Concrete Strength Gain Calculator (Strength at 3, 7, 14 Days by Eurocode 2 or ACI 209)
The strength a concrete has reached at a given age as a share of its 28-day value, and how many days it takes to reach a share you choose, such as 70 % for striking formwork.
Results
The dashed line marks the value you entered.
What this tool does
Concrete does not reach its rated strength overnight: it gains most of it in the first week and keeps creeping up for months. Eurocode 2 gives the mean strength at age t as e^(s(1 − √(28/t))) times the 28-day value, with s = 0.20, 0.25 or 0.38 for fast, normal or slow cement, so normal cement stands at about 78 % after 7 days; ACI 209 uses t ÷ (4 + 0.85 t) for moist-cured Type I. Cold weather slows all of this, and the result is a mean strength, not the guaranteed characteristic value (about 8 MPa lower in Eurocode terms), so treat it as a guide to planning and rely on tests for striking and loading.
Formula
Eurocode 2: f(t) = f₂₈ × e^(s(1 − √(28 ÷ t))) · ACI 209: f(t) = f₂₈ × t ÷ (a + b t)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
f | Mean compressive strength at 28 days | MPa |
t | Age of the concrete | d |
m | Curve and cement | — |
p | Share of the 28-day strength you want | % |
ST | Strength at that age | MPa |
PC | Share of the 28-day strength | % |
DY | Age at which it reaches that share | d |
Worked example
- Mean compressive strength at 28 days38 MPa
- Age of the concrete7 d
- Curve and cement2
- Share of the 28-day strength you want70 %
- Strength at that age29.6 MPa
- Share of the 28-day strength77.9 %
- Age at which it reaches that share4.8 d
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.