USPS Shipping Limits: What You Can Send and How

If you’ve ever wondered why a package gets bounced back or why the price jumps at checkout, the answer is usually in the USPS shipping limits. Knowing the weight, size and content rules saves you time, money and a lot of frustration. Below you’ll find the basics for the most common services – First-Class, Priority Mail, Priority Mail Express and Parcel Select – plus a quick cheat‑sheet for items you can’t mail at all.

Weight and Size Rules for Every USPS Service

First‑Class Mail is the go‑to for letters, postcards and lightweight parcels under 13 oz. The max thickness is ¼ inch for a letter and ¾ inch for a flat. Anything heavier or thicker moves up to Priority Mail.

Priority Mail handles packages up to 70 lb and 108 in combined length and girth (girth = 2 × width + 2 × height). The longest side can be 108 in, but if the package is longer than 36 in you must use a flat‑rate box or a special parcel service.

Priority Mail Express, the overnight option, shares the same 70 lb limit but the size cap drops to 70 lb and 130 in combined length + girth. It’s the only service that guarantees delivery by 8 am or 10 am in many areas, so you’ll pay extra if the package is near the limit.

Parcel Select is for bulk shippers. It also caps at 70 lb and 108 in combined dimensions, but you get a slower, ground‑only route. If you need a bigger box, consider Freight services – they’re separate from standard USPS limits.

Flat‑rate boxes are a shortcut: any weight up to 70 lb fits, as long as it fits inside the box. That’s why many people choose a Medium Flat Rate when shipping heavy books or electronics – you pay a set price, not a weight‑based one.

Common Items That Can’t Go Through the Mail

Even if your package fits the size and weight rules, some items are outright prohibited. Liquids over 3 oz (like perfume or sauce) need special packaging and can’t be sent in regular parcels. Batteries are another gray area – dry cell lithium batteries are allowed in small quantities, but large lithium‑ion packs must be shipped as hazardous cargo.

Explosives, flammable gases, and weapons are strictly banned unless you have a special permit. This includes fireworks, dry ice (only in limited amounts) and firearms. Check the USPS prohibited items list if you’re unsure.

Perishable foods can be mailed, but they must be insulated and clearly marked. If the item spoils quickly, consider a faster service like Priority Mail Express to keep it fresh.

Finally, remember that the destination matters. Some countries have stricter import rules, so a package that’s fine domestically may be stopped abroad. Always verify the recipient country’s restrictions before you seal the box.

By keeping these limits top of mind, you’ll avoid surprise surcharges, delayed deliveries and returned shipments. The next time you head to the post office or print a label online, give the size and weight numbers a quick glance, double‑check the contents, and you’ll be on the fast track to a smooth delivery.