To calculate cubic inches, multiply a box’s length × width × height, with every measurement in inches: a 12 × 10 × 8 inch box is 960 cubic inches. To calculate girth in inches, add the two shorter sides and double the result, girth = 2 × (width + height), so the same box has a girth of 36 inches and a length plus girth of 48 inches.
Those two numbers decide most of what a carrier does with your package: cubic inches drive dimensional weight and cubic pricing, and length plus girth decides whether the box is within size limits. This guide works as a girth calculator in inches and a cubic inch reference, with worked examples and the current USPS and UPS thresholds.
How do you calculate cubic inches step by step?
The formula for any rectangular box is Volume (in³) = Length × Width × Height. Measure the outside of the box, because carriers bill the space the package takes up, not the space inside it.
- Measure the length, the longest side, in inches.
- Measure the width, the second-longest side, in inches.
- Measure the height, the shortest side, in inches.
- Convert any feet or centimeters to inches first (1 foot = 12 inches, 1 inch = 2.54 cm).
- Multiply the three numbers. The result is the volume in cubic inches.
Example: a box measuring 14 × 10 × 5 inches is 14 × 10 × 5 = 700 cubic inches. If one side is given as 1.5 feet, convert it to 18 inches before multiplying, so 18 × 10 × 6 = 1,080 cubic inches. Our guide on how to read dimensions correctly explains which number is which on a box label.
How do you calculate girth in inches?
Girth is the distance around the thickest part of the package, measured at right angles to the length. For a rectangular box, girth = 2 × width + 2 × height. Carriers then add the length to get length plus girth, the figure their size limits use.
Use this table as a quick girth calculator in inches for common box sizes. It also shows which services the box fits by size alone.
| Box (L × W × H, in) | Cubic inches | Girth (in) | Length + girth (in) | Fits by size |
|---|---|---|---|---|
| 12 × 10 × 8 | 960 | 36 | 48 | Priority Mail, Ground Advantage, UPS |
| 18 × 14 × 12 | 3,024 | 52 | 70 | Priority Mail, Ground Advantage, UPS |
| 24 × 18 × 18 | 7,776 | 72 | 96 | Priority Mail, Ground Advantage, UPS |
| 36 × 20 × 20 | 14,400 | 80 | 116 | Ground Advantage, UPS (over the Priority Mail limit) |
| 48 × 24 × 20 | 23,040 | 88 | 136 | UPS only (over both USPS limits) |
USPS limits combined length and girth to 108 inches for Priority Mail and 130 inches for USPS Ground Advantage. UPS allows up to 165 inches in length plus girth and up to 108 inches in length, as covered in our guide to UPS size and weight limits. For irregular shapes, our girth calculator guide for package measurements walks through more cases.
How do you convert cubic inches to cubic feet?
Divide cubic inches by 1,728 to get cubic feet, because one cubic foot is 12 × 12 × 12 = 1,728 cubic inches. To go the other way, multiply cubic feet by 1,728.
| Cubic feet | Cubic inches | Why it matters |
|---|---|---|
| 0.5 | 864 | Maximum size for Priority Mail Cubic |
| 1 | 1,728 | USPS dimensional weight threshold; maximum for Ground Advantage Cubic |
| 2 | 3,456 | USPS Ground Advantage nonstandard volume threshold |
| 6 | 10,368 | UPS Additional Handling cubic trigger |
| 10 | 17,280 | UPS Large Package cubic trigger |
Cubic feet are also the unit for freight. A 48 × 40 × 60 inch pallet is 115,200 cubic inches, or 66.7 cubic feet, and LTL carriers use that volume with the weight to work out density and freight class.
How do cubic inches affect dimensional weight?
Dimensional weight converts a box’s cubic inches into a billable weight, and carriers charge whichever is higher: the actual weight or the dimensional weight. The formula is cubic inches ÷ divisor, rounded up to the next pound.
- USPS: since July 12, 2026, USPS divides by 139 for USPS Ground Advantage, Priority Mail, Priority Mail Express and Parcel Select parcels over 1 cubic foot, after rounding each dimension up to the next whole inch. Parcels of 1 cubic foot or less are billed on actual weight.
- UPS: UPS uses a divisor of 139 for its daily rates.
Example: a 20 × 15 × 8 inch box is 2,400 cubic inches. Divided by 139, that is 17.3, which rounds up to 18 lbs of dimensional weight. If the item inside weighs 3 lbs, you are billed for 18 lbs. Repacking it in a 16 × 12 × 6 inch box (1,152 cubic inches, under 1 cubic foot) means USPS bills the actual weight instead. Our cubic inch calculator guide has more worked conversions.
Which USPS and UPS rules use cubic inches?
