Weight to Length Calculator

Metal is bought by weight and cut to length, so a delivery note in kilograms does not tell a fabricator how many metres of bar are on the truck. This calculator turns weight into length, or length into weight, for solid bars: pick a material, enter the cross-section as an area, a round diameter, or a rectangle, and it returns the length, the total weight, and the weight per metre.

Quick Answer

How to use the weight to length calculator

Start with the direction. Weight to length answers the delivery question: how many metres did this weight buy? Length to weight answers the ordering question: how heavy will this length be? The two direction buttons swap the weight and length fields, and everything else stays put.

Then describe the section. If you know the area from a table, choose Area and type it in square centimetres. If you have a round bar, choose the diameter option and type millimetres. If you have a flat or rectangular bar, choose the rectangular option and type width and height in millimetres. Only the fields for the chosen shape affect the answer; the hidden fields are ignored.

Pick the material, check the pre-filled example if you want to see the tool work, and press Calculate. The result shows the headline answer plus weight per metre, which is the number worth keeping: multiply it by any length to get that length in weight, without reopening the tool. Reset clears every field and returns the tool to steel, area input, and weight-to-length mode, exactly as the original tool behaved.

Do not confuse this tool with its near-namesake. The weight to mass calculator converts a force in newtons into a mass in kilograms under a chosen gravity. This page converts a mass in kilograms into a length in metres for a bar of known section, which is a different job with different units.

The formula, with the unit conversions spelled out

Length equals weight divided by density times cross-section area, and weight equals density times area times length. Weight per metre equals density times area, which is why it appears in both directions. All three formulas are the same relationship rearranged, not three separate rules.

Units decide whether the answer is right. Density is in kilograms per cubic metre, so area must be in square metres before it is multiplied. The tool converts for you: square centimetres divided by 10,000, and square millimetres divided by 1,000,000. A round section uses pi times radius squared, with the diameter first halved and converted from millimetres to metres. A rectangular section uses width times height, each converted from millimetres to metres.

For a quick sense check, the density calculator covers the underlying density relationship (mass divided by volume) on its own, while the mass to volume calculator handles the case where you know a mass and a density but have no bar length in the question at all.

Worked example: 50 kg of steel bar, checked against this exact tool

Take a steel bar with a 10 cm² section and a 50 kg weight. First convert the area: 10 divided by 10,000 is 0.001 m². Then divide the weight by density times area: 50 divided by (7,850 times 0.001) is 6.3694 m. The weight per metre is 7,850 times 0.001, which is 7.8500 kg per metre. Enter 10, steel, and 50 in the tool and press Calculate to reproduce those two numbers exactly.

Run it backwards as a check. In length-to-weight mode, 6.3694 m of the same bar weighs 7,850 times 0.001 times 6.3694, which the tool rounds to 50.00 kg. The tiny difference between 50.00 and the original 50 is rounding in the length display, not an error: the length carried into the reverse calculation was rounded to four decimals.

Two more cases show the shape options. A 20 mm round steel bar has an area of 314.2 mm² and weighs 2.4662 kg per metre, so 50 kg of it measures 20.2745 m. A 50 mm by 10 mm rectangular steel bar weighs 3.9250 kg per metre, so a 6 m length weighs 23.55 kg. Both figures come from the same density-times-area step, with the area worked out from the dimensions you typed.

The d²/162 shortcut, and how close it really is

Site staff often quote steel round-bar weight per metre as diameter squared divided by 162, with diameter in millimetres. It is not a separate physical law. It is the exact formula with steel density and the circle area folded into one constant: pi divided by 4, times 7,850, divided by 1,000,000, which is 0.006165 per square millimetre, or 1 divided by 162.2. Rounding 162.2 to 162 makes the shortcut slightly high, by about 0.1%.

For a 20 mm bar the shortcut gives 400 divided by 162, which is 2.4691 kg per metre, against the exact 2.4662 kg per metre. On a tonne of bar that gap is roughly 1.2 kg, small for a site estimate and large enough to matter if you are invoicing by weight. Use the shortcut on the shop floor and the tool for ordering, delivery checks, and anything priced by the kilogram.

Common steel round bars: weight per metre from this tool

These values use the tool's steel density of 7,850 kg per cubic metre and the exact circle formula, not the 162 shortcut. The shortcut column is included so you can see the size of the gap at a glance.

What competing calculators miss

Most bar-weight tools found in search run in one direction only: enter a diameter and a length, get a weight. That leaves the delivery question unanswered, because a weighed bundle has no length typed in anywhere. They also tend to assume steel and a round section, so a rectangular flat bar or a copper bar needs a different tool or a hand calculation.

This tool runs both directions, takes a direct area for sections already looked up in a table, and covers ten materials instead of steel alone. It also reports weight per metre in both modes, which is the figure a buyer actually compares across suppliers. The gap it does not close is quantity: it calculates one bar, not a bundle, so a delivery of many bars still needs the per-bar result multiplied by the bar count.

Limits worth knowing before you order from the answer

The tool assumes a solid bar with the same section along its full length. A hollow tube breaks that assumption, unless you enter the net area yourself: outer area minus inner area, in square centimetres, using the direct-area option. A tapered bar, a bar with a welded attachment, or a bundle with spacers needs a different method, because no single section describes it.

Densities are typical values, not mill certificates. Published tables put iron at 7,850, copper near 8,940 to 8,960, and gold near 19,300 to 19,320 kg per cubic metre, so the tool's rounded presets sit inside the published ranges, but a specific alloy can sit a little off the preset. Stainless steel is the widest case: grades commonly range from about 7,480 to 7,980 kg per cubic metre, while the tool presets 7,900. For a certified weight, weigh the actual bar; for estimating, cutting, and checking a delivery, the preset is close enough that the error stays well under 1% for most grades. Flat and plate work follows the same density-times-area idea with a different section, and the steel plate mass calculator is the right tool when the piece is a plate rather than a bar.

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