Vladimir Sukhachev/Shutterstock

When two people book the same flight, they can get wildly different carbon footprints from online calculators. Many carbon calculators leave out big chunks of climate impact or rely on oversimplified assumptions.

Here’s what’s missing, why it matters and a practical checklist you can use to judge any flight estimate.

1. CO₂ isn’t enough

If a tool only reports in kilograms of carbon dioxide (CO₂), it misses two other groups of emissions:

Carbon dioxide equivalent (CO₂e) is a metric that converts the impact of other greenhouse gases such as methane to an equivalent of carbon dioxide using science-based data that shows their global warming potential. If a calculator only shows CO₂, it undervalues the footprint.

A calculator that references CO₂e includes CO₂ as well as those other greenhouse gas emissions so it is more comprehensive. Good practice cites the metric and links to the emissions table used.

Non-CO₂ impacts are the ways that aircraft warm the climate via nitrogen oxides, water vapour and contrails. These trap heat, effectively acting like a blanket that reflects heat from the Earth back down to the ground. Research shows that these non-CO₂ effects can be comparable to or larger than CO₂ alone on common time horizons.

This means that a CO₂e-only number still undervalues total climate impact for aviation because it doesn’t take into account the effect of nitrogen oxides, water vapour and contrails. The magnitude of non-CO₂ impacts varies with altitude, latitude, weather and time of day, but it’s too large to ignore.

Calculators should quantify it transparently, with uncertainty. The best calculators take into account CO2e and non-CO2 categories.

2. Planes don’t really fly in straight lines

Another source of undervaluation is distance. Many tools assume a great-circle path (shortest distance between the origin and destination airports), plus a small, fixed distance. Real flights are shaped by winds, storms, congestion, military zones and airspace closures that force long detours. Even regular routes between A and B can change on a daily basis due to these variables.

Studies show average detours of several percent even in “normal” times, let alone the current geopolitical state of the world. Increased distance means more fuel consumption and more emissions. So if a calculator doesn’t adjust for route deviations, it will undershoot fuel burn.

By using recent historical route data to adjust pre-flight estimates, the number better reflects how that route is flown now, not how it flies in idealised conditions.

3. Scope: what counts?

Even when distance and emissions are handled well, many tools leave the system boundary, which defines which sources of emissions are included or excluded, too narrow. Many tools disregard the emissions involved in mining for the materials, extracting the fuel and constructing the aircraft themselves.

A comprehensive, standards-aligned footprint includes well-to-tank emissions (fuel extraction, refining, transport), in-flight services and the aircraft and airport life cycle.

ISO 14083, the international standard for the quantification and reporting of greenhouse gas emissions from transport chains (2023) sets common rules for quantifying and reporting transport emissions. Good calculators align to it to make their scope explicit and comparable.

But better calculators go beyond it to cover the full scope of flight emissions. If a carbon footprint excludes upstream emissions or infrastructure, expect it to be lower than a comprehensive life cycle view.

yellow background, white toy plane and white notebook with pen
Some aviation carbon calculators make too many generalised assumptions. eamesBot/Shutterstock

4. How your emissions are allocated

Per-passenger emissions accounting involves dividing aircraft-level impacts across seats and cargo. Cabin class and seat density change the space per passenger. Seats in business or first class can have multiple-times the per-capita impact of economy class on the same aircraft.

Load factors (how full a plane is) change the denominator too. Passengers on a plane that’s half full should be allocated twice the emissions. Good methods use load factors and aircraft seating plans that are route, airline and season specific, rather than generalised global assumptions.

5. Baggage and belly cargo

From a traveller’s perspective, it feels unfair if a hand-bag-only passenger gets the same carbon allocation as someone checking 30kg of luggage. Most calculators assume a single average baggage weight, so light travellers subsidise heavy ones. Better approaches allow users to declare baggage or apply route specific baggage factors.

Most passenger flights also carry freight in the belly of the plane. If a calculator attributes all aircraft emissions to passengers, it overstates per-passenger numbers. Fair practice is to split emissions between passengers and airline cargo using load data.

What ‘better’ looks like

Based on recent peer-reviewed work, here is a checklist for any flight calculator. When you next see a carbon footprint for your flight, ask whether it is:

  • comprehensive – does it include CO₂, CO₂e and non-CO₂ as well as aligning with ISO 14083 or better, covering the full scope of emissions for an air travel passenger?

  • accurate – does it use modern, data-driven methods to predict flight path and fuel consumption, and specific, up to date data for all variables, including load factors and seating configurations?

  • transparent – are the methods and data sources published with its limitations and is it peer-reviewed?

  • effectively communicated – is there a breakdown of emission sources and is it easy to understand and make targeted climate decisions?

When calculators tick all these boxes, the debate shifts from arguing over a single figure to acting on the biggest drivers. That’s how footprints become tools for change rather than just numbers on a screen.

Don’t have time to read about climate change as much as you’d like?
Get a weekly roundup in your inbox instead. Every Wednesday, The Conversation’s environment editor writes Imagine, a short email that goes a little deeper into just one climate issue. Join the 47,000+ readers who’ve subscribed so far.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Finn McFall, University of Surrey and Xavier Font, University of Surrey

Read more:

Finn McFall works on a Knowledge Transfer Partnership between Therme Group and the University of Surrey, co-funded by UKRI through Innovate UK.

Xavier Font works for the University of Surrey. He is part of the academic team working on a Knowledge Transfer Partnership between Therme Group and the University of Surrey, co-funded by UKRI through Innovate UK.