About the Rebar Calculator
Reinforcing bar — rebar — gives concrete the tensile strength it lacks on its own. Concrete is excellent in compression and poor in tension, so slabs, footings and walls that bend or crack under load are reinforced with a grid of steel bars cast inside them.
Working out how much rebar a slab needs is a counting problem with a few traps. Bars are counted by the gaps between them plus one. They stop short of the edges by the concrete cover. Runs longer than a stock bar need laps where two bars overlap, and those laps add steel. And rebar is sold and priced by weight, which has to be calculated from the bar size.
This rebar calculator handles all four. It lays out a grid over a rectangular slab or footing, counts the bars each way, adds laps where needed, and gives the total length, the number of stock bars and the weight — in US bar sizes or metric.
How to Use the Rebar Calculator
Choose feet and inches or metres and millimetres.
Enter the slab length and width and the bar spacing, measured centre to centre.
Pick the bar size. US bars are numbered in eighths of an inch, so #4 is half an inch; metric bars are named by diameter in millimetres.
Adjust the edge cover (how far bars stop from the edges), the stock bar length you can buy, and the lap length in bar diameters if your design specifies them.
How the Rebar Is Worked Out
inside dimensions = slab size − 2 × cover
bars each way = (inside span ÷ spacing, rounded up) + 1
lap length = lap diameters × bar diameter
run length = bar length + laps × lap length
weight (kg/m) = π/4 × d² × 7,850 ÷ 10⁶ ≈ d² ÷ 162Step-by-Step Example
A 10 × 4 m slab, 12 mm bars at 200 mm, 75 mm cover, 6 m stock, 40d laps.
Inside: 9.85 × 3.85 m
Bars lengthwise: 3.85 ÷ 0.2 = 19.25 → 20 spaces + 1 = 21 bars
Bars across: 9.85 ÷ 0.2 = 49.25 → 50 spaces + 1 = 51 bars
Lap: 40 × 12 mm = 480 mm
Lengthwise runs: 9.85 m + one 0.48 m lap = 10.33 m each
Total: 21 × 10.33 + 51 × 3.85 = 413 m
Weight: 413 × 0.888 kg/m = 367 kgThat is about 69 six-metre bars, before allowing for offcuts that cannot be reused.
Spaces Plus One
The single most common rebar mistake is counting the spaces instead of the bars. A run of 3 metres at 300 mm spacing has ten spaces, but eleven bars, because there is a bar at each end — exactly like fence posts. Dividing the span by the spacing counts the spaces. The calculator always rounds the spaces up (a partial space still needs a bar at its end) and then adds one.
Where the Weight Comes From
Steel has a density of about 7,850 kg per cubic metre. A metre of bar is a cylinder, so its weight is its cross-sectional area times that density:
π/4 × (d / 1000)² × 7,850 = d² ÷ 162 kg per metre (d in mm)That is the figure steel fixers quote from memory: 12 mm bar is 144 ÷ 162 = 0.888 kg/m, 16 mm is 1.58 kg/m. The same formula gives the published US weights: #4 bar, 12.7 mm, is 0.668 lb per foot.
Laps and Stock Lengths
Rebar is sold in fixed lengths — 20 feet in the US, 6 or 12 metres in metric markets. Where a run is longer than that, two bars are overlapped and tied, and the overlap is the lap. A common rule of thumb is 40 bar diameters, but the real figure depends on bar size, concrete strength, and where the lap falls, so it should come from the design.
Laps should also be staggered — not all in the same line across the slab — so there is no single weak band.
Cover and Chairs
Cover is the concrete between the steel and the surface. It protects the bars from moisture, which would corrode them and crack the concrete, and from fire. Around 3 inches (75 mm) is typical where concrete is cast against the ground.
Bars must also sit at the right height, not lie on the ground. Chairs or spacers hold the grid up, usually in the lower part of a ground-bearing slab where tension is greatest.
Buying and Cutting Rebar
Rebar is usually bought in stock lengths and cut to size on site, so a little planning cuts waste.
Plan cuts around the stock length. If the bars running across a 3.85 m slab come from 6 m stock, each stock bar yields one full bar and a 2.15 m offcut. Two offcuts do not make a bar, so that offcut is waste unless it can be used as a short bar elsewhere. Choosing 12 m stock, where available, can fit three bars per length instead.
Order a little extra. Allow for offcuts, a bar or two bent in handling, and tie wire. Around 5 to 10 per cent on top of the calculated length is usual.
Ask about cut-and-bend. Many suppliers will cut and bend bars to a schedule, which saves site time on larger jobs and reduces waste.
Understanding Your Result
Rebar needed gives the total number of bars and the total length.
The grid shows the bars in each direction, their lengths, and any laps.
Stock bars estimates how many stock lengths to buy.
Weight gives the steel weight, which is how rebar is usually priced.
Worth knowing covers laps, chairs and who sets the design.
When Should You Use This Calculator?
Estimating a slab, footing or driveway that needs reinforcement.
Comparing bar sizes and spacings for cost.
Checking a supplier's quote against the weight of steel.
Not to decide the design. Bar size, spacing, cover and laps are set by the engineer or local code.
Common Mistakes
Counting spaces, not bars. Add one.
Running bars to the edge. Leave the cover.
Forgetting laps. Long runs need more steel than their length.
Lining laps up. Stagger them.
Laying bars on the ground. Chairs keep them where they do their job.