ACI 301-96 References 1 specifies a k-factor, by which the Standard Deviation is increased in the calculation to obtain the Required Average Compressive Strength, according to the number of Test Strengths for which the Standard Deviation is calculated. These factors are listed in the table below.
| Number of Test Strengths | k-Factor |
| 15 | 1.16 |
| 20 | 1.08 |
| 25 | 1.03 |
| 30 or more | 1.0 |
These values are taken from Table 4.2.3.3.a of ACI 301-. FirstBreak employs linear interpolation, as permitted by ACI 301-96, for intermediate values of the number of Test Strengths.