Evaporation Rate Calculator
How fast does water evaporate? Two engineering models in one free tool: the Rohwer formula for open water (pools, ponds, lakes, tanks) in mm/day and liters/day, and the ACI 308 nomograph for fresh concrete in kg/m²/h — with a plastic-shrinkage cracking risk warning. Metric and imperial units supported.
Units:
Use the water temperature, not the air temperature.
Measured just above the surface. 1 m/s ≈ 2.24 mph.
Pool, pond or tank surface area.
Evaporation rate
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Fresh concrete temperature at the surface.
10 mph ≈ 16 km/h.
Evaporation rate —
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⚠ Evaporation exceeds 1.0 kg/m²/h — high risk of plastic shrinkage cracking. Take precautions: fog spray, windbreaks, evaporation retarders, or reschedule the pour.
How the calculation works
1. Open water — Rohwer formula
E = 0.484 × (1 + 0.6 × V) × (es − ea)- E — evaporation rate in mm/day
- V — wind speed just above the surface, in m/s
- es — saturation vapor pressure at the water temperature, in kPa, from the Tetens equation:
es = 0.6108 × exp(17.27 × T / (T + 237.3)) - ea — actual vapor pressure =
RH/100 × es
The engine is the vapor-pressure deficit (es − ea): dry air pulls water off the surface, wind replaces the humid boundary layer, and total volume is simply E (mm/day) × area (m²) = liters/day, because 1 mm of depth over 1 m² is exactly 1 liter.
Worked example — Rohwer
Pond: water 25 °C, RH 50%, wind 2 m/s, area 100 m².
- es = 0.6108 × exp(17.27 × 25 / 262.3) = 3.168 kPa
- ea = 0.50 × 3.168 = 1.584 kPa; deficit = 1.584 kPa
- E = 0.484 × (1 + 0.6 × 2) × 1.584 = 0.484 × 2.2 × 1.584 = 1.687 mm/day
- Total = 1.687 × 100 = 168.7 liters/day (≈ 44.6 US gallons/day)
2. Fresh concrete — ACI 308 nomograph
ER = 5 × [(Tc + 18)2.5 − r × (Ta + 18)2.5] × (V + 4) × 10−6- ER — evaporation rate in kg/m²/h
- Tc — concrete temperature (°C), Ta — air temperature (°C)
- r — relative humidity as a decimal (50% → 0.5)
- V — wind speed in km/h
Worked example — ACI 308
Slab pour: concrete 30 °C, air 25 °C, RH 50%, wind 16 km/h.
- (30 + 18)2.5 = 482.5 = 15,963; (25 + 18)2.5 = 432.5 = 12,123
- Bracket = 15,963 − 0.5 × 12,123 = 9,902
- ER = 5 × 9,902 × (16 + 4) × 10−6 = 0.99 kg/m²/h
- Just under the 1.0 threshold — moderate risk: have precautions ready.
Reference tables
Saturation vapor pressure (Tetens)
| Water temp | es (kPa) | Water temp | es (kPa) |
|---|---|---|---|
| 0 °C (32 °F) | 0.611 | 25 °C (77 °F) | 3.168 |
| 5 °C (41 °F) | 0.872 | 30 °C (86 °F) | 4.243 |
| 10 °C (50 °F) | 1.228 | 35 °C (95 °F) | 5.623 |
| 15 °C (59 °F) | 1.705 | 40 °C (104 °F) | 7.376 |
| 20 °C (68 °F) | 2.338 |
Wind effect on evaporation (25 °C water, 50% RH)
| Wind speed | Evaporation rate | Loss per 100 m² |
|---|---|---|
| 0 m/s (calm) | 0.767 mm/day | 77 L/day |
| 1 m/s | 1.227 mm/day | 123 L/day |
| 2 m/s | 1.687 mm/day | 169 L/day |
| 3 m/s | 2.146 mm/day | 215 L/day |
| 5 m/s (breezy) | 3.066 mm/day | 307 L/day |
| 8 m/s | 4.446 mm/day | 445 L/day |
| 10 m/s (windy) | 5.366 mm/day | 537 L/day |
ACI 308 plastic-shrinkage risk levels
| Evaporation rate | Risk | Action |
|---|---|---|
| < 0.5 kg/m²/h | Low | Normal curing practice. |
| 0.5 – 1.0 kg/m²/h | Moderate | Precautions ready: fog spray, windbreaks. |
| ≥ 1.0 kg/m²/h | High | High cracking risk — evaporation retarders, reschedule pour. |
Example: concrete 30 °C / air 25 °C / 50% RH gives 0.198 kg/m²/h at calm, 0.99 at 16 km/h, and 1.44 (HIGH) at 25 km/h — wind is the dominant risk lever on site.
Factors that affect the evaporation rate
- Temperature — warm water has a much higher saturation vapor pressure (roughly doubles every 10 °C), so hot days evaporate far more.
- Relative humidity — dry air creates a large vapor-pressure deficit; at 100% RH net evaporation stops.
- Wind speed — strips the humid boundary layer; the single biggest lever for both pools and concrete.
- Surface area — rate per m² is fixed by weather, but total liters lost scale with area.
- Sunlight and shading — solar heating raises surface temperature and the deficit.
