Type how much petrol you are mixing and the ratio your engine’s manual asks for. The oil comes out below, in ounces and millilitres, with the whole ratio as a chart you can print for the shed wall.
5 gal of petrol at 50:1
Add 12.8 fl oz of oilThat is 379 ml. About one standard 12.8 fl oz bottle.
- Oil
- 12.8 fl oz (379 ml)
- Petrol
- 5 gal (18.93 L)
- In the tank when mixed
- 5.1 gal
- Oil by volume
- 1.96%
| Petrol | Oil | Oil, the other way |
|---|
Your engine’s manual outranks this page. If it names a ratio, use that one — and if fuel or a carburettor is a job beyond you, it is worth paying somebody who does it every day.
Oil standard and fuel notes checked 2026-09-20.
Quick answer: at 50:1 — the ratio most modern outboards ask for — one US gallon of petrol takes 2.6 fl oz (76 ml) of oil, and five gallons take 12.8 fl oz (379 ml). At 40:1 a gallon takes 3.2 fl oz, at 32:1 it takes 4 fl oz, and at 25:1 it takes 5.1 fl oz. The ratio is petrol to oil by volume, so the oil goes on top of that much petrol rather than being part of it. Use the ratio your engine’s manual names, and use oil marked TC-W3 — the certification for water-cooled marine engines. Air-cooled two-stroke oil from the garden shelf is a different product.
How to use it
Pick the ratio your manual asks for, type how much petrol you are about to mix, and read the oil figure. It comes out in both fluid ounces and millilitres, because pumps and bottles rarely agree on which one they use. If you have already emptied a bottle into the can and lost track, switch to The oil I poured and it tells you how much petrol that oil is mixed for instead.
The chart underneath is the whole ratio at the amounts people actually mix, laid out to print. Stick it inside the shed door and you will not need this page again.
The mix ratio chart, at a glance
| Ratio | Per US gallon | Per 5 US gallons | Per litre | Per 20 litres |
|---|---|---|---|---|
| 100:1 | 1.3 fl oz | 6.4 fl oz | 10 ml | 200 ml |
| 50:1 | 2.6 fl oz | 12.8 fl oz | 20 ml | 400 ml |
| 40:1 | 3.2 fl oz | 16 fl oz | 25 ml | 500 ml |
| 32:1 | 4 fl oz | 20 fl oz | 31 ml | 625 ml |
| 25:1 | 5.1 fl oz | 25.6 fl oz | 40 ml | 800 ml |
| 24:1 | 5.3 fl oz | 26.7 fl oz | 42 ml | 833 ml |
| 16:1 | 8 fl oz | 40 fl oz | 63 ml | 1,250 ml |
Those are the same numbers the calculator gives, rounded the same way. Nothing on this page is a recommendation for a particular motor: the ratio comes from your manual, and the calculator does the arithmetic.
Which oil an outboard actually wants
Two-stroke oil is not one product. An outboard is water-cooled, and the oil made for it is certified to the NMMA’s TC-W3 standard: bench tests for fluidity, for mixing properly with the fuel, and for rust, plus engine tests for carbon build-up and ring sticking. Oil for a chainsaw or a strimmer is built for an air-cooled engine that runs far hotter, with a different ash and detergent package. It is cheaper, it is on the same shelf, and it is not the same thing.
If a bottle does not say TC-W3 on it, it is not certified for your outboard, whatever the front label suggests. The NMMA publishes the list of registered oils.
Why a mixed tank goes bad over the winter
Most pump petrol in the US is E10 — up to 10% ethanol — and ethanol absorbs water out of the air. Take on enough and the ethanol and the water drop out of the mix and sink to the bottom of the tank, which is exactly where the fuel pick-up sits. The engine then draws a corrosive water-and-ethanol layer instead of fuel. This, far more often than anything mechanical, is why a motor that ran perfectly in September will not start in April.
Three things help, in order of how much: mix only what you will actually burn, use ethanol-free fuel where you can get it, and add a stabiliser to anything that will sit. None of them un-does a tank that has already separated — that fuel has to come out.
Mixing it properly
- Mix in the can, never in the tank. Pouring oil into a tank of petrol leaves you with oil on the bottom and petrol going through the engine.
- Oil first, then petrol. The fuel going in does the mixing for you.
- Rock the can. Ten seconds, lid on. Do it again if the can has been standing.
- Write the ratio on the can. A can of 50:1 and a can of 32:1 look identical in three weeks’ time.
- Do it outside, away from anything hot. Petrol vapour is heavier than air and it travels.