Work out the oil
For engines whose manual says to mix oil into the gasoline. Pick the ratio it gives.
Calculators
Two-Stroke Oil Mix Calculator
Enter how much gasoline you're mixing and the ratio from your outboard's manual. You get the oil in fluid ounces or milliliters, and the chart below covers the common ratios.
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2-stroke oil mix chart in gallons
Fluid ounces of oil for each amount of gasoline. Find your manual's ratio in the top row.
| Gasoline | 25:1 | 32:1 | 40:1 | 50:1 | 100:1 |
|---|---|---|---|---|---|
| 1 gal | 5.1 | 4.0 | 3.2 | 2.6 | 1.3 |
| 2 gal | 10.2 | 8.0 | 6.4 | 5.1 | 2.6 |
| 2.5 gal | 12.8 | 10.0 | 8.0 | 6.4 | 3.2 |
| 3 gal | 15.4 | 12.0 | 9.6 | 7.7 | 3.8 |
| 5 gal | 25.6 | 20.0 | 16.0 | 12.8 | 6.4 |
| 6 gal | 30.7 | 24.0 | 19.2 | 15.4 | 7.7 |
| 10 gal | 51.2 | 40.0 | 32.0 | 25.6 | 12.8 |
2-stroke oil mix chart in liters
Milliliters of oil for each amount of gasoline.
| Gasoline | 25:1 | 32:1 | 40:1 | 50:1 | 100:1 |
|---|---|---|---|---|---|
| 1 L | 40 | 31 | 25 | 20 | 10 |
| 2 L | 80 | 63 | 50 | 40 | 20 |
| 5 L | 200 | 156 | 125 | 100 | 50 |
| 10 L | 400 | 313 | 250 | 200 | 100 |
| 20 L | 800 | 625 | 500 | 400 | 200 |
| 25 L | 1,000 | 781 | 625 | 500 | 250 |
Which ratio does your outboard use?
The one in its owner's manual, and only if the manual says to premix. Ratios differ by maker and model, and a new engine often takes a richer mix while it breaks in. Two examples:
- Mercury 2.5 and 3.3 hp two-strokes: 25:1 (4%) for the first tank of fuel, then 50:1 (2%). Mercury says the oil must be NMMA-certified TC-W3 two-cycle oil.
- Tohatsu M9.9D2, M15D2 and M18E2: 25:1 during break-in, then 50:1, with Tohatsu's own two-stroke oil or an NMMA-certified TC-W3 oil.
How to mix two-stroke fuel and oil
Find the ratio
Look in your owner's manual. Use the break-in ratio if the engine is new.
Work out the oil
Enter the gasoline and the ratio in the calculator, or read the chart. By hand, gallons times 128 divided by the ratio gives fluid ounces.
Pour the oil and some gasoline into an approved container
Mercury says to pour the full amount of oil and about one gallon of gasoline into an approved container, then shake them together until they are thoroughly mixed.
Add the rest of the gasoline
Pour in the remaining gasoline and shake the container again.
Fill the tank
Stop the engine first, fill outdoors away from sparks and open flames, and don't smoke. Tohatsu says to discharge static electricity from your body before refueling and to wipe up spilled gasoline immediately. Don't overfill: Mercury warns that fuel expands as it warms and can leak under pressure from a full tank.
Mixing in a portable or built-in tank
Mercury's manual for its 2.5 and 3.3 hp engines says to mix the fuel and oil in an approved container before it goes into the motor's tank, and never to pour them into the tank separately. Tohatsu's manual for its M9.9D2 to M18E2 allows mixing in a portable fuel tank: oil first, then gasoline, then close the cap and air vent tightly and shake.
For a tank built into the boat, Tohatsu recommends pre-mixing in a separate container. If you must mix in the built-in tank, it says to pour the oil in little by little while the gasoline goes in.
Why the makers' charts round
Mercury's chart lists 3 fluid ounces (89 mL) of oil for a gallon at 50:1 and 16 ounces for 6 gallons. The exact figures are 2.56 and 15.36 ounces, which is what the calculator shows. Tohatsu's liter figures are exact: 0.2 liters of oil for 10 liters of gasoline at 50:1.
Tohatsu warns against getting it wrong either way. Too little oil can cause severe problems such as piston seizure, and too much can shorten spark plug life and increase noxious exhaust.
Common questions
2.56 fluid ounces, about 76 mL. Mercury's chart rounds this to 3 fluid ounces.
12.8 fluid ounces, about 379 mL.
15.36 fluid ounces, about 454 mL. Mercury's chart lists 16 fluid ounces for 6 gallons.
Fifty parts gasoline to one part oil, so the oil is 2% of the gasoline. At 25:1 it is 4%.
The one in your manual. Mercury's 2.5 and 3.3 hp two-strokes take 25:1 for the first tank, and Tohatsu's M9.9D2, M15D2 and M18E2 take 25:1 during break-in. Both then run 50:1.
Mercury says its 2.5 and 3.3 hp engines need NMMA-certified TC-W3 two-cycle oil. Tohatsu calls for its own two-stroke oil or an NMMA-certified TC-W3 oil.
Tohatsu says too little oil can cause severe engine problems such as piston seizure, and too much can shorten spark plug life and increase noxious exhaust.
Sources
Mercury, 2.5/3.3 TwoStroke Operation and Maintenance Manual; Tohatsu, M9.9D2, M15D2 and M18E2 Owner's Manual.