You've booked a flight to Thailand for £400. What you may not have considered: the return journey produces roughly 1.5 tonnes of CO₂e per passenger — approximately the same as driving a petrol car for six months.
Aviation accounts for around 2.5% of global CO₂ emissions, but because of high-altitude warming effects, its total climate impact is estimated to be around double that. For frequent flyers, a single transatlantic return trip can represent a significant portion of a sustainable annual carbon budget.
A flight carbon offset calculator gives you the actual number — and tells you exactly what it costs to compensate for it through a verified offset project.
The raw CO₂ figure understates a flight's climate impact. At cruising altitude, aircraft emit not just CO₂ but also water vapour, nitrogen oxides, and contrails — all of which have a warming effect. This is captured by the concept of radiative forcing: the total warming impact of aviation emissions is roughly 2× the CO₂ alone.
The calculator uses DEFRA/BEIS emission factors that already include radiative forcing. The figure you see is the real-world impact — not the optimistic ground-level CO₂ number.
The 0.115 kg per passenger-kilometre figure is the most widely used standard for individual flight carbon estimates. It reflects economy class on a modern commercial aircraft, with radiative forcing included.
Emissions are allocated per seat — and seats in premium cabins take up far more space than economy seats. Fewer passengers per aircraft means each one is allocated a larger share of the total fuel burn:
| Class | Multiplier | Reason |
|---|---|---|
| Economy | 1.0× (baseline) | Standard pitch and width — most passengers per flight |
| Premium / Business | 1.5× | Larger seat footprint, lie-flat beds, fewer passengers |
| First Class | 2.0× | Effectively occupies the space of two economy seats |
Flying first class produces twice the emissions per journey as economy. The seat upgrade is also a carbon upgrade.
Route: London Heathrow → JFK (5,570 km each way, 11,140 km return)
| Class | CO₂e per passenger | Offset cost (~£12/tonne) | Tree equivalent |
|---|---|---|---|
| Economy | 1,281 kg (1.28 t) | ~£15 | ~58 trees/year |
| Premium/Business | 1,922 kg (1.92 t) | ~£23 | ~87 trees/year |
| First Class | 2,562 kg (2.56 t) | ~£31 | ~116 trees/year |
Tree equivalent based on ~22 kg CO₂ absorbed per mature tree per year.
Economy class London–New York return costs around £15 to offset. Less than a pint at Heathrow Terminal 5.
Offsetting funds projects that reduce or remove CO₂ elsewhere — compensating for emissions that can't currently be avoided. The most common project types:
Quality varies enormously. The calculator links to four providers with independent verification: Gold Standard, South Pole, Terrapass, and atmosfair — all certified to internationally recognised standards.
It's a fair challenge. The honest answer: certified offsetting is better than nothing, but flying less is always better than offsetting more.
The most reliable offsets are those verified to Gold Standard or Verra/VCS certification. These are independently audited and have a clear methodology for measuring actual carbon reduction. Cheaper, uncertified "offset certificates" are often worth little.
For unavoidable flights — family commitments, essential work travel, genuine once-in-a-lifetime journeys — a verified offset is a meaningful action. Just don't use it as a mental licence to fly more than you otherwise would.
The most effective interventions, in order of impact:
Aviation is one of the hardest sectors to decarbonise. Unlike road transport, there is no electric alternative at scale for long-haul flights. Sustainable aviation fuel (SAF) exists but currently accounts for less than 1% of global jet fuel consumption. Hydrogen and electric aircraft remain a decade or more away for commercial routes.
In this context, voluntary offsetting — imperfect as it is — represents a meaningful bridge: a way for individuals to take financial responsibility for their emissions while the industry works towards structural solutions.
The flight happened. The emissions are real. The question is only what you do next.