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.
PourDay calculates evaporation rate automatically using live weather data — no manual inputs needed.
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.
ACI 305R — the American Concrete Institute's guide for hot weather concreting — sets 0.2 lb/ft²/hr as the evaporation rate at which precautions are necessary for conventional concrete. With modern blended cements like Type 1L that produce less bleed water, ACI guidance drops that trigger to 0.1 lb/ft²/hr, and many experienced contractors treat it as their working caution level. The calculator above scores the same way the PourDay app does — a conservative band between the two ACI numbers: below 0.15 is GO, 0.15–0.20 is CAUTION, and 0.20 or above is WARNING.
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, CAUTION, or WARNING 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. Temperature is only half the story — see how the concrete curing temperature range affects strength once the slab is down.
Enter four numbers from your job site — air temperature, relative humidity, wind speed at slab level, and concrete temperature. The calculator runs the ACI 305R formula and returns the rate in lb/ft²/hr with a GO, CAUTION, or WARNING decision.
ACI 305R calls for precautions at 0.2 lb/ft²/hr for conventional concrete — and at 0.1 lb/ft²/hr for low-bleed blended-cement mixes like Type 1L. PourDay starts its CAUTION band at a conservative 0.15; at or above 0.20, plastic shrinkage cracking risk is high and the pour should be mitigated or postponed.
Yes — PourDay is a free evaporation rate calculator app for iOS and Android that runs this calculation automatically using live weather for your exact job site.
Completely free, no signup. The PourDay app is also free for automatic, real-time calculations in the field.
Go deeper on the topics behind the calculator.
The formula, the variables, and the thresholds that matter.
Read articleThe standard behind this calculator — broken down for the field.
Read articleWhat happens when evaporation rate exceeds the threshold — and how to prevent it.
Read article