Why rebar is detailed in feet and bought in tons
A reinforcement drawing tells you bar marks, sizes, shapes and lengths. A mill or a service centre sells reinforcing steel by weight. The bar list is the bridge, and the only arithmetic in it is unit weight multiplied by total length, done size by size and then summed.
Unit weight is fixed by the bar's nominal cross-sectional area and the density of steel. It does not depend on grade: a Grade 60 #5 bar and a Grade 80 #5 bar weigh the same 1.043 lb per foot, because grade changes the chemistry and the yield strength, not the section. Nor does it depend on whether the bar is epoxy coated, except by the negligible mass of the coating itself, which is why coated bar is still ordered against the same nominal weights.
The word nominal matters. A deformed bar has ribs and lugs standing proud of the core, so it has no single true diameter. ASTM A615 defines the nominal diameter as that of a plain round bar of the same mass per unit length, which makes the unit weight the primary property and the diameter the derived one. That is why an A615 table is a list of masses, and why any weight you calculate from the diameter agrees with the table to three figures.
For bar counts and spacing rather than weight, use the rebar spacing calculator; it produces the lengths and quantities that feed into this one.
Where 2.67 and 162 come from
Both constants are the same physics with different units. Steel has a density close to 7,850 kg/m³, or 490 lb/ft³, or 0.2836 lb/in³.
In metric, take a bar of diameter d millimetres. Its area is (π/4)d² mm². A metre of bar is 1,000 mm long, so its volume is 1,000 × (π/4)d² mm³. Multiplying by density in kg/mm³ and simplifying gives mass per metre = d² × 7850π / (4 × 10⁶) = d² / 162.28. The trade writes it as d²/162, which is accurate to better than two tenths of a percent.
In inch-pound units, take a bar of diameter d inches. Its area is (π/4)d² in², a foot is 12 in, and steel is 0.2836 lb/in³, so pounds per foot = (π/4)d² × 12 × 0.2836 = 2.6726 d². For a #8 bar, d = 1 in exactly, so the answer is 2.673 lb/ft against a tabulated 2.670 - the difference is rounding in the density.
The calculator uses the tabulated ASTM values for standard bar sizes rather than the formula, because those are the numbers a supplier will invoice against, and falls back to 2.6726 d² for any diameter not in the table. On the metric side it uses d²/162.2 throughout.
Total weight follows directly: unit weight × length of one bar × number of bars. Short tons are pounds divided by 2,000; tonnes are kilograms divided by 1,000. Those two are not the same unit and mixing them costs 10 percent, which is a large error on a steel invoice.
Worked example: a slab mat in #5 bar
A slab is reinforced with #5 bars at 12 in centres each way. The bar list calls for 50 bars 20 ft long in one direction, and the quoted price is $0.85 per pound.
- Unit weight. A #5 bar is 5/8 in = 0.625 in nominal diameter. From the ASTM table, 1.043 lb/ft. Check it against the formula: 2.6726 × 0.625² = 2.6726 × 0.390625 = 1.0440 lb/ft, which agrees to three figures.
- Total length. 50 bars × 20 ft = 1,000 ft.
- Total weight. 1.043 × 1,000 = 1,043 lb.
- Short tons. 1,043 ÷ 2,000 = 0.5215 tons.
- Metric. 1,043 × 0.45359237 = 473.1 kg, or 0.4731 tonnes. Note that 0.4731 tonnes is not 0.5215 tons - the two units differ by a factor of 1.1023.
- Cost. 1,043 × $0.85 = $886.55 for this bar mark.
Now do the same for the transverse bars and any dowels, and add the sums. A bar list is nothing more than this calculation repeated once per bar mark, which is why an error in one unit weight propagates quietly into the tonnage total.
Reading the result before you order
Compare the tonnage with the concrete volume as a sanity check. Residential slabs and footings typically carry a low reinforcement ratio; heavily reinforced columns, transfer beams and mat foundations carry much more. If your tonnage per cubic yard is wildly outside what similar jobs have produced, the likely cause is a missed lap allowance or a bar mark counted twice, not an exotic design.
Laps are the most common omission. Splice lengths are set by ACI 318 development-length rules and depend on bar size, concrete strength, cover and coating, and they can add a substantial share to the steel on a job with many stock-length bars. This calculator takes the cut length you give it, so add laps to the bar length before you enter it, or add a separate bar mark for them.
