25:1 Fuel Mix — Gas to Oil Ratio Calculator & Chart
A 25:1 two-stroke fuel mix means 5.12 oz of 2-stroke oil for every US gallon of gas, or 40 ml of oil per litre. Use the calculator to get the exact amount for any quantity of fuel, then check the full 25:1 mix chart in gallons and litres below. It works the same for "2-cycle", "2-stroke" and "two-stroke" engines — it's all the same premix.
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At 25:1 you add 5.12 oz of oil per gallon (40 ml per litre). For a 5-gallon can of gas that's 25.6 oz of 2-stroke oil.
Full mix chart
25:1 ratio
| Gallons of gas | oz of oil |
|---|---|
| 0.5 | 2.56 |
| 1 | 5.12 |
| 1.5 | 7.68 |
| 2 | 10.24 |
| 2.5 | 12.8 |
| 3 | 15.36 |
| 4 | 20.48 |
| 5 | 25.6 |
| 6 | 30.72 |
| Litres of gas | ml of oil |
|---|---|
| 1 | 40 |
| 2 | 80 |
| 3 | 120 |
| 4 | 160 |
| 5 | 200 |
| 10 | 400 |
| 15 | 600 |
| 20 | 800 |
| 25 | 1,000 |
Where 25:1 still shows up
Twenty-five to one is the richest ratio in general use. It is called for by older outboards (many Johnson, Evinrude and Mercury manuals from the 1960s specified 24:1 or 25:1), by vintage chainsaws and industrial two-strokes, and by some break-in schedules where the first tanks through a new or rebuilt engine are double-oiled.
If your two-stroke outboard has oil injection, do not premix: the system meters oil on its own. A 25:1 premix applies only to premix motors, or to one whose injection system has been deliberately bypassed.
24:1 and 25:1 are not really different ratios
At 25:1 you add 5.12 oz per gallon; at 24:1 it is 5.33 oz. The gap is about a quarter of an ounce per gallon, well inside the error of any shop measuring cup.
Older manuals used 24:1 because a 16 oz US pint treats exactly 3 gallons at that ratio. If your plate says 24:1, mixing 25:1 is fine, and the reverse is fine too.
What a 25:1 engine behaves like
A healthy engine at 25:1 smokes visibly, especially at idle and when cold, and leaves oily residue in the exhaust. That is normal at this ratio and not a fault. What does need watching is the spark plug, which fouls sooner, and the exhaust staying clear.
Match the oil to the cooling type: TC-W3 for water-cooled outboards, a JASO-rated oil for air-cooled equipment. A 5-gallon can at 25:1 takes 25.6 oz of oil, and in metric it is 40 ml per litre.
Common questions
[01] What is 25:1 in ounces per gallon?
25:1 is 5.12 fluid ounces of oil per US gallon of gas (128 ÷ 25 = 5.12).
[02] What is 25:1 in ml per litre?
25:1 is 40 ml of oil per litre of gas (1000 ÷ 25 = 40).
[03] What happens if I add too much oil at 25:1?
Too much oil makes the engine run rich — it can smoke, foul the spark plug and lose power. A little extra is safer than too little, but stay close to 25:1 and follow your owner's manual.
[04] Is 24:1 the same as 25:1?
In practice, yes. 25:1 is 5.12 oz per gallon and 24:1 is 5.33 oz, a quarter-ounce apart and well inside the error of any measuring cup. If your plate says one, the other is fine.
[05] Why does my older outboard call for 25:1?
Because it was designed around the oils and tolerances of its era, and many 1960s manuals specified 24:1 or 25:1. Some modern motors also call for 25:1 during break-in, for the first few tanks.