Sustainable aviation fuel (SAF) is a “drop-in” fuel that can be mixed with fossil jet fuel and used in existing fueling infrastructure and engines. Once SAF is mixed with conventional jet fuel, it cannot be physically traced. So how do airlines know how much SAF is used on their flights? And how does SAF use by airlines affect your personal carbon footprint? Welcome to the world of SAF accounting.

I’ll tackle the first question, the easier one, by explaining mass-balance accounting. This practice is used when chemically identical feedstocks with unique environmental characteristics are blended. In mass-balance accounting, the mass of SAF blended into the fuel system is matched to an equivalent mass of fuel being withdrawn by the fuel purchaser even though the original fuel molecules have not been directly traced. This allows a SAF purchaser to claim SAF use even if most of the fuel molecules in shared fuel supply infrastructure are of fossil origin.

But what if SAF is not even supplied to a specific airport/location? Can SAF use still be claimed for departing flights? This is where book-and-claim accounting comes in. In this practice, a fuel supplier mixes SAF into the shared jet fuel infrastructure at another airport on behalf of the airline making the purchase. Even though other airplanes, likely from other carriers, are the ones receiving the SAF molecules in their fuel, the airline purchasing the SAF gets credit for its use.

If an airline gets credit for SAF uplifted into aircraft from a different carrier, how can we be sure that the greenhouse gas (GHG) emission reductions from SAF use are properly tracked and the other airline isn’t also claiming credit (i.e., double counting)? For this the aviation industry is developing a system of SAF “proof of sustainability” certificates that document each batch of SAF production and its environmental characteristics. If SAF crediting is further checked and verified against a central trusted registry—for example, the International Air Transport Association (IATA)’s recently announced SAF Registry—users can be more confident in the correct attribution of fuel.

Figure 1. System being developed to track SAF use and assign GHG savings

Source: Adapted from IATA.

As shown in the figure, SAF sustainability certificates are tracked in registries that log SAF purchases. Each certificate also records information on the volume and type of SAF produced and its life-cycle GHG emissions intensity. Airlines purchasing the SAF are given custody of a corresponding certificate that indicates the purchased SAF was mixed into the aviation fuel system for use. If there is a one-to-one ratio between the quantity of certificates purchased and the volume of SAF uplifted, then there has been no double counting within corporate GHG reporting. And if life-cycle analysis best practices are followed, it’s reasonable to assume that GHG reduction estimates on SAF certificates are robust.

Now we can turn to the second question about personal carbon footprint. Importantly, there’s a second component to the SAF certificate system that allows for the allocation of SAF GHG reductions to individual travelers or air-freight shippers. These end-user certificates are known as Scope 3 certificates because a business’s air travel emissions are covered under the Scope 3 emissions category of the GHG Protocol corporate emissions accounting framework. The Scope 3 certificates determine which passengers can claim a lower carbon footprint based on the use of SAF. The recently released IATA accounting and reporting methodology states that airlines can only represent lower per-seat emissions to customers if the Scope 3 certificates have been retired by the airline on their customers’ behalf. If, instead, these Scope 3 certificates are sold to corporations to offset emissions from a business’s travel or shipping activities, only the corporation purchasing the Scope 3 certificates can claim a reduced carbon footprint.

So, what happens if you purchase a SAF upgrade to offset CO₂ emissions on a personal flight? According to IATA accounting principles, the airline should retire Scope 3 SAF certificates on your behalf and use your fare premium towards SAF purchases. Note, though, that reduced GHG fare options may also potentially combine a carbon offset from another sector with a SAF purchase to arrive at the total GHG reduction value on the ticket.

Is SAF reducing the climate impact of my travel?

You may be wondering if SAF is likely to be reducing your personal carbon footprint right now. Unless you’re purchasing SAF upgrades or flying in Europe, the answer is probably no. In the European Union and the United Kingdom, SAF blending mandates came into effect at the start of 2025, but in other jurisdictions, the vast majority of SAF use is supported by selling SAF certificates to business customers and thus should not be claimed by other passengers.

It’s also important to remember that, on a global basis, the current impact of SAF on aviation emissions is minimal. In 2024, SAF accounted for 0.3% of global jet fuel production. So, for now, it’s safe to assume that the majority of fuel on most flights is conventional jet fuel derived from petroleum.