Rebar
Quick answer: A 4 × 5 m slab with Ø8 bars at 20 cm both ways needs about 219 m of rebar (87 kg), i.e. 37 bars of 6 m.
Calculate reinforcement for concrete elements
How the calculation works
Add an element (slab, foundation slab, beam, column), enter dimensions, bar spacing and diameter, and a waste percentage. The calculator works out length, mass and the number of 6 m bars, and for meshes the number of sheets.
Formula and assumptions
For a slab with bars in two directions: bars = ⌈dimension ÷ spacing⌉ + 1, length = bars × span. Total × (1 + waste). Mass = length × kg/m of bar (e.g. Ø8 = 0.395 kg/m). 6 m bars = ⌈length ÷ 6⌉. For meshes: sheets = ⌈area ÷ effective sheet area × (1 + waste)⌉ × layers. The kg/m values come from the table in the app.
Worked example
Slab 4 × 5 m, Ø8 at 20 cm both ways, 1 layer, 5 % waste: along the 5 m direction ⌈4 ÷ 0.2⌉ + 1 = 21 bars = 105 m; along the 4 m direction ⌈5 ÷ 0.2⌉ + 1 = 26 bars = 104 m. Total 209 m × 1.05 = 219.45 m. Mass 219.45 × 0.395 = 86.7 kg. 6 m bars: 219.45 ÷ 6 = 36.6 → 37.
More worked examples
- Slab 3 × 3 m, Ø8 at 15 cm in both directions, 1 layer, waste 5 %: ⌈3 ÷ 0.15⌉ + 1 = 21 bars per direction = 63 m, both directions 126 m. With waste 132.3 m. Mass 132.3 × 0.395 = 52.3 kg. 6 m bars: 132.3 ÷ 6 = 22.05 → 23.
- Slab 6 × 4 m, Ø10 at 20 cm in both directions, 1 layer, waste 5 %: along 6 m ⌈4 ÷ 0.2⌉ + 1 = 21 bars = 126 m; along 4 m ⌈6 ÷ 0.2⌉ + 1 = 31 bars = 124 m. Total 250 m × 1.05 = 262.5 m. Mass 262.5 × 0.617 = 162 kg. 6 m bars: 262.5 ÷ 6 = 43.75 → 44.
Mass of ribbed bars (standard values)
| Diameter | kg per m | kg per 6 m bar |
|---|---|---|
| Ø6 | 0.222 | 1.33 |
| Ø8 | 0.395 | 2.37 |
| Ø10 | 0.617 | 3.7 |
| Ø12 | 0.888 | 5.33 |
| Ø14 | 1.208 | 7.25 |
What affects real consumption
- Laps, hooks and anchorage are not included unless you add them through the waste percentage.
- Concrete cover and spacing according to the design.
- Actual mass differs between ribbed and plain bar.
Common mistakes
- Forgetting the extra bar at the edge (⌈dimension ÷ spacing⌉ + 1).
- Not allowing for laps and waste; laps are not counted separately.
- Treating the quantity as a structural design; the layout is set by the structural engineer.
Frequently asked questions
Does the calculation include bar laps?
Laps are not counted separately; account for them through the waste percentage or according to the reinforcement design.
Is the result a structural calculation?
No – the calculator only quantifies the layout you enter; the amount and layout of reinforcement is set by the structural design.
How many bars cover 4 m at 20 cm spacing?
⌈4 ÷ 0.2⌉ + 1 = 21 bars.
How heavy is a Ø8 bar?
0.395 kg per metre, so about 2.37 kg for a 6 m bar.
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Publisher: Art-Visum d.o.o. · Content reviewed:
Calculations use common consumption rates and manufacturers' technical data sheets; results are estimates. Methodology · Standards · About