The Tenth of an Ounce That Costs 87¢
Postage looks like it should be smooth. Heavier letter, slightly more money — that is how every child assumes it works, and it is how it works right up until the moment it does not.
The postage calculator has a limit built into it, and crossing that limit is the single most expensive tenth of an ounce in the whole system.
What the numbers actually do
| Weight | As a letter | As a large envelope | What it really is |
|---|---|---|---|
| 1 oz | $0.82 | $1.69 | Letter |
| 2 oz | $1.11 | $1.98 | Letter |
| 3 oz | $1.40 | $2.27 | Letter |
| 3.5 oz | $1.69 | $2.56 | Letter |
| 3.6 oz | — | $2.56 | Large envelope |
| 5 oz | — | $2.85 | Large envelope |
| 8 oz | — | $3.72 | Large envelope |
| 13 oz | — | $5.24 | Large envelope |
A plain letter can weigh 3.5 oz and no more. At 3.5 oz it costs $1.69. At 3.6 oz it is not an expensive letter — it is not a letter. The post office moves it into the large-envelope class, where the first ounce alone costs as much as the whole 3.5 oz letter did, and the price becomes $2.56.
That is 87¢ for a tenth of an ounce: roughly the weight of a single extra sheet of paper, and about a 51% increase.
Why the calculator warns you instead of just answering
Put 3.6 oz into the calculator as a letter and it does something slightly unusual — it gives you a price and tells you not to trust it. The note says the post office would move the item to a heavier class, so the figure is a floor rather than an estimate.
That is deliberate. A tool that quietly returned $1.69 for a 3.6 oz letter would be arithmetically consistent and practically wrong, and the person paying for it would be a child at a counter being told a number they did not expect.
What weighs what
A single sheet of ordinary paper is about 0.16 oz. A standard envelope is roughly 0.2 oz. So a plain letter runs to something like:
One sheet and an envelope: a little over a third of an ounce — comfortably inside the first-ounce charge of $0.82, with room to spare.
Five sheets and an envelope: around an ounce. Still one stamp.
Twenty sheets and an envelope: about 3.5 oz — right on the edge.
The things that actually push a letter over are not extra pages. They are the additions: a sheet of stickers, a pressed flower, a friendship bracelet, a folded drawing on thicker paper, a photograph. Each is small, and three of them together are the difference between $1.69 and $2.56.
What to do about it
Weigh it before you go. A kitchen scale is accurate enough. This is the one part of posting a letter where guessing costs real money.
If you are near the line, take something out. A letter at 3.5 oz and a letter at 3.6 oz feel identical in your hand and differ by 87¢. Removing one sticker sheet is the cheapest decision available.
If you are well over, stop resisting. Once the item is genuinely a large envelope, the additional-ounce charge is gentle — $2.56 at 3.6 oz rises only to $3.72 at 8 oz. The expensive part was crossing the line, not what you do afterwards.
Do not fold something to make it lighter. Folding changes the size, not the weight. It may change which envelope you need, which is a separate and genuinely useful thing, but the scale will read exactly the same.
The same cliff exists in the other classes
A large envelope has its own limit, and past it the item becomes a package. The pattern repeats: each class is cheap inside its range and the jump between classes is abrupt, because the classes describe different ways of physically handling the mail rather than different amounts of it.
A letter goes through automated sorting machinery. A flat is handled differently. A package is handled differently again. You are not paying for weight so much as for which machine the item fits through — and that is why the price is a staircase rather than a slope.
The useful habit
Before sealing anything, put it on the scale and read the number. Then check it in the calculator and glance at the class it names. If the class says "Letter", you are fine. If it says anything else, or if the note mentions a limit, you have crossed a staircase step and the price is no longer the one you assumed.
It takes ten seconds and it is the difference between posting a letter and posting a letter you have to re-do at the counter.
Weight is not the only thing a class cares about
A letter has to be thin, flexible and rectangular as well as light: at least 3.5 by 5 inches, no larger than 6-1/8 by 11-1/2 inches, and no more than 1/4 inch thick. An envelope under the weight limit that is thicker than that is priced as a large envelope, and one that is square, rigid or lumpy pays a $0.49 nonmachinable surcharge on top of the letter price, because it cannot go through the sorting machinery.
That is the invisible version of the same cliff. Nothing on the scale warns you, because the problem is not mass. A birthday card with a badge pinned inside it weighs almost nothing and is emphatically not an ordinary letter any more.
The three things that push a letter over
A second card. Greetings cards are printed on heavy stock — a single one can be 0.3 oz, and two of them plus pages is most of an ounce before anything else goes in.
Anything with a spine or a clasp. A small notebook, a keyring, a badge. These fail on thickness long before weight, and they are exactly what a child wants to send.
Glitter, confetti and loose beads. Weightless and genuinely unwelcome: they escape inside the machinery, and some postal services treat a leaking envelope as undeliverable.
The honest limits of the estimate
The prices in this post are USPS retail prices effective July 12, 2026. Check the current rate before you post: prices usually change each January and July.
The calculator reads the same USPS price list, and the price at the counter on the day you post is the one that counts.
What does not change is the shape of the thing: a flat charge for the first unit, a smaller one for each unit after, and a hard edge where the class ends. Learn the shape and the exact numbers matter much less, because the shape is what tells you where the expensive decisions are.