How To Measure A Box The Right Way Every Time

Getting a box’s measurements wrong is an easy mistake to make, and an expensive one to fix. Order a custom box using the wrong dimensions and your products won’t fit. Get a shipping box’s size wrong and you’ll pay more for freight than you need to, or worse, the item arrives damaged because there was too much room for it to move around inside.

The good news is that measuring a box correctly only takes a ruler or tape measure and a few minutes, once you know the right technique. Most people are never taught this, so they guess, and the guesswork shows up later as returns, wasted packaging and higher shipping costs than necessary.

This article walks through exactly how to measure a box step by step, when to use inside measurements versus outside measurements, how different box types are measured, and what to do if you don’t have a tape measure handy. By the end, you’ll be able to measure any box accurately enough to order the right size, ship it for the right price, or get an accurate custom quote.

Here’s Everything You Need To Know In Under A Minute

Table Of Contents

How Do You Measure A Box In The Right Order?

To measure a box, use a ruler or tape measure and record the Length x Width x Height (L x W x H), always in that exact order. This is the industry standard, and getting the order right matters just as much as getting the numbers right.

Every measurement should be recorded in millimetres (mm). Australian suppliers, including custom cardboard boxes makers like The Boxman, work in millimetres by default, so converting from inches or centimetres upfront saves confusion later.

A Worked Example

Take a standard A4-sized shipping carton as an example. Measured correctly, it comes out at 300mm x 215mm x 150mm, written as 300 x 215 x 150 (mm) in L x W x H order.

Should You Measure A Box From The Inside Or Outside?

Whether you measure from the inside or the outside depends on what you’re using the measurement for. Getting this mixed up is the most common measuring mistake, and it’s an easy one to avoid once you know the rule.

Internal dimensions are the usable space inside the box, measured from the middle of the crease (the indented score line) on one side to the middle of the crease on the opposite side. This is what matters when you’re checking whether a product will actually fit, and it’s the measurement most Australian manufacturers use when building a box to order.

External dimensions are measured from the outside edge of the assembled, sealed box. Couriers use external size, combined with cubic weight, to calculate freight charges, so this is the number that matters for shipping.

A sturdy, thicker-walled box can have a noticeable gap between its inside and outside measurements, sometimes 10 to 20mm per side. That difference rarely matters for storage, but it can be the difference between a snug custom fit and a box that’s too tight or too loose. For more on this convention, see why boxes use inside measurements.

How Do You Measure Different Types Of Boxes?

Not every box is measured exactly the same way, because flaps and openings sit in different places depending on the box style. Here’s how to measure the most common categories.

Shipping Boxes And Cartons

Standard shipping boxes have flaps on the length and width panels. Measure length along the panel below the longest flap, width along the shorter flapped panel, and height up the standing side, excluding the flaps.

Mailing Boxes And Self-Locking Boxes

Self-locking mailing boxes fold shut without tape, so their length and width are measured the same way as a shipping carton, flap side for length and width, vertical side for height.

TV And Art Shipping Boxes

Oversized or awkward items like screens and artwork need TV boxes and art boxes measured with a bit more care. Length and width are still taken across the flapped panels, but height should account for the item’s actual thickness, since these boxes are often slimmer than standard cartons.

Packing Boxes And Trays

General-purpose packing boxes follow the standard method: length is the longest side, width the shorter side, and height the vertical panel from top to bottom.

Tubes And Non-Rectangular Packaging

Not every item ships in a rectangular box. Triangular mailing tubes and similar non-rectangular packaging are measured differently again: diameter (or width across the triangular face) first, then length along the tube itself. If you’re shipping posters, prints or rolled documents, check whether a tube from the tri boxes and mailing tube range suits the item better than a flat box.

Tips For Getting An Accurate Box Measurement

A few small habits make the difference between a measurement you can trust and one that causes problems later.

• Close the flaps. Always close the top and bottom flaps before measuring height, so you don’t accidentally include them.
• Measure at the widest point. If a packed box bulges anywhere, measure across that point rather than the box’s resting shape.
• Round up, not down. For shipping and custom orders, round any fraction up to the next whole millimetre, so your box or freight estimate has a small margin instead of falling short.
• Check the parcel size and weight guidelines for your courier before finalising a shipment, since external dimensions and cubic weight both affect what you’ll pay. Australia Post’s parcel size and weight guidelines are a useful starting point.
• Measure twice. A five-second re-check catches most mistakes before they turn into a wrong order.

External size also feeds into cubic weight, the formula couriers use alongside actual weight to price a shipment. Get your outside measurements right and you’ll avoid paying for space you didn’t need.

How Do You Measure A Box Without A Tape Measure?

No tape measure on hand? A few household items will get you close enough for most purposes.

• A4 paper. A standard A4 sheet is exactly 210mm x 297mm, so it doubles as a rough ruler against the box.
• A ruler app. Most smartphones have a built-in measuring tool that uses the camera to estimate length reasonably accurately.
• String and a fixed ruler. Lay string along each dimension, mark it, then measure the string against any ruler or fixed-length object you do have.
• A known object. A credit card (85.6mm long) or a sheet of standard printer paper both work as rough reference points.

None of these replace an actual tape measure for a final custom order, but they’re enough for a workable estimate, or to double-check a measurement you’re unsure about. If you’re planning to make your own custom box rather than order one, a proper tape measure is worth having on hand from the start.

Frequently Asked Questions

Ready To Get Your Box Measurements Right

Measuring a box correctly comes down to three things: the right order (Length x Width x Height), the right reference point (inside or outside, depending on what you need it for), and a few extra millimetres of allowance where it counts.

Once you’ve got those numbers, you’re ready to order with confidence, whether that’s picking a size from our Australian box size chart or getting a box built to your exact specifications.