Measure the usable space inside a box to check what fits. Measure the outside of the packed, closed box when checking a delivery service’s parcel limits.
Start with what goes inside
Measure the item in the position in which you plan to pack it. Include any parts that stick out. For several items, arrange them as intended and include the space needed for separation or protection.
Compare the complete arrangement with the box’s internal dimensions—the usable space inside. Keep length, width and height in the same order, using the same unit throughout.
What order are box dimensions written in?
Box listings commonly use length × width × height. Read the supplier’s dimension labels or drawing so you know which edges each number describes. Keep the order and units consistent when comparing the item and the box.
Convert units before comparing: 10 mm equals 1 cm, so a size of 300 × 200 × 150 mm is 30 × 20 × 15 cm. Changing units does not change the size.
Use the internal-size calculator below this guide.
Include your chosen protection
A box that only just fits a bare item may leave no room for cushioning. Select protection suitable for the contents, then measure the complete packing arrangement. There is no single allowance that works for every item.
Read the listing carefully
Look for a clear statement of whether sizes are internal or external. Check the box construction, pack count and any stated tolerance. “Medium” and “large” are useful labels, but they cannot replace measurements.
If the title and specification disagree, ask the supplier to confirm the size before ordering.
Measure the finished parcel
Once the box is packed and closed, measure its external dimensions and weigh the entire parcel, including its packaging. Compare those measurements with the current rules for the delivery service you intend to use.
Royal Mail’s weighing and measuring guidance ↗
Before ordering: confirm the internal space, finished external size, box construction, number of boxes in the pack and total delivered cost.
Keep a repeat-order packing specification
Can I calculate external dimensions from internal dimensions?
You need the actual board thickness and assembled box design. Folded flaps, overlaps and the packing arrangement can change the result. Measure a closed sample or request confirmed external measurements instead of adding a universal allowance.
Once you have your measurements, compare cardboard box types by use and construction.