From the ISTAT indices to the amount due to the contractor, one type of works at a time.
Article 60 of Legislative Decree 36/2023 requires Italian public contracts to include a price revision clause. The clause does not operate continuously: it comes into play when the variation exceeds 3% against the award date, and recognises 90% of the excess alone (article 3, paragraphs 2 and 3, of Annex II.2-bis). These are the figures for works: for services and supplies the threshold is 5% and the recognised share 80%. The calculation is the one set out in Annex II.2-bis, implemented by Ministry of Infrastructure decree no. 743 of 30 March 2026, and applies to works started from 27 April 2026.
Revision does not follow a single construction index: it follows twenty homogeneous types of works — the T.O.L. — listed in Table A.1 of Annex II.2-bis. They range from building works on non-listed structures to mechanised tunnelling, from electrical systems to waste disposal. Table A.2 splits them into nine specialist and eleven general types: where a work item could sit in two of them, article 4, paragraph 3, letter a) gives precedence to the specialist one.
ISTAT publishes a monthly cost index for each type, roughly sixty days after the month it refers to.
The path is the following, and each step has its own provision:
1. The whole contract amount is classified. Every work item is assigned a type (article 4, paragraph 3, letter a). For five types — earthmoving, traditional and mechanised tunnelling, rail infrastructure and special foundations — the waste disposal component is separated out and referred to type 20 (article 4, paragraph 4).
2. The weights are computed. The weight of a type is its amount over the total contract amount (article 4, paragraph 3, letter b).
3. The indices are rebased to 100. Each index is taken against its own value in the month of the award decision, which is the reference month (article 4, paragraph 2, and point 3 of Table B).
4. The synthetic index is assembled. It is the weighted average of the rebased indices: Is = Σ pi × Ii. For an interim valuation covering several months, the average of the indices for the period is used (point 6, letter b).
5. The coefficient is (ISpx − ISmo) / ISmo, rounded to the fourth decimal (point 4). Above 3%, 90% of the excess is applied to the interim valuation at contract prices, safety costs included and gross of retentions. Revision covers the works still to be executed after the clause is activated (article 3, paragraph 3): interim valuations accrued before that stay at contract prices.
Classifying only part of the bill. This is the dangerous one, because nothing shows. If the weights are computed over the classified amount alone they still sum to one hundred, the coefficient looks plausible, and the unclassified share has been redistributed among the others. The figure is not imprecise: it is a different figure, and it ends up in a signed accounting document. Article 4, paragraph 3, letter a) requires the entire contract amount to be classified, and while anything is left out the calculation should not be made.
Using the latest published index. The index to use is the one for the reference month of the interim valuation, not the most recently released. Since ISTAT publishes two months late, the two almost never coincide.
The type is assigned from the bill item, and the program computes the weights with the waste component already separated out. ISTAT indices are downloaded inside the program and kept in the archive; a missing reading can be entered by hand. The calculation is made per interim valuation, multi-month ones included, under the Table B method or Table C where the project declares it.
The result does not stay on screen: the price revision statement prints as PDF or Word, with one line per type — weight, index of the award month, index of the period, the same index rebased to 100 and its contribution to the synthetic index — closing with the coefficient, the threshold check and the amount due. Whoever signs it can redo the arithmetic with a calculator.
If part of the bill has no assigned type, the program refuses to calculate and states how many items and what amount are still unclassified.
Two points of the Annex remain open pending the Ministry guidelines, which are not yet published: the rounding direction of a negative coefficient, and whether the weights are renormalised after removing types below 4%. Both concern secondary cases — a downward revision and an option that has to be switched on — and both are documented inside the program. We will check them against the guidelines when they appear.
Read also: Interim valuations and payment certificates · How to prepare a bill of quantities · Regional price lists
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