Reinforcement Steel Calculator – Rebar Weight in kg and Tonnes

US bar sizes #3 to #11 at a glance

Each bar number is eighths of an inch across: #4 is 4 eighths, which is half an inch, or 12.7 mm. Weight per metre comes from diameter squared divided by 162, so a bar twice as thick weighs about four times as much per metre.

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Understanding rebar weight

Weight per metre: diameter squared over 162

The weight of a reinforcement bar per metre follows a single trade formula: weight per metre in kilograms equals the diameter in millimetres, squared, divided by 162. The divisor 162 is not arbitrary: it comes from steel’s density of 7,850 kilograms per cubic metre, combined with the circular cross-section and the unit conversions, compressed into one constant for site use. The formula is an approximation that rolled bars follow closely but not exactly, because real bars vary slightly in cross-section and density, and the page’s figures should be read with that tolerance in mind.

The default bar shows the formula directly. A #4 bar has a diameter of 12.7 mm, and 12.7 squared is 161.29, divided by 162 gives about 0.996 kilograms per metre, which is the page’s reported figure. The “about” matters: the page computes from unrounded values and displays rounded ones, so hand multiplication from displayed figures may differ in the last digit from the page’s outputs. Quote the page’s figures for ordering, and use hand arithmetic as a check, not as a replacement.

Diameter dominates the result because it is squared. Doubling the diameter quadruples the weight per metre, not doubles it, which is why bar size selection matters so much for total tonnage. A #8 bar at 25.4 mm, exactly double the #4 diameter, weighs 3.982 kilograms per metre against the #4’s 0.996, which is four times as much, as the squaring predicts. When comparing bar sizes, think in squares, not in diameters: a small step up in size is a large step up in weight.

The formula also explains why the page asks for diameter rather than bar weight directly. Diameter is the measurable, orderable property, stamped and specified for every bar, while weight per metre is derived and varies slightly by mill. Entering the diameter and letting the page apply the standard formula gives a consistent basis for comparing bars, and the approximation error, a fraction of a percent for most rolled bars, is smaller than the variation introduced by cutting, laps and waste on a real job.

From per-metre to per-bar to total

Total weight builds in three multiplications: per-metre weight times length gives per-bar weight, per-bar weight times bar count gives total weight, and total weight divided by 1,000 gives tonnes. The default job walks all three. At about 0.996 kilograms per metre, a 6-metre bar weighs about 5.97 kilograms, ten such bars weigh 59.74 kilograms in total, and that total is 0.0597 tonnes. Each step is simple, but the chain matters, because an error in any step propagates to the total, and the total is the figure orders are placed against.

Total length is the intermediate figure that ties the chain together: bar count times length per bar. For the defaults, 10 bars at 6 metres give 60.00 metres of bar in total, and the total weight is the per-metre rate applied to that length. Checking total length first is the fastest sanity test on any result: if the length looks wrong, the weight will be wrong in proportion, and the error is in the count or the length input, not in the formula. The page reports total length alongside total weight for exactly this checking purpose.

Tonnes are the ordering unit for larger jobs, and the conversion is division by 1,000: 59.74 kilograms is 0.0597 tonnes, displayed to four decimal places because small jobs are fractions of a tonne. For large jobs the tonne figure is the one suppliers quote against, while kilograms remain the figure for handling and for bar-by-bar checks. Keep both in view: kilograms for the bars in front of you, tonnes for the order as a whole, and the conversion between them as a division that should always be checked when an order is placed.

Rounding accumulates through the chain, which is why the page’s displayed figures may differ in the last digit from hand multiplication of displayed figures. The page computes from unrounded per-metre rates and rounds only for display, so a hand calculation from 0.996 may give 5.98 where the page gives 5.97, because the unrounded rate is slightly below 0.996. The difference is negligible for ordering, but it explains the apparent mismatch, and the rule is to trust the page’s chain over a hand chain built from rounded intermediates.

