As per IS : 10262: 2009, the standard deviation for M25 concrete is:
Concept:
The assumed value of standard deviation for initial calculations As per IS : 10262: 2009 are given below in the table:
Sl. No 
Grade of Concrete 
Characteristic compressive strength (N/mm2) 
Assumed standard deviation (N/mm2) 
1. 
M10 
10 
3.5 
2. 
M15 
15 

3. 
M20 
20 
4.0 
4. 
M25 
25 

5. 
M30 
30 
5.0 
6. 
M35 
35 

7. 
M40 
40 

8. 
M45 
45 

9. 
M50 
50 

10. 
M55 
55 