Unit of measure converter — length, mass, volume, area
Convert length, mass, volume and area with exact factors, plus ERP case and pallet maths that shows the rounding remainder behind stock discrepancies.
Free · no signup · runs in your browser·Updated
Short answer
A unit of measure conversion multiplies your quantity by the ratio between the two units. 1 inch is exactly 25.4 mm and 1 pound is exactly 0.45359237 kg, so 12.5 in is 317.5 mm. ERP conversions work the same way: 5,000 each at 24 per case is 208.33 cases, and rounding that to 208 loses 8 units.
This unit of measure converter handles the four measures that fill up ERP item records — length, mass, volume and area — using exact definitional factors rather than the rounded ones that circulate in spreadsheets. Pick a measure, type a value, choose the units, and it also lists the same quantity in every other unit of that measure.
The second mode is the one purchasing and warehouse teams need: base unit to case to pallet, driven by a conversion factor. It shows the exact quantity, the whole-unit quantity, and the remainder left over — because that remainder is what turns into an inventory variance three weeks later.
Your numbers
Conversion type
Base unit for the maths: metre.
Decimals and thousands separators are both fine.
Result
12.5 in converts to
317.5 mm
length · base unit metre
Conversion factor1 in = 25.4 mm
Reverse factor1 mm = 0.03937007874 in
Value in metres0.3175
Rounded to 2 decimals317.50 mm
Lost to that rounding0 mm
12.5 in × 0.0254 metre per in = 0.3175 metre
0.3175 ÷ 0.001 metre per mm
12.5 in = 317.5 mm
12.5 in in every unit of length.
Unit
Same quantity
metres per unit
millimetre (mm)
317.5
0.001
centimetre (cm)
31.75
0.01
metre (m)
0.3175
1
kilometre (km)
0.0003175
1,000
inch (in)
12.5
0.0254
foot (ft)
1.0417
0.3048
yard (yd)
0.347222
0.9144
mile (mi)
0.000197285
1,609.344
1 in is 25.4 mm, derived from definitional factors rather than measurement, so the only error possible here is rounding you introduce afterwards. Put 0.0254 on the item as its conversion factor rather than writing 317.5 into a field and losing the ability to change units later.
Everything is computed in your browser. Nothing you type is sent anywhere or stored.
The formula
Result = Value × (Factor of source unit ÷ Factor of target unit)
Value
The quantity you are converting.
Factor of source unit
How many base units one source unit equals. For length the base is the metre, so an inch has a factor of 0.0254.
Factor of target unit
The same figure for the unit you want out. Millimetre is 0.001 metres.
Every factor in this tool is exact by definition, not rounded. The inch has been defined as exactly 25.4 mm since 1959 and the pound as exactly 0.45359237 kg. That means the conversions here introduce no error of their own — any drift you see afterwards came from rounding the result, not from the factor.
Worked example
Measure
Length
Value
12.5
From
inch (in)
To
millimetre (mm)
Result
12.5 in = 317.5 mm
12.5 × 0.0254 = 0.3175 metres. Divide by 0.001 metres per millimetre and the answer is 317.5 mm exactly, because the inch is defined in terms of the millimetre rather than measured against it. Store 0.0254 as the conversion factor on the item and let the system convert, rather than storing 317.5 and losing the ability to change units later.
Exact conversion factors worth knowing
Most unit conversions in commercial use are exact, defined relationships rather than measurements. Using a rounded version of an exact factor introduces a fixed percentage error into every costing and every shipping weight that touches it — small per unit, and identical in sign every time, so it never averages out.
Unit
Exactly equals
Where the definition comes from
1 inch
25.4 mm
Defined exactly, by international agreement, since 1959.
1 foot
0.3048 m
12 inches, so it follows from the inch.
1 mile
1,609.344 m
5,280 feet.
1 pound
0.45359237 kg
Defined exactly against the kilogram.
1 short ton
907.18474 kg
2,000 pounds.
1 US gallon
3.785411784 L
231 cubic inches, exactly.
1 imperial gallon
4.54609 L
Defined exactly. It is about 20% larger than the US gallon.
