Concrete Evaporation Rate Calculator
Enter slab-level weather and concrete conditions to estimate evaporation in US and metric units. The calculator also estimates total water loss and solves for the maximum wind speed that meets your selected planning limit.
Evaporation rate (US)0.11
- Evaporation rate (metric)
- 0.53
- Evaporation risk band
- Caution: 0.10 to below 0.20 lb/ft2/hr
- Equivalent water loss
- 13.06
- Maximum wind speed for target
- 4.38
- Mitigation message
- Remeasure at the slab, reduce concrete temperature if practical, erect windbreaks, fog the air above the slab, use an evaporation reducer or begin curing promptly because the selected target is exceeded.
Rates are in lb/ft2/hr and kg/m2/hr; water loss is in US gallons.
Risk bands are planning bands, not guaranteed cracking boundaries.
Weather-station wind can differ substantially from wind measured about 20 inches above the slab.
How to use this calculator
- Enter air temperature, concrete temperature, relative humidity, and wind measured near the slab.
- Enter the exposed area and how long the conditions are expected to last before curing protection begins.
- Choose either the cautious 0.10 lb/ft2/hr target or the 0.20 lb/ft2/hr upper reference target.
- Review the risk band, equivalent water loss, maximum wind speed, and mitigation message together.
How the evaporation rate is calculated
This calculator uses the ACI 305R/Menzel evaporation-rate relationship. The inputs are first converted to metric units: air temperature and concrete temperature are converted from F to C, and wind speed is converted from mph to km/h. Relative humidity is used as a decimal.
The temperature and humidity term is:
D = (Tc + 18)^2.5 - r x (Ta + 18)^2.5
- Ta is air temperature in C.
- Tc is concrete temperature in C.
- r is relative humidity divided by 100.
- D is the temperature-humidity driving term.
The metric evaporation rate is then:
Ekg = max(0, 0.000005 x D x (V + 4))
V is wind speed in km/h. The US rate is Eus = Ekg x 0.204816, reported in lb/ft2/hr. Equivalent water loss multiplies the US rate by exposed area and exposure time, then divides by 8.3454 lb per US gallon.
How to read the risk band
The calculator labels rates below 0.10 lb/ft2/hr as lower, rates from 0.10 to below 0.20 as caution, and rates at or above 0.20 as high. These are planning bands. They are not guaranteed cracking boundaries because plastic shrinkage cracking also depends on bleed rate, mixture proportions, finishing timing, solar radiation, subgrade absorption, and curing practice.
Maximum wind speed for a target
For a selected target rate T, the wind-speed equation is inverted:
Vmax = T / (0.204816 x 0.000005 x D) - 4
The result is converted from km/h back to mph. If D is zero or negative, the reverse equation is undefined. If the target is already exceeded at zero wind, no nonnegative wind speed can meet that target under the entered temperatures and humidity.
What moves the result most
Concrete temperature, relative humidity, and wind speed usually move the answer the most. Hotter concrete raises the vapor-pressure term at the surface. Drier air and faster wind increase the drying demand. Large exposed areas do not change the rate, but they can make the total gallons lost much larger during a long finishing window.
What this calculator leaves out
The estimate does not model solar radiation, cloud cover, mixture bleed rate, cementitious materials, admixtures, evaporation reducers, surface crusting, delayed finishing, or curing effectiveness. Use measurements taken at the slab whenever possible; airport, rooftop, or phone-weather values may not describe the actual pour conditions.
Worked example
Suppose the air temperature is 80 F, concrete temperature is 85 F, relative humidity is 50%, and wind speed measured above the slab is 5 mph. The converted values are about 26.7 C, 29.4 C, and 8.05 km/h.
The temperature-humidity term is about 8,837.7, so the metric rate is 0.000005 x 8,837.7 x (8.05 + 4) = 0.532 kg/m2/hr. Converted to US units, the evaporation rate is about 0.109 lb/ft2/hr, which falls in the caution band.
For 1,000 ft2 exposed for one hour, equivalent water loss is 0.109 x 1,000 x 1 / 8.3454, or about 13.1 gallons. With a 0.10 lb/ft2/hr target, the maximum wind speed is about 4.38 mph, so the entered 5 mph wind exceeds that target.
Common questions
What evaporation rate can cause plastic shrinkage cracking?
Many hot-weather concreting references use 0.10 lb/ft2/hr as a cautious planning level and 0.20 lb/ft2/hr as a higher reference level for severe drying. Cracking is not controlled by evaporation rate alone. Bleed rate, set time, finishing, solar exposure, base absorption, and curing timing all matter.
How is the ACI concrete evaporation rate calculated?
The method estimates the drying demand from concrete temperature, air temperature, relative humidity, and wind speed. This calculator computes the metric Menzel equation, clamps negative evaporation to zero, and converts the result to lb/ft2/hr. It also multiplies the rate by slab area and time to show equivalent gallons lost.
Where should wind speed be measured above a concrete slab?
Use wind measured near the evaporating surface when possible, commonly about 20 inches above the slab. Airport or rooftop wind observations can be much higher or lower than the wind around forms, walls, equipment, and windbreaks at the pour. Remeasure when site conditions change.
Why does concrete temperature matter more than air temperature?
The concrete surface supplies the evaporating water, so its temperature strongly affects vapor pressure at the surface. Warm concrete under cooler or dry air can still evaporate quickly. Cooling aggregates, mixing water, or delivered concrete can reduce the calculated rate before placement.
Can water lost to evaporation simply be added back to the surface?
No. The equivalent gallons output describes drying demand; it is not a recommendation to add water to the surface. Adding water during finishing can weaken the surface and increase defects, so use curing, fogging, windbreaks, evaporation reducers, or project-approved procedures instead.
When should mitigation begin?
Mitigation should begin before the surface dries enough to disrupt finishing or curing. If the entered conditions exceed the project target, plan windbreaks, fogging, evaporation reducer, temperature control, and prompt curing before placement starts. Follow the project specification and the responsible engineer's hot-weather concreting plan.