From its creation until the third quarter 2021 included,
the Commercial Rent Index (CRI) is calculated as the weighted sum of three indices:
- Consumer Price Index, excluding tobacco and rents, published for all households in Metropolitan France and French Overseas Departments (CPI)
- Cost-of-construction (CCI) index
- Retail Trades Index, in value, corrected for seasonal variations and calendar effects (RTI).

Each of the three indices making up the ILC is calculated as a mean:
- Over 12 consecutive months for consumer prices (mCPI) and the turnover (mRTI), which are monthly indices
- Over 4 quarters for the cost of construction, an index calculated on a quarterly basis (mCCI).
In order to calculate these means, the last month (or quarter) considered corresponds to the last month of the quarter (or to the quarter) of calculation of the CRI. The CRI, like each of its constituent indices, is expressed in relation to the reference 100 in the first quarter of 2008.

These three indices are used to calculate the CRI with the following weighting:
Quarterly Commercial Rent Index (CRI) = 50% mCPI + 25% mCCI + 25% mRTI

From the date of fourth quarter 2021, the Commercial Rent Index is deduced by the index for the same quarter of the previous year by applying to it the change between these two periods of the sum of components defined in I and II of article 2, weighted with the following formula:
75 % mCPI + 25 % mCCI.

The quarterly Commercial Rent Index is not revised.