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I am browsing olive oil on Google Shopping (the following problem also happens when I shop in local supermarkets). I found the information about unit price of olive oil is inconsistent. Sometimes only volume is available in liters and gallons, and sometimes it is weight in ounces and pounds. So I wonder how you can compare the unit prices between available information in volumes and weights? Do we have to know some common sense of the density of olive oil in order to convert between volume and weight using physics? Thanks!

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4 Answers 4

According to this, a gallon of olive oil weighs 7.6 lbs.; 1l weighs .91kg.

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The density of vegetable oils is within a few percent of that of water. For the purpose of rough price comparisons, treating it as such will give you sufficiently accurate comparisons. So use a pint a pound, or a kilo a liter. Don't drive yourself crazy with it.

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Thanks! How do you get 1 pint is 1 pound? –  Tim Feb 22 at 19:45
    
From the nursery rhyme, a pint is a pound, the world around. Really. But you can find references: en.wikipedia.org/wiki/Pint –  SAJ14SAJ Feb 22 at 19:49
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Unfortunately, the "few percent" is about 9, which makes quite a difference. –  Roddy Feb 22 at 22:13
    
That depends on what sort of pint you mean. A US liquid pint is 473ml. But an imperial (English) pint is 568ml. So 1 imperial pint of water is quite a bit more than 1 pound (454g). –  vclaw Feb 23 at 0:16
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@SAJ14SAJ "the world around" : that's "world" as in "world series" then ;-) I agree for cooking that it's in the noise, but for price comparisons 10% may well be significant. And food packagers often use every trick in the book to make 'apples vs. apples' comparisons difficult. –  Roddy Feb 23 at 13:47
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It's easiest to do this in metric because in the metric system 1 liter of water is 1 kilogram (under standard temperature and pressure, but let's not get complicated). If olive oil had the same density as water then you could say 7 kilograms of oil would be 7 liters of olive oil, however oil is less dense which is why it floats on top of water. You need to know the density of olive oil expressed in the term "specific gravity", which is density compared to water. In the case of olive oil it is .91 or .92, meaning it's 91-92% the density of water. Using this factor here's how to make the conversions:

  • volume to weight: multiply the volume in liters by .91 (or .92)
  • weight to volume: multiply the weight by 1.1 (1/.91)

If you are buying small amounts of olive oil then you could just use a like for like measurement, however if you are buying a lot of it then using these conversions could end up saving you money.

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You can use this tool for any conversions you need. I'd imagine this is about right averaged out.

It also allows you to calculate the measurement for extra light olive oil and extra virgin

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