- Water temperature vs air temperature — use the water/surface temperature in the formula, not the air temperature.
Scope boundary — what this tool does and doesn't cover
This calculator estimates evaporation rates: open-water loss via the Rohwer model and fresh-concrete evaporation via the ACI 308 nomograph. It does not compute humidity ratios, dew points, or air density — those live in dedicated tools:
- Air Density Calculator — moist-air density from temperature, pressure and humidity.
- Concrete Calculator — volume, bags and mix quantities for a pour (pairs with the ACI 308 evaporation mode above).
- Aquarium Calculator — tank volume and stocking; use the Rohwer mode here to estimate top-off water for open-top tanks.
Frequently asked questions
What is evaporation rate?
Evaporation rate is how much water leaves a surface per unit of time. For open water it is usually expressed in millimeters per day (mm/day) — the depth of water lost from the surface — and for fresh concrete in kilograms per square meter per hour (kg/m²/h).
How do I calculate the evaporation rate of water?
This calculator uses the Rohwer formula: E = 0.484 × (1 + 0.6 × V) × (es − ea), where V is wind speed in m/s, es is the saturation vapor pressure at the water temperature, and ea is the actual vapor pressure from relative humidity. Enter water temperature, humidity, wind speed and surface area above to get the answer in mm/day and liters/day.
What is the Rohwer evaporation formula?
The Rohwer formula estimates open-water evaporation as E = 0.484 × (1 + 0.6 × V) × (es − ea), with E in mm/day, wind speed V in m/s, and the vapor-pressure deficit (es − ea) in kPa. Saturation vapor pressure es comes from the Tetens equation: es = 0.6108 × exp(17.27 × T / (T + 237.3)).
How do you calculate the evaporation rate for fresh concrete?
This calculator uses the ACI 308 nomograph (Menzel) formula: ER = 5 × [(Tc + 18)^2.5 − r × (Ta + 18)^2.5] × (V + 4) × 10^-6, where Tc is concrete temperature and Ta is air temperature in °C, r is relative humidity as a decimal, and V is wind speed in km/h. The result is in kg/m²/h.
What evaporation rate is dangerous for fresh concrete?
An evaporation rate above 1.0 kg/m²/h on fresh concrete is the ACI 308 warning threshold: water leaves the surface faster than bleed water can replace it, which causes plastic shrinkage cracking. Rates of 0.5–1.0 kg/m²/h call for precautions, and rates below 0.5 are generally low risk.
How fast does pool water evaporate per day?
A typical outdoor pool loses about 2–6 mm per day, depending on weather. For example, water at 25 °C with 50% relative humidity and a 2 m/s breeze evaporates at about 1.7 mm/day; a 100 m² pool surface therefore loses roughly 169 liters per day. Try the pool evaporation mode above with your own numbers.
Does wind increase the evaporation rate?
Yes. Wind sweeps humid air away from the surface and replaces it with drier air, keeping the vapor-pressure deficit high. In the Rohwer formula the wind factor is (1 + 0.6 × V): doubling wind speed from 2 to 5 m/s roughly doubles the evaporation rate.
Why does low humidity speed up evaporation?
Evaporation is driven by the difference between the saturation vapor pressure (es) and the actual vapor pressure (ea). Low relative humidity means ea is small, so the deficit (es − ea) is large and water escapes faster. At 100% relative humidity the deficit is zero and net evaporation stops.
How many liters evaporate from one square meter per millimeter of evaporation?
Exactly one liter. One millimeter of depth over one square meter is one liter (1 mm × 1 m² = 1 L), so total water loss in liters per day is simply the evaporation rate in mm/day multiplied by the surface area in m².
What is the difference between evaporation and evapotranspiration?
Evaporation is water loss from open surfaces such as pools, ponds and lakes. Evapotranspiration adds the water that plants release through transpiration, so it applies to fields, crops and watersheds. This calculator covers open-surface evaporation only; irrigation planning usually needs evapotranspiration models such as Penman-Monteith.
Can the evaporation rate be zero or negative?
Yes, it can be zero: at 100% relative humidity the vapor-pressure deficit is zero, so net evaporation stops. Apparent negative values can occur when humid air deposits moisture (dew/condensation); this calculator reports those as approximately zero.
How accurate is the Rohwer evaporation formula?
Rohwer is an empirical wind-and-vapor-deficit model that works well for open water surfaces in typical weather. Like all empirical formulas it is an estimate: local shading, water depth, salinity and gusts are not modeled. For reservoir management, calibrate against local pan-evaporation measurements.
How do I reduce evaporation from a pool or pond?
The most effective measures are a pool cover (cuts losses by 90% or more), windbreaks such as fences or hedges, and lowering water temperature. Higher humidity cannot be controlled outdoors, but shading the surface reduces the saturation vapor pressure and therefore the evaporation rate.
What units does the ACI 308 concrete evaporation rate use?
The ACI 308 formula takes concrete and air temperature in °C and wind speed in km/h, and gives the evaporation rate in kg/m²/h. In imperial mode this calculator converts the result to lb/ft²/h (1 kg/m²/h ≈ 0.2048 lb/ft²/h) and the 1.0 kg/m²/h plastic-shrinkage warning still applies.