Rebar Weight Calculator: Convert Bar Lengths to Tonnes

To work out rebar weight, multiply the total length of each bar size by its mass per metre, then add the sizes together: 10M weighs 0.785 kg/m, 15M 1.570 kg/m, 20M 2.355 kg/m and 25M 3.925 kg/m. Divide the total by 1,000 and you have tonnes. The tables below do most of the arithmetic for you, and the worked example shows how a real bar list rolls up into a tonnage.

This post is about converting lengths you already have into weight. If you still need to count the bars from a drawing, start with our guide on estimating rebar quantities by hand, then come back here to convert.

The numbers behind every rebar weight calculator

Every online calculator, spreadsheet and estimating program uses the same input: the nominal mass per metre for each bar size in CSA G30.18, the Canadian standard for carbon-steel reinforcing bar. These values come from the bar's nominal cross-sectional area and the density of steel, which our rebar weight formula post explains step by step.

The kg/m values are the CSA G30.18 nominal masses. The per-foot figures follow the Reinforcing Steel Institute of Canada (RSIC) Manual of Standard Practice. "Metres per tonne" is simply 1,000 kg divided by the mass per metre, rounded.

A quick sanity check worth remembering: one 15M bar weighs the same as two 10M bars of the same length, and one 35M bar weighs exactly 7.850 kg per metre, which is the density of steel showing through (1,000 mm² of steel in a metre-long bar).

Weight per bar at common lengths

Most questions on site are about single bars: what a bundle weighs, or what a crew can carry. This table gives the weight of one straight bar at three common lengths.

The RSIC manual lists standard mill lengths of 12 m for 10M and 18 m for 15M and larger, which is why those columns are there. A cut-to-length order will rarely use full mill lengths, though, and the bar list sets the real lengths.

Bars per tonne

Going the other way, here is roughly how many bars make up one tonne:

This is handy for picturing a delivery or checking whether a quantity on a quote looks sensible. It is not a way to order: a fabricated order is cut and bent to the bar list, so the pieces are rarely uniform lengths.

Converting a bar list to tonnes: worked example

Say a small foundation package has four marks on its bar list. The lengths are the developed (cut) lengths, so any hooks and bends are already included.

Step 1: total metres by size.

• 10M: 128.0 + 88.8 = 216.8 m
• 15M: 139.2 m
• 20M: 144.0 m

Step 2: multiply by mass per metre.

• 10M: 216.8 × 0.785 = 170.2 kg
• 15M: 139.2 × 1.570 = 218.5 kg
• 20M: 144.0 × 2.355 = 339.1 kg

Step 3: add them up. 170.2 + 218.5 + 339.1 = 727.8 kg, or about 0.73 tonnes.

Keep the weight split by size. Suppliers and fabricators think in weight per size because bar size affects handling and fabrication effort, and a single blended number hides that.

Working in feet

If your lengths are in feet, either convert to metres first (feet × 0.3048) or use the per-foot column. For 300 ft of 15M:

• 300 × 1.055 = 316.5 lb, or
• 300 × 0.3048 = 91.44 m, then 91.44 × 1.570 = 143.6 kg

Both routes give the same answer (316.5 lb is 143.6 kg). Watch for drawings that mix US bar sizes with metric lengths. A US #5 bar is not a 15M bar, even though they are close, and our Canadian vs US rebar sizes chart has the US weights if you need them.

Rebar mat weight per square metre

For slabs and mats, weight per square metre of area is a faster way to estimate than counting bars. For a single layer of bars in one direction:

kg/m² = mass per metre ÷ spacing in metres

For a mat with the same bar and spacing each way, double it. Here is what that gives for common layouts:

Two mats (top and bottom) double the figure again. To use it, multiply by the slab area: a 60 m² slab with 10M @ 300 each way is about 60 × 5.2 = 312 kg of bar in the field of the slab.

That number is the floor, not the total. A mat calculator does not see:

• Laps where bars are longer than a stock length or the pour is split
• Edge, corner and trim bars around openings and re-entrant corners
• Thickened edges and turn-downs with their own bars
• Dowels into walls, columns or the next pour
• Bar supports, which RSIC estimating practice treats as a separate item anyway

On a typical job those items add a real amount. Which is why the arithmetic is a good check on a quote but a poor basis for ordering. For how slab layouts are chosen in the first place, see rebar spacing for concrete slabs.

Theoretical weight vs what the truck scale says

Canadian rebar is bought and sold on theoretical weight. The RSIC manual describes rebar as sold by the fabricator on the basis of theoretical mass, calculated from the approved bar list, rather than by weighing material. Three practical consequences:

• Bent bars are weighed along their full developed length. A 15M bar with two 90° hooks is billed for the straight run plus both hook allowances, because that is the steel in the piece.
• Actual bars vary slightly. Rolling tolerances mean a real bar can weigh a little more or less than its nominal mass, and CSA G30.18 sets the permitted variation. A truck scale reading will rarely match the paperwork to the kilogram, and it is not meant to.
• Coatings change the picture. Epoxy-coated and galvanized bar are separate items on a quote. GFRP bar is far lighter than steel and its weight comes from the manufacturer's data, not from steel tables. See our GFRP rebar sizes and weights chart.

If you are comparing quotes, make sure each one is pricing the same tonnage from the same drawings. A quote that comes in light on weight has usually missed laps, dowels or bent bars, not found a cheaper way to build. Our article on how rebar is priced explains what else goes into the number.

Get an exact tonnage from your drawings

A weight calculator is only as good as the lengths you feed it. The most accurate tonnage comes from a bar list where every piece is detailed, counted and measured along its developed length, which is exactly what a rebar detailer produces.

We prepare free take-offs and quotes from your structural drawings, detail the bar list, and cut, bend, tag and bundle the bar for delivery across the GTA and Ontario. Send your drawings for a take-off, or see our rebar products for straight bar, fabricated dowels and stirrups, and GFRP.