When is a “cup” not a cup?

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“So What’s the Deal?”

  • There is no internationally-agreed standard for “cup”.
  • Modern volume ranges are between 200 and 284 milliliters.

“Why Should I Care?”

“These aren’t the cups you’re looking for.”

If your Instant Pot inner liner has markings labeled “cups”, ignore those markings completelyunless you are familiar with the extra-special measuring magic of a rice cooker.

If you need “real” cups, use a “real” measuring cup.

That’s it, that’s all you need to know. Nothing else to see here. Move along. 🙂

Now, if you want to know why these markings aren’t the cups you think they are, then keep reading!

Fun Facts

  • The United States is one of only three nations that have not adopted the metric system as their official system of weights and measures.
    • The other two are Liberia and Burma.
    • USA! USA! We’re #1! We’re #1! (*insert giant eyeroll here*)
  • The U.S. government passed the Metric Conversion Act of 1975, which made the metric system “the preferred system of weights and measures for U.S. trade and commerce.”
    • The legislation states that the federal government has a responsibility to assist industry as it voluntarily converts to the metric system.
    • *cough*MeaningNever*cough*
  • U.S. customary units are widely used on consumer products and in industrial manufacturing — while Metric units are standard in science, medicine, as well as many sectors of industry and government, including the military.

Important Instant Pot Specific Info

Duo Plus & Ultra

LUX, Duo & Smart

If you have an inner liner that indicates “cups”, you need to know that these are indicator lines for Rice Cups, and not for US Cups!

  • Fortunately, newer versions of the inner liner now indicate Quarts instead of Cups.

Note how on the photos shown here that for the Lux, Duo and Smart, the “10 cup” line is right below the 3.5 L line, and for the Duo Plus and Ultra the “4 quart” line is right above the 3.5 L line.

  • If you fill your your pot to this “10 cup” line — you’ve actually got closer to 16 cups of liquid, not 10 cups of liquid!
  • Long story short: if you need “real” cups, use a “real” measuring cup — not the indicator lines on the inside of your pot.

Some Not-so-Fun Facts

It’s highly likely that the majority of Americans don’t know diddly squat about there being more than one “cup” measurement. (I sure didn’t until this “rice cup” thing came up regarding the Instant Pot!)

So, like me, you all probably have always just gone out and purchased measuring cups willy-nilly. Surprise! Not all measuring cups are created equal.

Double Surprise! Unless you know what you’re reading in the product description, you could purchase two different sets that don’t even measure the same way!

Why you might now feel the need to be a more-informed consumer…

Links to Amazon product listings are all non-affiliate

  • This product is 1 cup = 250 mL — a “Metric” Cup (details below)
  • This product is 1 cup = 236 mL — a US “Customary” Cup (details below)
  • This product (which I have) is 1 cup = 240 mL — a US “Legal” Cup (details below)
  • And these Pyrex 1 cup containers (which I also have) are 1 cup = 236 mL
    • Imagine my surprise just now to learn that if I used my 1 cup measuring cup to fill one of these 1 cup storage containers, that I’d overfill it!!
  • And finally, this Pyrex glass measuring cup set (also in my collection) has US and Metric on opposite sides — so I don’t even know what kind of “cup” it is!

…and why you probably don’t actually have to worry too much about that

  • The difference between a Metric Cup (250 mL) and a US Legal Cup (240 mL) is just 10 mL — or 2 US Teaspoons (1 tsp = 5 mL)
  • The difference between a Metric Cup (250 mL) and a US Customary Cup (236 mL) is just 14 mL — or a little less than 1 US Tablespoon (1 Tbsp = 15 mL)

So for most measurement quantities dealt with by the home cook, these slight differences “scale up” slowly.

