Check Evaporation Conditions Before The Pour
Estimate fresh-concrete surface evaporation using air temperature, concrete temperature, relative humidity and wind speed. Compare weather scenarios, estimate surface moisture loss and choose a more favorable pour window before placement begins.
Use measured or forecast conditions for planning. Results remain closed until you press Calculate.
Estimate fresh-concrete surface evaporation using the temperature, humidity and wind variables referenced by ACI hot-weather guidance.
Use wind at/near the concrete surface when possible.
Planning estimate only. Hot-weather procedures, acceptance limits, mixture requirements, curing and field decisions must follow the project specifications and qualified concrete professionals.
Estimate the theoretical quantity of surface water represented by an evaporation rate over a known area and duration.
Planning estimate only. Hot-weather procedures, acceptance limits, mixture requirements, curing and field decisions must follow the project specifications and qualified concrete professionals.
Compare current jobsite conditions with a proposed lower-temperature, higher-humidity or lower-wind scenario.
Planning estimate only. Hot-weather procedures, acceptance limits, mixture requirements, curing and field decisions must follow the project specifications and qualified concrete professionals.
Enter three expected jobsite condition sets and compare estimated evaporation rates to identify the lowest calculated exposure window.
Planning estimate only. Hot-weather procedures, acceptance limits, mixture requirements, curing and field decisions must follow the project specifications and qualified concrete professionals.
A Concrete Hot Weather Calculator is a planning tool for conditions that can accelerate moisture loss from freshly placed concrete and make batching, transport, placing, finishing and curing more demanding. Hot-weather concreting is not defined by air temperature alone. A hot day with high humidity and little wind may create a different evaporation condition than a somewhat cooler day with very dry air and strong wind. Fresh concrete temperature matters as well.
The most useful calculation for flatwork planning is the estimated surface evaporation rate. ACI guidance uses air temperature, concrete temperature, relative humidity and wind velocity as the key environmental variables for estimating evaporation. This page also converts that rate into an area-and-time moisture-loss estimate and lets you compare multiple scenarios before committing to a pour schedule.
Fresh concrete temperature directly affects estimated evaporation and hydration rate.
Wind across the slab surface can substantially increase moisture loss.
Dry air generally increases the vapor-pressure difference driving evaporation.
Air temperature, solar exposure and changing site conditions influence the pour plan.
The main calculator uses the environmental inputs associated with the ACI hot-weather evaporation procedure. Internally, Fahrenheit temperatures are converted to Celsius and mph wind speed is converted to km/h. The calculator then applies the Uno-style evaporation relationship referenced in ACI hot-weather specifications.
E = estimated evaporation in kg/m²/h; Tc = concrete temperature °C; Ta = air temperature °C; r = relative humidity as a decimal; V = wind speed in km/h.
The result is also converted to pounds per square foot per hour. Because the formula is an estimate, real jobsite conditions should be measured as close as practical to the concrete surface and at the time of placement. Conditions can change rapidly as cloud cover, wind, shade and temperature change.
ACI's current hot-weather guide explains that high ambient temperature, low humidity, high wind, high concrete temperature, or combinations of these can negatively affect mixing, transporting, placing and early curing. The goal of the calculator is to make those combined inputs easier to evaluate before the truck arrives.
Do not interpret one number as a universal safe/unsafe boundary. Project specifications may define their own trigger values, measurement method and required evaporation-control procedures. Historically, 1.0 kg/m²/h (about 0.20 lb/ft²/h) has been widely cited as a level associated with plastic-shrinkage precautions, but ACI literature also discusses research showing cracking can occur at lower evaporation rates. That is why this calculator labels the result as a planning concern rather than a structural approval.
| Estimated Rate | Planning Interpretation | What to Review |
|---|---|---|
| < 0.50 kg/m²/h | Lower calculated evaporation | Still follow curing and placement requirements; conditions can change. |
| 0.50–0.99 kg/m²/h | Elevated planning concern | Review wind, concrete temperature, crew readiness and evaporation-control measures. |
| ≥ 1.00 kg/m²/h | High calculated evaporation | Do not rely on the calculator alone; confirm project-specific procedures and mitigation. |
These bands are intentionally described as planning labels, not code acceptance limits. Concrete mixture proportions, bleed rate, member geometry, finishing operations and curing procedures also influence cracking risk.
Low humidity, wind and high concrete temperature can increase evaporation from the exposed surface. If surface water disappears too quickly while the concrete is still plastic, the slab can become more vulnerable to plastic-shrinkage cracking.
