Difference between Cost Performance Index (CPI) and Schedule Performance Index (SPI)

Cost Performance Index (CPI): Cost Performance Index (CPI) is the measure of the cost efficiency of project. It is expressed as a ratio of earned value to actual cost. 

Schedule Performance Index (SPI): Schedule Performance Index (SPI) is the measure of schedule efficiency of the project. It is expressed as the ratio of earned value to planned value. Example: A project is going to be completed in 6 months and the budget for the project is 50, 000 dollar. After the 3 months it is found that only 40% of the total work has been done to date and 30, 000 dollar has been spent. Then the Cost Performance Index (CPI) and the Schedule Performance Index (SPI) is calculated as below:

Actual Cost, 
= 30, 000 dollar

Planned Value, 
= 50% of 50, 000 dollar 
= 25, 000 dollar

Earned Value, 
= 40% of 50, 000 dollar 
= 20, 000 dollar

Cost Performance Index, 
= Earned Value / Actual Cost 
= 20, 000 / 30, 000 
= 0.67

Schedule Performance Index (SPI), 
= Earned Value / Planned Value 
= 20, 000 / 25, 000 
= 0.8 

Conclusion: It can be concluded that the project is over budget and behind schedule. Difference between Cost Performance Index (CPI) and Schedule Performance Index (SPI):

Cost Performance Index	Schedule Performance Index
It actually measures the performance regarding the budget of the project.	It measures the performance regarding the scheduled time of the project.
It describes the amount of money spent on the project.	It describes the amount of time consumed on the project.
CPI also tells about the remaining cost of the project.	SPI tells about how much more time will be consumed on the project.
CPI is the measurement of deviation from the estimated cost of the project.	SPI is the deviation from the scheduled time for project.
CPI = Earned Value / Actual Cost	SPI = Earned Value / Planned Value
If CPI is less than 1 then project is over budget.	If SPI is less than 1 then project is behind schedule.
If CPI is greater than 1 then project is under budget.	If SPI is greater than 1 then project is ahead of schedule.
If CPI is equal to 1 then project is on estimated budget.	If SPI is equal to 1 then project is on schedule.