AKI (Anti-Knock Index): the number on every US pump
AKI, the Anti-Knock Index, is the octane number shown on US and Canadian pumps, printed as (R+M)/2. It's the average of RON and MON, and it's required by US federal rule to appear on a yellow pump sticker.
In this entry
What AKI is
Anti-Knock Index (AKI) is the octane number posted on every fuel pump in the United States and Canada. It's the average of two separate lab measurements, Research Octane Number (RON) and Motor Octane Number (MON), taken under different test conditions.
AKI exists because it's the specific figure US and Canadian regulation requires retailers to display, not because it measures anything RON and MON don't already capture between them.
Printed on US and Canadian pumps as (R+M)/2. Full detail on the two tests it averages is in RON and AKI.
Why it's on the pump by law
AKI isn't just a convention; it's a specific line item in US federal rules for how fuel must be labelled.
In the United States, a federal rule requires every pump to display a yellow octane sticker showing the AKI figure as (R+M)/2. It's one of the few places a fuel quality number is a legal labelling requirement rather than just a marketing figure.
Regular, Midgrade and Premium
US and Canadian pumps generally offer three grades, named for what they're for rather than their exact number.
US grade names and their AKI figures
The altitude exception
The AKI number for Regular isn't fixed nationwide.
Thinner air at high altitude reduces engine knock, so some fuel retailers in Rocky Mountain states sell an 85 AKI Regular grade instead of 87. It's a legal, intentional regional variation, not a mislabelled pump.
Where AKI is used
AKI is a regional convention, not a universal one.
How this shows up on FuelTheGuide
We show US and Canadian prices with their own AKI grade names, not converted onto the RON scale, so the number matches what the pump in that country actually displays.
We label these prices with their US grade names, not converted to RON, so they match what a US or Canadian pump actually shows.
Questions and answers
Is a higher AKI number always a better fuel?
Not in a way that matters for most drivers. It means the fuel resists knock better at a given compression and timing, not that it holds more energy or improves general engine performance. Use the grade your manufacturer specifies.
Why do some US regions sell 85 AKI instead of 87 as Regular?
At high altitude, thinner air reduces the knock tendency of petrol, so some states in the Rocky Mountains allow 85 AKI as the standard Regular grade. It is a legal regional variation tied to altitude, not a lower-quality product. The US DOE fueleconomy.gov addresses altitude and octane and confirms that most modern vehicles run normally on 85 AKI at elevation.
Does AKI apply to diesel?
No. Diesel quality is rated by cetane number, which measures ignition quality under compression rather than knock resistance. AKI and RON are petrol-only ratings.
Is the (R+M)/2 sticker the same everywhere in the US?
The formula is federal, so the calculation is the same nationwide, but the actual number on the sticker can vary slightly by region and by season because refiners adjust the blend. The number shown is what the pump is dispensing at that moment.
Sources and further reading
- Fuel Rating Rule (octane posting requirement) Federal Trade Commission, ftc.gov
- Gasoline explained: octane in depth US Energy Information Administration, eia.gov
- Fuel prices tool and grade lists OpenVan.camp, openvan.camp/compare/fuels/fuel-prices