USPS's 108-inch combined length and girth rule is the strictest among major US carriers. It also has the most nuance — different services have different limits, the measurement method has some quirks, and there are exception categories that allow you to push past 108 in specific circumstances.
The base rule
For USPS Priority Mail and Ground Advantage (the everyday domestic services most people use), the combined length plus girth limit is 108 inches. This applies to virtually all retail customers shipping at the post office or via Click-N-Ship.
Length is the longest side. Girth is the distance around the thickest part of the package — for a rectangular box, that's 2 × (width + height).
For a package measured at 109 inches combined length plus girth, USPS will refuse it at the retail counter. No surcharge — actual refusal. The recipient simply will not get it.
Where the exceptions hide
USPS Retail Ground. The older Retail Ground product (still used by some business customers, less common since 2024) accepts up to 130 inches length plus girth, with a separate oversize pricing tier.
USPS Bound Printed Matter. For book and printed material shipments (qualifying media), the length plus girth limit increases. This is a niche category and most consumer shippers won't qualify.
USPS Media Mail. Limit is 108 inches but oversize pricing kicks in earlier. Media Mail is for books, CDs, DVDs, and educational materials only — strict eligibility.
USPS oversized rate. Between 84 and 108 inches combined length plus girth, USPS treats the package as "oversized." This doesn't refuse the package — it just prices it differently. The package is rated as if it weighed at least 20 pounds (Priority Mail) or 50 pounds (Ground Advantage), even if it weighs less. So a 5-pound oversize package ships at the 20-pound rate.
How USPS measures
USPS measures the package as presented. They don't unwrap it, don't separate handles or labels, and don't account for soft items being compressible. If your bow on top adds an inch, the bow counts.
USPS rounds up to the next whole inch. A package at 108.1 inches is treated as 109 — over the limit, refused. Even 108.0 is sometimes counted as 109 if the measurement tape moves slightly. In practice, give yourself at least an inch of margin: aim for 107 or less if you want certainty.
For non-rectangular packages, USPS measures the longest dimension and the circumference at the thickest cross-section. For a guitar-shaped package (irregular outline), they may measure the bounding box or use the actual outline — discretion varies by clerk.
Why USPS is more restrictive than UPS or FedEx
USPS operates a different network than the private carriers. Where UPS and FedEx have purpose-built oversized handling at their major hubs, USPS relies more on standard mail processing equipment, much of which can't handle packages over 108 inches. The 108-inch limit is essentially the equipment limit.
Anything beyond that has to be hand-sorted at distribution centers, which USPS isn't priced or staffed to do at scale. Hence the refusal rather than a surcharge.
If you're at 109-115 inches
You're not going to ship it USPS. Switch to UPS or FedEx Ground — both accept up to 165 inches combined length plus girth.
USPS pricing is usually competitive under 84 inches; once you're between 84 and 108, the oversize pricing closes much of the gap with UPS Ground. Once you're over 108, USPS isn't an option at all.
If you're at 105-108 inches
You're in the danger zone. The package can ship USPS, but with very little measurement margin. Repack into a slightly smaller box if possible, or budget for switching to UPS or FedEx if USPS measures it differently than you did.
Use the USPS calculator to verify your specific dimensions, and run the package through the multi-carrier comparison to see your other options.
A practical example
A guitar in a hard case measuring 48 × 18 × 6 inches:
length = 48 in
girth = 2 × (18 + 6) = 48 in
length + girth = 96 in
That's 96 — between USPS's 84-inch oversize threshold and the 108-inch absolute limit. The package ships USPS Priority Mail with oversize pricing. For a 12-pound guitar in a case weighing 8 pounds, total 20 pounds, the package would be rated as 20 pounds — but since it's already at 20, oversize rules don't add an extra charge in this case.
For the same guitar in a larger gig bag measuring 50 × 20 × 8:
length = 50 in
girth = 2 × (20 + 8) = 56 in
length + girth = 106 in
Still under 108 — ships USPS. Two inches of margin. If you'd used a 52 × 21 × 8 box (just a touch larger), you'd be at 110 inches — refused at the counter.