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CPI is earned value over actual cost: below 1.0 means you are spending faster than you are earning. Dividing the budget by CPI gives an estimate at completion assuming current performance continues, and netting the remaining contingency against the projected overrun shows whether the project can still be delivered within its approved funding.
Cost performance index
CPI = Earned value / Actual cost
Estimate at completion
EAC = Budget at completion / CPI
Overrun after contingency
Net overrun = (EAC - Budget at completion) - Contingency remaining
Research on large projects shows cumulative CPI stabilises by around 20% completion and rarely improves after that, which makes early CPI a strong warning signal.
Anything above about 1.10 is generally regarded as unrealistic. If TCPI exceeds that, the budget needs revising rather than the team exhorting.