I made measuring helps for accurately mixing only one tankful of 2-cycle fuel at a time for our weed trimmer. My tank holds 8 fluid ounces US. The specified ratio is 1:40.
I have a MIG welder and made my own measuring devices from scrap metals. Anyone without a welder can use corks and tin cans to do the same tasks in an entirely satisfactory way without welding.
You may be wondering, “Why bother? Just buy pre-mixed fuel. It has a stabilizer and will keep.” That is certainly an option, but I have been mixing individual small batches of fuel for 2-cycle engines since the 1970s. Old habits die hard. I also like mixing my own small batches. The price per gallon is certainly much less.

Supplies

1/2 inch EMT tubing

1/8 inch steel rod

20 gauge sheet steel

Or—

Used soup can

A cork or a wooden plug whittled from anything

Step 1: A Handy Thing to Know

In metric measurements, liquid measurements can be calculated from linear measurements. 1 cubic centimeter = 1 milliliter. It is also important to know 1 cubic centimeter = 1,000 cubic millimeters. (1 centimeter = 10 millimeters. 1 cubic centimeter = 10 x 10 x 10 millimeters = 1,000 cubic millimeters.)

Your browser will automatically give you conversions. I entered, “8 fluid ounces US = milliliters” in my browser and it immediately told me the figure is 236.58 milliliters. (Be careful. A browser may default to Imperial fluid ounces and the answer will be a bit different.) I am using a 1:40 ratio. 236.58 divided by 40 equals 5.91 milliliters or cubic centimeters, or 5,900 cubic millimeters (slightly rounded).

Step 2: Calculate the Area of the Vessel Opening and the Height of the Vessel

I set my digital caliper to read in millimeters. I also reamed the end of the tubing so it is the same diameter at the end as away from the end. Surprise! This tubing is a little egg-shaped. I chose an average figure between the high and the low.
The area of a circle equals π multiplied by the square of the radius. 15.7 mm is the diameter. Divided by 2 it gives the radius as 7.85 mm. That squared is 61.62 square mm. Multiply by π and the area of the opening is 193.48 square millimeters.
I need a measuring vessel that contains 5.9 milliliters of oil when level full. That is 5,900 cubic millimeters. 5,900 cubic milliliters divided by 193.48 square millimeters yields the desired height of the tube to be 30.49 millimeters.*
In the second photo I am marking the metal tube to a length between 30 and 31 millimeters. I cut it fairly precisely with a tubing cutter as shown. A tubing cutter is really for softer metal than the steel in this tube, but it does work.

* I am welding a bottom over the end of the tube. If someone were using a wooden or a cork plug, the length of the tube to be made would need to add length for the amount the plug that goes into the tube.

Step 3: Welding

Welding for this project involves relatively thin materials. It is easy to let a little too much heat build up and a hole suddenly appears where a nice weld should have been. The adage, “Weld a little, cool a lot.” applies. Stop often and let the metal cool. In addition, by all means use back step welding. This link explains how it is done. The photo is a practice example for illustrating this Instructable. You can see the first bead is short. It begins near the right edge and moves right. The next bead began to the left of it and moved right up onto it a little. You are welding items that are not to leak fluids. Back step welding makes it easy to go a little up on the previous bead for a solid seam.

I am using 1/2 inch electrical metallic tubing, or conduit for the tubing. Before I welded, I ground off the galvanized coating for a better weld and for fewer toxic fumes.

Step 4: More on the Oil Vessel

Once I mixed fuel in 10 ounce quantities. For that I needed a measuring vessel that holds 7.4 Milliliters. When I was finished I engraved “7.4 ml” on the metal. I also added a little handle I made with thin steel rod. These little vessels are very stable and easily rest on a flat surface without tipping.

Step 5: The Gasoline Container

I used a soup can to estimate the size of container needed for 8 plus ounces. I cut a rectangle I rolled around a form and welded the ends together. Then I welded on a bottom. I put water into a measuring cup from our kitchen measuring cups to total 8 ounces. I marked the level in the container I had made. See the second photo. I added a metal tab. When fuel reaches the bottom of the tab, 8 ounces is in the container.
I made another smaller rectangle and bent it to make a spout. I had to trim so it meets the top edge of my 8 ounce container. I welded my spout in place.
Check your containers or vessels for leaks. Fill them and set them on newspaper. Watch for wet spots.
I like to fill the oil vessel to the top with oil. I fill the gasoline container a bit less than half full. I pour the oil into the gasoline and slosh the vessel around to mix the oil and the gasoline well. Then I add gasoline until it reaches the 8 ounce level. I slosh it more to mix well and to get all of the oil out of the little vessel. Then I use the spout to pour fuel into my machine’s gas tank.

Step 6: Using a Soup Can

We have old 20 ounce soup cans. I used pliers to crimp a smooth pouring spout in the rim of this one. Pour 8 ounces of water into the can to know the level. Bend a piece of wire to hang over the top edge. Let it extend down into the can so the bottom edge of the wire is at the top of the gasoline when 8 ounces have been poured into the can.

I cover these things with a rag to keep dust and grit from getting into my containers. I would not claim my method is as precisely mixed as a can of premix, but the end result is very, very close to the desired 40:1 ratio.

Step 7: Doublecheck

What I really like is that by converting liquid measurements to milliliters I can make measurement vessels to fit any requirement. As long as I have been careful and done the mathematics accurately, I know their size will be correct. Still, when possible, it is a good thing to check my work.

The photo shows three things in our house graduated in milliliters. First is a set of measuring spoons for cooking. Although in Tablespoons and teaspoons, milliliter equivalents are given, but they are not exactly precise. They are probably more precise for dry measure than for liquid measure. I also have a small disposable hypodermic needle a nurse friend gave me for filling ink cartridges I use in a fountain pen. I thought it would be precise, but got varying results. The most accurate measure is the little dosage cup that comes with nighttime cold medicine. Pour water into what you use to check your measuring device, not 2-cycle oil.

A sure and certain way to double check accuracy is to fill the little oil measuring container with water and dump it into the larger container. If your oil to fuel ratio is 1:40 do this 40 times to know exactly where the fill level for gasoline should be.

Step 8: Bonus

If you want to mix 32 ounces at one time using an empty pre-mix can, the vial for measuring the oil is easy to make from a piece of nominal 3/4 inch electrical conduit. The vial in the photo has a flat steel bottom I welded in place. If you do not have a welder, you can use a rubber stopper from a big box store, like Lowe’s in the USA. They are in the screw and bolt drawers in the hardware section. Just measure and cut so you have a column of oil 53mm high. I double-checked my calculations by filling my vial 51 times and pouring into a measured 32 ounce container. That is for a 50:1 mixture ratio, which is 51 parts. That is one part 2-cycle oil and fifty parts gasoline.