Rebar Weight Calculator

Where the unit weight comes from

Every US bar size is a count of eighths of an inch: a #5 bar is 5/8 of an inch across, a #4 is 1/2, an #8 is a full inch. Steel weighs about 490 pounds per cubic foot, so a round bar's weight per foot is just its cross-sectional area times that density. Work it through and it collapses to one line: unit weight = 2.67 × d × d, with d the diameter in inches. For #5 that is 2.67 × 0.625 × 0.625 = 1.043 lb per foot.

You do not have to trust the shortcut. ASTM A615, the standard specification for deformed carbon-steel reinforcing bars, prints the nominal dimensions and weights in its Table 1, and state transportation departments reprint the same table. The New York State DOT engineering instruction carries the full list in both systems: 0.376 lb/ft for #3 up to 13.60 lb/ft for #18, each with its kilogram-per-meter twin. The calculator uses those table values, not the rounded shortcut, so its output matches mill certificates and supplier quotes.

One caution on the shortcut itself: 2.67 is a rounded constant, so it lands within a fraction of a percent of the table for #3 through #8 but drifts slightly on the bigger sizes. When money changes hands, the table value is the number of record.

Unit weights by bar size

The sizes sold for driveways, patios, walkways and footings are #3 through #6, with #7 and #8 showing up in footings and walls. This is the whole table the calculator draws from, with the weight of one 20 ft stick added, since that is how retail rebar is sold:

The metric column is the soft conversion: a #5 is sold as a 16 mm bar on metric drawings, even though its actual diameter is 15.9 mm. The shortcut kg/m = d²/162.2, with d in millimeters, reproduces the column within about one percent: 15.9² ÷ 162.2 = 1.56 against 1.552 for #5, and 12.7² ÷ 162.2 = 0.994 against 0.994 for #4. The CRSI product catalog (2024) publishes the same nominal dimensions along with hook and lap details that the next sections touch on.

Worked example: twenty sticks of #5

Take the default job: 20 bars of #5 at 20 ft each, with a 10 percent allowance for laps, offcuts and bars that arrive bent. The allowance lifts 20 bars to 22. Total length is 22 × 20 = 440 ft. Weight is 440 × 1.043 = 458.92 lb, which is 0.229 tons. At $12.00 a stick the cost is 22 × 12 = $264.00, and 264 ÷ 458.92 = $0.58 per pound.

Size is the whole ballgame at the same bar count. Swap the size and the batch weight moves with the square of the diameter, because area does:

The same 22 sticks of #8 weigh 1,174.8 lb, about two and a half times the #5 batch. If your quote is per pound, the size decision is most of the money, long before spacing or lap details enter the picture.

Weight per square foot of a mat

Spacing turns unit weight into a per-area number, which is how mats get estimated. A mat with bars every 12 inches in both directions carries one foot of bar under every square foot of slab, so the mat weight equals the unit weight: 1.043 lb per square foot for #5 at 12 inches. Tighten the spacing and the number climbs in proportion:

Now a real slab. A 12 by 20 foot patio covers 240 square feet. At #4 on 18 inch centers, the mat weighs 0.668 × 12/18 = 0.4453 lb per square foot, so the steel in the slab is 240 × 0.4453 = 106.9 lb. If the slab is 4 inches thick, that is 240 × 1/3 = 80 cubic feet of concrete, or 2.96 cubic yards, so the patio carries about 36 lb of steel per cubic yard. Estimators quote steel in exactly these two units, pounds per square foot for mats and pounds per cubic yard for slabs, and both come from the same unit-weight line.

Spacing is also where a drawing beats a rule of thumb. Sliding #4 from 18 to 12 inch centers raises the mat from 0.445 to 0.668 lb per square foot, a 50 percent jump in steel for the same slab, and the weight calculator shows the cost of that decision before the truck arrives.