  • Unless you accidentally measure using Rice Cups
  • The difference between a US Customary Cup (236 mL) and a “Rice Cup” (180 mL) is 56 mL — or about 4 US Tablespoons (59 mL)
  • So the difference between 4 US Customary Cups (940 mL) and 4 Rice Cups (720 mL) is a whopping 220 mL — or nearly another whole 1 US Customary Cup (236 mL)

The Complete Dirty Details

Liquid Measure NameUS Customary Fluid OuncesBritish Imperial Fluid OuncesMilliliters (approx.)Usage LocationUsage Notes
US Legal Cup8.12 fl oz8.45240 mLUnited StatesNutrition Labeling, as stipulated by the FDA
US Customary Cup8.0 fl oz8.33236.59 mLUnited StatesTraditional cooking recipes
US Coffee CupA customary "cup" of coffee in the U.S. is usually defined as 4 fluid ounces, brewed using 5 fluid ounces of water.150 mLUnited StatesCoffee "cup" measurements account for volume lost to evaporation during brewing. So a "12-cup" US coffeemaker makes 57.6 US customary fluid ounces of coffee (not 8 x 12 = 96 customary fluid ounces).
Canadian Cup7.686 fl oz8227.30 mLCanadaNo longer in use; deprecated in favor of the Metric Cup
UK Imperial Cup9.61 fl oz10284.13 mLBritish CommonwealthNo longer in common use, but appears in older recipe books
Metric Cup8.45 fl oz8.8250 mLCanada, UK, Australia, New Zealand, South Africa, and some other members of the Commonwealth of NationsAlthough derived from the metric system, "cup" it is not actually an official metric unit
Japanese gō 合 ("Rice Cup")6.09 fl oz6.34180 mLJapanTraditional unit of measure for Rice and Sake
Japanese Standard Cup6.76 fl oz7.04200 mLJapan
Latin American CupIn Latin America, the amount of a "cup" varies from country to country -- from anywhere between 200 mL and 250 mL (Spanish: taza or vaso), or 236 mL to 240 mL (US legal or customary measures).



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  • Heather

    This is fascinating! I knew professional bakers typically preferred to measure by weight, and this difference in cup sizes is obviously the reason. Makes me wonder what I have at home! And whether there’s a difference between the two sets I have.

    • Jen Neefer

      Dry measure opens up a whole different can of worms! The more recipes I use from Amy & Jacky (they are in Canada) the more I like the way the rest of the civilized world measures, compared to us. Milliliters for liquid measure, and grams for dry measure.

      But we illogically use cups for everything, and then need to indicate defining characteristics such as “packed brown sugar” or “sifted flour”. Because 1 cup of packed brown sugar is obviously a lot more volume than unpacked brown sugar…. and 1 cup of pre-sifted flour is a lot less weight than 1 cup of scooped-from-the-bag flour. And particularly with flour, you then have to sift a whole bunch into an extra bowl so you can then measure what you need, hope you don’t pack it down too much while measuring, and then put the rest back — instead of putting the main bowl on a scale and sifting flour into it until you get the correct weight.

      Jill (This Old Gal) has a really good explanation about the problems with American’s measuring flour on her blog that is an excellent read https://thisoldgal.com/how-to-properly-measure-flour/

  • Katy

    Very interesting!

  • Mary Neese


  • Beth Ann Fuchs

    I had been aware of the differences in “cup” sizes, and was hoping that you would mention the coffee cup size. I would suggest a slight difference in what you show though. A “cup” of coffee is assumed to be 4 fluid ounces and it is brewed from 5 fluid ounces of water–not like they can confuse things more. The reference for this is in your first link (https://en.wikipedia.org/wiki/Cup_(unit)), at the bottom of the section labeled “customary cup”.

    I know that there is a lot of confusion as to conversion of measurements from the Imperial system used by the US and the metric system, used by most of the rest of the world. Something that I found years ago that is a huge help to me is a small piece of software that can be downloaded to convert one system of measurement to another. You can find it at : https://joshmadison.com/convert-for-windows/

    • Jen Neefer

      The “coffee” cup one is soooo confusing! I read that part of the wiki page a whole bunch of times and still couldn’t quite make heads or tales of it. But you’re right, that particular statement needs to be included, so I’ll go tweak it to fit that in there.

      For quick conversions, without installing software, this page that I listed in my references was amazingly handy. You can click on any of the measurement names and immediately switch to using conversion in those units. https://www.convert-me.com/en/convert/volume/cup.html?u=cup&v=1

  • Love this! Thanks for sharing!

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