Why people use a stamp calculator
Guessing postage is expensive. Too little and the piece comes back or arrives postage due. Too much and you waste money on every piece you send. A simple calculator removes the guesswork: you match your envelope to USPS letter rules, round weight up to the next whole ounce, and add any surcharges that apply.
This tool focuses on domestic First-Class letter mail—the kind of mail most people send from home: bills, letters, greeting cards, and light documents in standard envelopes. If your piece fits within letter limits and you tell us whether it is square, rigid, or lumpy, we show the non-machinable surcharge when it applies so you are not surprised at the counter.
What counts as a letter (for this calculator)
USPS treats a letter as a rectangular piece that weighs up to 3.5 ounces, is not thicker than one quarter inch, and fits within maximum length and height. If anything is outside those bounds—too heavy, too thick, or too large in the other dimensions—USPS may classify it as a large envelope (flat). This MVP tells you when you have crossed into that category and shows flat pricing when we add it later.
Weight is always rounded up to the next whole ounce for postage. That is why a 1.1 ounce piece pays the same as 2 ounces. The calculator states the rounded weight in the notes so you can double-check against a kitchen scale or postage scale.
How we show “stamps to use”
Retail stamps are sold in fixed denominations. A Forever stamp covers the first ounce of a standard letter at the current First-Class letter rate. Each extra ounce uses an “additional ounce” stamp (or equivalent postage). When a non-machinable surcharge applies—often with square envelopes, rigid mailers, or odd shapes—you need extra postage beyond the ounce ladder. We spell that out in the breakdown and in the stamp line so you can buy the right combination or use a postage label online.
Rates change. We keep the numbers in one place in the codebase so you can update them when USPS announces new prices. Always verify critical mail at usps.com or at the post office if the piece is valuable or time-sensitive.
What we do not do yet
Large envelope (flat) pricing, international letters, and bulk commercial mail use different rules. We structured the project so those can plug in later. For now, if your inputs do not qualify as a letter, you will see a clear message instead of a wrong number.
If you are comparing this tool to the retail counter, remember that clerks may weigh and measure slightly differently than your home equipment. Small differences in thickness—especially near the quarter inch limit—can change classification. When your mail is borderline, round up postage or ask USPS to verify. The goal of this calculator is to get you in the right ballpark fast, with enough detail that you understand what drives the price: ounces, shape, rigidity, and the non-machinable surcharge when automation cannot handle the piece.