Concrete Curing Time Calculator

1. Enter the 28-day strength of the mix in psi.
2. Enter the age of the concrete in days since it was placed.
3. Choose the cement type and curing method.
4. Optionally set a target percentage to see how many days it takes to get there.

Here is what each result line means:

A concrete curing time calculator estimates how much strength concrete has gained a given number of days after it was poured. Concrete does not dry to strength; it hardens through a chemical reaction between cement and water that continues for weeks, fast at first and then more slowly.

Specifications usually quote strength at 28 days, but crews need to know the strength sooner, to strip forms, saw-cut joints or put loads on a slab. This tool uses the ACI 209 strength-gain curve to estimate that early strength. Timing the saw cuts is covered by the control joint spacing calculator, sealing a cured slab by the concrete sealer calculator, and what a finished slab can carry by the slab load capacity calculator.

ACI Committee 209 published a simple curve for how concrete strength develops with age. The constants a and b depend on the cement and the curing: Type III, high early strength cement, and steam curing both gain strength faster in the first days.

1. Pick the constants for your cement and curing: moist Type I a = 4.0, b = 0.85; moist Type III a = 2.3, b = 0.92; steam Type I a = 1.0, b = 0.95; steam Type III a = 0.70, b = 0.98.
2. Divide the age in days by a plus b times the age for the fraction of the 28-day strength.
3. Multiply that fraction by the 28-day strength.
4. For days to a target fraction p, solve t = a times p divided by (1 minus b times p).

Suppose a slab uses a 4,000 psi mix with Type I cement, kept moist, and you want to know its strength at 7 days.

Calculation: with a = 4.0 and b = 0.85, the fraction at 7 days is 7 / (4.0 + 5.95) = 0.704. The estimated strength is 4,000 x 0.704 = 2,814 psi, reported as 2,810 psi. At 3 days it is about 1,830 psi, and the curve reaches 75 percent at about 8.3 days.

The lesson: a 7-day cylinder at around 70 percent of the specified strength is on track, not a failure.

Using the ACI 209R-92 constants, the share of the 28-day strength at common ages is:

The curve is a fit, so it can read slightly above or below 100 percent at 28 days. Type III cement and steam curing mostly change the early days; by 28 days all four are close to the specified strength.

Temperature

Warm concrete gains strength faster and cold concrete slower. Near freezing, strength gain almost stops, which is the biggest reason real slabs depart from a standard curve.

Cement Type and Supplementary Materials

Type III cement gains strength early. Fly ash and slag usually slow the early gain but keep adding strength later.

Moisture

Cement needs water to keep reacting. Concrete that dries out early stops gaining strength at the surface, which is why curing matters.

Water-cement Ratio and Admixtures

A wetter mix reaches a lower strength overall, and accelerators or retarders shift the early part of the curve.

Planning Form Stripping and Loading

Get a first estimate of when the concrete might reach the strength the engineer requires, then confirm it with field-cured cylinders.

Reading Early Cylinder Breaks

Compare a 3 or 7-day break with the curve to see whether the concrete is on track for its 28-day strength.

Comparing Cement and Curing Choices

See how much sooner Type III cement or steam curing reaches a target, when schedule matters.

1. Loading on the Estimate Alone

A curve is not a test. Stripping, loading and traffic decisions should rest on field-cured cylinders or the tests the engineer specifies.

2. Ignoring Temperature

The ACI 209 constants assume ordinary curing conditions. In cold weather, real strength can lag well behind the curve.

3. Confusing Drying with Curing

Concrete gains strength by hydration, not by drying out. Letting it dry early reduces the strength it reaches.

4. Judging a 7-day Break Against F’c

A 7-day result is expected to be below the 28-day specified strength. Compare it with the curve, not with f’c.

5. Using the Wrong Cement Setting

Type III and steam curing gain much faster early. Pick the setting that matches the mix and the curing actually used.

The equation is applied exactly; how well it matches your concrete depends on temperature, mix and curing.

On the ACI 209 curve, moist-cured concrete with Type I cement reaches about 70 percent of its 28-day strength at 7 days, so a 4,000 psi mix is near 2,810 psi. Type III cement reaches about 80 percent, and steam curing more.

About 46 percent of the 28-day strength for moist-cured Type I concrete on the ACI 209 curve, or about 59 percent with Type III cement. A 4,000 psi Type I mix is near 1,830 psi at 3 days.

28 days is the reference age used in specifications, and the ACI 209 curve puts concrete close to 100 percent then. Concrete keeps gaining some strength after 28 days, especially mixes with fly ash or slag.

For moist-cured Type I concrete the ACI 209 curve reaches 75 percent at about 8.3 days, and for Type III at about 5.6 days. Set the target in the tool for other percentages.

ACI 209R-92 estimates strength at t days as the 28-day strength times t divided by (a plus b t). For moist-cured Type I cement a = 4.0 and b = 0.85.

No. The estimate is a planning guide. Decisions on stripping forms or loading should be based on field-cured cylinders or the tests the engineer specifies, especially in cold weather.

The cement reaction slows as temperature drops and nearly stops near freezing, so concrete gains strength much more slowly than the standard curve assumes. Protect and heat concrete in cold weather.

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

• WSDOT PGSuper time-dependent material models (ACI 209R-92) (Washington State DOT; ACI 209R-92 Table 2.2.1 constants).
• American Concrete Institute (ACI; Committee 209 and concrete strength standards).
• National Ready Mixed Concrete Association (NRMCA; concrete strength and curing practice).