25:1 (Old outboards / Break-in)
32:1 (Vintage engines)
40:1 (Standard high-load)
50:1 (Standard modern outboard)
100:1 (Specific modern outboards)
Amount of 2-Stroke Oil to Add:
0.00
Mastering Your 2-Stroke Outboard Oil Mixture
Unlike automotive engines that have a dedicated oil sump, 2-stroke outboard motors rely on oil mixed directly into the gasoline to lubricate the crankshaft, connecting rods, and cylinder walls. Getting the ratio wrong isn't just a minor error—it can lead to catastrophic engine failure or foul your spark plugs beyond use.
Common 2-Stroke Marine Ratios
50:1 Ratio: The industry standard for most modern 2-stroke outboards produced since the late 1960s (Evinrude, Johnson, Yamaha, Mercury).
25:1 Ratio: Often used for vintage outboards (pre-1960s) or during the "break-in" period of a newly rebuilt engine.
100:1 Ratio: Rare, but specified by some later model low-emission 2-stroke engines.
Example Calculations
If you have a standard 6-gallon portable boat tank and your engine requires a 50:1 mix, the math works as follows:
6 Gallons × 128 ounces per gallon = 768 total ounces of fuel. 768 / 50 = 15.36 ounces of oil.
Why Precision Matters
Too much oil (Rich Mix): While "richer" mixtures provide better lubrication, they create excessive smoke, carbon buildup on the piston rings, and can foul spark plugs, causing the engine to sputter or stall at low idle.
Too little oil (Lean Mix): This is the most dangerous scenario. Without enough oil, friction generates intense heat, which can cause the piston to expand and seize against the cylinder wall, effectively destroying the engine.
Pro-Tip: Mixing the Right Way
Always pour your 2-stroke oil into the tank before adding the gasoline. The force of the fuel coming from the pump or a jerry can will help agitate the mixture, ensuring the oil is thoroughly suspended in the gas rather than sitting at the bottom of the tank.
function updateLabels() {
var fuelLabel = document.getElementById('fuelLabel');
var isMetric = document.getElementById('unitMetric').checked;
if (isMetric) {
fuelLabel.innerText = "Total Fuel (Liters)";
} else {
fuelLabel.innerText = "Total Fuel (Gallons)";
}
// Clear previous result when switching units to avoid confusion
document.getElementById('resultArea').style.display = 'none';
}
function calculateOilMix() {
var fuelAmount = parseFloat(document.getElementById('fuelAmount').value);
var ratio = parseFloat(document.getElementById('oilRatio').value);
var isMetric = document.getElementById('unitMetric').checked;
var resultArea = document.getElementById('resultArea');
var oilResult = document.getElementById('oilResult');
var formulaNote = document.getElementById('formulaNote');
if (isNaN(fuelAmount) || fuelAmount <= 0) {
alert("Please enter a valid fuel amount.");
return;
}
var oilNeeded = 0;
var unitName = "";
if (isMetric) {
// Calculation in Milliliters (1 Liter = 1000ml)
oilNeeded = (fuelAmount / ratio) * 1000;
unitName = "Milliliters (ml)";
formulaNote.innerText = "Calculation: (" + fuelAmount + " L / " + ratio + ") × 1000 ml";
} else {
// Calculation in Ounces (1 US Gallon = 128 fl oz)
oilNeeded = (fuelAmount / ratio) * 128;
unitName = "Fluid Ounces (fl oz)";
formulaNote.innerText = "Calculation: (" + fuelAmount + " gal / " + ratio + ") × 128 oz";
}
oilResult.innerHTML = oilNeeded.toFixed(2) + " " + unitName + "";
resultArea.style.display = 'block';
}