Sides first
Divide each centimeter measurement by 2.54 to get inches. A 40 cm side becomes about 15.75 in; keep two decimal places rather than rounding early, since a small error on three sides multiplies.
Weight & size
Calculating dimensional weight in kilograms
The metric version of dimensional weight runs length times width times height in centimeters, divided by 5000, rounded up to the next whole kilogram. That wording comes from FedEx's own dimensional weight pages, read 2026-08-20, and the same math with a different divisor is what US carriers run on a box measured in inches. Below is one carton put through both versions, then priced for real on a US lane.
Carrier wording, quotedRead 2026-08-20One box, priced live
Same box, two divisors
A spec sheet written in centimeters and a US carrier that bills in pounds are reading the same carton two different ways.
The metric formula, in the carrier's own words
FedEx's Japan customer support page states the metric version plainly: “Dimensional weight (kg) = Length x Width x Height (centimeters or inches) / 5000 for centimeters.” The same page gives the inch and pound version used domestically in the United States: “Dimensional weight (pound) = Length x Width x Height (inches) / 139 for inches.” FedEx's German shipping-costs page states the same 5000 divisor and rounds every measurement up before the division happens, walking through a 36 x 25 x 16 cm carton that comes out to 3 kg. Both pages read 2026-08-20.
| Unit system | Formula | Divisor | Rounds up to |
|---|---|---|---|
| Metric | Length × width × height in centimeters, divided by the divisor | 5000 | Next whole kilogram |
| US customary | Length × width × height in inches, divided by the divisor | 139 | Next whole pound |
A cubic inch holds about 16 cubic centimeters, and a kilogram is about 2.2 pounds; both of those conversions are folded into the jump from a 139 divisor to a 5000 one. Neither number is arbitrary on its own; each is scaled to the unit it works in.
One carton, run through both formulas, then priced
A 40 x 30 x 20 cm carton at 6 kg is a common size for a compact kitchen appliance, and it is the kind of spec sheet that arrives from an overseas manufacturer in centimeters with no inches printed anywhere. Converted, that box measures roughly 15.75 x 11.81 x 7.87 in and 13.23 lb. We ran both versions of the math, then quoted the converted box through the SMKlog calculator on 2026-08-20, Berkeley Heights, NJ 07922 to Denver, CO 80202.
| Version | Box | Dimensional weight | Actual weight | Which one bills |
|---|---|---|---|---|
| Metric | 40 × 30 × 20 cm | 24,000 ÷ 5000 = 4.8, rounds to 5 kg | 6 kg | Actual |
| US customary | 16 × 12 × 8 in (rounded up) | 1,536 ÷ 139 = 11.05, rounds to 12 lb | 13.23 lb | Actual |
The two formulas agree on the outcome even though the raw numbers look nothing alike: this carton is dense enough that its actual weight, not its size, sets the bill in either unit system. That will not hold for every box; a large, light one can flip to dimensional under both formulas at once, for the same reason it flips under one.
| Carrier | Service | Price | Delivery window |
|---|---|---|---|
| USPS | Ground Advantage | $21.25 | 4-5 business days |
| UPS | Ground Saver | $25.50 | 5 business days |
| FedEx | Ground Economy | $25.51 | 6-7 business days |
| UPS | Ground | $30.71 | 5-6 business days |
| FedEx | Ground | $39.31 | 5-6 business days |
None of these five prices are billed on the dimensional figure from either table above; the carton's actual weight carried it the whole way. The formula still matters, because it is the check every one of these rates runs before landing on a number, and on a lighter or bigger box it would have been the number that won.
Converting a metric spec sheet into a US quote
Weight second
Multiply kilograms by 2.2046 to get pounds. 6 kg becomes about 13.23 lb. This is the number that gets compared against whichever dimensional weight comes out of the box.
Skip the manual math
Our own calculator takes centimeters and kilograms directly on the measurement screen and runs the conversion itself before it prices a US lane, so a spec sheet from an overseas factory can go in exactly as printed.
One unit choice does not change what a box actually costs to move; it only changes which numbers you are staring at while you figure that out. A US domestic rate is always set on the inch and pound math in the end, whatever the carton's original spec sheet said.
Where this formula comes from
- FedEx, How do I calculate dimensional weight of a package? — both the metric and the inch and pound formulas, quoted directly. Read 2026-08-20.
- FedEx, Understand dimensional weight to calculate costs — the 5000 divisor and a worked centimeter example. Read 2026-08-20.
Carriers can revise a published divisor without notice, and different services from the same carrier sometimes use different ones. Check the figure against the carrier actually moving your box before you rely on it for a quote.
Common questions
What is the formula for dimensional weight in kilograms?
Length times width times height in centimeters, divided by 5000, rounded up to the next whole kilogram. That is FedEx's own wording for the metric divisor, read 2026-08-20.
Why is the metric divisor 5000 and the US divisor 139?
They describe the same density assumption in two different units. Multiply the US divisor, 139, by roughly 36 and it lands close to 5000, because a cubic inch holds about 16 cubic centimeters and a kilogram is about 2.2 pounds, and both of those conversions are baked into the jump between the two divisors.
Do USPS, UPS and FedEx bill US parcels using the metric formula?
No. Domestic US parcels are billed on the inch and pound formula, divided by 139. The metric formula belongs to international and cross-border services priced in kilograms.
My supplier's spec sheet only lists centimeters. How do I get a US rate?
Convert each side to inches (divide by 2.54) and the weight to pounds (multiply by 2.2046), then hand those numbers to a calculator built for US parcels. Ours takes centimeters and kilograms directly and does the conversion for you.
Does a box ever bill lower under the metric formula than the imperial one?
No, not for the same physical box. Both formulas are scaled versions of the same volume-to-weight assumption, so a box that clears on actual weight under one clears under the other, and a box that trips dimensional pricing under one trips it under the other.