Quick answer: Indoors at about 20 °C and 50 % RH with still air, an open water surface loses roughly 2 mm of depth per day. So a thin puddle (about 2 mm deep) dries in about a day, a wiped-down wet floor in an hour or two, and a small drop in roughly 10–30 minutes. Outdoors on a warm, sunny, breezy day, evaporation is several times faster, often 5–10 mm a day. Evaporation speeds up with warmer water, drier air, moving air and a larger surface area.
Quick answer (indoors, 20 °C, 50 % RH, still air)
- Small drop of water on a surface: about 10–30 minutes.
- Thin puddle, 2 mm deep: about 1 day.
- 5 litres in a bucket: about a month; spread 5 mm deep over 1 m²: about 2–3 days.
Evaporation times at a glance
These are estimates for still indoor air at 20 °C and 50 % RH, with the water at room temperature. A fan typically makes them 2–3 times faster.
| Water | Depth or size | Approximate time |
|---|---|---|
| Small drop on a countertop | about 10 µL | 10–30 minutes |
| Large drop or splash | about 0.1 mL | about 1–2 hours |
| Wiped floor (thin film) | about 0.1 mm | 1–2 hours |
| Thin puddle | about 2 mm | about 1 day |
| Deep puddle | about 10 mm | about 5 days |
| Glass of water (8 cm wide, 10 cm deep) | 0.5 L | about 7 weeks |
| 5 L bucket (30 cm wide) | 5 L | about 5 weeks |
| 5 L spread over 1 m² | 5 mm deep | about 2–3 days |
| 1 US gallon (3.8 L) in a 30 cm bucket | 3.8 L | about 4 weeks |
| 1 US gallon spread over 1 m² | about 4 mm deep | about 2 days |
Outdoors in warm, sunny, windy weather, the same puddle can be gone in a few hours.
Water evaporation rate by temperature and humidity
Still water at the same temperature as the air, indoors (mm of depth per day; 1 mm over 1 m² = 1 litre):
| Air and water temperature | 30 % RH | 50 % RH | 70 % RH | 90 % RH |
|---|---|---|---|---|
| 10 °C | 1.5 | 1.1 | 0.6 | 0.2 |
| 15 °C | 2.1 | 1.5 | 0.9 | 0.3 |
| 20 °C | 2.8 | 2.0 | 1.2 | 0.4 |
| 25 °C | 3.8 | 2.7 | 1.6 | 0.5 |
| 30 °C | 5.1 | 3.7 | 2.2 | 0.7 |
(Calculated with the equation below, activity factor 0.5.) Use these for puddles, wet floors and open tanks; for a fan or breeze, multiply by about 2–3.
How the estimate is calculated
Evaporation from a water surface depends on the difference between the water vapour pressure at the surface and in the air. An engineering equation used for indoor pools by ASHRAE gives, in SI units:
evaporation (kg per m² per hour) ≈ 0.144 × (p_water − p_air) × F
- p_water = saturation vapour pressure at the water temperature (kPa)
- p_air = vapour pressure of the room air (kPa)
- F = activity factor; about 0.5 for undisturbed water
At 20 °C, p_water = 2.34 kPa. Air at 20 °C and 50 % RH has p_air = 1.17 kPa. So:
0.144 × 1.17 × 0.5 ≈ 0.084 kg per m² per hour, which is 0.084 mm of depth per hour, or about 2 mm per day.
To estimate any case: time (hours) ≈ depth (mm) ÷ 0.084, or for a volume, time ≈ litres ÷ (0.084 × surface area in m²).
This is an estimate. Evaporation cools the water a little below room temperature, which slows it; drafts, sunlight and warm floors speed it up.
What changes the speed
| Change | Effect on the 2 mm/day baseline |
|---|---|
| Warmer: 30 °C water and air, 50 % RH | about 1.8× faster |
| More humid air: 20 °C, 80 % RH | about 2.5× slower |
| Drier air: 20 °C, 30 % RH | about 1.4× faster |
| Fan or breeze across the surface | about 2–3× faster |
| Sunlight on the water | Much faster; sun heats the water |
| Larger surface, same volume | Proportionally faster; spread water thin |
Humidity matters more than most people expect: in a closed, damp room, the evaporating water raises the humidity, and drying slows down. That is why a dehumidifier speeds up drying even though it does not heat the water.
