Box Size Calculator

Box size = items along each side × item size + 2 × padding, rounded up. Four 8 × 6 × 4 in items with 1 in padding need a 10 × 14 × 10 in box.

Box to order (inside) 10 × 14 × 10 in 4 items as 1 × 2 × 2 · 45.1% void

Measured by USPS11 × 15 × 11 in

USPS billable14 lb

Length + girth59 of 108 in

Item length (in)

Item width (in)

Item height (in)

Items per box

Padding per side (in)

Weight per item (lb)

Box wall, packaging weight and DIM divisor

Wall allowance per side (in)

Box and void fill weight (lb)

DIM divisor (in³ per lb)

The wall allowance is an example; use the board thickness your supplier quotes. The divisor starts at the USPS Priority Mail figure. Replace it with the one on your own rate card.

Tight inside size—

Arrangement—

Box volume—

Outside size—

Actual weight—

DIM weight at your divisor—

USPS Priority Mail check

Measured size (whole inches)—

Cubic inches—

USPS DIM weight—

Billable weight—

Length + girth—

Within USPS limits—

USPS rules as published on usps.com Priority Mail, read 2 October 2026: fractions of an inch round up, DIM pricing above 1,728 in³ at ÷139, 108 in length plus girth and 70 lb maximum.

Match against your stock carton sizes

One box per line as length × width × height, inside dimensions, in the units selected above. Add a name after a comma if you like.

Stock carton list

Box	Inside	Measured (USPS)	Void	Items it holds	USPS billable	Length + girth
Box to order (custom)	—	—	—	—	—	—
Smallest stock carton that fits	—	—	—	—	—	—
Next stock carton up	—	—	—	—	—	—

The calculation runs in your browser. Stock sizes start from a sample of one supplier's published range; paste your own.

Formula

How to Calculate Box Size in 4 Steps

Measure one item. Use the widest points, including any retail packaging, in inches or centimeters.
Pick the grid. Choose how many items sit along the length, width and height. Their product must be at least your quantity.
Add padding twice per side. Inside size on each side = items along it × item size + 2 × padding.
Round up, then add the wall. Round each inside side up to a whole unit to order. Add the board on both faces for the outside size.

What you want	Formula	Worked value (example below)
Inside size, one side	n × item side + 2 × padding	2 × 6 + 2 × 1 = 14 in
Box volume	L × W × H	10 × 14 × 10 = 1,400 in³
Void share	1 − item volume ÷ box volume	1 − 768 ÷ 1,400 = 45.1%
Outside size	inside + 2 × wall	10 × 14 × 10 → 10.25 × 14.25 × 10.25 in
USPS DIM weight	L × W × H ÷ 139, rounded up	1,815 ÷ 139 → 14 lb
Length + girth	longest + 2 × (other two)	15 + 2 × (11 + 11) = 59 in

Why the calculator tries six orientations. An item can sit on any of its faces, and the best grid for a 8 × 6 × 4 in item is often not the upright one. Every item in the box sits the same way.

Worked example

Four Boxed Mugs From Austin, Texas to Columbus, Ohio

A home goods brand in Austin ships a four-mug gift set to a customer in Columbus by USPS Priority Mail. These are the calculator's default values.

8 × 6 × 4 ineach boxed mug, 1.2 lb

4 items, 1 in padding0.7 lb of box and fill (assumed)

10 × 14 × 10 ininside box to order, 1 × 2 × 2 grid

14 lb billedagainst 5.5 lb on the scale

Option	Inside (in)	Measured (in)	Cubic in	Void	USPS DIM	Billable	Length + girth
Box to order (custom, rounded up)	10 × 14 × 10	11 × 15 × 11	1,815	45.1%	14 lb	14 lb	59 in
Smallest stock carton that holds 4	14 × 12 × 10	15 × 13 × 11	2,145	54.3%	16 lb	16 lb	63 in
Box to order with 0.5 in padding	9 × 13 × 9	10 × 14 × 10	1,400	27.1%	Not applied	5.50 lb	54 in

The inside box is 1,400 in³, under the 1,728 in³ line. The 0.125 in wall and the whole-inch round-up take the measured box to 1,815 in³, so DIM pricing applies.
That one step adds 415 in³ and moves the billed weight from 5.50 lb to 14 lb.
Cutting padding to 0.5 in gives a 9 × 13 × 9 in box billed at 5.50 lb, 8.50 lb less. Only do it if the mugs still pass your drop test.

