Ordering the wrong box size is one of the most common and most frustrating packaging mistakes. The product doesn't fit, or it rattles around inside, or the box is fine but the courier charges you far more than expected.
Most of these problems come down to one thing: misunderstanding how box dimensions work. In this guide we'll explain how boxes are measured, the difference between internal and external dimensions, and how to choose the right size for what you're shipping.
How Box Dimensions Are Written
Box sizes are almost always written in this order:
Length x Width x Height (or Depth)
For example, a box listed as 310 x 230 x 105mm is 310mm long, 230mm wide and 105mm high. In Australia, box sizes are usually given in millimetres.
- Length is the longer side of the opening (the top of the box).
- Width is the shorter side of the opening.
- Height (sometimes called depth) is the distance from the bottom of the box to the opening.
The order matters. A box that's 300 x 200 x 100mm and one that's 300 x 100 x 200mm use similar amounts of board, but they open differently and suit very different products. One is a shallow tray you load from the top. The other is a tall, narrow box.
Internal vs External Dimensions
Every box has two sets of measurements, and they're not the same.
Internal dimensions
The internal dimensions are the measurements of the space inside the box. This is what determines whether your product fits.
External dimensions
The external dimensions are the measurements of the outside of the box. This is what couriers use to calculate cubic (volumetric) weight, and what matters for fitting boxes on shelves, in pallets or inside a satchel.
Why they're different
The difference is the thickness of the cardboard, plus any overlapping flaps. Thicker board means a bigger gap between internal and external sizes. As an approximate guide:
| Board type | Approx. thickness | Typical use |
|---|---|---|
| E flute | About 1.5mm | Diecut mailing boxes, eCommerce mailers |
| B flute | About 3mm | Shipping cartons, general-purpose boxes |
| C flute | About 4mm | Larger cartons that need more stacking strength |
| Double wall (BC flute) | About 7mm | Heavy duty cartons and removal boxes |
So a single wall box is usually around 5 to 10mm bigger on the outside in each direction, and a double wall box can be 15mm or more bigger. Flaps that overlap, such as the top flaps of a regular slotted carton, can add a little more height. You can read more in our guides to corrugated flutes and regular slotted cartons.
Which one do suppliers list? Across the packaging industry, box sizes are most commonly quoted as internal dimensions, because that's what buyers need to know to fit their product. If a listing doesn't say, ask before you order in bulk.
How to Measure a Box You Already Have
If you're reordering a box you already use, or trying to match a sample, here's how to measure it properly:
- Assemble the box and open the top.
- Measure the length along the longer side of the opening, from the inside of one wall to the inside of the opposite wall.
- Measure the width along the shorter side of the opening, again inside wall to inside wall.
- Measure the height from the inside of the base to the top edge of the box walls (the crease where the flaps fold).
- Write it down in order: L x W x H.
Use a rigid ruler or tape measure, and measure in a couple of places, because boxes can bow slightly. For external dimensions, measure the closed box from outside edge to outside edge.
How to Measure Your Product for a Box
To choose a new box, start with the product, not the box:
- Measure the product at its widest points, including handles, packaging or anything that sticks out.
- Decide how much protection it needs. A boxed product going in a mailer may only need a snug fit. A fragile item needs room for bubble wrap or void fill on every side.
-
Add the allowance to each dimension:
- Snug fit for sturdy items: add about 5 to 10mm to each dimension
- Light cushioning (a layer or two of bubble wrap): add about 20 to 30mm to each dimension
- Fragile items: allow about 50mm of cushioning on every side, which means adding around 100mm to each dimension
- Find the nearest box with internal dimensions at or just above that size.
Example: a candle in a gift box measures 100 x 100 x 120mm. You plan to wrap it in a layer of bubble wrap, so you add about 25mm to each dimension and need a box of roughly 125 x 125 x 145mm internally.
Why the Right Size Saves You Money
Many Australian couriers charge by cubic weight when a parcel is large but light. Cubic weight is calculated from the box's external dimensions:
Length (m) x Width (m) x Height (m) x cubic factor = cubic weight (kg)
A cubic factor of 250 is common in Australia. For example, a box measuring 400 x 300 x 200mm externally has a cubic weight of 0.4 x 0.3 x 0.2 x 250 = 6kg, even if the contents only weigh 1kg. You're charged whichever is higher.
That's why an oversized box can quietly double your shipping costs. Cutting just a few centimetres from each dimension can drop a parcel into a cheaper weight band. Our guide to DIM weight reduction goes into more detail.
Common Box Measuring Mistakes
- Mixing up the order. Always check which side is the opening before you order.
- Using external sizes to fit a product. The box will be too small inside.
- Using internal sizes for courier quotes. Your quote will come in too low.
- Forgetting the packing material. Bubble wrap and void fill take up more room than you'd think.
- Not allowing for bulging. Soft items like clothing push the walls out and can stop flaps closing neatly.
- Measuring in centimetres and ordering in millimetres. It happens more often than you'd think.
Need Help Finding the Right Box?
eBPak stocks hundreds of box sizes, from small diecut mailing boxes to medium and large shipping cartons and heavy duty boxes. Browse all shipping boxes and cartons, or if you can't find the exact size you need, we can make one for you. See our guide to custom printed boxes or request a custom box quote.