1 acre
4,046.8564224 m²
4,840 square yards.
Exact by definition. There is no reason to carry a rounded version of any of these in a spreadsheet.
Using the unit of measure converter for ERP case and pallet quantities
ERP unit-of-measure conversion is the same arithmetic wearing a warehouse hat. You buy in pallets, receive in cases, pick in eaches and count in whichever unit the operator happens to be holding. The item's conversion factor is what ties those together: 24 eaches per case, 12 cases per pallet, so 288 eaches per pallet.
The trouble starts when a quantity does not divide evenly. 5,000 eaches is 208.33 cases. If the transaction is entered in cases and the system stores whole cases, 8 eaches vanish from the record and stay on the shelf.
On-hand quantity
Cases (exact)
Cases (whole)
Eaches unaccounted for
5,000 each
208.3333
208
8
4,992 each
208.0000
208
0
1,000 each
41.6667
41
16
12,000 each
500.0000
500
0
24 eaches per case. Only quantities that are exact multiples of the factor survive a round trip through case units.
Where UOM conversions break inventory
Changing the factor after transactions exist. Historic quantities were converted with the old factor and are not restated. On-hand still balances; the audit trail no longer explains it.
Rounding on the document instead of the record. A transfer order that rounds cases to whole numbers moves less stock than the paperwork says. Round for display, never for posting.
Buying in one unit and costing in another. If the purchase price is per case and standard cost is per each, a wrong factor shows up as a purchase price variance that no amount of negotiation will fix.
Weight and volume are not interchangeable. Converting litres to kilograms needs a density, which is a property of the material and not of the units. Any tool that offers that conversion without asking for density is guessing.
Catch-weight items. Products sold by count but priced by weight need two quantities per line, not one converted quantity. Forcing them through a single conversion factor guarantees the invoice and the pick disagree.
Auditing conversion factors across an item master is a query, not a spreadsheet: which items have a case factor of 1, which have factors that changed in the last year, which have on-hand quantities that are not whole multiples of their case factor. With ERPray you ask for exactly that and see the SuiteQL it ran. Pair it with the landed cost calculator when the units in question are what freight is billed on.
Frequently asked questions
How many millimetres are in an inch?
Exactly 25.4 mm. It is a definition rather than a measurement, agreed internationally in 1959, which is why the number never carries an approximation sign. That also makes every derived imperial-to-metric length factor exact: a foot is precisely 0.3048 m and a mile is precisely 1,609.344 m.
Is a pound exactly 0.45359237 kg?
Yes. The international avoirdupois pound is defined as exactly 0.45359237 kilograms, so an ounce is exactly 0.028349523125 kg and a 2,000-pound short ton is exactly 907.18474 kg. Rounding it to 0.45 is 0.8% light, which understates a tonne of product by about 8 kg.
What is a UOM conversion factor in ERP?
The number of base units contained in an alternate unit — 24 eaches per case, 12 cases per pallet. The system stores quantities in the base unit and multiplies or divides by the factor to show any other unit. Get it wrong and stock, cost and price are all wrong by the same ratio.
Why does my inventory quantity not match after a UOM change?
Because existing transactions were converted with the old factor and are not recalculated when you change it. The on-hand total is still internally consistent, but the case and pallet figures derived from it no longer agree with the receipts that created them. Correct forward and reconcile the base-unit quantity, not the display unit.
What is the difference between a US gallon and an imperial gallon?
A US gallon is exactly 3.785411784 litres, defined as 231 cubic inches. An imperial gallon is exactly 4.54609 litres, about 20% larger. Fuel, chemical and beverage quantities quoted in gallons across a US–UK supply chain need the unit stated explicitly, or every volume is out by a fifth.
Can I convert weight to volume?
Not with units alone. You need the density of the material: litres × kg per litre gives kilograms. Water is convenient at roughly 1 kg per litre, but resin, oil and powders are not, and bulk density differs from solid density for anything granular. Hold density as an item attribute, not a conversion factor.
This calculator needs you to find the inputs first. ERPray pulls them from your own ERP account and computes the answer live — with the exact query shown so you can check it.