Identify the current period index value
Identify the base period index value
Use the formula: CPI = (Current period basket cost ÷ Base period basket cost) × 100
If using index numbers, use: CPI = (Current index ÷ Base index) × 100
Subtract the base value from the current value if you need the change in CPI
Divide the change by the base value if you need the percentage change
Multiply the result by 100 to convert it to a percentage
Round the final CPI value if required