One Thick Letter Costs Less Than Two Thin Ones
Postage is not charged by weight. It is charged by the first ounce, and then by weight — and those are very different things.
The first ounce of a domestic letter costs $0.82. Every additional ounce costs $0.29, which is about 35% of the first. That ratio is the whole story.
The cost of an ounce, by which ounce it is
| Total weight | Total cost | Added cost | Cost per ounce |
|---|---|---|---|
| 1 oz | $0.82 | — | $0.82 |
| 2 oz | $1.11 | $0.29 | $0.56 |
| 3 oz | $1.40 | $0.58 | $0.47 |
| 3.5 oz | $1.69 | $0.87 | $0.48 |
The prices in this post are USPS retail prices for stamped letters, effective July 12, 2026. Check the current rate before you post: prices usually change each January and July.
Look at the last column. A one-ounce letter costs $0.82 an ounce. A 3.5 oz letter costs $0.48 an ounce, about 59% as much. The heavier the letter, the cheaper each ounce inside it becomes.
Which settles the commonest question
A child wants to send a letter and a drawing to the same friend. Two envelopes, or one?
Two separate one-ounce letters: $1.64.
One two-ounce letter: $1.11.
Splitting costs 53¢ more — 48% — for delivering exactly the same paper to exactly the same address. You pay the first-ounce charge twice, and the first ounce is the expensive one.
And the second commonest
"Can I add one more page?" Almost always yes, and often for nothing at all. A sheet of paper is about 0.16 oz, so six sheets fit inside a single ounce. Going from one sheet to five costs precisely nothing, because you have not crossed into the second ounce yet.
The calculator shows this clearly: the price is a flat line across each whole ounce and then steps up. Anywhere on a flat stretch, more paper is free.
How the rounding works, and why it matters
The additional-ounce charge is applied to whole ounces, rounded up. A letter weighing 1.1 oz is charged as two ounces and costs $1.11, the same as a letter weighing a full 2 oz.
So the expensive moment is not reaching 2 oz — it is reaching 1.01 oz. Once you have crossed, everything up to the next whole ounce comes free, which is the practical version of the advice above: if you have gone slightly over, go properly over and use the room you have paid for.
What this means for a pen pal
Write longer letters, less often. Four short letters a month cost $3.28. One long letter a month costs $0.82 if it stays under an ounce and $1.11 if it does not. The long letter is also more fun to receive, which is rare agreement between arithmetic and sentiment.
Put the extras in with the letter. Stickers, a drawing, a photograph — they all travel inside the ounce you have already paid for, provided you stay under the 3.5 oz class limit.
Do not send two envelopes to the same house. Ever, unless the two things genuinely need to arrive separately.
Where the logic reverses
All of this assumes one destination. Two letters to two different friends are two letters, and no amount of arithmetic combines them.
It also assumes the letter class. Once an item becomes a large envelope or a package, the first-unit charge is bigger and the trade-offs shift — a package's base price covers the first eight ounces rather than the first one, which makes small packages disproportionately expensive and medium ones surprisingly reasonable.
A worked example
A birthday envelope: a card (0.3 oz), two sheets of letter paper (0.32 oz), a sheet of stickers (0.2 oz) and the envelope itself (0.2 oz). That is about 1.02 oz — just over, and therefore charged as two ounces at $1.11.
Take out the sticker sheet and it is 0.82 oz, charged as one ounce at $0.82. Or leave the stickers in and add another page and a drawing, because you have already bought the whole second ounce and are currently using about a fiftieth of it.
The second option is better. It is the same price as the first mistake and a much better letter.
When two envelopes genuinely are right
The arithmetic says combine, and there are three cases where it is wrong.
Different people at the same address. Two children in one house should each get their own envelope with their own name on it, and the 53¢ is the price of that, cheerfully paid.
Things that must not be folded together. A photograph and a bulky craft item in one envelope produces a creased photograph.
Anything that needs to arrive on a different day. Posted together, arriving together — that is the whole point of one envelope, and occasionally it is exactly what you do not want.
Why the first ounce is the expensive one
Almost everything the postal service does to a letter happens once, regardless of weight: collecting it, reading the address, sorting it, routing it, carrying it to a door. The marginal cost of the paper inside is close to nothing.
So the price is mostly a fixed charge for handling one item, with a small weight component bolted on. Once you see it that way, the $0.29 additional ounce stops looking like a bargain and starts looking like what it is: the only part of the price that is actually about the letter rather than about the journey.
It also explains why the advice never changes. Fewer, fatter envelopes beat more, thinner ones — because you are paying per journey, and the paper is very nearly free.
The rule, in one sentence
One envelope per person per occasion, filled as full as the class limit allows.
Everything else in this guide is a consequence of that. It is also, conveniently, the version that produces the best letters: an envelope somebody has bothered to fill is a nicer thing to receive than two thin ones sent a week apart, and it happens to be the cheaper option by a comfortable margin.
Run your own weights through the calculator once and the boundaries stop being abstract. After that you will know, by feel, whether an envelope is on a flat stretch or about to step up — which is the only postal skill worth actually acquiring.