Boat Motor Gas to Oil Ratios

19 January 2013 On The Water

Here is an easy way to quickly calculate how much two-stroke boat motor oil you need to put in the gas tank whether you are in Canada or the U.S.

Two-Stroke Boat Motor Oil and Gas Mixing Tips

How many times have you been at the marina or fishing camp getting gas for the boat and been stumped trying to figure out how much outboard motor oil you need to add to the gas tank?

Pre-mixing the gas and oil in a container to be used for your boat or smaller two-stroke motors is relatively easy to sort out. The challenge arises when you have to make a quick calculation with an odd number and there is a line-up of people at the dock waiting to use the pump.

How do you know how much two-stroke engine oil to mix with the gas?

Regarding boat motors, it can be a bit confusing because you have to figure out the calculation on the spot, and it is especially difficult when you take your boat over the border because Canada uses litres and millilitres and the U.S. uses gallons, quarts, and fluid ounces.

An Easy Gas to Oil Mixture Calculation
Here are some tips to help you figure out the right gas to oil mixture when you are filling the boat, whether you are in Canada or the U.S.

Most two-stroke engines that are used for boats require a 50:1 ratio although there are a number of exceptions. If you are the new owner of a used two-stroke boat motor, always check the manufacturer’s recommendation for the gas-to-oil ratio before you take the boat out for a test drive.

For new two-stroke boat motors, there is usually a break-in time that requires a richer oil to gas mix, meaning you need to add more oil during the initial period of operation. The break-in specification is often a 25:1 ratio. Be sure to get the details from the dealer, the owner’s manual, or the manufacturer before you take the new motor out for the first time.

Canada’s Metric System
How do you calculate the gas to oil mixture in Canada?
If you are going on a fishing trip to Canada you will need to know how to mix the gas and boat motor oil using the metric system. In Canada, gas is sold by the litre (L), and you normally add oil in millilitres (ml). There are 1000 millilitres in a litre.

With a 50:1 ratio, for 50 litres of gas you need 1 litre (1000 millilitres) of oil.

The easy way to figure it out, especially if you have an odd number, say 35 litres of gas, is to double the number of litres and add a zero (an extra decimal place) to get the number of millilitres of oil you need.

Here is how you do the calculation:

35 X 2 = 70.0 so you need 700 ml of two-stroke engine oil to go with the 35 litres of gas.

Here is a quick chart for a 50:1 ratio

Gasoline (L) Two-Stroke Oil (ml)
50 1000
47.5 950
45 900
42.5 850
40 800
37.5 750
35 700
32.5 650
30 600
27.5 550
25 500
22.5 450
20 400
17.5 350
15 300
12.5 250
10 200
7.5 150
5 100
2.5 50
2 40
1 20

Many of the other two-stroke motors we use around the cottage require a 40:1 ratio. For every 40 litres of gas you need 1 litre of oil, so the motor requires more oil per litre. This is often the case with chainsaws and some trimmers. For these smaller engines we pre-mix the gas and oil in a container and then use it as needed. Again, always check the manufacturer’s recommendation. Often, the required ratio for the motor is written right on, or near the fuel cap.

For a 40:1 ratio there are two easy ways to figure it out.

The first is to multiply the number of litres by 2.5 and then add a decimal place to get the number of millilitres of oil that you have to add.

The second is to multiply the number of litres of gas by 100 (add two zeros) and then divide by 4 to get the number of millilitres of oil that you need.

Using the same example of 35 litres of gas, here is the calculation using each method:

35 X 2.5 = 87.5 so you need 875 ml of two-stroke engine oil.

3500 / 4 = 875 so you need 875 ml of two-stroke engine oil.

Here is a quick chart for a 40:1 ratio

Gasoline (L) Two-Stroke Oil (ml)
40 1000
38 950
36 900
34 850
32 800
30 750
28 700
26 650
24 600
22 550
20 500
18 450
16 400
14 350
12 300
10 250
8 200
6 150
4 100
3 75
2 50
1 25

Converting to the U.S. System
How do you calculate the gas to oil mixture for a two-stroke boat motor in the United States?

1 U.S. Gallon (Gal) = 4 quarts (qt) = 128 fluid ounces (fl oz) = 3.785 Litres (L)

For a 50:1 ratio each gallon of gas requires 2.6 oz of two-stroke engine oil.

Here is a quick chart for a 50:1 ratio

Gasoline (Gal) Two-Stroke Oil (oz)
25 64
22.5 57.6
20 51.2
17.5 44.8
15 38.4
12.5 32
10 25.6
7.5 19.2
5 12.8
2.5 6.4
2 5.12
1 2.56

For a 40:1 ratio each 1 gallon (gal) of gas requires 3.2 ounces (oz) of oil.

Here is a quick chart for a 40:1 ratio

Gasoline (Gal) Two-Stroke Oil (oz)
25 80
22.5 72
20 64
17.5 56
15 48
12.5 40
10 32
7.5 24
5 16
2.5 8
2 6.4
1 3.2

Using these quick gas to oil ratio calculation tips and charts should make your visit to the gas pump at the marina a stress-free event.

Remember to use two-stroke boat motor oil that is specifically produced for boat engines, and always follow the user’s guide and manufacturer’s recommendations for your boat motor. Not only will your motor run more efficiently, it will also last longer.

Go to the Boat Motor Maintenance Tips page.
Go to the Boat Maintenance Checklist page.
Go to the Boat Motor Positioning Tips page.

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