Nominal weight is not delivered weight. ASTM A615 permits a tolerance on mass per unit length, so a bundle weighed at the gate will differ from the calculated figure by a small percentage. Suppliers invoice against theoretical weight in most markets precisely to avoid that argument; ask which convention your quote uses before comparing prices.
Cutting and bending is priced separately from the steel and is usually charged per piece or per tonne of fabricated bar, with extras for small quantities, unusual shapes and short lengths. A tonnage figure alone will not predict an invoice.
ASTM A615 nominal bar properties
| Bar size | Nominal diameter (in) | Diameter (mm) | Area (in²) | Weight (lb/ft) | Weight (kg/m) |
|---|---|---|---|---|---|
| #3 | 0.375 | 9.53 | 0.11 | 0.376 | 0.560 |
| #4 | 0.500 | 12.70 | 0.20 | 0.668 | 0.994 |
| #5 | 0.625 | 15.88 | 0.31 | 1.043 | 1.552 |
| #6 | 0.750 | 19.05 | 0.44 | 1.502 | 2.235 |
| #7 | 0.875 | 22.23 | 0.60 | 2.044 | 3.042 |
| #8 | 1.000 | 25.40 | 0.79 | 2.670 | 3.973 |
| #9 | 1.128 | 28.65 | 1.00 | 3.400 | 5.060 |
| #10 | 1.270 | 32.26 | 1.27 | 4.303 | 6.404 |
| #11 | 1.410 | 35.81 | 1.56 | 5.313 | 7.907 |
| #14 | 1.693 | 43.00 | 2.25 | 7.650 | 11.385 |
| #18 | 2.257 | 57.33 | 4.00 | 13.600 | 20.240 |
Diameters and areas are the ASTM A615 nominal values; the metric column is derived from the lb/ft column, not measured independently.
Where a rebar takeoff goes wrong
- Forgetting lap splices. Every joint between stock lengths adds a development length of bar that appears nowhere on the plan dimensions.
- Confusing short tons with tonnes. A short ton is 2,000 lb and a tonne is 2,204.6 lb. Quoting one and buying the other is a 10 percent error.
- Assuming a metric bar matches an ASTM size. A 16 mm bar is 1.578 kg/m; a #5 bar is 1.552 kg/m. They are close but not interchangeable, and a substitution changes the area as well as the weight.
- Using coated-bar weight as if it were different. Epoxy and galvanised coatings add a negligible mass; order against the same nominal weights and pay the coating premium separately.
- Measuring bent bars along the chord. A stirrup's cut length runs around the shape, including the hook extensions, not across it.
- Pricing tonnage without fabrication. Cut-and-bend, tags, ties and delivery are separate lines, and on a small order they can rival the steel.
Which standard defines these weights
The inch-pound bar sizes, nominal diameters, areas and masses in the reference table are from ASTM A615/A615M, the specification for deformed and plain carbon-steel bars for concrete reinforcement. Low-alloy weldable bar is ASTM A706 and carries the same nominal dimensions and masses. Metric bars named by diameter follow the d²/162 convention derived from the same steel density. Bar numbers up to #8 are simply the diameter in eighths of an inch; #9, #10 and #11 correspond to the old 1 in, 1-1/8 in and 1-1/4 in square bars and have areas of exactly 1.00, 1.27 and 1.56 in².
Where the weight figure is actually used
Three people need this number and they need it for different reasons. The estimator wants tonnage to price the package. The detailer wants it to check a bar list against a schedule. The site engineer wants it to know what is arriving on a truck and whether the crane, the trestles and the lay-down area can take it - a bundle of #11 bar at 60 ft is nearly two tons on its own.
The weight also drives handling decisions that are easy to overlook until delivery day. A single 20 ft #5 bar weighs 21 lb and one person carries it. A 40 ft #11 bar weighs 213 lb and needs three or a machine. If your schedule mixes sizes, look at the per-bar weight before you plan the placing sequence.
On the concrete side, the same job needs volumes: use the concrete slab calculator or the concrete footing calculator for the pour, and the Sonotube column calculator for piers. If the reinforcement is going into a masonry wall rather than a concrete member, the cell layout comes from the concrete block calculator, and the grout to fill those cells is a separate quantity again.