Concrete Evaporation Rate Calculator

1. Pick US or metric units, then enter the air temperature and relative humidity measured at the site.
2. Enter the concrete temperature from a thermometer in the fresh mix, and the wind speed over the surface.
3. Read the evaporation rate and the colored band, then compare it with the 0.1 and 0.2 lb/ft2/h levels before you place or finish.

Here is what each result means:

The concrete evaporation rate is the estimated mass of water that leaves each unit of fresh concrete surface per hour, written in pounds per square foot per hour (lb/ft2/h) or kilograms per square metre per hour (kg/m2/h). It matters because plastic shrinkage cracks form when surface moisture evaporates faster than rising bleed water can replace it.

Contractors check it on hot, dry or windy days, before a slab, deck or pavement is placed and again while it is being finished. It is a short-term placement check, not a measure of strength, curing time or mix quality. The rate describes the first hours while the concrete is still plastic; it does not say how long to cure or what water-cement ratio to use.

E is the evaporation rate in lb/ft2/h, Tc is the concrete (water surface) temperature in degrees Fahrenheit, Ta is the air temperature in degrees Fahrenheit, r is the relative humidity divided by 100, and V is the wind speed in miles per hour. The calculator converts metric entries to these units first, so the result is the same in either unit system.

1. Convert temperatures to deg F and wind to mph if you chose metric.
2. Raise the concrete temperature and the air temperature to the power 2.5, and weight the air term by the humidity fraction.
3. Subtract the air term from the concrete term, multiply by (1 + 0.4 V), then multiply by one millionth.
4. Multiply by 4.88243 to give kg/m2/h, and compare with the 0.1 and 0.2 lb/ft2/h levels.

The same source also prints a metric version, E = 5 ([Tc + 18]^2.5 – r [Ta + 18]^2.5)(V + 4) x 10^-6, with temperatures in deg C and wind in km/h. For the cases we checked it lands within about 4 percent of the English-units form, so this tool uses one form for both unit systems rather than letting the answer drift when you switch.

Suppose the concrete and the air are both 80 F, the relative humidity is 50% and the wind is 10 mph.

Calculation: 80^2.5 = 6,400 x 8.944 = 57,243. The air term is 0.5 x 57,243 = 28,622, so the difference is 57,243 – 28,622 = 28,622. The wind factor is 1 + 0.4 x 10 = 5. E = 28,622 x 5 x 10^-6 = 0.143 lb/ft2/h, which is 0.699 kg/m2/h.

That is 72 percent of the 0.2 lb/ft2/h level and sits in the 0.1 to 0.2 band where plastic shrinkage cracking may occur. Dropping the wind to 5 mph brings the rate to 0.086, below the 0.1 level, which shows how much a windbreak can matter.

Four inputs drive the number, and small changes in any one of them can move it a long way. The table starts from the example above (0.143 lb/ft2/h) and changes one input at a time.

Wind Speed

Wind scales the whole result through the (1 + 0.4 V) term, so doubling the wind from 5 to 10 mph raises the rate by two thirds in this example. A windbreak is one of the measures the NRMCA sheet lists.

Relative Humidity

Drier air pulls more water from the surface. Falling from 50 to 30 percent humidity raises the rate by about 40 percent with nothing else changed.

Concrete Temperature

Hot concrete evaporates faster. Cooling the mix, or shading the subgrade and forms, lowers the first term of the formula directly.

Air Temperature

By this formula, warmer air at the same humidity holds more moisture, so on its own it lowers the rate. In practice hot air also warms the concrete, so enter both readings you actually measure.

If you are using a fast-setting product, follow its data sheet and see the fast setting concrete calculator for quantities. For a tall pour, the concrete wall calculator sizes the volume so you can plan crews and timing.

Three nearby calculators answer different questions, and mixing them up is a common source of confusion.

A mix can have an excellent water-cement ratio and a long curing plan and still crack in the first hour if the surface dries too fast. The evaporation rate is the early warning; curing starts afterwards and covers a different period.

Planning a Hot-weather Pour

Before ordering, run the forecast conditions through the tool. If the result is near or above 0.2 lb/ft2/h, schedule the pour for early morning or evening, and line up windbreaks, sunshades or fog sprays.

