2-Stroke Fuel/Oil Mix Calculator
Enter how much gas you are mixing and the ratio your equipment calls for, and the calculator returns the oil to add — in US fluid ounces and milliliters at the same time. It also works in reverse: tell it how much oil you have and it computes how much gas that oil will treat. If you want a paper reference for the shop wall, the mix charts below cover 16:1 through 100:1 and are laid out to print cleanly.
Worked example: 2.5 gallons at 40:1
Suppose you are filling a 2.5-gallon can and the saw's manual specifies 40:1. The arithmetic, start to finish:
- Convert the fuel to fluid ounces. One US gallon is 128 fluid ounces, so 2.5 gal × 128 fl oz/gal = 320 fl oz of gas.
- Divide by the ratio. 40:1 means 40 parts gas to 1 part oil by volume, so 320 fl oz ÷ 40 = 8.0 fl oz of oil.
- Convert for a metric measuring cup if needed. One US fluid ounce is 29.574 mL, so 8.0 fl oz × 29.574 ≈ 237 mL.
Result: add 8.0 fl oz (237 mL) of two-stroke oil to 2.5 gallons of gas. Every cell in the charts below is this same calculation with different numbers plugged in.
How the math works
A mix ratio is parts of gas per one part of oil, measured by volume. 50:1 means fifty equal volumes of gasoline for every one volume of two-stroke oil. Read it carefully: the bigger the first number, the less oil ends up in the tank. 100:1 is half the oil of 50:1, not double.
The whole calculation is one division with a unit conversion attached: oil equals fuel divided by ratio, with both quantities in the same unit. The wrinkle is that gas and oil are conventionally measured in different units — gallons for gas, fluid ounces for oil. NIST defines the US gallon as 231 cubic inches, which divides into exactly 128 US fluid ounces, so the US-unit formula becomes: oil (fl oz) = gallons × 128 ÷ ratio.
That conversion is also why 50:1 works out to the familiar 2.6 ounces per gallon: 128 ÷ 50 = 2.56 fl oz, which oil-bottle instructions round up to 2.6. Stihl and Husqvarna operator's manuals, for example, specify 50:1 for their consumer saws and trimmers when run on the manufacturers' own two-stroke oils — which is why 2.6 oz per gallon is the figure printed on so much oil packaging.
Metric is cleaner because liters and milliliters share a base of 1,000: oil (mL) = liters × 1000 ÷ ratio. At 50:1 that is exactly 20 mL of oil per liter of gas; at 40:1, exactly 25 mL.
Running the formula backwards answers the other common question — how much gas a given amount of oil will treat: fuel (gal) = oil (fl oz) × ratio ÷ 128. A 6.4 fl oz bottle at 50:1 treats 6.4 × 50 ÷ 128 = 2.5 gallons, which is why 6.4 oz is a common bottle size: one bottle, one standard 2.5-gallon can.
The ratio itself is not a matter of preference — it comes from the equipment manufacturer, and it varies with engine design and era. Manuals for older two-stroke equipment often specify much richer mixes (32:1, 24:1, even 16:1 on some vintage engines), so check the operator's manual, the fuel-cap marking, or the tank decal for your specific machine rather than assuming the modern 50:1 default.
Mix charts
Both tables are generated at build time by the same tested function the calculator runs — one source of truth for every number on this page. Print this page and the tables keep their borders while the navigation and calculator drop out.
US units — fl oz of oil per gallons of gas
| Ratio | 0.5 gal | 1 gal | 1.5 gal | 2 gal | 2.5 gal | 3 gal | 4 gal | 5 gal |
|---|---|---|---|---|---|---|---|---|
| 16:1 | 4.00 | 8.00 | 12.00 | 16.00 | 20.00 | 24.00 | 32.00 | 40.00 |
| 20:1 | 3.20 | 6.40 | 9.60 | 12.80 | 16.00 | 19.20 | 25.60 | 32.00 |
| 24:1 | 2.67 | 5.33 | 8.00 | 10.67 | 13.33 | 16.00 | 21.33 | 26.67 |
| 25:1 | 2.56 | 5.12 | 7.68 | 10.24 | 12.80 | 15.36 | 20.48 | 25.60 |
| 32:1 | 2.00 | 4.00 | 6.00 | 8.00 | 10.00 | 12.00 | 16.00 | 20.00 |
| 40:1 | 1.60 | 3.20 | 4.80 | 6.40 | 8.00 | 9.60 | 12.80 | 16.00 |
| 50:1 | 1.28 | 2.56 | 3.84 | 5.12 | 6.40 | 7.68 | 10.24 | 12.80 |
| 80:1 | 0.80 | 1.60 | 2.40 | 3.20 | 4.00 | 4.80 | 6.40 | 8.00 |
| 100:1 | 0.64 | 1.28 | 1.92 | 2.56 | 3.20 | 3.84 | 5.12 | 6.40 |
Metric — mL of oil per liters of gas
| Ratio | 1 L | 2 L | 3 L | 4 L | 5 L | 10 L | 15 L | 20 L | 25 L |
|---|---|---|---|---|---|---|---|---|---|
| 16:1 | 63 | 125 | 188 | 250 | 313 | 625 | 938 | 1250 | 1563 |
| 20:1 | 50 | 100 | 150 | 200 | 250 | 500 | 750 | 1000 | 1250 |
| 24:1 | 42 | 83 | 125 | 167 | 208 | 417 | 625 | 833 | 1042 |
| 25:1 | 40 | 80 | 120 | 160 | 200 | 400 | 600 | 800 | 1000 |
| 32:1 | 31 | 63 | 94 | 125 | 156 | 313 | 469 | 625 | 781 |
| 40:1 | 25 | 50 | 75 | 100 | 125 | 250 | 375 | 500 | 625 |
| 50:1 | 20 | 40 | 60 | 80 | 100 | 200 | 300 | 400 | 500 |
| 80:1 | 13 | 25 | 38 | 50 | 63 | 125 | 188 | 250 | 313 |
| 100:1 | 10 | 20 | 30 | 40 | 50 | 100 | 150 | 200 | 250 |
FAQ
What if I don't know my mix ratio?
Find it rather than guessing it. The ratio is usually printed on the fuel-filler cap, on a decal near the tank, or in the operator's manual, and manufacturers publish manuals online searchable by model number. If you genuinely cannot find it, at least know the trade-off before picking a number: more oil than specified (a richer mix) tends to foul spark plugs and build exhaust deposits, while less oil than specified (a leaner mix) starves the engine of lubrication and can destroy it. The failure modes are not symmetric — which is exactly why this page points you at the manual instead of inventing a universal ratio. There isn't one.
40:1 vs 50:1 — how much difference is it really?
Per US gallon of gas, 40:1 takes 3.20 fl oz of oil and 50:1 takes 2.56 fl oz — a difference of 0.64 fl oz per gallon. In metric, it is 25 mL versus 20 mL per liter. Small in absolute terms, but it is a 25% difference in oil content, so run the ratio your equipment specifies rather than splitting the difference between two numbers you have seen elsewhere.
Does premixed fuel go bad?
Yes — treat premix as perishable. Two-stroke oil manufacturers' guidance is that mixed fuel has a limited shelf life, with roughly 30 days a commonly published figure, and gasoline itself degrades as it sits, mixed or not. The conservative practice: mix only what you expect to use soon, label the can with the date and the ratio, and when in doubt, mix fresh instead of running old fuel.