CBS indices for price indexation: which series, which figure, which month

How indexation with a CBS figure works

The CBS publishes index figures: a series showing how prices or wages develop against a base year set at 100. A contract uses two figures from such a series, usually the same month in two consecutive years. The rule is always the same: new figure divided by old figure, times 100, minus 100. That is the price increase in percent.

A fictional example. The index for August this year is 120.0, for August last year 115.4. The increase is 120.0 / 115.4 × 100 − 100 = 4.0%. Your price goes up by 4.0%.

The base year makes no difference to the result, as long as both figures come from the same series. Never mix figures from an old and a new series.

New figure divided by old figure, from the same series. Everything else is detail, but those details decide your margin.

The series you meet in contracts

CPI or wage index: the choice that matters most

The CPI is the broadest figure, which is why buyers often choose it. But it measures what households pay, not what your staff cost. If your contract is mainly labour, such as cleaning, security or care, the CPI and your costs can drift apart for years. Sometimes in your favour, often not.

For the labour share, a wage index is the logical choice. There are two. Collective wages leave out employer costs. Contractual labour costs include them, and so track your real labour costs more closely. Both are available per sector: many Dutch tender documents name, for example, the collective wages for construction or for the metal and electrical industry.

If your contract combines labour and materials, you often see a mix: for example 85% collective wages and 15% producer prices. The closer that split matches your own cost structure, the smaller the gap.

Which month, and which figure?

• MonthOne month or an averageThe increase from August to August, or a yearly or quarterly average. An average moves more smoothly, but lags further behind.
• Reference dateWhen is the figure taken?A clause from a public tender: as of 1 January, using the August figure, taken in the first week of November. If August is not yet published, a fixed fallback order applies.
• ProvisionalFirst published or finalThe first CBS figure is provisional and revised later. The contract must say which figure counts. The first published figure is faster, the final one more accurate.

The CPI appears in steps. A flash estimate comes on the last working day of the month. Between the first and second Thursday of the next month the index figures follow, still provisional. A month later they are final. The collective wage index appears within a week after the month and is revised monthly, until the previous year’s figures become final in the second quarter.

When does your cost increase show up in the figure?

This is the risk overlooked most often. Say your collective wages rise on 1 January. The contract indexes as of 1 January, using the August figure against August a year earlier. Then the wage increase of 1 January only appears in that year’s August figure, and you only get it on 1 January of the following year. For a whole year you carry that cost yourself.

Add a price level older than the contract start, or a first indexation only in year two or three, and the gap grows every year. Work it out with the indexation calculator.

A wage increase on 1 January that only reaches your price a year later, you pay for yourself for a year.

What changed in 2026

Since 2026 the CPI has a new base year: 2025 = 100 instead of 2015 = 100. There is also a new classification of goods and services, closer to the European HICP. If a running contract refers to the old series, check how the contract handles a change of series, and never calculate with a figure from the old series against one from the new.

What do you do with it?

1. 1Read the clause sentence by sentenceWhich series, which sector, which month, which figure, on which date, from which year, with which limits. How to build it into your pricing, read in price indexation.
2. 2Hold it against your own costsHow large is your labour share? When do your wages rise? Does the chosen series follow that, and in time?
3. 3Ask a question if it does not fitA different series, a different moment or indexation from the first year. Other bidders read the answer too, and are pointed to the same risk. How to go about it, read in the clarification round.