Stamp and Postage Cost Calculator

US postage is priced in tiers rather than by a smooth rate, which is why a letter that weighs a fraction over an ounce costs the same as one at a full two ounces.

Postage = base rate for the first ounce + (additional ounces × per-ounce rate)

Weight always rounds up to the next whole ounce. One ounce and a hair is two ounces as far as the counter is concerned.

The four things that set the price

1. Shape, and this matters more than weight. A postcard, a letter, and a large flat envelope are three different products at three different prices, and the jump from letter to flat is the big one. A letter must be rectangular, under a quarter inch thick, and no larger than about 11.5 by 6.125 inches. Exceed any of those and it is priced as a flat, roughly double.
2. Weight, in whole ounces rounded up. A standard letter tops out at 3.5 oz before it has to go as a flat.
3. Destination, domestic or international.
4. Rigidity and squareness. A square envelope, or one with something lumpy inside, cannot be machine-sorted and picks up a non-machinable surcharge. Wedding invitations get caught by this constantly.

Why this page asks for the current rate

Postage rates change, usually more than once a year, and any figure hardcoded into a web page is wrong within months. Enter the current first-class rate from a stamp or from usps.com and everything else scales from it. The relationships between the tiers hold far better than the absolute numbers do.

Those relationships, for the record, so you can see what the page is doing with your number: a postcard is about 0.77 of a letter, a large envelope about 2.05, and an international letter about 2.26. Each additional ounce runs about 0.38 of the letter rate. Check those against a current rate sheet once a year or so. They have held for several increases in a row, but nothing about postage pricing is guaranteed to stay put.

Forever stamps

A Forever stamp is always worth exactly one ounce of first-class domestic postage, whatever it cost when you bought it. That makes buying them ahead of an announced increase a small, genuine, riskless return. It also makes them the wrong thing to put on a two-ounce letter: you need the base stamp plus an additional-ounce stamp, not two Forever stamps, or you overpay.

The cheapest thing most people get wrong

Padded envelopes and small boxes are almost always cheaper as Priority Mail Flat Rate than by weight, and anything book-shaped and heavy should go Media Mail, which is a fraction of the price in exchange for slower delivery and the possibility of inspection. If you are posting a hardback, Media Mail is usually the difference between a few dollars and a great many. Neither of those is priced here, because neither follows a per-ounce formula.

Not sure which class you are in? This page assumes you already know, and multiplies. The postage class calculator does the other half: give it the weight and the three dimensions of one item and it tells you whether that item is a letter, a flat or a package, which is the decision that sets the price before any rate is applied. It is the page to use when the envelope is an awkward shape. Come back here when you know the class and want to know what forty of them cost.