Chargeable weight: actual, volumetric and which one the line bills

Measure the box before you measure the contents. Three numbers decide what a line charges: the weight on the scale, the weight the dimensions imply, and whichever of the two is larger. Enter the three sides, the scale weight and the divisor, and this gauge returns all three figures side by side plus the gap between them as a weight and as a percentage.

The defaults describe one pair of trainers in its box: 40 × 30 × 20 cm and 1.2 kg. We keep that basket on screen because it is the most common shape we see in the 85 items on this site — a shoe-sized carton that is nowhere near full. The divisor opens at 6000, the middle of the three values lines publish; 5000 flatters bulky goods and 8000 flatters dense ones, so check which figure your line uses before you read the result as a bill.

Conversions run before any rounding. Inches become centimetres at 2.54 and pounds become kilograms at 0.45359237, so an imperial entry and its metric twin produce identical answers. Rounding then happens on the half-kilogram step the lines use: 1.2 kg displays as 1.5 kg, and a raw volumetric figure of 3.33 kg displays as 3.5 kg. The chargeable figure is the larger of those two stepped numbers, and the gap is measured against the stepped actual weight rather than the raw one, which is why the percentage can look smaller than the ratio you expected.

Work one comparison by hand to see why shape matters more than weight. Load 3.2 kg into a flat 50 × 40 × 10 cm carton and the volume is 20,000 cm³, or 3.33 kg, stepped to 3.5 kg — the same step the scale weight lands on, so the gap reads 0.0 kg and the divisor stops mattering. Move the same 3.2 kg into a 30 × 30 × 30 cm cube and the volume rises to 27,000 cm³, or 4.5 kg, and you have bought a kilogram of nothing. That single kilogram is £4.20 on our recorded line structure (reviewed 2026-10-02).

The three worked cases above cover the three readings this tool produces. The default box is volume-driven at 4.0 kg against 1.5 kg of stepped actual weight, a 2.5 kg gap that is 62.5% of the bill. The flat box is the shape to copy, where volume and weight agree and the gap falls to zero. The imperial case shows the same geometry labelled in pounds: 2 lb reads like a light parcel, but 4.5 kg of volumetric weight is what the line will bill, and the gap is 77.8% of the chargeable figure.

Three packing moves repay the effort in order of size. Repack into the flattest carton the contents allow, because height is the cheapest dimension to remove — dropping the default box from 20 cm to 12 cm takes volumetric weight from 4.0 kg to 2.5 kg. Ask for shoeboxes and retail packaging to be taken out before the parcel is sealed, which removes air rather than goods. Vacuum-bag soft goods so a puffer stops holding its own shape. Do these before the parcel is packed, because once it is sealed the measurement is fixed and the charge follows it.

Four boundaries produce a named error instead of a number. A side of zero or less returns "Length must be greater than 0." or the same sentence with the offending side named. Any side above 200 cm returns "Each side must be 200 cm or less — split the parcel above that." A scale weight of zero or less returns "Actual weight must be greater than 0." And an actual weight above 50 kg returns "Above 50 kg most lines require a split — check with the line first." The tool never guesses a divisor you did not choose, and it never reports a chargeable figure below the actual weight.

Send the chargeable figure straight into the landed-cost gauge rather than retyping it, and send both parcels into the merge gauge when a consolidation is on the table. One limit is worth stating plainly: which divisor a given line applies on the day you submit is not something this page can verify, so treat 6000 as a working default and confirm it against the quote on your own parcel page. Where a line publishes 8000, the same box bills a third lighter — the geometry does not change, only the rule applied to it.