Several carrier rules are written in cubic feet or cubic inches, so knowing your box’s volume tells you which pricing and handling rules apply before you buy a label.
| Rule | Volume threshold | Other conditions |
|---|---|---|
| Priority Mail Cubic | Up to 0.5 cubic foot (864 cu in) | Up to 20 lbs; longest side up to 22 inches from July 12, 2026 |
| USPS Ground Advantage Cubic | Up to 1 cubic foot (1,728 cu in) | Up to 20 lbs; longest side up to 22 inches |
| USPS dimensional weight | Over 1 cubic foot (1,728 cu in) | Divisor 139 since July 12, 2026 |
| USPS Ground Advantage nonstandard volume | Over 2 cubic feet (3,456 cu in) | Separate nonstandard fees also apply over 22 and 30 inches long |
| UPS Additional Handling | Over 10,368 cubic inches | Cubic trigger added January 26, 2026 |
| UPS Large Package | Over 17,280 cubic inches | Also applies over 130 inches in length plus girth |
Cubic pricing is priced by volume tier rather than weight, so a small, heavy box under 20 lbs often prices best as cubic. To see how a box prices across carriers, enter its dimensions and weight in the ParcelPath rate calculator, which compares UPS and USPS at up to 85% off UPS and up to 89% off USPS.
How do you calculate cubic inches for cylinders and odd shapes?
Non-rectangular items use different volume formulas, but carriers bill most packages on the box they ship in. Calculate the item’s volume to choose a box, then calculate the box’s cubic inches and girth for shipping.
- Cylinder (tube, bottle): π × radius² × height. A tube with a 2-inch radius and 18-inch height is 3.14 × 4 × 18 ≈ 226 cubic inches.
- Sphere: (4 ÷ 3) × π × radius³.
- Cone: (1 ÷ 3) × π × radius² × height.
- Irregular objects: submerge in water and measure the displaced volume, or measure the smallest box that would contain the object.
For tubes and rolls, girth is the circumference, π × diameter, rather than 2 × (width + height). Choosing a standard box size helps, and our list of standard Amazon box sizes gives common dimensions to start from.
What mistakes cause wrong cubic inch and girth numbers?
Most errors come from measuring the wrong thing or mixing units. Check for these before you buy a label.
- Mixing units: convert every side to inches before multiplying.
- Using inside dimensions: carriers measure the outside, including bulges and flaps.
- Forgetting rounding: USPS rounds each side up to a whole inch for dimensional weight, so a 10.2-inch side counts as 11 inches.
- Measuring girth on the wrong sides: girth always uses the two shorter sides, never the length.
- Ignoring padding: extra cushioning can push a box over a cubic or girth threshold.
For more ways to cut shipping costs with better packaging, see our frugal living guide to shipping for less, or check how size affects a heavier box in our guide to how much it costs to ship a 25 lb package.
Frequently Asked Questions
What is the formula to calculate cubic inches?
The formula to calculate cubic inches is length × width × height, with all three measurements in inches. A box measuring 10 × 8 × 6 inches is 480 cubic inches. Convert any feet to inches first by multiplying by 12, and measure the outside of the box, because carriers bill the space the package occupies rather than the space inside it.
How do I calculate girth in inches?
Calculate girth in inches by adding the width and height of the package and multiplying by two: girth = 2 × (width + height). Then add the length to get length plus girth. A 20 × 12 × 10 inch box has a girth of 44 inches and a length plus girth of 64 inches, well within the USPS Priority Mail limit of 108 inches.
How many cubic inches are in a cubic foot?
There are 1,728 cubic inches in a cubic foot, because a cube measuring 12 inches on each side is 12 × 12 × 12 = 1,728 cubic inches. To convert cubic inches to cubic feet, divide by 1,728. This threshold matters for shipping because USPS applies dimensional weight only to parcels larger than 1 cubic foot.
How do cubic inches turn into dimensional weight?
Divide the package’s cubic inches by the carrier’s divisor and round up to the next pound. Since July 12, 2026, USPS uses a divisor of 139 for parcels over 1,728 cubic inches, and UPS daily rates also use 139. A 2,400 cubic inch box therefore has a dimensional weight of 18 lbs, and you pay for that or the actual weight, whichever is higher.
Should I measure inside or outside dimensions for cubic inches?
Measure the outside dimensions when you calculate cubic inches or girth for shipping. Carriers measure the space your package takes up on their trucks, including any bulges, handles or flaps, and they bill on that. Use inside dimensions only when you are checking whether an item will fit in a box.
Sources
- USPS Postal Bulletin, Dimensional weight, cubic pricing and size changes effective July 12, 2026
- USPS, USPS Ground Advantage (130 inch length plus girth limit)
- USPS, Priority Mail (108 inch length plus girth limit)
Use the ParcelPath shipping calculator guide to compare carriers once you know your box’s size and weight.
Part of our How to Ship for Less guide.
