Steel Unit Conversion Calculator: US & Metric Units

Switching between inch-pound drawings and metric documents is easier when the quantity stays clear. This Steel Unit Conversion Calculator converts dimensions, section properties, mass, forces, stresses, moments, and line loads. Choose the quantity first so that a mass value cannot accidentally be treated as a force.

Steel Unit Conversion Calculator

Enter decimals or fractions such as 0.375, 3/8, or 1 1/2. For feet and inches, first use one unit: 12 ft 8 in = 152 in. Commas and unit symbols are not accepted in the value field.

How to use the converter

Select the quantity, enter the source value, and choose the starting and target units. Results update immediately. The table beneath the answer shows the same value in every supported unit for that quantity. “Swap units” reverses the conversion and carries the converted value into the input.

Start with Length to convert a plate thickness. Use Stress / pressure for ksi to MPa, Mass per length for a member weight listed in lb/ft, and Line load for a force distributed along a beam. Signed inputs are accepted to preserve the sign convention of analysis results; a negative output is not a valid physical thickness or mass.

Common steel unit conversion chart

Displayed values are rounded where needed. The calculator derives factors from 1 in = 0.0254 m, 1 lb = 0.45359237 kg, and standard gravity g₀ = 9.80665 m/s². It uses the international foot, not the historical U.S. survey foot. [1, 2]

Five examples from steel drawings and calculations

1. Plate thickness: 3/8 in to mm

Select Length, enter 3/8, and convert in to mm. The result is 9.525 mm. That is the converted dimension. Choosing a commercially available 10 mm plate is a separate specification decision; the calculator does not authorize a substitution.

2. Stress: 50 ksi to MPa

Select Stress / pressure and enter 50 ksi. The result is approximately 344.737865 MPa. Unit conversion changes the number and unit together. It does not establish that two steel grades have equivalent chemical, mechanical, testing, or delivery requirements.

3. Member mass per length: 35 lb/ft to kg/m

Select Mass per length. A value of 35 lb/ft converts to approximately 52.085738 kg/m. For an illustrative 20 ft member, multiply 35 × 20 to obtain 700 lb, or 317.514659 kg. Use the published property for the actual section when preparing a takeoff.

4. Distributed force: 2 kip/ft to kN/m

Select Line load. A load of 2 kip/ft converts to approximately 29.187806 kN/m. Do not select Mass per length for this calculation: kip is a force unit, while the lb in the mass category is a mass unit.

5. Second moment of area: 100 in⁴ to mm⁴

Select Second moment of area. The result is 41,623,142.56 mm⁴. Each of the four length factors must be converted, so the multiplier is 25.4⁴. Multiplying by 25.4 alone would give the wrong section property.

Why the exponent matters

Volume and section modulus share length-cubed units, but describe different quantities. They have separate menus to keep the purpose visible. The second moment of area is not a mass moment of inertia. A warping constant with length-to-the-sixth units is outside this calculator’s scope.

Mass, force, and steel self-weight

In this tool, lb means mass only when explicitly labeled as mass. The force menu uses lbf and kip; 1 kip is 1,000 lbf. Kilograms and kilonewtons cannot be interchanged through a unit-only conversion. To obtain gravitational force from mass, use W = m × g. [1, 2]

For example, a mass per length of 52.085738 kg/m corresponds to approximately 0.510787 kN/m under standard gravity: multiply by 9.80665 and divide by 1,000. This extra physics step is intentionally separate from the unit converter. Read a source document carefully when it uses the shorthand “plf”: determine whether it means a mass listing or a force line load.

Decimal feet are not feet and inches

A dimension of 12.649 ft does not mean 12 ft 6.49 in. Multiply the fractional foot by 12: 0.649 × 12 = 7.788 in. Thus 12.649 ft is 12 ft 7.788 in, approximately 12 ft 8 in when rounded to the nearest whole inch. A CAD dimension can look different simply because its display precision is lower.

Precision and checking the result

Answers display up to 10 significant digits; small or large values may use scientific notation. Conversion uses unrounded factors internally. Extra displayed digits do not improve the accuracy of the original measurement. Choose the final drawing precision to suit the project, and avoid repeatedly converting rounded values.

This tool converts quantities only. It does not check member capacity, connections, welds, bolt strength, fabrication tolerances, or code compliance. It also does not select an equivalent metric section. For drawing changes, keep the original section designation and specification identifiable until the responsible designer approves any substitution.

Frequently asked questions

Is MPa the same as N/mm²?

Yes. One MPa equals one N/mm². Both options appear in the stress menu to match common drawing notation.

Is a US ton the same as a metric tonne?

No. The mass menu identifies a US short ton as 2,000 lb and a metric tonne as 1,000 kg. A British long ton is not included.

Can this calculator convert bolt diameter into a metric bolt size?

It can convert the diameter numerically. It cannot match thread pitch, fit, grade, or connection requirements; a converted dimension is not a replacement bolt specification.

Does it need a steel density?

No. Unit conversion does not depend on material density. The Mass density category converts a density you supply; it does not assume a value or calculate plate mass from dimensions.

Sources and conversion basis

[1] NIST SP 811 (2008), Appendix B.8 — Conversion factors listed alphabetically.

[2] NIST SP 811 (2008), Appendix B.9 — Conversion factors by quantity. Factors for compound units are derived from the base conversions. These references support units, not structural design acceptance. Reviewed October 19, 2026.