014 and its predecessor Directive 95/12/EC, for the spin-drying efficiency classes defined on remaining moisture content. eur-lex.europa.eu — Regulation (EU) 2019/2014
• NIST Chemistry WebBook, enthalpy of vaporisation of water, for the 2.454 MJ/kg at 20 °C and 2.257 MJ/kg at 100 °C used in the energy comparison. webbook.nist.gov
• Alduchov and Eskridge, Improved Magnus form approximation of saturation vapour pressure, Journal of Applied Meteorology 35, 601–609 (1996), for the coefficients 610.94 Pa, 17.625 and 243.04 °C. journals.ametsoc.org — Alduchov & Eskridge 1996

Frequently asked questions about laundry drying time