Concrete Strength by Age Calculator
Estimate concrete compressive strength at 1, 3, 7, 14 or any number of days from its 28-day f′c, using the ACI 209 strength-gain curve for normal or high-early cement, moist or steam cured — in MPa, kg/cm² and psi.
How it works
f′c(t) = t ÷ (a + b·t) × f′c(28) · Type I moist: a = 4.0, b = 0.85 · Type III moist: a = 2.3, b = 0.92 · Type I steam: a = 1.0, b = 0.95 · Type III steam: a = 0.70, b = 0.98 · days to reach p = a·p ÷ (1 − b·p) (ACI 209R-92)
Concrete gains strength quickly in the first week and slowly afterwards as cement keeps hydrating. ACI Committee 209 fitted this with a simple hyperbolic curve whose constants depend on the cement type and curing: ordinary cement that is kept moist reaches about 70% of its 28-day strength in 7 days and keeps creeping up past 100% for months, while high-early cement and steam curing get there much faster. Use the estimate to plan form stripping and loading, or to judge whether a 7-day cylinder test is on track — but the real answer for a given pour comes from cylinder or core tests, and cold weather slows everything down.
Worked example
A 25 MPa (≈ 250 kg/cm²) mix with normal cement, moist cured, tested at 7 days: 7 ÷ (4 + 0.85 × 7) = 0.704, so about 70.4% of f′c — 17.6 MPa, 179 kg/cm² or 2,552 psi. It reaches the 70% target after 4 × 0.7 ÷ (1 − 0.595) = 6.9 days.
Sources: ACI 209R-92 — Prediction of Creep, Shrinkage and Temperature Effects in Concrete Structures · PCA — Design and Control of Concrete Mixtures, Chapter on strength development
Frequently asked questions
What percentage of strength does concrete have at 7 days?
About 65–70% of its 28-day strength with ordinary cement and good moist curing (70.4% on the ACI 209 curve), and around 80–90% with high-early cement or steam curing.
What strength is concrete at 3 days and 14 days?
On the ACI 209 curve for normal cement, about 46% at 3 days and 88% at 14 days; at 28 days it is 100% by definition and it keeps gaining slowly afterwards.
When can I remove formwork?
Side forms of beams and walls after 1–2 days; slab and beam soffits usually when the concrete reaches about 70% of f′c — roughly 7 days with normal cement in warm weather — and keep back-props in place until full strength for long spans.
My 7-day test is low — should I worry?
Compare it with the expected share: a 25 MPa mix testing 15 MPa at 7 days (60%) is a little slow but may still pass at 28 days. Low results in cold weather are common; wait for the 28-day cylinders before deciding.
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This calculator is for educational purposes. Double-check important results before acting on them.