SMKlog
Tape measure procedure

How to measure a box for shipping

Pack the box, tape it shut, then measure it: longest side is the length, then width, then height, outside edge to outside edge, each figure rounded up to the next inch. Measuring the empty carton or the item inside it is the mistake that quietly changes what you owe.

Below is the full procedure, the girth arithmetic every carrier limit is written in, and three live quotes that show what a few extra inches are worth in dollars.

Girth formula explainedCarrier limits citedSame weight, 2.7x the price
Price a box by its measurements
Tape measure laid across the longest side of a sealed carton
Sealed first, measured second

A packed carton bulges. The carrier measures the bulge, so you should too.

The procedure, in the order that matters

  1. 1. Finish the packing firstFill the voids, close the flaps, run the tape. A carton measured before it is loaded reads smaller than the one that leaves your house, and the difference is exactly what a carrier bills for.
  2. 2. Longest side is the lengthLay the tape along the longest edge and call that number length. Then the second longest is width, and the remaining one is height. Carriers write their rules in that order, so using any other order makes every published limit read wrong.
  3. 3. Measure the outside, including the bulgeRun the tape over the outer face at its widest point, not along a straight edge that a full box no longer has. If the sides bow, measure across the bow. If a handle or a rim sticks out, it counts.
  4. 4. Round every figure upTake each of the three numbers to the next whole inch. Half an inch of rounding down on two dimensions is enough to move a parcel into a different billing bracket on a bulky shipment.
  5. 5. Work out length plus girthUSPS gives it as length plus twice the width plus twice the height. UPS says the same thing from the other direction: girth is twice the width plus twice the height, and you add the length. This single number is what every size limit in the industry is written against.
  6. 6. Write all four numbers on the packing listLength, width, height and weight, on the slip that goes inside the box. If anything is queried later, you have your own record of what left the house.

Three boxes, one weight, three very different prices

Nothing makes the case for measuring properly like watching the price move without the contents changing. All three rows below hold exactly 5 lb and travel the identical route, quoted through the SMKlog calculator on 2026-07-28, Newark, NJ 07102 to Atlanta, GA 30303.

Identical 5 lb contents in three box sizes, Newark, NJ 07102 to Atlanta, GA 30303, quoted 2026-07-28. Cheapest returned service shown.
BoxCubic inchesLength plus girthCheapest price
12 × 10 × 6 in72044 in$13.52
18 × 14 × 10 in2,52066 in$20.50
24 × 18 × 14 in6,04888 in$36.75

From the smallest to the largest, the price climbed by a factor of 2.7 while the scale never moved. That is what carriers mean by charging for space. Our dimensional weight page does the arithmetic that produces those jumps; this page is only about getting the three inputs right.

The cubic inches and the length-plus-girth columns are multiplication and addition of the measurements themselves, shown so you can check the working rather than take our word for it.

Quote your measured box Try a bulky one

The numbers your measurements are being checked against

  • USPS: 70 lbs maximum on every service. Ground Advantage, Media Mail, Library Mail and Parcel Select allow up to 130 inches of length and girth combined; Priority Mail and Priority Mail Express stop at 108 inches (Notice 123 and usps.com, checked 2026-07-28).
  • UPS: 150 lbs maximum, 108 inches maximum length, and 165 inches of length and girth combined (ups.com, checked 2026-07-28).
  • FedEx applies its oversize charge to packages that “exceed 96 inches in length or 130 inches in length and girth”, and bills them on a minimum of 90 lbs (fedex.com, checked 2026-07-28).
  • UPS additional handling is triggered by a longest side over 48 inches, a second-longest side over 30 inches, packaging not fully encased in corrugated cardboard, or a cubic size above 10,368 cubic inches (ups.com, checked 2026-07-28). The carrier publishes the criteria without the amounts, so no figure is quoted here.

Run the three boxes from the table against that list and all of them pass comfortably, the largest at 6,048 cubic inches and 88 combined inches. The price still nearly tripled, which is the part worth remembering: you can be well inside every published limit and still be paying for air.

Measuring questions

How do you measure a box for shipping?

Measure the box after it is packed and taped shut. Take the longest side as the length, then the width, then the height, from outside edge to outside edge. Round every figure up to the next inch, because a bulging carton measures larger on a carrier belt than a flat one does on your table.

How do you calculate girth?

Girth is the distance around the parcel at its thickest point, and USPS gives the arithmetic as length plus twice the width plus twice the height for the combined figure. UPS describes girth itself as twice the width plus twice the height, added to the length for the combined measurement. Both arrive at the same number.

What are the size limits for a parcel?

USPS Ground Advantage allows up to 130 inches of length and girth combined, while Priority Mail and Priority Mail Express stop at 108 inches. UPS publishes 108 inches of length, 165 inches of length and girth combined, and 150 lbs. FedEx applies an oversize charge above 96 inches of length or 130 inches of length and girth.

Does box size change the price if the weight is the same?

Dramatically. On 2026-07-28 the same 5 lb of contents quoted at $13.52 in a 12 by 10 by 6 inch box, $20.50 in an 18 by 14 by 10, and $36.75 in a 24 by 18 by 14, all from Newark to Atlanta. Nothing changed but the air around the contents.

Next steps

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