The price of a product is denoted by p, the price reference period is denoted by 0, and the comparison period is denoted by t. Weights (w) are expenditure shares of a period (b) prior to the price reference period, and are adjusted to reflect the prices of the price reference period 0;
Cuirtear praghas táirge in iúl le p; cuirtear tréimhse tagartha na bpraghsanna in iúl le 0, agus cuirtear an tréimhse chomparáide in iúl le t. Is ionann ualaí, (w), agus scaireanna caiteachais de thréimhse (b), roimh an tréimhse tagartha, agus déantar iad a choigeartú chun léiriú a thabhairt ar phraghsanna thréimhse tagartha na bpraghsanna, is é sin 0;