You’re standing in the kitchen, flour on your apron, phone on the counter, and the recipe suddenly demands precision you weren't prepared for. You need to know how much is 2/3 cup in oz right now. No fluff.
The quick, "get me back to cooking" answer is 5.33 fluid ounces.
But wait. If you just tilt a dry measuring cup into a bag of flour and call it a day, your cake might come out like a brick. Why? Because "ounces" is a tricky word in the United States. It’s a linguistic trap that catches even seasoned pros. There is a massive difference between weight and volume, and in the world of 2/3 cup measurements, that distinction is everything.
The Fluid Ounce vs. Dry Ounce Tug-of-War
Fluid ounces measure volume—how much space something takes up. Weight ounces (Avoirdupois) measure mass—how heavy something is. When people ask how much is 2/3 cup in oz, they are usually looking for the liquid conversion.
Standard U.S. kitchen math dictates that 1 cup equals 8 fluid ounces.
Mathematically, that means:
$$2/3 \times 8 = 5.333...$$
In a professional kitchen, we usually round this to 5.3 fluid ounces for liquids. If you are using a glass measuring jug, you’ll be squinting at the lines trying to find that sweet spot just above the 5-ounce mark. It’s annoying. It’s imprecise. It’s why the rest of the world uses grams.
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However, if you are measuring "ounces" of flour, 2/3 of a cup isn't 5.33 anymore. Not even close. According to King Arthur Baking, a well-regarded authority in the industry, a cup of all-purpose flour weighs about 120 to 125 grams. If you do the math, 2/3 cup of flour is roughly 2.8 to 3 ounces by weight. See the problem? If you use the liquid conversion (5.33) for your flour weight, you’ll be adding nearly double the required amount.
Why Your 2/3 Cup Measurement Might Be Lying to You
Accuracy is a ghost in most home kitchens.
Think about your measuring cups. Are they the plastic ones from the dollar store? The stainless steel set from your wedding registry? Believe it or not, many consumer-grade measuring cups are notoriously inaccurate. A study by Cook’s Illustrated once found that some dry measuring cups varied by as much as 10% in actual volume. When you’re dealing with a fraction like 2/3, which already feels "messy" compared to a clean 1/2 or 1/4, those errors compound.
Then there is the human element.
How do you fill the cup? If you scoop the cup directly into a bin of sugar, you are packing the granules down. You might end up with 6 ounces of volume in a space meant for 5.33. The "spoon and level" method—where you spoon the ingredient into the cup and scrape the top flat with a knife—is the only way to get close to the truth.
The Liquid Reality: Water, Oil, and Milk
When we talk about liquids, the conversion for how much is 2/3 cup in oz stays relatively stable. Water has a density that allows us to use volume and weight almost interchangeably in the metric system (1ml of water = 1 gram), but in the U.S. Customary system, we stay at that 5.33 fl oz mark.
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- Milk: Slightly denser than water, but for a 2/3 cup measurement, the difference is negligible. Stick to 5.33 fl oz.
- Honey or Molasses: These are "heavy" liquids. While they fill 5.33 fluid ounces of space, they weigh significantly more. If a recipe asks for 2/3 cup of honey "by weight," you’re looking at about 8 ounces on a scale.
- Oil: It’s lighter than water. 2/3 cup of vegetable oil will sit right at that volume mark but will feel lighter if you’re weighing it.
Honestly, if you're making a vinaigrette, the 5.33 vs 5.4 debate doesn't matter. If you’re making a delicate soufflé or a tempered chocolate ganache? It matters.
Converting 2/3 Cup to Other Units
Sometimes you don't have an ounce-marked glass or a 2/3 measuring cup at all. It’s a common kitchen crisis. Most sets come with 1/4, 1/3, 1/2, and 1 cup.
If you're missing the 2/3 cup, you just use your 1/3 cup twice. Easy.
But what if you only have tablespoons?
There are 16 tablespoons in a full cup.
To find 2/3 of that, you’re looking at 10 tablespoons plus 2 teaspoons.
It’s a tedious way to live, but it works. Here’s the breakdown:
- 1/3 cup = 5 tablespoons + 1 teaspoon
- 2/3 cup = 10 tablespoons + 2 teaspoons
- 2/3 cup = 32 teaspoons (if you really want to punish yourself)
The Metric Shift: Why 157 Milliliters is Better
In the UK or Australia, they don't really mess around with "how many ounces in 2/3 cup" because they use milliliters. A standard U.S. cup is technically 236.59 ml.
So, 2/3 cup is approximately 157.7 ml.
If you have a digital scale—and honestly, you should—switching to grams or milliliters removes the guesswork. You don't have to wonder if you packed the flour too tightly or if the surface tension of the water is giving you a false reading. You just pour until the number hits 158. Done.
Common Pitfalls with 2/3 Cup Calculations
One of the weirdest things about American cooking is the "scant" and "heaping" terminology.
If a recipe calls for a scant 2/3 cup, they want you to be just under the line. Maybe 5.1 or 5.2 ounces. This is common in older recipes where the author knew their flour was particularly thirsty or their oven ran hot.
Conversely, a heaping 2/3 cup is only possible with dry ingredients. You can't "heap" water. If you see this, you’re likely looking at closer to 6 ounces of material. It’s imprecise, frustrating, and exactly why modern recipe developers like Stella Parks (BraveTart) or the team at America's Test Kitchen have moved toward weight-based measurements.
Impact of Altitude and Temperature
You wouldn't think temperature changes how much is 2/3 cup in oz, but physics is a stickler.
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Water expands as it gets warmer. While the change from room temperature to boiling isn't going to ruin your Sunday gravy, it does technically change the volume. If you are measuring boiling water for a gelatin-based dessert, 2/3 cup of hot water contains slightly less actual water mass than 2/3 cup of ice-cold water.
Altitude is a bigger jerk. In Denver, the air is thinner. Liquids evaporate faster. Often, high-altitude bakers will slightly increase their liquid measurements—sometimes by just a teaspoon or two—to compensate for the dry air. In that case, your "perfect" 5.33 oz might need a tiny splash more to keep your bread from drying out.
Actionable Steps for Perfect Measurements
Stop guessing. If you want to master the 2/3 cup measurement and ensure your kitchen results are consistent, follow these steps:
- Buy a Digital Scale: This is the single best thing you can do for your cooking. Switch the unit to ounces or grams. If the recipe says 2/3 cup of flour, look up the weight (usually 80-85g) and weigh it.
- Use Eye Level for Liquids: When measuring 5.33 oz of liquid in a glass jug, don't look down at it from above. Get your eyes level with the counter. Parallax error is a real thing that makes you misread the meniscus (the curve of the liquid).
- The Spoon-and-Level Method: If you refuse to buy a scale, at least stop scooping. Spoon your dry ingredients into the 1/3 cup measure, do it twice, and level it off with a flat edge.
- Check Your Math: Remember that how much is 2/3 cup in oz is always 5.33 fl oz for volume, but the weight will vary wildly based on the ingredient's density.
Consistency is the difference between a "good cook" and a "great cook." By understanding that 2/3 cup isn't just a mark on a plastic cup—but a specific volume of 5.33 ounces—you're already ahead of the curve. Next time you're mid-recipe and the math starts to blur, just remember: two thirds is a little more than five, and the scale never lies.