To estimate rebar, count the bars, work out the length of each one, add laps and hooks, then multiply the total metres of each size by its weight per metre (0.785 kg/m for 10M, 1.570 kg/m for 15M). That is all a rebar calculator does. Doing it by hand takes a few minutes per element and shows you exactly where the numbers come from, which makes it easier to check a quote or a weight schedule.
This guide covers the method and three worked examples. It deals with quantity, not price: the bar size, spacing, cover and lap lengths always come from your drawings or your designer, and the examples below use values picked for illustration only.
What you need before you start
Pull these from the structural drawings (or for a house, the approved drawings) for each element:
- Concrete dimensions: length, width, depth
- Bar size and spacing in each direction, and the number of layers
- Concrete cover, or at least the edge distance to the bars
- Lap length for each bar size, and any hooks, dowels or corner bars shown on typical details
- Bar weights from the Canadian rebar size chart:
| Bar size | Weight (kg/m) | Standard mill length |
|---|---|---|
| 10M | 0.785 | 12 m |
| 15M | 1.570 | 18 m |
| 20M | 2.355 | 18 m |
| 25M | 3.925 | 18 m |
Standard mill lengths in Canada are 12 m for 10M and 18 m for 15M and larger. Those lengths decide where laps are needed and how much offcut waste you get if you cut bars yourself.
The method in five steps
- Bar length. Concrete dimension minus the cover at each end. Add hooks if the drawings show them.
- Bar count. Divide the width the bars are spread across (also minus cover) by the spacing, round up to a whole number of spaces, then add one bar.
- Laps. If a bar is longer than the stock length, or the run is continuous, add one lap length for every splice.
- Total metres. Count times length for each mark, summed by bar size.
- Weight. Metres times kg/m for each size. Divide by 1,000 for tonnes.
For hooks, the Reinforcing Steel Institute of Canada (RSIC) estimating practice is to add the hook allowance from its hook tables at each hooked end. For a 15M Grade 400W bar, a standard 90° hook adds about 250 mm. If the hook type is not specified, a 90° hook is assumed.
Example 1: slab reinforced each way
A 6.0 m by 9.0 m slab (about 20 by 30 feet) with 10M bars at 400 mm each way, one layer. For the example we use a 75 mm edge allowance at every edge. Your drawings set the real cover.
Bars running the 9 m direction
- Length: 9.00 m minus 2 × 0.075 m = 8.85 m
- Spread across: 6.00 m minus 0.15 m = 5.85 m
- Spaces: 5.85 ÷ 0.40 = 14.6, round up to 15, so 16 bars
- Total: 16 × 8.85 = 141.6 m
Bars running the 6 m direction
- Length: 6.00 m minus 0.15 m = 5.85 m
- Spread across: 8.85 m
- Spaces: 8.85 ÷ 0.40 = 22.1, round up to 23, so 24 bars
- Total: 24 × 5.85 = 140.4 m
Weight: 282.0 m × 0.785 kg/m = about 221 kg of 10M.
Both bar lengths fit inside a 12 m stock bar, so no laps are needed. Now look at waste. If you bought 12 m bars and cut them on site, each 8.85 m bar uses a full stock bar and leaves a 3.15 m offcut, while two 5.85 m bars fit in one stock bar. That is 28 stock bars, or 336 m and about 264 kg, to place 221 kg. Ordering the bars cut to length avoids carrying that offcut.
For how spacing is chosen in the first place, see our guide to rebar spacing in concrete slabs.
Example 2: continuous footing bars with laps
A strip footing under a 10 m by 12 m building, with two continuous 15M bars along the footing. The centreline perimeter is 2 × (10 + 12) = 44 m. For this example, assume the drawings call for 600 mm laps. Lap length is set by the engineer and depends on bar size, concrete strength and position, so use the value on your drawings.
- Each 18 m stock bar adds 18.0 minus 0.6 = 17.4 m of net length once lapped.
- Pieces per run: 44 ÷ 17.4 = 2.5, round up to 3 pieces, so 3 laps in a closed loop.
