Rebar Calculator

Reinforcement is ordered by the bar and measured by the metre, and those two numbers never quite meet. A 6 m by 4 m slab at 400 mm centres carries 125.68 m of bar, which means 21 whole bars of 6 m — and the invoice follows the 21 bars, not the metres. This calculator works out all of it from the slab size and the grid: the dimensions the mesh follows, the running length of bar, how many stock bars to buy and what they cost.

Three jobs bring people to this page, and they all use the same five results:

• Quoting a simple rectangular slab. A contractor or concrete crew pricing the mesh for a slab on grade before the formwork goes down, with the supplier's stock length and price to hand.
• Checking a number someone else sent. A builder, buyer or site engineer confirming that a supplier's bar count and total agree with the spacing marked on the drawing.
• Ordering for one pour. A patio, shed base, garage apron or small extension where the bars come from a merchant that sells whole lengths and charges by the bar.

One thing the calculator deliberately does not do is design the reinforcement. The spacing, the edge distance and the bar size are decisions that belong to the drawing or to whoever signs off the slab. Given those numbers, it counts, rounds and prices them exactly — and shows its working, so a surprising figure can be checked by hand.

• The grid figures are inside the slab. They are the slab dimensions with one edge spacing removed at each end, so they are always smaller than the slab. Bars follow the grid, not the concrete edge.
• The total length is the mesh, not a cut list. It is the sum of every bar in the mesh before laps, anchorage and waste are added.
• The bar count is always a whole number. A fractional bar is not a thing a merchant can sell, so the count rounds up and the cost follows the rounded count.

Step one: pull the grid in from the edges

The first bar cannot sit on the corner of the slab, because it needs concrete cover on both sides. One edge spacing is taken off each end of each direction, which is why the slab length $L$ and width $W$ shrink to a grid of $g_{\mathrm{L}}$ and $g_{\mathrm{W}}$:

The edge spacing is measured to the centre of the first bar, so it already includes the cover plus half the bar diameter. That is the number to take from the drawing — not the clear cover on its own.

The same relation read the other way is what lets you type a grid dimension:

Type a grid length you already know — a clear span between walls, say — and the slab length it implies is solved from it. The edge spacing stays exactly where you put it, because it is shared by both directions: if it moved, a grid typed on one axis would quietly rewrite the other.

Step two: count the bars in each direction

Every line of bars gets one bar at each end of its grid span, and the gaps between them must be no wider than the spacing you entered. The count is therefore the number of bays, rounded up, plus the closing bar:

This is the detail most square-metre estimates miss. The spacing behaves like a ceiling rather than a fixed pitch, so a 3.84 m span at 400 mm needs 11 bars, not 10: ten bays of 400 mm would need 4.0 m, and the ten bays that do fit work out at 384 mm centres. Sixteen bars over the 5.84 m direction land at 389 mm centres — just inside the spacing you asked for. The calculator counts bars; it does not dictate where the setter snaps the final line.

Step three: add up the running length

Bars running along the length of the slab are counted across the width, and bars running across the width are counted along the length. Each one is as long as the grid it follows:

Step four: buy whole bars, then price them

The total length is divided by the stock length and rounded up, because the supplier sells complete bars. The price of one bar is the stock length times the price per unit length, and the order cost is whole bars times that price:

Notice what this does not say: the cost is not $T \times p$. The difference is the off-cuts, and on a small slab it is not small — 125.68 m of bar at 2.00 per metre is 251.36, while the 21 whole bars cost 252.00. On a large pour the gap shrinks; on a tight one it can be most of a bar.

The supplier boxes solve both ways

Stock length, price per unit length and price per bar are a single relationship. Type any two and the third is worked out. Put a supplier's per-bar quote into the per-bar box — 15.00 for a 6 m bar, say — and the per-metre price drops to 2.50, while the order cost is recalculated from the quote. The three totals below stay outputs: they are read, not typed into.

Units and currencies

Slab dimensions, both spacings, the stock length and every length result keep independent unit menus, so a feet-and-inches drawing with a metric bar order is fine — every value is converted internally and the answers stay consistent. The three money boxes each carry their own currency menu from twenty options, and each one converts on its own. To compare a quote with your budget directly, put both in the same currency; the exchange rates behind the menus are refreshed daily, so a saved calculation can shift slightly when the rates move.

Where the model stops

• Twice the edge spacing has to fit inside both slab dimensions. If it does not, the grid dimension turns negative and is flagged in red, and the edge spacing is called out alongside it.
• A total running length past 100,000 m is flagged as a reality check. That figure is reached by typing kilometres where metres were meant, or a spacing in the wrong unit.
• The geometry is one uniform rectangular grid: no openings, thickened edges, beams or second layer, and no allowance for lapped bars.

It is also worth checking what the number does not include before it becomes an order: bar chairs and spacers, tie wire, tying labour, cutting and bending, and any additional layer or trimmer bars around openings. None of those are in the results.