# How to calculate monthly inflation from CPI

I have data that consists of dates and monthly CPI belong to that date. And I'm trying to find Inflation belongs to specific date, for instance inflation at 2015-02.

I used this formula to calculate for example inflation rate in 2016-01

((i/l)-1)*100 = inflation rate
i = CPI value of the current month
l = CPI value from 12 month ago


( (-1,90 / -1,60) - 1 ) * 100 = 18,75

But it doesn't make sense because result of formula is 18,75 but the real inflation rate at 2016-01 is different.

Also, for some other month I found -73,68 value from that formula, It also doesn't make sense at all.

How can i find monthly inflation according to specific CPI value belongs to that month?

• You're probably supposed take actual inflation from the starting month then sum the values in the column you presented to the ending month to get ending month inflation as is typical with the monthly CPI published tables; but it's not clear where your data came from so its hard to say for sure.
– quid
Commented Dec 22, 2020 at 17:08

Looks like you have monthly inflation percentages, so you should compound the values, e.g.

Feb   = 100.  * (1 + 0.002) = 100.2
March = 100.2 * (1 + 0.006) = 100.801

month     %     index
2015-01    -    100
2015-02   0.2   100.2
2015-03   0.6   100.801
2015-04   0.9   101.708
2015-05   0.5   102.217
2015-06   0.3   102.524
2015-07  -0.9   101.601
2015-08  -0.3   101.296
2015-09  -0.3   100.992
2015-10   0.6   101.598
2015-11   0.4   102.005
2015-12  -0.3   101.699
2016-01  -1.9   99.7663


Then the annual inflation since Jan is 99.7663/100 - 1 = -0.23374 %