Free packing calculator
Box size calculator: what size box do I need?
Enter your item’s size, how many you are packing and how much padding you want. Get the smallest box that fits, its outside size and dimensional weight, stock box sizes that fit and which USPS Flat Rate boxes fit.
What are you packing?
Box you need
Enter the item size to find the smallest box that fits.
Get the real rate for this box
Enter the box size and weight in our shipping calculator to compare discounted USPS, UPS and FedEx labels.
How to choose a shipping box
Measure the item
Measure the longest side, the width and the height, including any retail packaging.Choose padding
Add space on every side for bubble wrap, air pillows or paper.Use the smallest box that fits
A snug box protects the item and keeps dimensional weight down.Price it
Enter the box size and weight in the shipping calculator to compare carriers.
Box terms
- Inside dimensions
- Length × width × depth of the space inside the box. Used to check fit.
- Outside dimensions
- The box’s external size, used by carriers for dimensional weight.
- Single-wall corrugated
- The common shipping box board, about 1/8 to 3/16 inch thick.
- Dimensional weight
- Length × width × height ÷ the carrier’s divisor. Billed when it exceeds the actual weight.
Box size FAQ
- What size box do I need?
- Measure the item, add padding on every side (about 1 to 2 inches for most products, more for fragile ones), and choose the smallest box whose inside dimensions are at least that big. For several items, the calculator tries every arrangement and orientation to find the smallest box.
- Are box dimensions inside or outside?
- Box makers list inside dimensions, length × width × depth, so you know what fits. Carriers measure the outside, which is a little larger, to calculate dimensional weight.
- Why does box size affect shipping cost?
- Carriers bill the greater of the actual weight and the dimensional weight (length × width × height divided by the carrier’s divisor). An oversized box can cost more than a heavier package in a snug box.
- How much padding should I leave?
- About 1 inch for sturdy items, 2 inches or more for fragile items, so cushioning can absorb a drop. The padding is added to every side.
- Will my item fit in a USPS Flat Rate box?
- The calculator lists the Flat Rate boxes whose inside dimensions are big enough. Flat Rate costs the same to any US address up to 70 lb, so it pays off for heavy, small items going far.
Found your box?
Compare discounted USPS, UPS and FedEx rates for its size and weight.
Free tools
More free packing tools
USPS Box Sizes
Every USPS Flat Rate box and envelope size with prices, plus a box finder.
Cheapest Way to Ship
Compare USPS Flat Rate with Ground Advantage and Priority Mail prices by zone.
Dimensional Weight Calculator
Find the billable DIM weight carriers will charge before the invoice lands.
Cubic Feet Calculator
Get cubic feet, cubic meters and freight density for boxes, cylinders and shipments.
Want us to find the money leaking out of your shipping? Get a free profit-leak audit.
Request a free auditResults are estimates from the sizes you enter. Box makers’ sizes and wall thicknesses vary; carriers measure and bill packages by their own measurements.