Bigger bars, longer bars: two ways to the same total

Two very different jobs can weigh the same, and the page’s probed cases demonstrate it. A #8 bar job, 25.4 mm diameter, 6 metres per bar, 10 bars, gives 3.982 kilograms per metre, 23.89 kilograms per bar, 60.00 metres total length, and a total of 238.95 kilograms, or 0.2389 tonnes. A #4 bar job with 20 bars of 12 metres each gives 0.996 kilograms per metre, 11.95 kilograms per bar, 240.00 metres total length, and the same total of 238.95 kilograms, or 0.2389 tonnes. Same total, different bars, different lengths, different counts.

The equality is a legitimate coincidence, not an error, and the arithmetic shows why. Total weight is per-metre rate times total length, and the two jobs balance a higher rate against a shorter length: the #8 job’s rate is about four times the #4 job’s, while its total length is a quarter of the #4 job’s, 60 metres against 240, so the products coincide. Coincidences of this kind are common when rates and lengths vary inversely, and recognising them prevents the false conclusion that one of the results must be wrong.

The practical lesson is that total weight alone does not describe a job. Two orders of 238.95 kilograms can contain different bar sizes, different lengths, different counts, and different handling requirements, and a supplier needs the breakdown, not just the total. Always order by size, length and count, with the total as a check, never by total alone. The page reports the full breakdown for this reason, and the breakdown, not the headline total, is what should be copied onto an order.

Length per bar also affects handling and waste in ways weight does not capture. Twelve-metre bars need longer transport and storage, produce different offcut patterns, and suit different layouts than six-metre bars, even when the total weight matches. A job’s logistics follow its lengths and counts, while its cost follows its weight, and both descriptions are needed to plan it. The page gives both; use both.

US bar sizes #3 to #11

US reinforcement bars are numbered by eighths of an inch: the bar number counts how many eighths of an inch make the nominal diameter. A #3 bar is 3 eighths of an inch, which is 9.525 mm. A #4 is 4 eighths, half an inch, 12.7 mm. The sequence continues through the page’s table, and the pattern is exact: each step adds one eighth of an inch, which is 3.175 mm, to the diameter.

The table’s figures are the page’s own, and the eighths rule lets you check any row: #7 at 7 eighths is 22.225 mm, #9 at 9 eighths is 28.65 mm, and #11 at 11 eighths is 35.81 mm. Larger bars beyond #11 exist in practice, but this page’s list runs #3 to #11, which covers the sizes most commonly used in ordinary reinforced concrete work. A bar outside that range should be entered by its custom diameter rather than forced onto the nearest listed size, because weight depends on the square of the diameter and a small diameter error becomes a larger weight error.

Nominal diameter is not measured diameter. A deformed bar’s measured diameter across its ribs exceeds its nominal diameter, and its weight per metre follows the nominal cross-section used in the standard, not a caliper reading over deformations. Enter the nominal size from the specification, not a field measurement over ribs, or the squared formula will overstate the weight. The page’s size selector exists to prevent that error: picking #4 enters 12.7 mm, the nominal value, regardless of what a rib measurement might suggest.

Metric practice differs by region, with bars specified directly in millimetres rather than by number, and the page’s custom diameter input covers that case. Enter the specified diameter in millimetres, and the same formula applies, because the formula takes diameter as its input whatever the naming system. A 20 mm metric bar and a #6 bar at 19.05 mm are close but not identical, and their weights differ accordingly, so keep the specification system straight when ordering: metric diameters for metric projects, US numbers for US-numbered projects, and no mixing between them.

Reading a bar schedule

A bar schedule is the document this page’s figures feed into, and reading it correctly is part of using the calculator well. A schedule lists bar marks, sizes, lengths, counts and shapes, typically in a table where each row is one bar type: its mark for identification, its size or diameter, its length, how many are needed, and its bending shape where applicable. The page takes one row at a time, size, length and count, and returns the weight for that row. A full schedule’s tonnage is the sum of its rows’ weights, computed row by row, never by averaging sizes or lengths across rows, because weight depends on the square of each row’s diameter individually.

Bar marks matter because they prevent the most expensive ordering error: right weight, wrong bars. Two rows can share a size and length but differ in shape or location, and their marks are what distinguish them on site. When transcribing a schedule into the calculator, work mark by mark, check each row’s size and length against the drawing, and keep the mark attached to the result, so that the weight computed for one mark is never accidentally assigned to another. The calculator does not know marks; the user must carry them alongside its outputs.