Laps and waste, counted in pounds

Bars come in 20, 30 and 40 ft sticks, and any run longer than the stick needs a lap splice. The old jobsite rule of thumb is 40 bar diameters, which for #5 is 40 × 0.625 = 25 inches. Printed tables run longer: the lap table in the CRSI catalog cited above starts at 29 inches for a #5 in Grade 60 with 3,000 psi concrete, and the exact length moves with concrete strength and the bar's position in the pour. Use the thumb for planning, and the spec for sign-off.

Laps are weight too. A 100 ft run of #5 built from 20 ft sticks needs 5 sticks and 4 laps of 25 inches, which is 100 inches or 8.33 ft of extra bar. Total length is 108.33 ft, and the weight is 108.33 × 1.043 = 113 lb, against the 104.3 lb the bare run would suggest. On a footing grid those laps multiply, which is why takeoff sheets carry a separate lap line instead of hiding it in a rounding. The 10 percent allowance in the calculator exists to absorb exactly this, plus offcuts and the odd bent bar; jobs with long runs should carry more, and the lap math above shows why.

From stick price to cost per pound

Home centers sell rebar by the stick, while steel yards quote by the pound, and the only way to compare them is to convert. A $12.00 stick of 20 ft #5 weighs 20.86 lb, so it costs 12 ÷ 20.86 = $0.58 per pound. Run the same $12.00 across sizes and the per-pound number moves a lot:

That spread is arithmetic, not a price prediction. Stick prices do not stay proportional across sizes, and small bars carry a bigger share of handling cost per pound. The safe comparison is always the whole batch: multiply your total feet by the unit weight, apply whichever price structure the seller uses, and compare totals rather than unit prices. For the default batch that means 440 ft × 1.043 = 458.92 lb of steel, and a per-pound quote times 458.92 should land within rounding of a per-stick quote times 22 sticks.

Trucks, tons and delivery

Weight decides logistics. A typical half-ton pickup carries around 1,500 lb of payload, and the 22 stick #5 batch above comes to 458.92 lb, an easy single load. Scale it up: 100 sticks of 20 ft #5 weigh 100 × 20.86 = 2,086 lb, or 1.04 tons, so the same truck needs two trips or the yard delivers. Tons also show up on invoices, since steel is often billed per 2,000 lb ton, and a ton of #5 is 2,000 ÷ 1.043 = 1,917.5 ft, roughly 95 sticks of 20 ft.

Ask the yard for the bundle weight before you promise to haul anything. Mills bundle bars in counts that vary by size and producer, and a bundle that looks manageable can weigh more than the truck does. The stick weight in the table keeps the math honest: count sticks, multiply by the stick weight, and compare with the payload plate in the driver's door jamb. The calculator's truck line makes the same check with a 1,500 lb payload assumption, and it rounds up, because a partial second trip still is a second trip.

Mistakes that make the weight wrong

Five errors account for most bad rebar weights:

• Reading the size as the weight. A #5 bar is not 5 lb per foot, and it is not its 0.625 inch diameter either; it weighs 1.043 lb/ft. Confusing the diameter figure with the weight understates #5 by 40 percent.
• Mixing lb/ft with kg/m. #5 is 1.043 lb/ft and 1.552 kg/m. The metric number is about 1.49 times the imperial one, and feeding it into an imperial batch doubles the weight.
• Forgetting laps and waste. The bare 100 ft run weighs 104.3 lb, the real one with four laps weighs 113 lb, and big grids hide whole tons in that gap.
• Counting a two-way mat once. Bars every 12 inches each way means one set along the length and one across the width. The bar count of a grid is roughly twice what a one-direction count suggests.
• Assuming grade changes weight. Grade 40, 60 and 75 bars share one size table; the grade sets yield strength, not density or diameter.

A last one is quiet but common: estimating weight from the concrete pour instead of the bar count. Steel per cubic yard varies by a factor of five across ordinary slab designs, so back-solving from the concrete volume is a guess. Count bars, multiply lengths, apply the table. That chain has one assumption in it, and you can see all three numbers.