The principle of a revision formula
A formula splits the price into parts, each following its own index between a reference date and the date of execution. The parts must sum to one.
| Part | What it follows |
|---|---|
| Labour | the movement of sector wage costs |
| Materials | the movement of material prices, by published indices |
| Non-revisable part | nothing, it stays fixed |
The weighting of those parts must reflect the real nature of the works. A formula calibrated on a standard split applied to an atypical contract produces a revision that does not match the actual movement of costs borne.
A labour-heavy contract and a materials-heavy contract should not carry the same weighting. The formula is calibrated on the actual structure of the items.
What the structure of the bill changes
Three effects, rarely anticipated at drafting stage. They touch the revised amount, cash flow and the risk of dispute.
The pace of execution of items. Revision applies to work at the moment it is executed. An item carried out at the start and one carried out eighteen months later do not undergo the same revision, even at identical rate.
Phasing. A contract divided into conditional stages will see its later stages revised more, being executed later.
Variations. An additional quantity recorded late is revised according to the period of its execution, not that of the original contract.
Hence a practical consequence: a slipping programme costs not only carrying charges, it mechanically alters the revised sum.
The indices used
Two families of index coexist in Belgium, and they do not serve the same purpose. One tracks sector costs, the other wages and charges.
The general construction index measures the movement of construction cost and serves notably to update estimates and index insured values. It is established half-yearly and stood at 1056 for the first half of 2026.
Sector indices for materials and labour serve contractual revision formulas. Their granularity reflects the real composition of a contract.
Confusing the two uses is a frequent error. Updating an estimate and revising a contractual price do not call on the same instruments.
What is not revisable
Three categories generally escape revision, and their treatment must be explicit. Their exclusion is stated in the contract to be enforceable.
The non-revisable part of the formula, whose percentage is fixed by contract.
Work executed outside the period attributable to the contractor, under the arrangements provided.
Items the contract expressly excludes, which presupposes that the particular specification says so.
An item whose revision regime is unstated is a source of dispute, particularly on long contracts.
The reference date
This is the element most often omitted, and without it the formula is unworkable. It is the reference date of the indices.
The reference date determines the starting index values. It is not necessarily the date of signature, and may be fixed at the tender deadline or another agreed date.
Three questions must find answers in the contract. They concern the indices, the reference date and the frequency.
What is the reference date for the starting indices.
What frequency of application, monthly, by interim valuation or otherwise.
Which indices precisely, designated unambiguously.
A formula lacking any of those three elements will be interpreted, and each party will interpret it in its own favour. The interpretation is then settled at the cost of a dispute.
What this means for a professional
Four rules.
Check the formula before comparing two tenders, a fixed and a revisable offer not committing to the same final sum.
Adjust the weighting to the nature of the contract, rather than adopting a standard split.
Provide for revision in proportion to foreseeable duration, rather than hoping it will be nil.
Keep the reference indices with the bill, or later reconstruction becomes hazardous.
This article reflects the position of the rules and indices at the date of checking and serves professional orientation. It does not constitute legal advice and does not replace assessment of the individual case.