Kitchen Scale Accuracy, Resolution and Calibration

Kitchen scales are sold on two numbers, the capacity and the smallest step the display shows, and a third idea, accuracy, is easy to confuse with the step. This guide explains the terms, when a 1 g step is enough and when you want 0.1 g, how to check a scale at home with US coins, and why a kitchen scale is the wrong tool for medicines. It is general information for adults. For the full set of buying checks, see our buying guide.

What the words on a spec sheet mean

Scale makers use a handful of terms. These are our plain-language versions, informed by the glossary Adam Equipment, a scale maker, publishes:

• Capacity: the heaviest load the scale is built to carry. A container on the platform uses up some of it, so a 1,000 g scale with a 400 g bowl has 600 g left for food (our example).
• Readability (resolution, increment): how fine the display's steps are. NIST's Handbook 44 defines it for a digital display as the difference between two consecutively indicated values.
• Accuracy: how close the reading is to the true weight of a known mass.
• Repeatability: whether the same mass gives the same reading each time you weigh it.
• Linearity: whether the readings stay correct from a light load up to full capacity.

Accuracy and readability are often used interchangeably, yet they differ: readability is the display's division, and accuracy is matching a known weight. A display that shows 0.1 g has a finer division, which does not by itself show that the last digit is right.

Maker pages show the difference. The Digitz DZ3-750 page lists a capacity of 750 g, a readability of 0.1 g and a 500 g calibration mass that is not included. The Culinarian page lists 22 lb or 9.9 kg, a readability of 0.03 oz or 1 g and a 5 kg calibration mass that is not included, although its page title says 0.1 oz, a reminder to read the specification table. Ozeri's page lists a 0.5 g minimum weight, so very small amounts may not register. None of the three pages states a tolerance for the readings.

What NIST covers

NIST Handbook 44 applies to commercial weighing and measuring devices: those used to establish the quantity of things bought or sold, to charge a fee for using the equipment, to decide an award or to compute a charge for services by weight. On our reading, a scale in your own kitchen is none of those, so no Handbook 44 tolerance is aimed at it. For a kitchen scale, then, the accuracy claim is the maker's own, and checking it is up to you.

When 1 g is enough, and when it is not

If a display rounds to the nearest gram, a reading of 5 g could stand for anything from 4.5 g to 5.5 g. For energy, the size of that error depends on the food. Using USDA FoodData Central values, a half-gram error is about 4.4 kcal for olive oil (884 kcal per 100 g), about 3 kcal for smooth peanut butter (598 kcal per 100 g) and about 2 kcal for granulated sugar (387 kcal per 100 g). Over a day's cooking that is small next to the other uncertainties, and labels themselves carry rounding and tolerance. For most food tracking, 1 g steps are enough.

A 0.1 g step earns its place for small amounts of strong ingredients:

• Salt and spices. FDA says the less-than-2,300 mg sodium limit in the Dietary Guidelines equals about 1 teaspoon of table salt, which FoodData Central lists at 6 g. A 1 g step is a sixth of that.
• Supplement scoops. A 3 g serving on a scale that rounds to the nearest gram could be anywhere from 2.5 g to 3.5 g, a swing of about 17% either way. A 0.1 g scale narrows it to 2.95 g to 3.05 g.

Check the capacity too, because the two usually trade off. Here are searches for kitchen scales that read to 0.1 g on Amazon (opens in a new tab) and digital pocket scales on Amazon (opens in a new tab); pocket scales are a separate category with small capacities.

A home accuracy check with US coins

Federal law, 31 U.S.C. 5112, sets the weight of new coins: a 5-cent coin 5 g, a quarter 5.67 g, a dime 2.268 g and a half dollar 11.34 g. Coins that have circulated may weigh slightly less (our caution, not a statement from the law), so treat this as a rough check, and use several coins.

1. Level and zero. Put the scale on a firm, level surface. Level the platform before you start, as scale makers advise. With nothing on it, the scale should read zero.
2. Known masses. Weigh one nickel, then a quarter, then both. Expect 5, 6 and 11 g on a 1 g scale, and about 5.0, 5.7 and 10.7 g on a 0.1 g scale.
3. Range. Add coins in steps. Twenty nickels come to 100 g and forty to 200 g, so the readings should rise in step with the coins you add.
4. Repeatability. Weigh the same coin three times, then again near each edge of the platform. The readings should agree.
5. Tare. Put a bowl on, press tare and add a coin. You should see the coin's weight alone.

If your scale has a calibration mode, the manual says what mass to use. Calibration weights are sold for this, and the class and tolerance on the listing matter more than the headline mass: calibration weights on Amazon (opens in a new tab) is a search for them. If a new scale fails these checks with fresh batteries, contact the seller or maker and ask what warranty applies; the AWS pages we read show both a 10-year and a 1-year warranty, so confirm the term for the unit you buy.

Supplement powders and dosing

Under 21 CFR 101.36, a Supplement Facts panel gives the serving size in a term that suits the product, such as capsules or teaspoonfuls, and the amount by weight of each dietary ingredient per serving. A teaspoonful is a volume and the weight is the dose, so a scoop and a label can disagree. Use the scoop supplied, and if you want to confirm what a level scoop holds, weigh it on a 0.1 g scale several times and compare it with the label, as our guide to how to take creatine suggests. Doses in micrograms are far below what a 0.1 g display can show, so a scale cannot help with those.

A kitchen scale is also not a pharmacy instrument. Follow the Drug Facts label and your pharmacist's directions for medicines, and do not weigh them out on a kitchen scale.

Keeping a scale accurate

Keep the platform clear when switching on, so it zeroes properly. Stay under the capacity, bowl included. Replace weak batteries and repeat the coin check above if readings seem to drift, and keep the scale dry: AWS says moisture getting inside is probably the biggest cause of scale failures. Scale makers such as Adam Equipment list ambient conditions among the factors that affect a scale, so avoid weighing next to a hot stove.

Cautions

Accurate numbers do not make weighing food right for everyone. NIDDK calls talking with a health care professional about your weight an important first step. NIMH notes that some people become fixated or obsessed with weight loss, body weight or shape, and controlling their food intake, and that these may be signs of an eating disorder. If measuring food starts to feel stressful, stop and talk to a health care provider.

The instructions that come with your scale are the authority on its capacity and care, and the label on a supplement is the authority on its dose. Ask your doctor or pharmacist if you take medicines or have a health condition.