What this calculator does
A two-stroke mix ratio of 50:1 means fifty parts petrol to one part oil, so the oil needed is the petrol volume divided by fifty. Five litres of petrol at 50:1 takes 100 ml of oil. The arithmetic is trivial and getting it wrong is expensive, which is the only reason a calculator helps.
One detail worth noticing: 50:1 does not mean the oil is one fiftieth of the mix. It is one part in fifty-one, or 1.96% of the total, because the ratio counts parts of petrol rather than parts of the finished fuel. The difference is small enough not to matter in practice and large enough to confuse anyone checking the arithmetic.
The formula
The oil volume is the petrol volume divided by the ratio number. Total mixed fuel is the two added together, and the oil share is the oil as a percentage of that total rather than of the petrol alone. The fluid ounce conversion uses the US fluid ounce of 29.5735 ml.
| Term | Meaning |
|---|---|
| Mix ratio | Parts petrol to one part oil, written 50:1 and read as fifty of petrol for every one of oil. |
| Two-stroke oil | Oil designed to burn with the fuel rather than circulate, since a two-stroke engine has no separate oil sump. |
| Pre-mix | Fuel with the oil already blended in, as opposed to an engine that injects oil separately. |
The inputs explained
| Field | What to enter |
|---|---|
| Petrol (L) | The volume of petrol you are mixing, in litres. |
| Ratio (petrol : 1 oil) | The first number of the ratio. 50 means 50:1. Use whatever the engine manufacturer specifies rather than a general figure. |
When to use it
Mixing fuel for garden equipment
Chainsaws, trimmers, blowers and older outboards all run on pre-mix, and each may specify a different ratio. Mixing a small batch to the right ratio is better practice than keeping a large one, since mixed fuel degrades.
Converting between ratios for the same equipment
Older engines often specify richer mixes such as 25:1 or 32:1 while modern ones with better oils specify 50:1. Comparing the oil volumes shows exactly how much difference the specification makes.
Checking a mix you have already made
If you know how much petrol and how much oil went in, the oil share figure tells you what ratio you actually produced, which is useful when a measure was misread.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much oil does five litres of petrol need at each ratio?
A fixed five litres of petrol across the ratios commonly specified for two-stroke equipment.
| Ratio | Oil needed (mL) | Oil needed (US fl oz) | Oil as a share of the mix |
|---|---|---|---|
| 25:1 | 200 mL | 6.76 fl oz | 3.85% |
| 32:1 | 156 mL | 5.28 fl oz | 3.03% |
| 40:1 | 125 mL | 4.23 fl oz | 2.44% |
| 50:1 | 100 mL | 3.38 fl oz | 1.96% |
| 100:1 | 50 mL | 1.69 fl oz | 0.990% |
Questions
What happens if I use too much oil, or too little?
Too little oil is the serious one. It leads to inadequate lubrication, scoring and in short order a seized engine, and the damage is usually terminal. Too much oil is messy rather than fatal: smoke, fouled plugs, carbon build-up and poor running. If you must be wrong, be wrong on the oil-rich side.
What ratio should I use?
Whatever the engine manufacturer specifies, which is in the manual and often on a label near the fuel cap. Modern equipment is usually 50:1, older equipment often 25:1 or 32:1. Do not apply a general figure to a specific engine, since the oil specification and the ratio are set together.
Does it matter which petrol I use?
Use fresh fuel at the octane the manufacturer specifies, and be careful with ethanol blends. Ethanol attracts water and degrades faster, and in equipment used seasonally it is a common cause of fuel system problems. Many manufacturers advise against blends above E10 for small two-stroke engines.
How long does mixed fuel keep?
Not long. Pre-mix is generally considered good for about 30 days in a sealed container, less in heat or with an ethanol blend. Mixing what you will use rather than filling a large can is the practical answer, and a fuel stabiliser extends it if you cannot.
For concentrate-to-water ratios in the same form, see the dilution ratio calculator. For working out fuel consumption from a fill-up, see the fuel consumption calculator.