Setting a carbon baseline year
6 min read · Updated 5 July 2026
The baseline year is one of the first decisions you make in a Carbon Reduction Plan, and one of the easiest to get slightly wrong. It is the reference point every future plan measures against, so the choice sticks with you. Get it right and your progress tells a clear story year on year. Get it wrong and you can end up comparing figures that were never really comparable. This guide explains what a baseline is, how to choose one, and when you are allowed to change it.
Close enough to today to reflect how the business operates now.
A full reporting period with enough evidence to calculate emissions.
Not distorted by a one-off closure, disposal or abnormal trading year.
Quick answer
Choose a baseline year that is recent, complete and representative. Use the same organisational boundary and reporting period when you compare later years against it, and only restate the baseline when the old one no longer gives a fair comparison.
What a baseline year is for
Your baseline is a snapshot of your emissions in a chosen year, covering Scope 1, 2 and the required Scope 3 categories. Every reduction target and every current-year figure is stated in relation to it. When a buyer looks at your plan, the baseline is what lets them see whether your emissions are genuinely coming down rather than just moving around. That is why the same baseline is carried forward from one plan to the next, instead of being reset each year.
How to choose the right year
A good baseline year is recent, complete and representative. Take each of those in turn:
- Recent, so the picture reflects how your business actually operates now.
- Complete, meaning you have solid data for the whole year rather than gaps you have to guess around.
- Representative, meaning it was a normal trading year rather than an unusual one distorted by a one-off event.
If your most recent year had something unusual in it, a site closure, a sudden expansion, a period of shutdown, consider whether an adjacent year gives a fairer picture. The aim is a starting point that a reader would accept as typical of your business.
When your baseline is also your current year
If this is your first Carbon Reduction Plan, you may not have historic data to draw on. That is fine. Your baseline year and your current reporting year can be the same year in a first plan. The important thing is to state clearly that they are the same, so a reader is not left wondering why two columns show identical figures. In later plans, the current year moves on while the baseline stays put, and the comparison starts to mean something.
First-plan example
If your first plan covers the financial year ending March 2026, that same year can be both your baseline and current reporting year. In the next annual update, the baseline remains 2025/26 while the current year moves forward.
Keeping the baseline consistent
Consistency is what makes a baseline useful. Measure the same sources, in the same scopes, using the same organisational boundary and the same reporting period each year, so that a change in your totals reflects a real change in your emissions rather than a change in how you counted. PPN 006 also expects the plan to use one reporting period throughout the submission. If you quietly add or drop a category between plans, your trend line stops being trustworthy.
When you can recalculate it
Occasionally the shape of your business changes enough that the original baseline no longer compares fairly. A major acquisition, a large disposal, or a significant change in what you measure can all justify restating the baseline so the old and new figures sit on the same footing. When you do this, say so in the plan and explain why. A recalculation you have documented is sensible housekeeping. A silent change looks like moving the goalposts.
Record the reason
If you restate a baseline after an acquisition, disposal or boundary change, keep the original year visible and explain what changed. The aim is to preserve a fair trend, not to make historic emissions disappear.
Frequently asked questions
Can my baseline year and reporting year be the same?
Yes, in a first Carbon Reduction Plan. If you have no earlier complete data, use the current reporting year as the baseline and say clearly that they are the same. In later annual updates the current year moves forward while the baseline stays fixed.
Can I change my baseline year later?
Only with a good reason, and openly. A major acquisition, disposal or a significant boundary change can justify restating the baseline so comparisons stay fair. Document what changed and why in the plan itself. Changing the baseline simply because a different year looks better is exactly what assessors are alert to.
Does the baseline need to include Scope 3?
Yes. The baseline covers the same boundary as the rest of the plan: all Scope 1, all Scope 2 and the five required Scope 3 categories. If Scope 3 data for the baseline year is thin, estimate it with a stated method rather than reporting a baseline that covers less than your current year.
What if my only complete year was an abnormal one?
Use it, but say so. If your best-documented year was distorted by a shutdown, a one-off contract or unusual trading, either pick an adjacent, more typical year if the data supports it, or keep the year and note the distortion so future comparisons are read fairly.
Where the baseline sits in the plan
The baseline is one of the core sections of a compliant plan, alongside the current-year figures and your targets. Our guide to the mandatory sections shows how it fits with the rest, and the step-by-step writing guide puts choosing a baseline in its proper place in the process. When you build a plan with Carbon Sorted, you set the baseline once and the tool carries it forward, keeping the year-on-year comparison consistent for you.
Practical next step
Use this worksheet to compare candidate baseline years and record why the chosen year is fair.
Download the baseline year worksheetReviewed for accuracy
Written by the Carbon Sorted editorial team and reviewed against current UK procurement and carbon-reporting guidance. Last reviewed 5 July 2026.
Sources checked