Dimensional Weight Calculator
The weight a carrier actually bills you on is the greater of what the box weighs and what its volume is worth. Enter the dimensions and the divisor from your rate sheet to see which one is winning — and the density at which that would change.
Measure the box
Picking a rule sets both the divisor and the units. It does not convert what you already typed — re-measure in the new units.
Measure the outside of the packed box at its widest point, bulges included.
The order of the three sides does not matter — the longest is treated as the length, which is how a carrier measures girth.
Each side is rounded up to a whole in before the multiplication.
The packed box on a scale, contents and packaging together.
Type over it if your contract says something else. Negotiated divisors above the published ones are common at volume.
18 × 12 × 10 in is 2,160 cu in, which at a divisor of 139 is 15.54 lb of space. That beats the 8 lb it weighs, so you are paying for the box, not the contents.
- Billed by
- Volume
- Dimensional weight
- 16 lb
- Actual weight, billed
- 8 lb
- Break-even weight
- 15.54 lb
- Break-even density
- 12.43 lb / cu ft
- Cubic size
- 2,160 cu in
- Girth
- 44 in
- Length plus girth
- 62 in
| Rule | Divisor | Dim weight | Billable |
|---|---|---|---|
| UPS or FedEx | 139 | 16 lb | 16 lb |
| USPS Priority Mail | 166 | 14 lb | 14 lb |
- Anything lighter than 12.43 lb / cu ft is billed by volume on this divisor — every box, not just this one. Lighter packing will not help; a smaller box will.
- Length plus girth is 62 in, inside the usual ground threshold, so no oversize surcharge applies.
- Each side is rounded up to a whole in, and both weights up to a whole lb, before anything is compared. This is the conservative reading of the published rules — it will not quote you under the invoice.
- Rates, zones, fuel and accessorial charges are not modelled. This is the weight your rate is looked up against, not a price.
Why carriers charge for space, and how they price it
A delivery van, a line-haul trailer and the belly of a passenger jet all share a problem: they fill up before they weigh out. A pallet of hardbacks reaches the axle limit with room to spare on the floor. A pallet of duvets fills the floor and leaves most of the payload unused — and the carrier cannot sell that floor space to anybody else while your duvets are sitting on it. Charging by the scale alone would mean the duvets travel almost free while displacing revenue the truck was built to earn.
Dimensional weight is the fix. Each rate has a dim divisor: the cubic units the carrier is willing to give you per unit of weight. Divide the cube of your box by it, and you have the weight that volume is worth. The bill is whichever of the two figures is larger.
dim weight = (L × W × H) ÷ divisor • billable = max(actual, dim)- L, W, H
- The outside of the packed box, each rounded up to a whole inch or centimetre
- divisor
- 139 or 166 for inches and pounds; 5000 or 6000 for centimetres and kilograms
- break-even
- 1,728 ÷ divisor, in pounds per cubic foot — the density at which the two cross
Two details in that formula do real damage if you skip them. Rounding comes first. Dimensions are taken to whole units before they are multiplied, so a box measured at 18.1 inches is priced as 19, and a tenth of an inch on the tape measure becomes a whole pound on the invoice. Both weights are then rounded up too, because carriers bill in whole pounds and whole kilograms.
Girth is measured on the longest side, not on whichever edge you called the length. Girth is twice the sum of the other two sides, and length plus girth is what oversize thresholds are checked against — about 130 inches for UPS and FedEx ground. This tool ranks your three dimensions before computing it, so the answer does not change depending on which box you typed each number into.
An 18 × 12 × 10 carton at 8 lb, on a 139 divisor
The cube is 18 × 12 × 10 = 2,160 cubic inches, which is 1.25 cubic feet. Divided by 139 that is 15.54 lb of space, rounded up to a dimensional weight of 16 lb. The box itself weighs 8 lb. The billable weight is the greater of the two, so you pay on 16 lb — double what the scale says.
The break-even density is 1,728 ÷ 139 = 12.43 lb per cubic foot. This carton is running at 8 ÷ 1.25 = 6.4 lb per cubic foot, barely half of that, which is why volume is winning so comfortably. Put another way: this exact box would have to weigh 15.54 lb before the scale took over.
Now watch the two fixes diverge. Replace two pounds of packing paper with half a pound of air pillows and the actual weight drops to 6.5 lb — and the bill does not move at all, because 16 lb is still the larger figure. Take an inch off every side instead, to 17 × 11 × 9, and the cube falls to 1,683 cubic inches, a dimensional weight of 13 lb. That is three pounds off the invoice on every unit you ship, from a change that costs nothing but a different carton.
