Last updated: 2026-07-02

What a tree carbon offset estimates

A tree carbon offset answers a simple question: how many trees would need to grow, and for how long, to absorb a given amount of carbon dioxide? Trees pull CO₂ out of the air through photosynthesis and lock the carbon into wood, roots, and soil. Counting the trees required to match a quantity of emissions is a common way to put a footprint into tangible terms.

How the estimate is built

The core relationship divides the CO₂ to be offset by the amount one tree captures over the chosen period:

Trees = CO₂ ÷ (sequestration rate per tree per year × years)

Each part uses a plain, editable input:

CO₂ to offset. The total mass of carbon dioxide, entered in tonnes, kilograms, or pounds.

Sequestration rate. How much CO₂ a single tree removes per year. The default of 21 kg reflects a commonly cited average for a maturing tree. Real values range widely — roughly 10 to 40 kg per year — depending on species, age, climate, and how the carbon is accounted for.

Time horizon. The number of years the trees grow. Because trees capture carbon gradually, a longer horizon means each tree removes more, so fewer trees are needed.

The result is rounded up to a whole number, since a partial tree cannot offset the leftover CO₂.

Worked example

Suppose you want to offset 4 tonnes of CO₂ over 10 years at the default rate of 21 kg per tree per year:

• Convert the CO₂: 4 tonnes = 4,000 kg.
• Per-tree total over the horizon: 21 kg/yr × 10 yr = 210 kg per tree.
• Exact trees: 4,000 ÷ 210 ≈ 19.05.
• Rounded up: 20 trees.

So roughly 20 trees, grown for a decade, would sequester about 4 tonnes of CO₂ at the assumed rate. Doubling the horizon to 20 years lets each tree capture 420 kg instead of 210 kg, so only about 10 trees would be needed for the same 4 tonnes.

Putting emissions in context

These figures assume every tree survives and grows at the average rate for the whole period, which real plantings rarely achieve.

Limitations

• Rates are averages. Per-tree sequestration varies by species, age, spacing, soil, and climate, and different accounting methods give different numbers. Adjust the rate input for a specific species or programme.
• Carbon is captured slowly. A newly planted sapling absorbs little at first; the full offset accrues over the whole horizon, not immediately.
• Storage can reverse. Fire, drought, disease, harvesting, or land-use change can release stored carbon back to the atmosphere.
• Survival is not guaranteed. Not every planted tree lives to maturity, so real programmes plant extra to account for losses.

Tree planting stores carbon over decades and can be undone, so it works best as a complement to reducing emissions at the source rather than a substitute for it. Use this calculator as a rough awareness and planning tool, not as certified carbon accounting.