40:1 Fuel Mix — Gas to Oil Ratio Calculator & Chart
A 40:1 two-stroke fuel mix means 3.2 oz of 2-stroke oil for every US gallon of gas, or 25 ml of oil per litre. Use the calculator to get the exact amount for any quantity of fuel, then check the full 40:1 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.
Advertisement
At 40:1 you add 3.2 oz of oil per gallon (25 ml per litre). For a 5-gallon can of gas that's 16 oz of 2-stroke oil.
Full mix chart
40:1 ratio
| Gallons of gas | oz of oil |
|---|---|
| 0.5 | 1.6 |
| 1 | 3.2 |
| 1.5 | 4.8 |
| 2 | 6.4 |
| 2.5 | 8 |
| 3 | 9.6 |
| 4 | 12.8 |
| 5 | 16 |
| 6 | 19.2 |
| Litres of gas | ml of oil |
|---|---|
| 1 | 25 |
| 2 | 50 |
| 3 | 75 |
| 4 | 100 |
| 5 | 125 |
| 10 | 250 |
| 15 | 375 |
| 20 | 500 |
| 25 | 625 |
What runs at 40:1
Forty to one is the older-equipment ratio and the sensible fallback when a spec is genuinely unknown. It covers chainsaws and trimmers built from roughly the 1970s into the early 1990s (Poulan, Homelite, Craftsman and McCulloch of that era), plenty of budget handheld tools sold today under the Ryobi, Troy-Bilt and Toro badges, and a large share of two-stroke dirt bikes and karts.
On bikes and karts the ratio is more a rider decision than a fixed rule: anywhere from 32:1 to 50:1 depending on the oil, the jetting and how hard the engine is worked. Some hedge trimmers and augers still ship with a 40:1 decal.
40:1 vs 50:1 in practice
At 40:1 you add 3.2 oz per gallon against 2.56 oz at 50:1. That is 25 percent more oil, not a different kind of oil.
Running richer than the decal (40:1 where 50:1 is specified) is tolerated by almost any air-cooled engine. Expect more visible smoke, a faster-loading spark-arrestor screen and a little more carbon over a season. Running leaner than the decal is what breaks engines: 50:1 in an engine that calls for 40:1 is 20 percent less oil than it was designed to receive.
If you inherit a machine with no decal and no manual, 40:1 is the safer of the two guesses.
Measuring 40:1
40:1 has the friendliest arithmetic of the common ratios: a 16 oz pint of oil treats exactly 5 gallons, and in metric it is 25 ml per litre, so a 1-litre jug of oil makes 40 litres of mix.
One gallon takes 3.2 oz and a 5-gallon can takes 16 oz. Watch the single-use bottles: nearly all of them are dosed for 50:1, so one 2.6 oz bottle per gallon leaves a 40:1 machine lean (it only treats 0.81 of a gallon at this ratio).
Common questions
[01] What is 40:1 in ounces per gallon?
40:1 is 3.2 fluid ounces of oil per US gallon of gas (128 ÷ 40 = 3.2).
[02] What is 40:1 in ml per litre?
40:1 is 25 ml of oil per litre of gas (1000 ÷ 40 = 25).
[03] What happens if I add too much oil at 40:1?
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 40:1 and follow your owner's manual.
[04] What equipment uses a 40:1 mix?
Chainsaws and trimmers from the 1970s through the early 1990s, plenty of budget handheld tools sold today, and many two-stroke dirt bikes and karts. It is also the safest guess when there is no decal and no manual.
[05] How much gas does a pint of oil treat at 40:1?
A US pint (16 oz) of 2-stroke oil treats exactly 5 gallons of gas at 40:1, which is what makes this ratio so convenient for larger cans.