Higher concrete temperatures can accelerate hydration and shorten the practical time available for transport, placement, consolidation and finishing. Crews may have less time to correct delays or equipment problems.
Finishers need to distinguish normal bleed-water behavior from rapid surface drying. Adding water to the surface simply to make finishing easier can create quality problems and should not be treated as a substitute for proper hot-weather procedures.
Curing should not be an afterthought. Materials, equipment and labor for the specified curing method need to be ready before concrete placement starts, particularly when drying conditions are severe.
Use a representative jobsite reading rather than a distant regional temperature when possible.
Use the fresh concrete temperature expected or measured near discharge and placement.
Use relative humidity for the pour period. Very dry air can increase evaporation.
Wind at the concrete surface is more useful than a remote weather-station value.
Review the result with project limits, mitigation procedures, labor and curing readiness.
The appropriate response depends on the project specification, mixture, equipment and environmental conditions. Common planning measures discussed in ACI hot-weather guidance include reducing concrete temperature, shortening haul or waiting time, improving placement logistics, protecting materials from excessive heat, scheduling for more favorable conditions and preparing curing measures in advance.
The second calculator multiplies the estimated evaporation rate by the exposed concrete area and duration. Because 1 kilogram of water is approximately 1 liter, the result can be expressed as a theoretical number of liters of surface moisture represented by the estimated evaporation rate.
This number is useful for visualizing how environmental conditions scale across a large slab, but it is not a recommendation to add that amount of water to the concrete. Do not add calculated evaporation loss back into the mix or onto the slab surface. Concrete water content, water-cementitious ratio, curing and finishing procedures must follow the approved mixture and project requirements.
The Pour Window calculator compares three sets of conditions. It is particularly useful when a contractor can choose between an early-morning, midday or later-afternoon placement. The best window is not automatically the coolest air temperature: concrete temperature, relative humidity and wind can change the ranking.
For example, a cooler afternoon with very strong wind and low humidity can produce a higher calculated evaporation rate than a warmer but calmer morning. Enter realistic values for each proposed period, then confirm them closer to placement. A weather forecast is only a planning input; the field crew should use actual jobsite measurements when the specification requires them.
Pour timing should also consider access, truck dispatch, pump availability, crew size, finishing duration and curing setup. Use the Concrete Pour Time Calculator to estimate placement duration and the Concrete Mixer Size Calculator when the project uses site mixing rather than ready-mix delivery.
Large exposed flat surfaces can be especially sensitive to evaporation because a broad surface is open to sun and wind. For pool-deck quantity planning, see the Concrete Pool Deck Calculator.
Long placements may span several hours, so environmental conditions can change during the pour. Coordinate truck spacing, placement rate, finishing and curing so later loads do not create avoidable delays.
Surface evaporation remains relevant, but formwork and geometry change exposure compared with an open slab. Temperature can still affect workability, setting and logistics.
Small-volume placements can heat and dry quickly, and crews sometimes underestimate the setup time required. For tapered repair quantities, the Concrete Wedge Calculator can help estimate volume before hot-weather scheduling.
One of the most common mistakes in hot-weather planning is entering the air temperature as the concrete temperature. Fresh concrete can be warmer or cooler than the surrounding air depending on constituent temperatures, plant operations, haul time, solar exposure and cooling measures. The evaporation equation treats these as separate inputs because the temperature of the fresh concrete surface affects vapor pressure.
If the concrete temperature is unknown during early planning, obtain an expected discharge temperature from the ready-mix supplier rather than assuming it will equal the weather forecast. For specification-controlled work, use the required test method and measurement timing.
Hot-weather concrete planning also needs a worker-safety plan. Concrete placement, screeding, finishing and pumping can involve sustained physical effort, protective equipment and direct sun exposure. OSHA's heat resources emphasize access to cool drinking water, rest and shade, and additional electrolyte-containing fluids for longer work periods. Heat illness can progress quickly, so crews should know the symptoms and emergency response procedures.
The concrete evaporation result is not a worker heat-stress index. A low concrete evaporation rate does not mean working conditions are safe for people. Worker heat exposure must be evaluated separately using the employer's safety program, applicable regulations and site conditions.
ACI PRC-305-20 — Guide to Hot Weather Concreting · ACI SPEC-305.1-14(20) · OSHA — Water, Rest, Shade
Good calculations depend on good inputs. For early planning, forecast data can help identify potentially difficult periods, but the values used for field decisions should match the measurement procedures required by the project. Air temperature should represent the placement area rather than a shaded office or a distant weather station. Relative humidity should be recorded for the same general location and time. Wind is especially sensitive to local obstructions, so the exposed slab surface may experience very different air movement than a regional forecast suggests.