Inside vs outside

Three Box Sizes, and Which One Each Decision Uses

Inside size

The space your items and padding occupy. Box suppliers list stock cartons by inside size, so this is the number you order.

Outside size

Inside plus the board on both faces. It is what sits on a pallet and what a tape measure reads on the dock.

Measured size

The outside size as the carrier counts it. USPS rounds every fraction of an inch up, so 12.2 in counts as 13 in.

The wall matters more than it looks. In the example, a 0.125 in wall adds 0.25 in per side, and rounding turns that into a full inch on every dimension: 10 × 14 × 10 in inside becomes 11 × 15 × 11 in measured.

Carrier rules

USPS Priority Mail Size and DIM Rules

These are the published USPS Priority Mail rules the calculator applies, read on 2 October 2026. Other carriers set their own divisors and limits, so check your rate card.

Rule	USPS published figure	What the calculator does
Measuring	Any fraction of an inch rounds up to the next whole inch	Rounds each outside side up before every USPS figure
When DIM applies	Packages larger than 1 cubic foot (1,728 in³)	Applies DIM only when measured volume is over 1,728
DIM formula	L × W × H ÷ 139 = DIM weight in pounds	Divides and rounds up to the next pound
Billed weight	DIM weight if it is greater than actual weight	Shows the greater of the two
Size limit	108 in combined length and girth	Flags any box over 108 in
Weight limit	70 lb	Flags any packed box over 70 lb

Source: USPS Priority Mail page, read 2 October 2026.

Box size chart

Common Box Sizes: Cubic Inches, DIM Weight and Girth

Sizes are measured outside dimensions in whole inches. Every figure is calculated with the same functions the tool uses, at the USPS Priority Mail rules above.

Box (in)	Cubic inches	Cubic feet	USPS DIM applies	USPS DIM weight	Length + girth	Size check
6 × 6 × 6	216	0.13	No	—	30 in	Within 108 in
8 × 8 × 8	512	0.30	No	—	40 in	Within 108 in
10 × 8 × 6	480	0.28	No	—	38 in	Within 108 in
12 × 9 × 6	648	0.38	No	—	42 in	Within 108 in
12 × 12 × 8	1,152	0.67	No	—	52 in	Within 108 in
12 × 12 × 12	1,728	1.00	No	—	60 in	Within 108 in
14 × 14 × 14	2,744	1.59	Yes	20 lb	70 in	Within 108 in
16 × 12 × 12	2,304	1.33	Yes	17 lb	64 in	Within 108 in
18 × 12 × 12	2,592	1.50	Yes	19 lb	66 in	Within 108 in
18 × 18 × 18	5,832	3.38	Yes	42 lb	90 in	Within 108 in
20 × 20 × 20	8,000	4.63	Yes	58 lb	100 in	Within 108 in
24 × 18 × 18	7,776	4.50	Yes	56 lb	96 in	Within 108 in

12 × 12 × 12 is the line. It is 1,728 in³, exactly 1 cubic foot, so USPS does not apply DIM to it. Any box one inch bigger on any side crosses the line.

Stock or custom

Stock Cartons, Custom Boxes and a Short Range

Stock carton

Ships from inventory at your supplier. It rarely matches your grid exactly, so the gap becomes void fill and billable volume.

Custom box

Made to the inside size this calculator returns. It removes the gap, but suppliers usually set a minimum order and a lead time.

Short range

Three or four sizes that cover your common orders. Packers choose faster, and you can still test each order against the list.

