A reinforced slab needs two orders: the concrete and the steel grid inside it. A 6 × 5 m slab, 15 cm thick, with two meshes of Ø10 bars at 15 cm, takes 4.73 m³ of concrete (with 5% extra) and about 900 m of bar, 555 kg. Type the slab size, choose the bar size (metric Ø6–Ø16 or US #3–#8), the spacing and the number of layers, and the calculator counts the bars each way, their total length and weight, the stock lengths to buy and the crossings to tie.
Bar weight and a 6 × 5 m slab
Two meshes at 15 cm spacing, 10% extra for laps:
| Bar | Mass per meter | Steel for the slab |
|---|---|---|
| Ø8 mm | 0.395 kg/m | 355 kg |
| Ø10 mm | 0.617 kg/m | 555 kg |
| Ø12 mm | 0.888 kg/m | 799 kg |
| Ø14 mm | 1.208 kg/m | 1,087 kg |
| Ø16 mm | 1.578 kg/m | 1,420 kg |
The mass grows with the square of the diameter: Ø16 weighs four times as much per meter as Ø8. Metric masses are steel at 7,850 kg/m³ times the bar’s cross-section, the nominal values used in European standards.
US bars: a 20 × 16 ft slab, one mesh at 12 in
| Bar | Size | Weight | Total length (+10%) | Slab weight |
|---|---|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft | 744 ft | 280 lb |
| #4 | 1/2 in | 0.668 lb/ft | 744 ft | 497 lb |
| #5 | 5/8 in | 1.043 lb/ft | 744 ft | 776 lb |
A US bar number is its diameter in eighths of an inch: #4 is 4/8 = 1/2 in. Weights per foot are from the ASTM A615 table.
How the bars are counted
The bars that run along the length are spaced across the width: 5 m ÷ 0.15 m = 33 gaps, so 34 bars. The other way, 6 m ÷ 0.15 m = 40 gaps, so 41 bars. Each mesh is 34 × 6 m + 41 × 5 m = 409 m. The calculator counts bars wall to wall, without taking off the concrete cover, so the lengths carry a small reserve; laps where bars join are the extra percentage.
The design – bar size, spacing, layers, cover and laps – belongs to an engineer or your local code. Use the results to order and budget. The calculator works offline on site.
Frequently asked questions
How do I calculate the rebar for a slab?
Count the bars each way: the width divided by the spacing, plus one, gives the bars that run the length, and the other way round. Multiply by their lengths, by the number of layers, add the laps and multiply by the mass per meter.
How much does rebar weigh?
A metric bar weighs about d² ÷ 162 kg per meter (d in mm): Ø8 0.395, Ø10 0.617, Ø12 0.888 kg/m. US bars, by ASTM A615: #3 0.376, #4 0.668, #5 1.043 lb/ft.
How much concrete for a 6 × 5 m slab, 15 cm thick?
6 × 5 × 0.15 = 4.5 m³; with 5% extra, 4.73 m³, or 6.18 cubic yards.
What spacing should rebar have?
The structural design or the local code sets it, along with the bar size, the cover and the laps. For planning, enter the spacing and size from your drawing to get the order quantities.
How many tie points will there be?
Every crossing of two bars: bars one way × bars the other way, per mesh. The 6 × 5 m slab at 15 cm has 34 × 41 = 1,394 crossings in each mesh.
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