What this calculator does
A two-way grid needs bars in both directions, each cut to the length of the opposite dimension. A 6 by 4 metre slab at 300 mm spacing takes 14 bars running the length and 21 running the width, for 168 metres of bar.
Spacing drives the quantity hard because it affects both directions at once. Tightening from 400 to 200 mm nearly doubles the bar length, from 130 to 250 metres, and more than triples the tie points from 176 to 651. Tying the grid is usually the slow part of the job.
The formula
Bars in each direction are the perpendicular dimension divided by the spacing, rounded down, plus one, since a grid needs a bar at each edge. Each bar is cut to the length of the dimension it runs along. Total length is the sum, and the bars to buy divides by the standard stock length and rounds up.
| Term | Meaning |
|---|---|
| Two-way grid | Bars running in both directions, tied at every intersection. |
| Cover | Concrete between the bar and the surface, protecting steel from corrosion. Not calculated here. |
| Chairs or bar supports | Holding the grid at the right height in the pour, which is what cover depends on. |
| Lap length | Overlap where bars are joined end to end, typically 40 bar diameters. |
The inputs explained
| Field | What to enter |
|---|---|
| Slab length (m) | Slab length in metres. |
| Slab width (m) | Slab width. |
| Grid spacing (mm) | Grid spacing in millimetres. 200 to 400 mm covers most domestic slabs. |
| Standard bar length (m) | Standard stock bar length, for the bars-to-buy figure. |
When to use it
Ordering reinforcement
Bar is bought in standard lengths, and the count determines the order.
Estimating tying time
The tie point count is a direct proxy for labour, since each intersection is wired by hand.
Comparing mesh against loose bar
Prefabricated mesh avoids the tying entirely, and the tie count shows what that saves.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does spacing change the quantity?
The same slab at a range of grid spacings.
| Grid spacing | Total bar length needed | Standard-length bars to buy | Grid intersections (tie points) |
|---|---|---|---|
| 200 mm | 250.0 m | 42 | 651 |
| 300 mm | 168.0 m | 28 | 294 |
| 400 mm | 130.0 m | 22 | 176 |
Questions
Does this include lap lengths?
No. Where a bar is too short to span the slab it must overlap the next by typically 40 bar diameters, about 480 mm for a 12 mm bar. On a slab larger than the stock length, add for every lap, which can be a meaningful quantity.
What about concrete cover?
Not calculated here, but it is critical. Bars must sit within the slab with adequate concrete around them, typically 40 to 50 mm from the bottom for ground-bearing slabs. Bar chairs hold the grid at height; steel lying on the ground is doing nothing useful.
Should I use mesh instead?
Often, for a domestic slab. Prefabricated mesh arrives as a welded sheet and removes the tying work entirely, which the tie point figure shows is substantial. Loose bar gives more control over spacing and suits irregular shapes or heavier reinforcement.
What spacing should I use?
It depends on the loads and the slab thickness, and for anything structural it should be designed rather than guessed. Domestic slabs commonly use 200 to 300 mm. This calculator gives quantities for a spacing you have already chosen.
For the concrete itself, see the concrete calculator. For deck footings, see the deck footing calculator.