---
title: Which CPI series does OWNER use and why does the result sometimes differ?
description: OWNER uses the CBS CPI series 2025=100. Learn why the result can differ from an old series and which method fits the ROZ terms.
---

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October 20, 2026

# Which CPI series does OWNER use and why does the result sometimes differ?

## OWNER uses the CBS series 2025=100, so the result can differ slightly from an old series.

Does your rental contract mention an old base year, or does your tenant arrive at a slightly different percentage via CBS (Statistics Netherlands)? That's because of the CPI series being used.

**In this article**

1. [Which series does OWNER use?](https://knowledge.owner.app/en/cpi-series#welke-reeks)
2. [What can you do?](https://knowledge.owner.app/en/cpi-series#wat-kun-je-doen)
3. [Indexing according to the ROZ terms](https://knowledge.owner.app/en/cpi-series#roz)

## Which series does OWNER use?

- The Monthly CPI and Annual average CPI methods get their figures automatically from CBS.
- OWNER only uses the 2025=100 series. Older series, such as 2000=100 or 2015=100, have been discontinued by CBS.
- CBS recommends the 2025=100 series for indexations over periods ending in January 2026 or later. OWNER follows that advice.
- You can't choose or display an old series. As a result, the outcome can sometimes differ slightly from a calculation with an old series, usually by just a few tenths of a percent.

## What can you do?

1. Simply choose Monthly CPI or Annual average CPI. You don't need to select a series.
2. Check what the rental contract and the general terms and conditions say about switching to a new series.
3. Want to use a different percentage, for example the result based on the old series? Then deviate from the scheduled indexation: [Can I deviate from a scheduled indexation?](https://knowledge.owner.app/en/deviate-from-scheduled-indexation?hsLang=en)
4. Want to explain the calculation to the tenant? Do that separately, for example by email. You can't change the text of the indexation letter itself.

## Indexing according to the ROZ terms

Does your rental contract contain a calculation using the index figure from 4 months before the indexation date, divided by the figure from 16 months before the indexation date (as in the Dutch ROZ standard lease terms)? Then choose **Monthly CPI**. That method calculates with these months.

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