Unit Converter
Pick the quantity, the unit you have and the unit you need, then type the value. The converter runs the constants from our calculation workbook, marks the exact ones, and offers an enquiry with the converted figure already in it.
Length
What this calculates
Metal is specified in a mix of unit systems. Sheet and plate thickness is quoted in millimetres in Saudi Arabia and in inches or gauge on American drawings; pipe comes in inch nominal sizes with millimetre walls; weight is in kilograms on the delivery note and in pounds on an imported mill certificate. This page converts between the pairs our engineers meet most, and nothing else — it does not carry every unit ever defined.
Five quantities are covered. Length between millimetres, inches, metres and feet. Area between square metres and square feet. Mass between kilograms and pounds. Mass per metre, the figure on a pipe or bar weight table, between kg/m and lb/ft. Mass per square metre, the figure on a sheet weight table, between kg/m² and lb/ft². Pressure, density and temperature have their own converter pages, linked from the tool strip above.
Some of these constants are definitions and some are rounded. An inch is exactly 25.4 mm and a pound is exactly 0.45359237 kg by international agreement, so those conversions are exact in either direction; the converter says so beneath the result. Metres to feet and kilograms to pounds are quoted to ten significant figures, which is more than any drawing needs but lets a long chain of conversions stay honest.
Show the formula and a worked example
The formula
- mm → in: × 0.03937007874015748 (exact)
- in → mm: × 25.4 (exact)
- m → ft: × 3.280839895
- ft → m: × 0.3048 (exact)
- m² → ft²: × 10.7639104167
- ft² → m²: × 0.09290304 (exact)
- kg → lb: × 2.2046226218
- lb → kg: × 0.45359237 (exact)
- kg/m → lb/ft: × 0.671968975
- kg/m² → lb/ft²: × 0.2048161436
Dimensions in millimetres, density ρ in kg/m³ — as the engine runs them.
Each conversion is the value multiplied by one constant from the workbook, listed above. Where the workbook gives only one direction — kg/m to lb/ft, for example — the reverse divides by the same constant, and the result is marked as run backwards. No second constant is introduced, so converting there and back returns the figure you started with.
The result is held at full precision and rounded to two decimals only for display, the same rule as every calculator on the site. If you need more digits, the enquiry carries the full value.
Worked example
| Quantity | Length |
|---|---|
| Value | 25.4 mm |
| To | in |
| Theoretical result | 1 in |
Computed by the calculation engine from these inputs at render time, rounded only for display.
Frequently asked
- 01Why is the result marked "exact" for some pairs and not others?
- Because some constants are defined rather than measured. The inch, the foot, the pound and the square foot are fixed by definition in metric terms, so converting to them is exact. The reverse constants — millimetres to inches, for example — are the reciprocals written to sixteen digits, which is exact for every practical purpose and marked as such in the workbook.
- 02Can I convert gauge to millimetres here?
- Not on this page. Sheet gauge is a lookup table, not a formula, and it differs between steel, stainless and aluminium. Send the gauge and the material on the enquiry and the estimator will quote in millimetres.
- 03What does the enquiry carry?
- The quantity, both units, the value you typed and the converted figure, written as one line in the requirement. The estimator sees what you started from, not only the result, so a misread unit is caught before it reaches a quotation.
- 04Why is kg/m separate from kg?
- Because they are different quantities. Kilograms is a mass; kilograms per metre is a mass per unit length, the figure a pipe or bar table lists so you can multiply by the length you are buying. Converting one to the other needs a length, which is a weight calculator's job.