The U chart measures the number of nonconformities per inspection unit in subgroups, which can have varying sample sizes. It is similar to C chart except that the number of nonconformities is expressed on a per unit basis.
Data Available
N = Subgroup size
T(1..K) = Total produced
K = Inspection Quantity
D(1..K) = Total Defects in sample
To be calculated:
Process Average Non conformities Per Unit,
= U(1..K) where UI = DI / TI
Sigma (Standard deviation )
SU =
Center Line and Control limits :
Center Line ( CLU ) 

Upper Control Limit (UCLU) 
CLU + 3 SU 
Lower Control Limit (LCLU) 
MAX(0,(CLU – 3 SU)) 
SPC chart calculations. 