Deciding When to Start Protection

On the day, re-enter the actual readings. Conditions change through the day, so a rate that was fine at 7 am can be well over the level by noon.

Reading a Specification

Many project specifications set an evaporation limit. The tool shows the same figure in lb/ft2/h and kg/m2/h so you can compare it with the limit your project uses. If the mix itself is the question, start with the concrete mix calculator, and size the pour with the concrete calculator.

1. Using the Air Temperature for the Concrete

The formula needs the concrete temperature as well. Fresh concrete on a hot day can be well above or below the air, and the difference changes the answer a lot.

2. Using a Forecast Wind Instead of Site Wind

A forecast or distant station may not match conditions over your slab. Measure at the surface where you are working.

3. Treating the Result as a Crack Prediction

The rate is an estimate of water loss from a free water surface. A low number does not guarantee no cracks, and a high one does not guarantee them.

4. Checking Only Once

Humidity, wind and sun change during a pour. Re-check when conditions shift and as finishing continues.

5. Mixing Up lb/ft2/h and kg/m2/h

The two units differ by a factor of about 4.88. The 0.2 lb/ft2/h level equals 0.98 kg/m2/h, which is why sources write it as about 1.0 kg/m2/h.

6. Skipping Protection Because the Rate Is Below the Level

Mixes with low bleeding, such as those with silica fume or a low water content, can crack at lower rates. Follow your mix supplier and specification as well as the number.

The arithmetic is exact for the readings you enter; the figure it produces is an estimate of water loss, not a forecast of cracking.

Cited guidance treats more than 0.2 lb/ft2/h (about 1.0 kg/m2/h) as the level where plastic shrinkage cracks are expected, and 0.1 to 0.2 lb/ft2/h as the range where they may occur. One FHWA report calls 0.5 kg/m2/h a condition that warrants caution.

E = (Tc^2.5 – r x Ta^2.5)(1 + 0.4 V) x 10^-6, with temperatures in deg F, wind in mph and r as humidity divided by 100. It gives lb/ft2/h and is the graphical nomograph written as an equation.

The first term of the formula depends on the concrete temperature raised to the power 2.5, so a warmer surface drives much more water vapor into the air. In the example, raising the concrete from 80 to 90 F lifts the rate from 0.143 to 0.241 lb/ft2/h.

Yes. Mixes that bleed little, such as those with high cementitious content, high fines, reduced water or silica fume, can crack at lower rates. The Caltrans manual notes the threshold for precautions can be much lower for modern concrete.

Erect temporary windbreaks, dampen the subgrade and forms, use fog sprays, and cover the surface between finishing operations, as the NRMCA sheet lists. The Caltrans manual also lists sunshades and cooling the mix in extreme conditions.

Yes. It converts metric entries to deg F and mph, applies one formula, then converts the result, so switching units changes only the units. The published metric version of the formula differs from it by about 4 percent in our checks.

The NRMCA sheet lists wind over 5 mph among the conditions that raise evaporation. In the example here, going from 5 to 15 mph more than doubles the rate, from 0.086 to 0.200 lb/ft2/h.

The formula can go negative when the air term exceeds the concrete term, for example cool concrete in warm, humid air. A negative rate would mean condensation rather than drying, so the tool shows zero and the good band.

Measure air temperature and humidity at the placement, concrete temperature in the mix, and wind at the surface. Handheld weather meters are used for this, as the FHWA brief notes.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• NRMCA CIP 5, Plastic Shrinkage Cracking (National Ready Mixed Concrete Association; causes and prevention).
• FHWA Tech Brief, Curing Concrete Paving Mixtures (FHWA-HIF-18-015; the 0.20 lb/ft2/hr critical rate and the Menzel nomograph).
• FHWA, evaporation and plastic shrinkage (FHWA; the 1.0 and 0.5 kg/m2/h levels and the limits of the nomograph).
• Caltrans Bridge Construction Records and Procedures Manual, BCM 90-6.0 (California DOT; plastic cracking precautions).
• How to Prevent Plastic Shrinkage Cracks (Arizona State University guest post; the formula in English and metric units).