The same carton on the USPS divisor of 166 comes out at 2,160 ÷ 166 = 13.01, billing at 14 lb, with a break-even density of 10.41 lb per cubic foot. The higher the divisor, the more space you are given per pound — which is exactly what a shipper is negotiating for when they push on the DIM factor rather than on the rate.
What this calculator assumes
- Each dimension is rounded up to the next whole inch or centimetre before multiplying. UPS and FedEx both publish this step as rounding to the nearest whole number; rounding up is the conservative reading, and it is the direction that cannot leave you quoting under the invoice. On a box with fractional sides this figure may come in a pound above what the carrier charges.
- Both weights are rounded up to a whole pound or kilogram, which is how parcel carriers bill. A tie between the two is reported as billed by the scale, because at that point the volume is costing you nothing.
- The longest of the three sides is treated as the length for girth, whichever field it was typed into.
- The oversize flag fires above 130 inches of length plus girth, the usual UPS and FedEx ground threshold; the metric equivalent of 330 cm is the same measurement restated. Thresholds, and the minimum billable weights that come with them, vary by service and change annually.
- Nothing here is a price. Rates, zones, fuel surcharges, residential delivery, additional handling and large-package minimums are all outside the formula. This is the weight your rate is looked up against.
- Which of your shipments dimensional pricing applies to is a contract question, not a geometric one. The tool computes the formula; your rate sheet says when it is used.
Dimensional weight FAQ
Why am I being charged for more weight than my package weighs?
Because a trailer runs out of room before it runs out of payload. A carton of pillows takes up the same shelf as a carton of books, and the carrier cannot sell that shelf twice. Dimensional weight prices the space: divide the cube of the box by the divisor in your rate sheet, and if that figure beats the scale, it is what you pay on. It is not a penalty and it is not an error — it is the carrier charging for the thing that is actually scarce.
What is a dim divisor, and where do I find mine?
It is the cubic units the carrier gives you per unit of weight. UPS and FedEx publish 139 cubic inches per pound for US domestic and international parcels; USPS uses 166 on the Priority Mail products it applies dimensional pricing to; most international and air-freight rates use 5000 or 6000 cubic centimetres per kilogram. Your own number is on your rate sheet or in your contract, usually near the top under a heading like dimensional weight or DIM factor. A higher divisor is better for you: it gives you more space per pound before volume starts driving the bill.
How do I stop paying dimensional weight?
Use a smaller box. That is genuinely the whole answer, and it is worth being blunt about because the intuitive fix is the wrong one. Swapping heavy packing paper for light air pillows takes weight off the scale and changes nothing about the shelf the box occupies, so on a shipment billed by volume it saves you exactly nothing. Cutting an inch off each side of an 18 × 12 × 10 carton removes 477 cubic inches, which is three pounds of billable weight on a 139 divisor.
What is the break-even density, and why does it matter more than the answer?
It is the weight per unit of volume at which the two figures cross: 1,728 divided by your divisor for inches and pounds, or a million divided by it for centimetres and kilograms. On a 139 divisor it is 12.43 pounds per cubic foot. Anything you ship that is lighter than that will be billed by volume — every carton, not just the one you typed in. It turns a single answer into a rule you can apply to a whole catalogue without opening a calculator again.
What is girth, and when does it start to cost me?
Girth is twice the sum of the two shorter sides — the distance a tape measure travels around the parcel. Carriers measure oversize limits on length plus girth, taking the longest side as the length whatever the shipper called it. Above roughly 130 inches, UPS and FedEx ground apply a large-package or oversize surcharge, and that surcharge usually brings a minimum billable weight with it that can be far above anything the dimensional formula produces. Above about 165 inches the package is generally refused.
Does dimensional weight apply to every shipment?
Not universally, though the exceptions are shrinking. UPS and FedEx apply it to all domestic and international package services. USPS applies it to its Priority Mail products above a certain cube and beyond the nearer zones, so a small dense box often escapes it. Freight, pallets and less-than-truckload move on a different basis again — density-based classification rather than a parcel dim divisor. Check your rate sheet: this tool computes the formula, it does not know which of your shipments the formula is applied to.
Sources and review notes
- UPS — shipping dimensions and weight, including the published dim divisor and the length-plus-girth limits
- FedEx — how to measure a package, and how dimensional weight is derived from those measurements
- USPS Domestic Mail Manual 123 — Priority Mail prices, including where dimensional weight pricing applies
The arithmetic is a division and a comparison, and it does not go stale. The divisors, oversize thresholds and the services each rule applies to are carrier terms that change — most often in January. Confirm yours against your own rate sheet before pricing anything on it.