To measure a parcel, measure the finished, sealed package rather than the product inside it. The longest side is the length, the next longest is the width, and what is left is the height or depth. Volumetric weight is then length x width x height in centimetres divided by the courier's divisor, which is 5,000 for both Parcelforce Worldwide and DPD, and you are charged on whichever figure is greater, the actual weight on the scales or the volumetric weight.
That one calculation explains why two parcels of identical weight can cost very different amounts to send. A cushion in an oversized box weighs almost nothing but fills the same van space as something heavy, so couriers charge for the space instead. The fix is rarely a cheaper courier. It is almost always a smaller box.
How do you measure a parcel correctly?
Measure the parcel once it is packed, taped and ready to leave. Take three measurements in centimetres, at the widest point of each side:
- Length: the longest side of the parcel, whichever way round it happens to be sitting.
- Width: the next longest side.
- Height or depth: the remaining measurement.
Include any bulge. A box that is slightly overfilled will bow outwards at the sides, and that bow is what a courier's automated scanner sees. Parcelforce measures awkward or bulky items by the smallest cubic shape the parcel would fit inside, so a protruding handle or a domed lid counts against you even though it holds no weight. Full guidance on this is set out in the Parcelforce measuring guide.
Round up to the nearest whole centimetre rather than down. Couriers scan parcels automatically before they leave the country or the hub, and a discrepancy between what you declared and what the scanner reads usually results in the higher figure being charged.
What is volumetric weight and how is it calculated?
Volumetric weight, sometimes called dimensional weight, converts the space a parcel occupies into a weight figure so that couriers can price by volume as well as mass. The formula is the same across most UK carriers, with only the divisor changing.
| Carrier | Volumetric formula | How it is charged |
|---|---|---|
| Parcelforce Worldwide | Length x width x height in cm, divided by 5,000 | The greater of the pre-advised weight, the actual weight and the volumetric weight, on international zones 4 to 12 |
| DPD | Length x height x width in cm, divided by 5,000 | Volumetric or actual weight, whichever is greater, on consignments sent by air |
A worked example makes it clear. A box measuring 40cm x 30cm x 50cm gives 60,000 cubic centimetres. Divide that by 5,000 and the volumetric weight is 12kg. If the parcel actually weighs 8kg on the scales, you are still charged as though it weighs 12kg, because the volumetric figure is higher.
The reverse is also true and often overlooked. Bring that same contents down into a 30cm x 25cm x 20cm box and the volumetric weight drops to 3kg, at which point the actual weight of 8kg becomes the billed figure. Nothing about the product changed. Only the box did.
Length and girth: the measurement most sellers forget
Volumetric weight is not the only dimension check a parcel has to pass. Many carriers also apply a combined length and girth limit, where girth is the four shortest sides added together. A long, slim parcel can sail through a weight limit and still be rejected or surcharged on this rule.
Evri, for example, publishes a simple version of the same test: add the two shortest sides together, multiply by two, then add the length. If the total comes to less than 245cm the parcel is within limits, with a maximum single side of 120cm through a ParcelShop or courier collection and an overall weight limit of 15kg. The full detail sits on the Evri parcel size and weight guide.
If you send long items such as prints, rolled artwork, curtain poles or lighting, run the length and girth calculation before you commit to a carton size. It is a quicker check than reworking a rejected consignment.
How Royal Mail sizing differs
Royal Mail does not use a volumetric divisor for domestic post. It prices in proportion instead, which means your parcel drops into a size band and the band sets the price. The bands are fixed, so a few millimetres of extra depth can move an order into a noticeably more expensive tier.
- Letter: up to 35.3cm x 25cm x 2.5cm and 750g.
- Small parcel: up to 45cm x 35cm x 16cm and 2kg.
- Medium parcel: up to 61cm x 46cm x 46cm, per the Royal Mail price guide.
Depth is the dimension that catches people out. An item that would post as a large letter at 25mm deep becomes a small parcel at 27mm, and the postage difference on every single order adds up fast across a month. Using boxes built to the PiP bands takes the guesswork out, because the carton is already engineered to sit inside the threshold.
Where oversized packaging quietly costs you money
Most sellers who overpay on volumetric weight are not doing anything reckless. They are usually running one or two box sizes for a whole product range because it keeps the packing bench simple. The cost shows up in three places:
- Higher billed weight on every parcel that ships in a box larger than it needs to be.
- More void fill per order, because a bigger box has more empty space to immobilise.
- More damage claims, since DPD notes that under-filled boxes are likely to collapse and overfilled ones risk bursting open. Both ends of that scale are avoidable with better sizing.
Stocking three or four well chosen sizes from a general mailing box and carton range usually pays for itself within a few weeks of despatch, before you account for the reduction in damaged goods.
How to right-size a parcel without under-protecting it
Right-sizing is not the same as squeezing everything into the smallest possible box. The aim is the smallest box that still leaves room for proper cushioning. As a working rule, allow 5mm to 10mm of clearance on each side of the item, and more if the contents are fragile.
The packing guidance published by carriers backs this up. DPD's packaging advice makes the point that the outer carton, not the contents, must provide the structural strength of the package, that goods should not touch the external packaging, and that if you can hear or feel the contents moving inside, the parcel is not packed properly. Correct sizing and correct void fill and bubble wrap work together, not against each other.
A pre-despatch measuring checklist
- Measure the packed and sealed parcel, not the product.
- Take all three dimensions in centimetres, at the widest point of each side.
- Round up to the nearest whole centimetre.
- Run length x width x height divided by 5,000 and compare it to the weight on the scales.
- Check length plus girth if the parcel is long or awkwardly shaped.
- Check depth against the Royal Mail band you are aiming for.
- Weigh the parcel with the packaging included, not the bare item.
Do this once for each product line rather than for every order. Record the finished parcel dimensions against the SKU, and the packing bench can then work from a list instead of re-measuring the same thing a hundred times.
Start with the right box
Most volumetric surcharges are a box problem, not a courier problem. If you are not sure which size you should be buying, our size finder takes the length, width and depth of your item and returns every box, bag and envelope it fits inside, closest fit first, using internal usable dimensions. From there you can browse the full range of cardboard postal boxes and order the sizes that actually match what you sell.
