Concrete evaporation rate calculator (ACI 305R)

Calculate your concrete evaporation rate using the ACI 305R formula. Enter your job site conditions below to find out whether it is safe to pour — and reduce your risk of plastic shrinkage cracking in ready mix concrete.

What is concrete evaporation rate?

Concrete evaporation rate measures how fast moisture leaves the surface of fresh concrete, expressed in pounds per square foot per hour (lb/ft²/hr). When evaporation outpaces the rate at which bleed water rises to the surface, the top layer dries and shrinks while the mass below stays wet — causing plastic shrinkage cracks. Monitoring evaporation rate is the most reliable way to predict and prevent these cracks before they form. Read our full evaporation rate guide for a deeper breakdown.

The ACI 305R threshold explained

ACI 305R — the American Concrete Institute's guide for hot weather concreting — sets 0.15 lb/ft²/hr as the evaporation rate above which precautions are necessary. Above this threshold, the risk of plastic shrinkage cracking increases significantly. With modern blended cements like Type 1L that produce less bleed water, many experienced contractors treat 0.10 lb/ft²/hr as their caution threshold. The calculator above uses conservative thresholds: below 0.10 is GO, 0.10–0.20 is CAUTION, and above 0.20 is NO GO.

What factors affect evaporation rate?

Four variables determine concrete evaporation rate: air temperature, relative humidity, wind speed at slab level, and concrete temperature. No single variable tells the full story — a hot day with high humidity and no wind can have a lower rate than a mild day with low humidity and high wind. The ACI 305R formula combines all four into a single number so you can make a clear GO or NO GO decision before your ready-mix truck leaves the yard. If conditions call for action, learn when to use a retarder vs accelerator and how to protect your curing process.