- Length per run: 44 + (3 × 0.6) = 45.8 m
- Two bars: 2 × 45.8 = 91.6 m
Weight: 91.6 m × 1.570 kg/m = about 144 kg of 15M.
If the details show corner bars, step bars or dowels into the wall above, count those as separate items. Dowels alone can add a surprising amount on a foundation. In practice the fabricator may also cut shorter pieces for handling, which adds laps, so a calculator that ignores laps will always come in light.
Example 3: stirrups in a beam
A 300 mm wide by 500 mm deep beam, 6.0 m clear span, with closed 10M stirrups at 200 mm and standard 135° hooks.
- Stirrup size. RSIC detailing practice, when the drawings do not say otherwise, makes stirrups 80 mm smaller than the beam in each direction: 220 mm by 420 mm out-to-out.
- Length per stirrup. Perimeter 2 × (220 + 420) = 1,280 mm. The RSIC stirrup hook table gives about 100 mm per standard 135° hook on 10M, so add 2 × 100 mm. Total about 1.48 m. Adding out-to-out legs slightly overstates the true bent length, which keeps the estimate on the safe side.
- Count. Assume the first and last stirrups sit 50 mm from the supports: (6,000 minus 100) ÷ 200 = 29.5, round up to 30 spaces, so 31 stirrups.
- Total: 31 × 1.48 = 45.9 m × 0.785 kg/m = about 36 kg of 10M.
Our guide to ordering fabricated stirrups covers the shapes and hook options in more detail.
Why calculators come in light
Online rebar calculators treat every element as a plain rectangle. Real drawings rarely are. A proper take-off also picks up:
- Openings and steps, with extra bars around openings and bent bars at steps
- Corner and intersection bars at wall and footing corners
- Dowels between footings and walls, and walls and slabs
- Laps wherever runs exceed stock length or pours are staged
- Hooks called for in typical details rather than on the plan
- Top and bottom layers where the drawings call for two mats
- Bar supports, which RSIC practice estimates as a separate item
Use a calculator, or the method above, for a budget or to sanity-check a number. Order from a take-off of the actual drawings. A detailed bar list from a detailer is the most accurate number you can get, because every bar is drawn, counted and measured along its full developed length.
From quantity to a quote
Rebar is sold on its theoretical weight, which is why the weight per metre matters. But weight is only part of the picture. A fabricator's quote also reflects the mix of bar sizes, how much of the steel is bent and how complex the shapes are, coatings or specialty bar, the number of deliveries and whether placing is included. Our article on rebar price in Canada explains those cost drivers.
If you would rather not do the take-off yourself, we prepare detailed take-offs and quotes from your drawings free of charge.
Send your drawings for a take-off, or see our services for estimating, detailing and cut and bend.
Frequently asked questions
How do I calculate how much rebar I need?
For each direction, divide the distance the bars are spread over by the spacing, round up and add one to get the bar count. Multiply by the bar length (dimension minus cover, plus hooks and laps), then multiply the total metres by the bar's weight per metre.
How do you calculate rebar length?
Take the concrete dimension the bar runs along, subtract the cover at each end, then add any hooks shown on the drawings and the lap length wherever bars have to be spliced because the run is longer than a stock bar.
How much does 10M and 15M rebar weigh per metre?
10M weighs 0.785 kg per metre and 15M weighs 1.570 kg per metre. Multiply total metres of each size by these figures to get the weight.
How accurate is an online rebar calculator?
Good enough for a first budget on a simple rectangle. Calculators miss openings, steps, corner bars, dowels, extra bars and stock-length waste, so they should not be used to order material. A take-off from the drawings is the number to order from.
How much rebar do I need for a 20 by 30 foot slab?
It depends on the bar size, spacing and cover on your drawings. As one example, a 6 m by 9 m slab (about 20 by 30 feet) with 10M bars at 400 mm each way and a 75 mm edge allowance works out to 40 bars, about 282 m and roughly 221 kg.
