How to measure box dimensions the right way

Start with the product, not the box

A box that fits is built from the product outwards. Before you measure any box, measure the item that goes inside it — at its widest, longest and tallest points, including caps, pumps, handles, hangers and any bulge in soft packs. If the product sits in a tray, insert or sleeve, measure that assembly instead.

• Use a steel ruler or calliper for small items and a rigid tape for large ones; soft tapes stretch and sag.
• Measure to the nearest millimetre or 1/16 inch and write down the unit you used.
• Measure three or four samples if the product varies (hand-filled pouches, glass, candles) and use the largest.

Length, width and height: the order matters

Box sizes are always written length × width × height (L × W × H), and the first two numbers describe the opening:

• Length — the longer side of the opening.
• Width — the shorter side of the opening.
• Height (also called depth) — the distance from the opening to the base, at right angles to the other two.

So a 10 × 4 × 2 inch box and a 10 × 2 × 4 inch box hold the same volume but open on different faces and are made from different dielines. Keep the order and say where the box opens, and a 6 × 4 × 2 box will come back the way you pictured it.

To measure an existing box, open it and measure each inside edge from wall to wall, at the widest point, with the flaps upright.

Inside vs outside dimensions

Inside dimensions decide whether the product fits. Outside dimensions decide how the box ships, stacks and fits on a shelf — carriers measure the outside. The difference is the board: every wall adds its thickness, so a box is always a little bigger outside than inside.

Box makers normally work from inside dimensions, so give those for a custom box and say so (“inside, in mm”). If you only have an outside size — for example a shelf or a shipping carton the box must slide into — give that and label it as outside; we allow for the board when we draw the dieline.

How much space to leave around the product

Clearance is the gap between the product and the inside walls. Too little and the product jams or crushes corners; too much and it rattles and you pay to ship air.

The last row follows carrier guidance: UPS asks for at least two inches of cushioning between fragile contents and the sides, top and bottom of the box. For glass, electronics and gift sets a fitted insert usually protects better than loose fill and keeps the box smaller.

How board thickness changes the size

Outside size ≈ inside size + the walls. Each wall adds one board thickness, and some styles double up where panels fold over each other:

See corrugated flute types for what each flute is good at. For a regular shipping carton the flaps also double up top and bottom, so the height grows by about four board thicknesses.

Which way L, W and H run on common styles

• Tuck end cartons open at the narrow end: L × W is the small end panel and H is the tall side. A lipstick carton might be 25 × 25 × 90 mm.
• Mailer boxes open from the top: L is the hinge side, W is front to back, H is the depth. A typical small mailer is 9 × 6 × 3 in.
• Shipping cartons (regular slotted cartons) open from the top with four flaps: L × W is the top, H the depth.
• Rigid boxes: give the inside of the base. For a lid-and-base box the lid is made slightly larger to slide over it.

Not sure which style you need? Box styles explained shows each one.

Volume: the L × W × H rule

Multiply the three dimensions to get the volume. A box 10 inches long, 7 inches wide and 9 inches high holds:

10 × 7 × 9 = 630 cubic inches (about 10.3 litres)

In metric: 25 × 18 × 23 cm = 10,350 cm³ = 10.35 litres. Use inside dimensions for how much the box holds and outside dimensions for how much space it takes on a pallet or in a van. Round and oval boxes use the area of the base instead: π × radius² × height for a round tube.

Girth and length plus girth

Girth is the distance around the box, measured around its thickest part and at right angles to its length: girth = 2 × (width + height), where length is the longest side. For a box 10 inches high and 2 inches wide:

2 × (2 + 10) = 24 inches

Carriers cap the size of a parcel by length + girth. A 30 × 12 × 10 inch carton has a girth of 2 × (12 + 10) = 44 in, so length + girth = 74 in. Published limits we checked in September 2026:

Carriers change surcharges and limits, so check your carrier’s current rules before you fix a size for a large parcel.

Dimensional (volumetric) weight

Carriers charge for the space a parcel takes as well as its weight. They work out a dimensional weight from the outside size and bill whichever is higher:

• Inches and pounds: round each outside dimension up to the next whole inch, multiply, divide by the carrier’s divisor and round up. UPS and FedEx use 139 for daily (account) rates; UPS uses 166 for retail rates. USPS Ground Advantage applies divisor 139 to parcels larger than one cubic foot (1,728 in³).
• Centimetres and kilograms: L × W × H in cm ÷ 5,000 is common for international express; some services use 6,000.

Example: a 12 × 10 × 8 inch box is 960 in³; 960 ÷ 139 = 6.9, so it is billed as at least 7 lb even if it weighs 3 lb. Trimming the same box to 11 × 9 × 7 in (693 in³) brings it to 5 lb. That is why a well-sized box saves money on every order.

Inserts, dividers and multi-packs

For a box that holds several items, arrange them first, then measure the group: items across × items deep × layers, plus the dividers between them and the clearance around the outside. Allow for the insert or divider board (often 1.5–3 mm) in every row. Try the arrangement both ways — four items in a row and two-by-two give very different boxes, and the smaller one is usually cheaper to ship.

Converting between inches and millimetres

• 1 inch = 25.4 mm exactly; 1 mm ≈ 0.03937 in.
• 1 cm = 10 mm; 1/16 in ≈ 1.6 mm; 1/8 in ≈ 3.2 mm; 1/4 in ≈ 6.4 mm.
• 1 cubic inch ≈ 16.39 cm³; 1 litre = 1,000 cm³ ≈ 61.02 in³.
• 1 kg ≈ 2.2046 lb.

Pick one unit for the whole job and state it. Millimetres make small differences easy to see; inches are what US carriers use for billing.

Why exact measurements matter

The size fixes almost everything else: the dieline and cutting die, how much board is used, how well the product is protected, how the box looks on the shelf and what every parcel costs to ship. A few millimetres too small means a reprint; a centimetre too big on every side can move a parcel into a higher dimensional-weight band for the life of the product.

Check your size before you order

The free box size calculator takes your product size and works out the inside and outside dimensions for your board, the best arrangement for multi-packs, dimensional weight for the main carriers and whether the box is inside their size limits.

When you are ready, request a quote with your inside dimensions. Our design team checks the size, sets up the dieline and sends a free digital proof (flat and 3D) before anything is printed — and there is no minimum order. Questions? Contact us.