Four job-site numbers, one formula, one decision. Here is exactly how the calculation works — with two worked examples you can check by hand — and a free calculator that does it for you.
ACI 305R estimates surface evaporation with a formula developed from Carl Menzel's work in the 1950s — the same math behind the paper nomograph chart generations of QC techs learned to read:
E = ( Tc2.5 − r × Ta2.5 ) × ( 1 + 0.4V ) × 10⁻⁶
The physics in one sentence: warm concrete pushes moisture out (the Tc term), humid air pushes back (the r × Ta term), and wind sweeps the moist boundary layer away so the process never slows down (the 1 + 0.4V multiplier). That last term is why wind is the variable that catches crews out — every 1 mph of wind adds 40% of the calm-air rate. A 10 mph breeze multiplies evaporation five times over.
1. Get your four inputs. Air temperature and relative humidity from a weather source for your job site. Wind speed at slab level — about 20 inches up, not the airport reading. Concrete temperature from the truck ticket, a probe thermometer, or your supplier's target (typically 65–80°F).
2. Raise both temperatures to the 2.5 power. This is the step that made the nomograph necessary in 1955 and makes a calculator necessary now — nobody computes 852.5 in their head on a tailgate.
3. Subtract the humidity-scaled air term, apply the wind multiplier, shift the decimal six places. The result is your evaporation rate in lb/ft²/hr.
4. Compare against the thresholds. ACI 305R calls for precautions at 0.2 lb/ft²/hr for conventional concrete, 0.1 for low-bleed blended mixes like Type 1L. PourDay scores it conservatively: below 0.15 is GO, 0.15–0.20 is CAUTION, 0.20 and above is WARNING.
Air 85°F, humidity 50%, wind 10 mph, concrete 75°F.
Well under 0.15 — a GO. Standard curing practice applies (if you're pouring a low-bleed Type 1L mix, note you're close to that 0.1 trigger).
Air 90°F, humidity 20%, wind 15 mph, concrete 80°F. Not obviously worse than example 1 — five degrees hotter, drier, breezier.
Nearly four times example 1 and well past the ACI threshold — a hard WARNING. This is plastic shrinkage cracking territory: postpone, or pour early with fogging, windbreaks, and evaporation retarder staged. The two examples are the whole argument for running the number instead of eyeballing the forecast — the difference between them is invisible from inside a truck.
The SI version of the same thresholds: 1 lb/ft²/hr ≈ 4.88 kg/m²/h, so ACI's 0.2 lb/ft²/hr precaution level is the familiar 1.0 kg/m²/h, and the 0.1 low-bleed trigger is 0.5 kg/m²/h.
Nobody runs exponents at 5am. The free PourDay evaporation rate calculator takes the same four inputs and returns the rate with a GO / CAUTION / WARNING call — same number the nomograph gives, no chart. And the PourDay app removes even the data entry: it pulls live weather for your saved job sites and recalculates all day, so you see the morning window closing before you're standing in it. For the deeper theory, the complete evaporation rate guide and ACI 305R explained pick up where this leaves off.
Run this calculation automatically, every morning, for your exact job site. PourDay is free on iOS and Android.
What each variable does, how they interact, and why wind is the one that sneaks up on you.
Read articleThe standard behind the formula — when it applies and what it requires on the job.
Read articleWhat happens when evaporation outruns bleed water — and how to prevent it on every pour.
Read articleReal-time evaporation rate. 16-day forecast. Pour logging. Free for contractors.