Carbon Budget Years Calculator
How long a remaining carbon budget lasts — at today's emissions, and if emissions fall by a steady percentage each year — with the year it runs out, the cut rate that would make it last to a target year, and the emissions still allowed.
A carbon budget is a fixed quantity of emissions compatible with a temperature limit; at a constant rate it lasts budget over rate.
How the carbon budget years calculator works
A carbon budget is a fixed quantity of emissions compatible with a temperature limit; at a constant rate it lasts budget over rate. If emissions fall by a fixed percentage a year, the cumulative total converges — to the current rate over the cut rate — and the budget may never run out if that total fits inside it. Solving for the cut rate that lands on a target year is the number climate plans are built around. The budget and the rate are inputs, because both are revised as the science and the accounts update.
Formula: years at constant rate = budget / rate; cumulative with cuts = rate × (1 − (1 − c)^t) / c; exhaustion when cumulative = budget
Worked examples
| Inputs | Years the budget lasts at today's rate | Note |
|---|---|---|
| 250 Gt at 40 a year, 5% cuts | 6.25 | still exhausted before 2040 |
| Steeper cuts | 6.25 | inside the budget |
| A larger budget | 12.5 | more room |
FAQFrequently asked questions
What is a carbon budget?
The total CO₂ that can still be emitted while keeping warming below a chosen limit with a chosen probability. Estimates for 1.5 °C are a few hundred gigatonnes; for 2 °C about a thousand. They are revised each assessment, so the figure is an input.
Why does a percentage cut change the picture so much?
Because a steady percentage cut makes the future total finite — today's rate over the cut rate. If that total fits inside the budget, the budget never runs out; if not, it just runs out later.
What cut rate is needed?
Rate over budget for the budget to hold: 40 Gt a year against 250 Gt is 16% a year, which is why 1.5 °C pathways look so steep. Two degrees allows a few per cent a year.
What does the target-year cut mean?
The steady annual reduction that brings emissions to a tenth of today by the year you choose — a proxy for net zero, since the last few per cent are usually handled by removals.
Is a steady percentage realistic?
It is the simplest pathway shape; real plans front-load or back-load the cuts. Cumulative emissions are what matter, and the page shows those for any rate.
Where these figures come from
- UK DESNZ / DEFRA — Greenhouse gas reporting: conversion factors — the flight, fuel and waste emission factors used here
- Scarborough et al. (2014), Climatic Change — Dietary greenhouse gas emissions — the per-diet daily emission figures
- US EPA — Technical Assistance Document for the Reporting of Daily Air Quality (AQI) — the PM2.5 breakpoints the air quality calculator uses
- Australian Government — National Greenhouse Accounts Factors — the official Australian emission factors
Last checked: September 2026. Factors are taken from published government and peer-reviewed sources, each named on the page with its year.