How to use it
- Type the Petrol volume in litres, for example 5.
- Enter the Ratio (1 : N) as the engine manual gives it. For a ratio of 1 : 50, type 50.
- Read Oil to add in millilitres and the Total mix in litres.
- Check Oil for other ratios if the manual or the oil label gives a different ratio.
How the oil is measured
A ratio of 1 : N means one part oil to N parts petrol, both by volume. The oil volume is the petrol volume divided by N: 5 L ÷ 50 = 0.1 L.
One litre is one cubic decimetre, which is 0.001 cubic metre (NIST SP 811, Appendix B.8). So 0.1 L is 100 mL.
Oil (L) = petrol (L) ÷ N
Oil (mL) = oil (L) × 1,000
Total mix (L) = petrol (L) + oil (L)Which ratio to use
- The engine manual or the oil label gives the ratio. Use that figure, because the calculator does not know your engine.
- Ratios differ between engines and oils, so an old manual or a different can of oil can give a different ratio.
- Mix the oil and the petrol in a clean container before you fill the tank, so the oil spreads evenly.
What this calculator does not include
- Engine wear, the quality of the fuel and the specification of the oil.
- Four-stroke engines, which have their own oil supply and do not take a fuel mix.
- Petrol that is already in the tank. The calculator works only on the volume you enter.
Worked example: 5 litres of petrol at 1 : 50
The oil is 5 ÷ 50 = 0.1 L, which is 100 mL. The total mix is 5 + 0.1 = 5.1 L.
The table gives the oil for the same 5 litres at 1 : 25, 1 : 40, 1 : 50 and 1 : 100: 200 mL, 125 mL, 100 mL and 50 mL.
| Ratio (1 : N) | Oil (ml) |
|---|---|
| 25 | 200 |
| 40 | 125 |
| 50 | 100 |
| 100 | 50 |
Frequently asked questions
How much two-stroke oil do I add to 5 litres of petrol at 1 : 50?
Add 100 mL of oil. Divide the 5 litres by 50 to get 0.1 litre, which is 100 millilitres.
What does a ratio of 1 : 50 mean?
It means one part of oil for every 50 parts of petrol, by volume. The oil is a fiftieth of the petrol volume, so 50 litres of petrol takes 1 litre of oil.
Where do I find the ratio for my engine?
In the engine manual, or on the oil label or the maker's instructions. Use that figure, because ratios differ between engines and oils.
Does the calculator need to know my engine or my oil?
No. It divides the petrol volume by the ratio you enter, so the result is right only when that ratio suits your engine and your oil.
Does the calculator keep the numbers I type?
The page works in your browser and sends nothing to a server. The numbers stay in the page address, so a copied link shows the same figures.
Sources
Figures and text last checked on Thursday, November 12, 2026.