Postal scale reads in grams — why USPS pricing needs ounces

Set a package on a postal or kitchen scale and it likely reads in grams by default — most scales sold today are calibrated to a global metric market. USPS pricing, however, is still built around whole-ounce brackets: First-Class Mail letters and flats are priced per ounce, with any fraction rounded up to the next bracket. Get the conversion wrong and you either overpay for extra postage you didn't need, or underpay and risk the piece being held for postage due.

How USPS ounce brackets work

USPS domestic letter and flat rates are published per ounce or per ounce range in Notice 123, the official USPS price list. A First-Class Mail letter has a base rate that covers up to 1 ounce, then an additional charge for each ounce (or fraction of an ounce) beyond that. Critically, the rounding always goes up: a letter weighing 1.02 oz is charged the 2-ounce rate, not a prorated 1-ounce-plus-a-little rate.

Converting your scale's reading correctly

The exact factor is 1 oz = 28.349523125 g. To find which bracket a gram reading falls into, divide by 28.349523125 and round the result up to the next whole number — not to the nearest whole number, which is the mistake that causes underpaid postage.

Notice the jump between 55 g and 57 g: both are under 2 ounces by a small margin or just over it, and the difference of a couple of grams — well within normal scale precision — decides which bracket you're billed at. When a reading is close to a bracket boundary, it's worth re-weighing or padding your estimate, since USPS scales at the counter are the ones that ultimately decide the postage.

Extra ounces and additional postage

Beyond the base First-Class Mail letter rate, USPS charges an additional per-ounce amount for each ounce (or fraction) past the first, up to the letter weight limit — after that, mail moves into a different pricing category entirely (large envelopes, or parcel services like Priority Mail, which price by weight and distance zone together rather than a flat per-ounce step). If your item is consistently landing right at a bracket boundary, it's often cheaper overall to trim a small amount of packaging weight than to pay for the next full ounce bracket, especially on mail sent in volume.

Why this differs from a food-label conversion

A food label's gram-to-ounce conversion rounds to the nearest ounce for readability. Postal pricing rounds up on any fraction, no matter how small — the two conventions solve different problems (labeling clarity vs. billing certainty) and shouldn't be assumed to match. If you're weighing mail, converting grams to ounces with the exact 28.349523125 g factor and then rounding up yourself is the safest way to avoid a postage-due surprise.