To get cubic feet, measure the length, width and height in inches, multiply them, and divide by 1,728, the number of cubic inches in a cubic foot (12 × 12 × 12). A box that measures 24 × 18 × 12 inches holds 5,184 cubic inches, and 5,184 ÷ 1,728 = 3 cubic feet. If you measured in feet, skip the division: 2 × 1.5 × 1 = 3.
The math is the easy part. The number matters because carriers, freight classes and Amazon all price off it, each with its own measuring rules, and a fraction of an inch can move a box into a higher bracket.
The formula for each unit
- Inches: length × width × height ÷ 1,728.
- Feet: length × width × height. No division needed.
- Mixed units (say, 2 ft by 18 in by 10 in): convert everything to inches first, then divide by 1,728.
- Centimeters: length × width × height ÷ 28,317, the number of cubic centimeters in a cubic foot.
- Cylinders and tubes: π × radius² × height in inches ÷ 1,728. A tube 12 inches across and 24 inches tall has a 6-inch radius, so 3.1416 × 36 × 24 = 2,714 cubic inches, or about 1.57 cubic feet.
The most common slip is dividing cubic inches by 12 instead of 1,728. Twelve converts one dimension from inches to feet; a cubic foot is 12 inches in all three directions.
Common box sizes in cubic feet
The DIM weight column uses the 139 divisor that UPS and FedEx apply to daily (account) rates. UPS retail rates use 166, which gives a lower figure. The USPS column assumes commercial postage, which is how cubic pricing is sold.
Box (inches) | Cubic inches | Cubic feet | DIM weight at 139 | What USPS does with it |
|---|---|---|---|---|
6 × 6 × 6 | 216 | 0.13 | 2 lb | 0.20 cubic tier; no DIM weight |
10 × 8 × 6 | 480 | 0.28 | 4 lb | 0.30 cubic tier; no DIM weight |
12 × 10 × 8 | 960 | 0.56 | 7 lb | Too big for Priority Mail Cubic; 0.60 tier in Ground Advantage Cubic |
12 × 12 × 12 | 1,728 | 1.00 | 13 lb | Exactly 1 cubic foot: top Ground Advantage Cubic tier, still no DIM weight |
16 × 12 × 12 | 2,304 | 1.33 | 17 lb | DIM weight applies (17 lb) |
24 × 18 × 12 | 5,184 | 3.00 | 38 lb | DIM weight applies, plus nonstandard fees for length over 22 in and volume over 2 cubic feet |
20 × 20 × 20 | 8,000 | 4.63 | 58 lb | DIM weight applies, plus the over-2-cubic-feet fee |
24 × 24 × 24 | 13,824 | 8.00 | 100 lb | Length plus girth is 120 in, past Priority Mail's 108 in limit; Ground Advantage charges its oversized price by zone, with no nonstandard fees |
DIM weight is rounded up to the next whole pound. You pay on it only when it's higher than what the box actually weighs.
Parcel shipping: dimensional weight
Cubic inches are the input to dimensional weight. UPS divides by 139 on daily rates and 166 on retail rates, and FedEx uses 139. Neither sets a minimum size the way USPS does; UPS says DIM weight may apply to any of its package services. The carrier bills whichever is greater, the actual weight or the DIM weight.
USPS works differently. For Priority Mail and Ground Advantage, DIM pricing applies only to parcels over 1 cubic foot. USPS rounds each side up to the whole inch, multiplies, and if the result is over 1,728 cubic inches, divides by 139 (DMM 123 and DMM 133). That makes 1 cubic foot the line worth knowing. A 12 × 12 × 12 box sits exactly on it and pays by actual weight. Make the box 12 × 12 × 12¼ and the height rounds up to 13 inches, the volume becomes 1,872 cubic inches, and the parcel is billed at 14 lb even if it weighs 3.
UPS rounds too: its measuring guide says to round each side up to the next whole inch, so 1.01 inches counts as 2. Plug your box sizes into our dimensional weight calculator to see the billable weight for each carrier side by side, or read the short dimensional weight definition if you're new to it.
USPS cubic pricing
USPS also sells postage priced by cubic feet instead of weight. Priority Mail Cubic covers parcels up to 0.50 cubic feet in five tiers of 0.10 each, and Ground Advantage Cubic runs to 1.00 cubic foot in ten tiers (DMM 223 and DMM 283). Either way, the parcel can weigh up to 20 lb and its longest side can't exceed 22 inches. Within those limits, the weight doesn't change the price, which is why cubic suits small, heavy products.
The measuring rule runs the opposite way to DIM weight. For a cubic tier, USPS rounds each side down to the nearest quarter inch, so 12⅜ inches counts as 12¼. The same box can measure slightly smaller for cubic pricing than it does for DIM weight. Our USPS cubic pricing guide walks through finding the tier.
Above 2 cubic feet (3,456 cubic inches), USPS adds a nonstandard fee to Priority Mail and Ground Advantage parcels. Another applies when the longest side is over 22 inches. A 24 × 18 × 12 box triggers both.
Cases and pallets: density and freight class
Inbound freight is where case cube and pallet cube matter. LTL carriers class most freight by density, which is weight divided by cubic feet. Under the NMFC scale that took effect on July 19, 2025, there are 13 density classes, from class 50 for freight of 50 lb per cubic foot or more down to class 400 for under 1 lb per cubic foot. Lower classes usually cost less to ship.
Take a pallet that stands 48 × 40 × 48 inches with the pallet included and weighs 500 lb. That's 92,160 cubic inches, or 53.33 cubic feet, so the density is about 9.4 lb per cubic foot, which lands in class 100. Measure the load as it will ship, at its widest points, pallet and any overhang included. The carrier's measurement is the one that counts, and NMFTA advises building in a margin above a class threshold.
Our freight class calculator does the density math for each pallet. To work out how many master cartons fit on a pallet before you book the freight, use the pallet calculator.
Amazon FBA storage
Amazon charges FBA monthly storage fees on your daily average volume in cubic feet, based on each unit's size when it's packaged and ready to ship. The rate per cubic foot depends on the size tier, and it's higher from October through December. Here cubic feet feed the bill directly: units × cubic feet per unit × rate. Trimming the packaging on a slow-selling SKU lowers its storage cost every month it sits.
Not every warehouse bills that way. Simpl, like some other 3PLs, bills storage by the space your inventory takes: a small bin, large bin, shelf or pallet. There, your carton sizes tell you how many bins or pallets you'll fill, and that sets the storage line on the invoice.
Measuring mistakes that change the number
- Measuring the empty box. Measure the packed carton at its widest point on each side. A bulge or a lid that doesn't sit flat adds to the dimension the carrier records.
- Using a supplier's listed size without checking. Box listings may give inside dimensions. The carrier measures the outside, and the walls add to every side.
- Ignoring the rounding rule. DIM weight rounds each side up to the next whole inch; USPS cubic tiers round down to the quarter inch. Run the number the way the carrier will.
- Mixing units. Multiplying feet by inches gives a meaningless figure. Convert first.
- Measuring a pallet without the pallet. Freight density uses the whole shipping unit, including the pallet and anything hanging over its edges.