Funnels. Per unit prices. Water. How many liters of Coke are in a 12-ounce can? 

English: A picture I took of various kinds of ...

Bottles and Cans of Tab

Linda liked to take her grandchildren grocery shopping with her.

She hoped they would learn from her healthy, frugal habits.

She bought lots of fruits and vegetables and taught them how to figure out if fruit was fresh or would soon spoil.

She saved coupons.

She compared prices.

Though she rarely bought sodas, one day she decided to buy her grandchildren this rare treat as part of a lesson about how to spot a good deal.

She let them pick out their favorite soda in a 2-liter bottle and a six-pack of 12-ounce cans.

When they got home, she asked them to set up an experiment.

It had two parts:

  1. Estimate how many 12-ounce cans it takes to fill up a two-liter bottle.
  2. Estimate which was the better deal, the 2-liter bottle or the 12-ounce can.

Conducting the Experiment


  • One 2-liter bottle, empty
  • Six 12-ounce cans, empty
  • Funnel
  • Water
  • Paper and pencil
  • Conversion rate of liters to ounces and ounces to liters


  1. Have each grandchild estimate how many times they will have to fill a 12-ounce can and pour it into the 2-liter bottle before the 2-liter bottle is full. Ask younger grandchildren whether they need to use the can to fill up the bottle or the other way around.
  2. They should write down their estimates.
  3. Each should then fill a 12-ounce can with water and pour it into the empty two-liter bottle.
  4. Each should write down every time they use a 12-ounce can, to keep count.
  5. Compare their estimates with how many cans it took.

Calculating the Results

  1. One ounce is about .03 liters.Click here to see exactly how many liters.
  2. One liter is about 34 fluid ounces. Click here to see exactly how many ounces.
  3. 12 ounces is a little more than one-third of a liter. Click here to see exactly how many liters.

Thus, it should take almost six 12-ounce cans to fill a 2-liter bottle.

Comparing the Cost

When you are measuring something using two different systems, you have to pick one and convert the other to it so you can compare them.

If a liter is 34 ounces, you could figure out how much one ounce costs, multiply it by 34 and know how that compares to one liter.

Have your grandchildren write down the cost you paid for the Coke when they went shopping with you.

For estimating purposes, a two-liter bottle of Coke costs between $1.40 and $1.70.

Though sometimes it’s on sale for 99 cents, let’s take the average of these prices and say it’s $1.55 ($1.40 + $1.70 divided by 2 = $1.55).

If there are 34 ounces in one liter, that means there are 68 ounces in two liters. The cost of a two-liter bottle is $1.55. When divided by 68 to get the cost of one ounce, you get a little over 2 cents. ($1.55 divided by 68 is 2.28 cents).

While store prices vary, a recent Wall Street Journal article put the price of Coke per ounce for 12-ounce cans at 37 cents.

This works out to $2.22 for a six-pack. The price at your store may be different.

Thus, an ounce of Coke costs 37 cents if you buy it in a 12-ounce can and a little over 2 cents if you buy it in a 2-liter bottle.

Thanks to for suggesting this activity.

Lessons in Shopping

Though you may not want to get your grandchildren to drink more sodas, understanding how much things cost is an invaluable lesson you can teach them.

Next time you go grocery shopping with your grandchildren, you can point out the per unit prices underneath grocery store shelves.

Now, they will know what that means.

Do you remember shopping before per unit prices were posted on grocery store shelves?

Did you shop with a calculator so you could figure out the best deal?

Can your grandchildren find products where bigger isn’t cheaper per unit?

Every Friday, we write about something fun and frugal to share with your grandchildren.

Click here to follow our Frugal Friday posts in your Reader.

To you and sharing the wisdom of a lifetime with your grandchildren.

Carol Covin, Granny-Guru

Author, “Who Gets to Name Grandma? The Wisdom of Mothers and Grandmothers

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