Where void comes from	Useful?	What to do about it
Padding you set	Yes, if your drop test needs it	Keep it, and re-test after any cut
Empty grid positions	No	Change the items per box so the grid fills
Gap to the next stock size	No	Add a stock size, or order a custom box
Whole-unit round-up	No, but unavoidable	Choose a grid whose sides land near whole units

The built-in stock list samples the Standard 200# (32 ECT) RSC range at packagingsupplies.com, inside dimensions, read 14 August 2026. It is a sample spread, not a catalog.

Avoid rework

Six Box Sizing Mistakes

Sizing to the product, not the pack

The box must hold the product plus padding on both sides. With 1 in of padding, every side grows by 2 in.

Ordering by outside size

Suppliers quote stock cartons by inside size. Ordering the outside figure gives you a box that is one wall too small on each face.

Rounding down

Carriers count up. USPS bills a 12.2 in side as 13 in, so round your own figures up too.

Forgetting the wall

In the example, the wall alone adds 415 in³ to the measured box and pushes it over the USPS DIM line.

Long, thin boxes

A 48 × 8 × 6 in box holds 2,304 in³ at 76 in length plus girth. An 18 × 16 × 8 in box holds 2,304 in³ at 66 in.

Mixing units

Item sizes in centimeters and a stock list in inches give a silent wrong answer. Switch the unit buttons and the calculator converts everything.

Next step

Related Shipping and Packing Calculators

Dimensional weight calculatorWork out DIM weight for a box you already have at any divisor.Chargeable weight calculatorCompare actual and volumetric weight to see which one is billed.Pallet calculatorTake your box size to a pallet: boxes per layer, layers and Ti-Hi.CBM calculatorTurn carton counts into cubic meters and container share.

See all free logistics tools, or read how Locad handles ecommerce fulfillment and shipping.

FAQ

Box Size Calculator Questions

Lay the items out as a grid. On each side, multiply the item size by the number of items along it, then add twice your padding. Round each result up to the next whole inch. 4 items of 8 × 6 × 4 in with 1 in padding need a 10 × 14 × 10 in box.

Enter the item length, width and height, how many go in one box and the padding you want per side. The calculator tests all six ways the item can sit and every grid that holds the quantity, then gives the smallest inside size to order.

A 12 × 12 × 12 inch box is 1,728 cubic inches, which is exactly 1 cubic foot. USPS applies Priority Mail DIM pricing only to boxes larger than 1,728 cubic inches, so a box measuring exactly 12 × 12 × 12 in does not trigger it.

Multiply the length by the width, then multiply the result by the height. A 14 × 10 × 8 in box is 14 × 10 = 140, and 140 × 8 = 1,120 cubic inches. Divide by 1,728 to get 0.65 cubic feet.

You cannot measure a physical box online, but you can calculate the size you need. Measure the item with a tape, enter it here, and the calculator gives the inside size to order. Then measure the outside of the packed box, because that is the size a carrier bills.

Paste the inside sizes of the boxes you stock into the carton list. For each box the calculator divides each inside side, less padding, by the item size in all six orientations and reports how many whole items fit. The smallest box that holds your quantity is recommended.

Use inside dimensions to check the items fit, and outside dimensions for shipping cost. A 0.125 in wall allowance turns a 10 × 14 × 10 in inside box into 10.25 × 14.25 × 10.25 in outside, which USPS rounds up to 11 × 15 × 11 in.

Twice the padding on every side. Padding goes on both faces, so 1 in of padding adds 2 in to each dimension. The right padding depends on how fragile the item is and how it travels, so set it from your own packing spec or drop test.

Carriers bill the greater of actual weight and dimensional weight. For USPS Priority Mail, a box over 1,728 cubic inches is billed at L × W × H ÷ 139, rounded up to the next pound. In the example on this page, that turns 5.5 lb of mugs into 14 lb billed.

USPS publishes a maximum combined length and girth of 108 inches and a maximum weight of 70 lb for Priority Mail (read 2 October 2026). Combined length and girth is the longest side plus twice the sum of the other two.

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