Lengths on a schedule are usually straight lengths before bending, with bending deductions and lap lengths handled by the detailing rules for the project. The page’s length input is the length per bar as scheduled, and if the schedule gives cut lengths that already account for bends, those are the lengths to enter. If it gives overall dimensions that bending will shorten, the cut length differs, and the difference belongs to the detailing computation, not to the weight formula. Enter scheduled lengths as scheduled, and resolve bending questions against the project’s detailing standard, not by adjusting the weight calculator’s inputs.

Counts on a schedule include the bars shown on the drawings, and laps, spares and waste are normally added as separate allowances by the scheduler, not by the weight calculator. A schedule count of 10 bars means 10 bars as drawn; the order quantity may be higher after allowances, and the page’s count input should be the quantity being weighed, whether drawn or ordered, stated explicitly. Mixing drawn counts with ordered totals, or weighing one and ordering the other without noting the difference, is how sites end up short or overstocked, and the distinction should be written on the order.

Use the page row by row, keep marks attached, sum the rows for the schedule total, and carry the breakdown, not just the total, onto the order. A schedule total without its rows cannot be checked, cannot be adjusted when a row changes, and cannot be verified on delivery. The page’s per-row figures, size, length, count, per-bar weight and row total, are the units of a verifiable schedule, and a schedule built from them, row by row, is one that can be audited, adjusted and trusted.

Coated and stainless bars deserve a note, because their weights differ slightly from plain carbon bars of the same nominal size. Epoxy coatings, galvanising and stainless alloys all add or change a small amount of material, and the standard formula on this page does not account for coatings. For ordinary estimating the difference is negligible beside detailing allowances, but for a large order of coated bars, or for verification against a delivery, confirm the applicable weights with the supplier or the mill certificate rather than assuming the plain-bar figures transfer exactly. The formula remains the right estimator; the certificate remains the right verifier.

Metric and imperial units meet on every site that uses both, and the conversion to keep straight is that one inch is exactly 25.4 millimetres. That single fact checks the whole bar table: a #8 bar at one inch is 25.4 mm, a #4 bar at half an inch is 12.7 mm, and a #3 bar at three eighths of an inch is 9.525 mm. When a drawing gives inches and the calculator takes millimetres, multiply inches by 25.4; when a specification gives millimetres and a supplier quotes inches, divide by 25.4. The conversion is exact, unlike the weight formula, so any discrepancy after converting is an error in transcription or in the size selected, not in the arithmetic. Keep one unit system through a calculation, convert once at the boundary, and the weights that follow will be consistent.

Custom diameters, and what the approximation means

Not every bar is a listed US size, and the page’s custom diameter input handles the rest. A 20 mm bar, entered directly, gives 2.469 kilograms per metre, 14.81 kilograms for a 6-metre bar, 148.15 kilograms for 10 bars, and 0.1481 tonnes. The computation is identical to the listed sizes: 20 squared is 400, divided by 162 is about 2.469, and the chain through per-bar and total follows as before. Custom input does not change the method; it only changes where the diameter comes from.

The approximation behind the formula deserves a final note, because it governs how much to trust the last digit. The divisor 162 encodes a density of 7,850 kilograms per cubic metre and a circular cross-section, while real rolled bars vary slightly in both, and their ribs add steel that the nominal cross-section does not fully capture. The result is that a computed weight and a weighed weight for the same bars can differ by a small percentage, in either direction, without either being wrong: one is a standard estimate, the other is a measurement of specific bars from a specific mill.

For ordering, the standard estimate is the right basis, because suppliers quote and mills certify against nominal weights, and the variation is smaller than the allowances a job normally carries for laps, hooks, bends and cutting waste. For verification, weighing delivered bars is the right check, and a discrepancy within a small percentage confirms the order rather than challenging it. Only a large discrepancy, well beyond rolling tolerance, suggests a wrong size or a short delivery, and that is a matter to raise with the supplier with the page’s breakdown in hand.

Keep the approximation in proportion. It does not affect bar choice, which is governed by structural design, and it does not affect layout, which is governed by drawings. It affects only the conversion from a specified bar schedule to an estimated tonnage, and for that purpose it is accurate enough that the industry uses it daily. Use the page’s figures for estimating and ordering, weigh for verification, and let the small gap between estimate and measurement be what it is: the difference between a standard and a sample.

