# We keep buying units. Should our reduction target use a fixed or a rolling base year?

Canonical: https://senseible.earth/answers/ghg-fixed-or-rolling-target-base-year-acquisitions

With frequent acquisitions a rolling target base year, usually moved forward one year, is the easier fit. When a unit joins, only the year before the change is recalculated. A fixed base year recalculates every year back to the base and may force you to estimate data the acquired unit never kept.

## The difference in one table

- Target wording. Fixed: "we will emit X% less in year B than in year A". Rolling: "we will cut emissions Y% every year compared with the previous year".
- Recalculation after a structural change. Fixed: all years back to the base year. Rolling: only the year before the change, or the year of the change, which then becomes the base year.
- Data burden. Fixed: an acquired unit needs data for the base year, which may mean backcasting and a less reliable base. Rolling: data is needed for the year before the acquisition, or only from the acquisition onward.
- Like-with-like view. Fixed: the whole series compares like with like. Rolling: only over two years at a time.

## What does not change

The triggers for recalculation are the same under both approaches, as set out in chapter 5 of the standard. The difference is how far back you go.

## The trade-off

A rolling base year never gives a clean line from the starting year to the completion year. You can still state reductions together over several years, for example "1% a year from 2026 to 2030", but you cannot restate the whole period on one footing.

## Choose fixed when

Acquisitions are rare and you have reliable data for the base year. The standard says to consider rolling when reliable, verifiable data for a fixed base year is hard to keep, for example because of frequent acquisitions.

## Example with made-up numbers: a stabilisation target (keep emissions flat)

- Company A emits 1,000 t. Company B, 200 t in year 1 and 260 t in year 2 because it grew.
- A buys B at the start of year 3. Year 3: A emits 1,000 t and B emits 260 t, total 1,260 t.
- Fixed base year: the base is recalculated to include B, 1,000 + 200 = 1,200 t. 1,260 is above 1,200, so the target is missed.
- Rolling base year: the comparison year is year 2 with B included, 1,000 + 260 = 1,260 t. 1,260 equals 1,260, so the target is met.
- B's growth before the purchase does not show as an increase for A under the rolling approach.

This follows the standard's own acquisition example. Growth or decline of a divested unit before the sale does count under the rolling approach.

## What Senseible does here

Senseible keeps dated energy and emission records per site, so a newly acquired unit can be brought in from its first bills without rebuilding old years.

- [When targets must be recalculated (SBTi)](https://senseible.earth/answers/sbti-target-recalculation-triggers-5-percent)
- [Falling grid factor and targets](https://senseible.earth/answers/falling-grid-factor-and-reduction-targets)
- [Carbon number for MSMEs](https://senseible.earth/msme-carbon-number)

Sources: [GHG Protocol, Corporate Accounting and Reporting Standard, chapter 11 (Setting a GHG target)](https://ghgprotocol.org/sites/default/files/standards/ghg-protocol-revised.pdf)

Last reviewed 2026-11-03.