Postage Calculator

What decides your postage is which class the item falls into, and that is settled by size and weight before any price is involved. Get the class wrong and the price is wrong by several dollars, which is the mistake this page exists to prevent.

The class rules, which change rarely:

Cross any one of those size limits and the item drops into the next class up, however little it weighs. A 1 oz greeting card in a rigid 7 inch square envelope is not a letter: it is too tall and too stiff, so it prices as a large envelope with a non-machinable surcharge on top.

Surcharges that catch people out:

• Non-machinable letter: rigid, square, lumpy, or clasped envelopes cannot run through the sorting machinery and carry a surcharge
• Oversize package: length over 22 inches, or length plus girth over 108 inches
• Dimensional weight: USPS applies it only to packages over one cubic foot (1,728 cubic inches), and divides by 166, not the 139 that UPS and FedEx use. Under a cubic foot, USPS bills actual weight regardless of how big the box is.

That one-cubic-foot threshold is worth remembering. It is why a bulky but modest-sized USPS parcel often beats UPS and FedEx on the same route: the other two start charging for air immediately.

Prices are the part that keeps moving. USPS raises rates most years, often more than once, and there are separate retail and commercial price sheets. So this calculator asks you for the first-ounce and additional-ounce prices rather than shipping a table that would be stale within months. Both numbers are on the front page of usps.com and take about ten seconds to find.

For reference only, and expect these to have moved: a First-Class letter has been running somewhere in the region of 70 to 80 cents for the first ounce with roughly 25 to 30 cents per additional ounce, and a large envelope starts around double the letter rate. Package prices do not follow a per-ounce formula at all, which is why this page reports the class and the billable weight for packages and sends you to look the rate up.

Worked example: A 6 oz item, 9 × 6 × 0.2 inches, going domestically. It is under 11.5 × 6.125 × 0.25, so it is letter-sized, but a letter maxes out at 3.5 oz. At 6 oz it prices as a large envelope. Enter the current flat rates, say $1.50 first ounce and $0.29 each additional, and it comes to 1.50 + 5 × 0.29 = $2.95.

Round up when you weigh it. USPS charges by the ounce and 6.1 oz is a 7 ounce item.

Posting a stack of the same thing? The stamp and postage cost calculator prices a batch: how many items, what each one costs, and how many stamps to buy. This page does the opposite job. It takes one item, measures it, and works out which class it actually falls into, which is the part that decides the price before any rate is applied. Classify here, then multiply there.