What is a baseline year?

The 12-month period you measure progress against. Your Carbon Reduction Plan shows your baseline emissions, your current emissions and your targets, all compared with the baseline.

Can my baseline year be the same as my reporting year?

Yes. If you are measuring for the first time, your baseline and current year can be the same year. Next year you will report a new current year against it.

Should I use an older year to make my reductions look bigger?

Only if you can evidence it. A baseline built on guesses is hard to defend, and evaluators look for a clear reason the year was chosen. If an older year was unusual, for example because the office was closed for much of it, a more typical year is the better choice.

Can I change my baseline later?

Yes, if there is a good reason, such as buying or selling part of the business, finding a significant error, or improving your method. Recalculate the baseline on the same basis as the current year and explain the change in the plan.

How to pick a baseline year when your records are patchy

Pick the earliest full 12-month period you can back up with real records, which for most small businesses measuring for the first time is simply their last financial year. Fill small gaps with explained estimates, write down why you chose the year, and recalculate the baseline later if the business changes significantly or you find a material error.

Updated 6 October 2026 by the Carbon Recycling team. Figures use the official UK government conversion factors and are checked against the current gov.uk guidance.

What the baseline is for

Your baseline is the yardstick for everything else in a Carbon Reduction Plan. Your targets are set against it, your current emissions are compared with it, and your reduction projects are judged by how far they move you from it. Our PPN 006 guide describes it as the year you first measured, or a typical year if that one was unusual, with a short explanation of why you chose it.

That gives you more freedom than people expect. You don't need the oldest possible year. You need a year you can stand behind.

The rule of thumb: the earliest year you can evidence

Go back as far as your records are complete and reliable, and no further. For a business with patchy older records, that is usually the most recent full financial year. A year counts as "evidenced" if, for each main source, you have something you could show a buyer:

electricity and gas: bills or your supplier's online statements covering all 12 months;
vehicles: fuel card statements, or mileage logs if there are no fuel records;
business travel: mileage claims and booking records;
waste: your contractor's reports or transfer notes;
commuting: a staff survey that describes how people travelled that year.

Choosing between years

Your situation	A sensible baseline
First time measuring, full records for last year only	Last financial year, also used as your current year
Good energy bills for three years, but no travel or waste data before last year	Last financial year. A baseline missing whole categories is not comparable with later years
The earliest complete year was unusual, such as a long closure or a temporary site	The next typical year, with a note explaining why
You measured years ago with a consultant and still have their report	That year, if the report shows the method and covers the same categories

Filling gaps without guessing

Even a good year usually has holes. Small gaps are fine as long as you fill them in a way you can explain:

A missing bill. Estimate the month from the months either side, or from the same month the following year, and note it.
A bill that spans two years. Split it by the number of days in each year.
No commuting data for the baseline year. Run a short survey now, asking staff how they usually travelled in that year, and say so.
Waste with no weights. Ask your contractor, who can often supply historic tonnages. Our post on waste contractor data covers what to do if they can't.

Write a line about each estimate in the baseline section of your plan. A baseline with explained estimates is far stronger than one that quietly leaves a category out.

Use the right factors for the year

Calculate the baseline with the government conversion factors for the baseline year, not this year's set. The grid electricity factor alone can change a lot from one year to the next: our guide to the 2026 conversion factors explains the drop in the latest set. Using each year's own factors keeps the comparison fair.

When to recalculate the baseline

The GHG Protocol's corporate standard expects a baseline to be recalculated when something significant changes, so that you keep comparing like with like. Common triggers are:

buying or selling a business, site or major activity;
finding a significant error in the original figures;
changing your method, for example moving from mileage to fuel card data, in a way that makes a real difference;
adding a category you previously left out.

What evaluators don't like is a baseline that changes every year with no explanation. If you recalculate, say what changed and why, and show the old and new figures if you can.

How Carbon Recycling helps

You choose your baseline year in the Carbon Reduction Plan, and the app calculates it from your activity data with the factors for that year. Bills that run across two years are split by day for you. If your baseline was measured elsewhere, you can enter its Scope 1, 2 and 3 totals by hand, and there is a notes field for the explanations evaluators look for. To get started on the data, read writing a Carbon Reduction Plan without a consultant.