Ordering, handling, and the limits of a weight calculator

An order needs more than a total weight. It needs the bar size, the length per bar, the count, and the total length, all of which this page reports, plus the detailing that the page does not model: laps where bars join, hooks and bends at ends, spacers and chairs, and cutting waste. Those detailing items add steel beyond the straight-bar total, and their amounts come from the drawings and the bar schedule, not from a weight formula. A total copied from this page without detailing allowances will understate the order, and the shortfall will appear on site, not on paper.

Handling follows the breakdown as well. Per-bar weights from about 5.97 kilograms for a #4 bar at 6 metres to about 23.89 kilograms for a #8 bar at 6 metres describe very different manual-handling tasks, and longer bars multiply the awkwardness even when the per-bar weight is modest. Plan transport, storage, cutting and placement around lengths and counts, not just tonnage: a tonne of short bars and a tonne of long bars are different jobs, and the page’s length and count figures are what distinguish them.

Most importantly, this page gives general engineering information only, and it does not design reinforcement. Bar sizes, spacing, cover, lap lengths, development lengths and detailing are structural design decisions that depend on loads, codes, exposure and the full structural system, and they require a structural engineer. Nothing on this page selects a bar size for a beam, a slab or a column, and no weight figure, however exact, substitutes for a design. Use the page to weigh a schedule that an engineer has specified, never to invent a schedule that no engineer has checked.

Used inside that boundary, the page is a fast, reliable estimator: enter the specified size or diameter, the length and the count, read the per-metre, per-bar and total figures, and carry the breakdown onto the order with detailing allowances added from the drawings. The arithmetic is standard, the approximation is honest, and the limits are stated. A weight calculator that knows it is not a design tool is more useful than one that pretends otherwise, because its results can be trusted exactly as far as they go, and no further.

The bar-size ladder: weight per metre from 10 mm to 25 mm

Weight per metre is diameter squared divided by 162 — one rule, every size.

The trade approximation behind this calculator is a single division: square the bar diameter in millimetres and divide by 162, which assumes steel at 7,850 kg/m³. A 10 mm bar is 100 ÷ 162 = 0.617 kg per metre; 12 mm is 0.889; 16 mm is 1.580; 20 mm is 2.469; 25 mm is 3.858. The growth is quadratic, not linear — doubling the diameter quadruples the weight per metre — which is why stepping up one bar size moves the total far more than adding a few bars of the same size.

Total weight is then per-metre rate × total length, where total length is bars × length each, and tonnes are kilograms divided by 1,000. The worked check on this page is a 12.7 mm bar at 0.996 kg per metre: 10 bars at 6 metres each weigh 59.74 kg, or 0.0597 tonnes, while a #8 bar at 3.982 kg per metre makes the same schedule 238.95 kg. The calculator weighs a specified schedule; it never designs one — sizes, spacing and detailing need a structural engineer. Full tables, US bar sizes and custom diameters: rebar explained.

Ordering: laps, waste and the allowance that is not in the formula

The formula weighs the bars on the drawing. The order must weigh the bars on the truck.

Bars are rarely used at exactly their drawn lengths. Lap splices, where two bars overlap to continue a run, consume length that the schedule’s bar count does not show; bends consume a little more; cutting produces offcuts too short to use. A small allowance over the calculated weight covers all three, and the allowance should be agreed with the supplier and the engineer before ordering, not discovered as a shortage on site. The calculator’s figure is the exact weight of the specified schedule — the order quantity is that figure plus the agreed allowance, rounded up to the supplier’s bundle or tonne pricing.

Two ordering traps deserve naming. First, mixing metric and imperial sizes on one order: a #8 bar and a 25 mm bar are close but not identical, and the weight difference across a full schedule is real money. Second, custom diameters: entering a diameter directly uses the same D²/162 rule as the listed sizes, so a 20 mm custom bar and a listed 20 mm bar weigh the same — the custom entry is a convenience, not a different formula. Confirm the diameter in millimetres before entering it; a centimetre entered as millimetres squares the error.

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