Fresh concrete temperature should be treated as its own measurement. Concrete arriving from a ready-mix plant carries the thermal history of its cement, water, aggregates, truck drum and haul. A hot aggregate stockpile or long haul in strong sun can influence the delivered temperature, while chilled water, ice or cooled materials can reduce it when the mixture and production plan allow. For controlled work, use the specified fresh-concrete temperature test method rather than estimating from touch or ambient air.
A large slab may begin under calm morning conditions and finish in hotter, drier, windier weather. One set of readings taken before the first truck may not describe the entire placement. If the pour lasts several hours, repeat the weather and concrete-temperature checks at the frequency required by the project or whenever conditions noticeably change. The Pour Window calculator can also be used before placement to compare expected periods and decide when the most demanding conditions are likely to occur.
Hot-weather problems often become worse when concrete waits. A well-planned delivery interval helps keep placement continuous without creating a queue of trucks that must remain on site while concrete temperature rises and workability changes. Coordinate batch-plant dispatch, travel time, site access, pump or chute location, placement rate and finishing capacity before the pour starts.
Ordering concrete faster than the crew can place it is not automatically productive. Trucks that stack up at the jobsite can increase waiting time, while ordering too slowly can create interruptions, lift interfaces or finishing problems. Use the Concrete Pour Time Calculator to estimate placement duration and compare that result with supplier dispatch capability. If using a site mixer, the Concrete Mixer Size Calculator can estimate batch production so the crew is not forced to work around an unrealistic mixing schedule.
Equipment breakdown, blocked access, pump problems and sudden weather changes can happen during any pour. In hot weather, the available response time may feel shorter because fresh properties can change quickly. Decide in advance who has authority to pause or reject a placement, how the supplier will be contacted, what the project documents require for delayed concrete and how partially completed areas will be protected. The calculator cannot make those decisions, but it can help show when environmental conditions leave less room for operational delays.
Finishing and curing are closely connected to hot-weather planning because the concrete surface can lose moisture while the crew is still working it. Finishing operations should follow the mixture behavior and project requirements rather than a fixed clock time. Surface appearance alone can be misleading when wind and dry air remove moisture quickly. Crews should avoid practices that simply mask rapid drying without addressing the underlying exposure conditions.
Curing materials should be staged before concrete arrives. Depending on the specification, this may involve curing compound, wet coverings, plastic sheeting, fogging equipment or other approved methods. The correct method and start time depend on the project, surface finish and concrete system. If evaporation control measures are permitted during placement, the crew should know exactly when and how they may be used.
The calculator's moisture-loss estimate describes environmental exposure at the surface. It is not a water-cement ratio adjustment and not a quantity to pour onto the slab. Adding water without authorization can change concrete properties and may violate the approved mixture. If workability or finishing becomes difficult, follow the ready-mix producer's instructions and the project procedure rather than compensating with unplanned water.
Hot-weather planning is strongest when production, placement, finishing and curing are treated as one continuous operation. Cooling concrete at the plant provides limited benefit if trucks then wait for long periods, just as a fast placement rate provides limited benefit if curing materials are not ready. Use the calculator result as one part of the pre-pour conversation among the contractor, producer, testing agency and design team.
Wind can vary substantially with buildings, barriers and site exposure. Use a representative field measurement when the project requires accurate evaporation assessment.
The calculator expects a percentage such as 35, not 0.35. Input validation prevents values above 100%.
High concrete temperature can affect slump loss, setting and workability even when humidity reduces calculated evaporation.
The governing project specification and responsible professionals determine placement requirements. A web calculator cannot approve structural concrete.
The surface moisture-loss output is not a water-addition instruction. Do not use it to alter the approved mixture or finishing procedure.
Hot-weather scheduling works best when quantity, production and placement time are already understood. These related tools can help complete the plan.
Calculate the concrete quantity before coordinating truck and crew timing.
Calculate concrete volume →Estimate how long placement may take with trucks, pumps or site mixing.
Estimate pour time →Plan mixer capacity, batches and production time for on-site mixing.
Size a mixer →Plan saw-cut layout and cutting quantities after flatwork placement.
Plan saw cuts →Convert specified bar spacing into practical layout quantities.
Plan rebar spacing →Estimate volume and material quantities for exposed pool-deck flatwork.
Calculate pool deck →Four environmental inputs combine to influence the estimated evaporation rate from fresh concrete.
Common questions about evaporation, temperature, humidity, wind and hot-weather concrete planning.