The Tertiary Activities Rent Index (TARI) consists of a weighted sum of three indices:
- the consumer price index for consumption excluding tobacco and rents (IPCL in French) of all households, on mainland and overseas departments;
- the "Cost of Construction Index" (CCI, ICC in French);
- the level of the gross domestic product in value measured from the quarterly Gross domestic product  in value adjusted for seasonal and working days (GDP, PIB in French) published by Insee.

The three indices contribute to the calculation of the TARI according to the following weightings: Quarterly tertiary activities rent index(TARI) = 50% mIPCL + 25% mICC + 25% mPIB

Each of the indices composing the TARI is calculated as a mean :
- over 12 consecutive months for consumer prices, which is a monthly index (mIPCL);
- over 4 consecutive quarters for the cost of construction index and Gross domestic product in value, which are quarterly (mICC and mPIB).

To calculate these means, the last month (or quarter) taken into consideration corresponds to the last month of the quarter (or to the quarter) for which the TARI is being calculated.

Each of the means calculated in this way and the TARI itself are expressed in reference 100 for the first quarter of 2010.