Octane Rating Converter
US AKI pump octane from RON and MON.
What it does
Octane math differs by ocean. Europe posts RON from the gentler lab test, while US pumps post AKI, the average of RON and harsher MON. A 95/85 fuel reads 95 in Europe and 90 in the US for the same gasoline.
The tool above averages the pair. Defaults of RON 95 with MON 85 give 90 AKI with a 10 spread. A 98/88 pair gives 93 AKI. Spread measures sensitivity to hard conditions, which is why two fuels with equal AKI can behave differently in hot engines.
Higher than needed rarely pays. Makers set a minimum to prevent knock; above that, power and mileage stay flat while price climbs. Ethanol blends post their blended AKI at the pump, so E10 numbers already include the alcohol lift.
This page converts ratings only. It does not test fuel, certify blends, or override the maker minimum. When in doubt, run the grade on the filler cap.
How to use
- Enter the RON value.
- Enter the MON value.
- Read AKI first for US pumps.
- Read spread for sensitivity.
- Read the lower grade for the weak side.
- Compare against the maker minimum.
Formula
- RON
- Research Octane Number, the European post.
- MON
- Motor Octane Number from the harder test.
- AKI
- US pump number, (RON+MON)/2.
Pump posted US number is the average of research and motor methods. At equal RON and MON the spread is zero and all three read together. Blends post their blended result.
Worked example
Example: RON 95 with MON 85
Defaults: RON 95 with MON 85 (regular US 87-90 style math shown on 90). Europe posts RON.
- Inputs: RON 95 with MON 85.
- AKI: (95 plus 85) / 2 is 90 AKI.
- Spread: 95 minus 85 is 10 spread.
- Lower: min of the pair is 85 lower.
- A 98/88 pair gives 93 AKI. Tool shows the same.
Reference table
| RON and MON | AKI | Spread |
|---|---|---|
| 95 with 85 | 90 AKI | 10 spread |
| 98 with 88 | 93 AKI | 10 spread |
| 91 with 82 | 86.5 AKI | 9 spread |
| 100 with 90 | 95 AKI | 10 spread |
| 92 with 82 | 87 AKI | 10 spread |
Limits
- Rating math only. No fuel testing.
- Blends post blended numbers. Components differ.
- Engine specific knock excluded.
- No ethanol content math.
Common mistakes
- Comparing RON with AKI directly.
- Buying premium over maker spec for power.
- Ignoring MON spread in hot engines.
- Reading spread as quality grade.
- Mixing E85 flex math with gasoline grades.
Try it yourself
Quick checks. Answers sit tucked under each question.
RON 98 with MON 88?
AKI is (98 plus 88) / 2 is 93 AKI with 10 spread.
RON 91 with MON 82?
AKI is (91 plus 82) / 2 is 86.5 AKI with 9 spread.
Questions
How is US pump octane figured?
AKI = (RON plus MON) / 2. Here 90 AKI.
What is RON?
Research method, gentler conditions. MON runs the harder test.
What is MON?
Motor method, harder conditions. The gap is sensitivity.
What is spread?
RON minus MON. Here 10 spread.
Is premium always better?
No. Use the grade the maker calls for. Higher than needed rarely helps.
Does ethanol change this?
No. Ethanol blends shift readings. E10 pumps post the blended AKI.