Outdoors: sun and wind change everything
Outdoors, sunlight heats the water and wind keeps replacing the humid air above it. On a warm, sunny, breezy day, open water typically loses 5–10 mm a day, and a thin puddle on warm pavement can disappear in an hour or two. On a cool, cloudy, still, humid day, the same puddle can last more than a day. The US National Weather Service measures this as pan evaporation; summer values in dry, hot regions are among the highest.
How long things take to dry
"How long does water take to dry" usually means a wet surface or object, not a pool of water:
| What's wet | Typical indoor drying time (20 °C, 50 % RH, some air movement) |
|---|---|
| Wiped countertop or tiled floor | Minutes to 1 hour |
| Mopped floor | 30 minutes to 2 hours |
| Splash on clothing | 1–3 hours |
| Wet carpet surface after extraction | 1–3 days, with fans and a dehumidifier |
| Wet drywall | Several days to weeks |
| Wet concrete slab | Weeks to months |
Surfaces dry at the evaporation rate; porous materials dry slower because water has to travel to the surface first.
Evaporation without heat: air drying
Drying at room temperature, with no heat added, is still evaporation. The energy comes from the water and its surroundings, which is why wet things feel cool. Air drying works as long as the surrounding air is drier than saturated: the drier the air, the faster it goes. That is the principle a dehumidifier uses in restoration drying: keep the air dry, and evaporation continues without heating.
Why water in materials takes longer
Free water on a surface follows the numbers above. Water soaked into carpet, plaster, wood or concrete must first move to the surface, so drying slows as the material dries out. Drywall and concrete can take days to weeks after the surface looks dry. See:
How to make water evaporate faster
- Spread it out: a thin layer over a large area dries far faster than a pool.
- Move the air: fans across the wet surface.
- Warm it: heat the room or the surface, if safe.
- Remove the moisture from the air: a dehumidifier keeps the room air dry so evaporation does not slow down. Closing windows in humid weather helps, because humid outdoor air slows drying.
- Mop up first: removing liquid water with a mop or wet vacuum saves hours or days of evaporation.
Use a mop or wet vacuum to remove standing water first, then fans and a dehumidifier to dry the remaining moisture in a closed, damp room.
Where the water goes
Every litre that evaporates indoors ends up in the room air. In a 50 m² room with a 2.5 m ceiling, evaporating just 1 litre can raise RH from 50 % to over 90 % at 20 °C if the air is not exchanged or dehumidified. On a large scale (water damage, wet floors after cleaning, pools, wet processes), this is the moisture load a dehumidifier must remove. See how much water a dehumidifier collects in a day and our industrial dehumidifiers.
Evaporation also absorbs heat: about 0.68 kWh per litre, which is why wet surfaces feel cold.
FAQ
Frequently asked questions
How long does it take for water to evaporate at room temperature?
About 2 mm of depth per day in still air at 20 °C and 50 % RH. A thin puddle dries in about a day.
How long does a puddle take to evaporate?
Indoors, roughly a day per 2 mm of depth. Outdoors in sun and wind, often a few hours.
How long does a drop of water take to evaporate?
A small drop takes roughly 10–30 minutes indoors; larger drops take an hour or more. Drops dry faster than the flat-surface rate suggests because vapour escapes from their edges.
How long does it take for 5 litres of water to evaporate?
In a bucket, about a month; spread 5 mm deep over 1 m², about 2–3 days indoors.
How long does it take for a gallon of water to evaporate?
In a 30 cm wide bucket indoors, about 4 weeks; spread over 1 m², about 2 days.
How long does it take for water to evaporate outside?
On a warm, sunny, breezy day, a thin puddle can go in an hour or two; open water loses roughly 5–10 mm a day.
What is the evaporation rate of water at room temperature?
About 2 mm a day (0.08 kg per m² per hour) in still air at 20 °C and 50 % RH.
Does water evaporate faster in low humidity?
Yes. Drier air has a lower vapour pressure, so water leaves the surface faster. Going from 80 % to 50 % RH at 20 °C makes it about 2.5 times faster.
Does water evaporate faster with a fan?
Yes, typically 2–3 times faster, because moving air replaces the humid layer above the surface.
Sources
- ASHRAE Handbook—HVAC Applications, Natatoriums (evaporation equation).
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- Perry's Chemical Engineers' Handbook, Psychrometry, evaporative cooling and solids drying.
- US Department of Energy, Consumer dehumidifiers.
- US National Weather Service, Pan evaporation.
