Just as much as you expect something to fall to the ground when you drop it, or for the sun to rise in the morning, you tend to assume that when you pull out a measuring cup, it’s actually going to measure the right amount.

Turns out, that’s not always the case.

Because I’m a bit of a nerd, and one who likes to be as close to 100 percent accurate as I can when it comes to baking, I recently decided to compare the two sets of metal measuring cups we have in our Food Lab. Just looking at them, I could tell there was no way they hold the same amount. And I was right.

I filled each with “1 cup” of flour and popped them on our scale. The difference was about ½ ounce, which is pretty significant given that I and others consider a cup to be 5 ounces. (See some other examples of bad measuring cups from Food 52 and King Arthur Flour.) If you’re using a few cups in a recipe, that discrepancy amplifies, and all of a sudden you’re looking at the kind of difference that can start to affect the outcome.

But variation is not limited to dry measuring cups. Start comparing the capacities of liquid measuring cups and measuring spoons and you’re certain to get the same frustrating inconsistency.

“We were as surprised as anyone to find that these were inaccurate,” says Lisa McManus, the executive tasting and testing editor — a.k.a the gadget expert — at America’s Test Kitchen. As part of their rigorous equipment testing program, ATK has evaluated dry measuring cups, liquid measuring cups and measuring spoons, testing their accuracy on a lab-quality, high-precision scale. They calculated the accuracy by comparing how much each vessel held against how much a known quantity of an ingredient weighs. The differences could be staggering — more than 30 percent for some measuring spoons, as much as 6 percent for some dry measuring cups and more than 2 tablespoons per cup of water (about 13 percent) in at least one liquid measuring cup.

So what’s the deal? “My theory is simple quality control,” McManus says. “If you’re printing the numbers and lines on the cup, it’s a machine.” If that goes out of alignment, the lines end up in the wrong place and voilà, a cup is no longer a cup.

(iStock)

When it comes to dry measuring cups, “manufacturers tend to think it’s a time for whimsy,” McManus says, so you start to get weird shapes, decorative flourishes and alternative materials that make it more difficult to get a universal measure.

Oxo, the maker of ATK’s winning dry measuring cups, sees a similar reason for the wide variation among brands.

“Because dry measuring cups don’t take up very much space, are fairly straightforward and are seen as an essential household tool, there is an opportunity to design based on user preferences,” says Francoise Vielot, the company’s category director for baking and measuring products. “Some people like stainless steel or copper, others prefer options that are more fun and colorful, or a less expensive plastic option. Still, some may want unique materials that provide additional functional benefits, like silicone, which can collapse for storage, or that have a traditional appeal, like ceramic.”

Joshua Redstone, a math enthusiast and former software engineer, started thinking about the size variations in measuring cups a few years ago. Eventually, his quest led him down the rabbit hole to design and manufacture one for liquids that he says maintains a high level of accuracy.

“I’ve never seen a measuring cup make a claim about accuracy” on their website, Redstone says, at least with any kind of numbers (like polls or scientific studies that give a margin of error). “You kind of wonder why.”

Redstone was determined to know how close to perfect his measurements were for his own measuring cup, the Euclid. That meant going to a third-party lab, an effort that can cost several thousand dollars a pop. (Redstone almost nailed it at an accuracy of within 1 percent but ultimately decided he’d want even more data before making it official on his website.)

To say the lab testers had their work cut out for them is an understatement. While most calibrating labs are geared toward straight-sided beakers and cylinders, the Euclid starts narrow and flares out toward the top as the measurements increase. The idea is that by increasing the surface area of the liquid as the amounts increase, you’re controlling for us being human and tending to over- or undershoot the target. Think of it this way: That same amount of over- or under-filling is going to be a lot more significant when you’re measuring ¼ cup, because it will account for a larger proportion of the total liquid you’re measuring. A hair above or below the 1 cup line will make less of a difference.

Granted, Redstone’s product is a more extreme example, but we really have no way of knowing whether manufacturers are pursuing accuracy with that same kind of doggedness, especially when money is a factor. “Cost wasn’t my number one concern” Redstone says of his pet project (funded by Kickstarter contributions), but McManus says ATK has heard from other engineers that there are times when they’re under pressure to cut production costs.

For its part, Oxo says it has quality-control processes in place to ensure accurate measurements, with several rounds of testing for all its measuring tools. The company tests by weight, except instead of using water, it employs tiny beads that make it easier to check capacity. But are companies boasting about this quality control? Hardly. And unless you’re a particularly stubborn consumer, or a reporter, you’re unlikely to find out.

(Stacy Zarin Goldberg for The Washington Post)

So that leaves the question of what a home cook can do to ensure they’re getting the most accurate measurements. You can try to get equipment that has been thoroughly tested by a company such as ATK, because not many of us have the resources or equipment to do that kind of vetting. Also look for tools that at least seem like they will be straightforward and reliable. Both ATK’s McManus and Oxo’s Vielot recommend looking for dry measuring cups that will be easy to level off smoothly at the top and won’t lose their markings over time. Durable markings that are easy to read are key in liquid measuring cups, too.

You can even do your own weight verification at home, if you have a scale. Or you can skip volume altogether and decide to rely entirely on weight alone, especially when it comes to baking. Do yourself a favor, too, by measuring properly. For dry ingredients, that means what ATK calls the “dip and sweep,” scooping up the ingredients and then leveling off the excess with a flat utensil, such as the non-blade end of a table knife. For liquids, place the measuring cup on the counter and bend down so you’re at eye level with it (ATK found some markings designed to be read from above that were either inaccurate or hard to read). Use the meniscus of the liquid, the underside of the curve of the surface, as your reference point for knowing you’ve reached the right level.

McManus would love to see standardized cups become the norm. “It would certainly make our job easier,” she says. That’s just a pipe dream, so the doubt will never completely go away, especially if you’ve been burned before. “After you’ve found a bad one, how do you trust any of them?” McManus says.

“It’s good to know that this is a possibility,” she says, so “buy something that’s going to support your cooking and not mess you up in ways you couldn’t even imagine.”

Clarification: This version has been updated to include the results of the third-party lab testing conducted on the Euclid measuring cup.

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