How Much Rebar Do I Need for a Slab, Footing and Beam – Bars, Mesh and kg
Short answer: a 6 × 4 m slab reinforced with Ø10 bars at 15 cm spacing in one layer needs 332 m of rebar, and with 10 % for waste and overlaps 365.2 m, which is 225.3 kg of steel and 61 bars of 6 m. A 5 m beam with a 25 × 40 cm cross-section needs about 40.7 kg. The rebar calculator works this out for a slab, footing slab, beam, column and footing strip – it counts bars, stirrups and ties and gives the weight and the number of 6 m bars or mesh sheets.
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When you need this calculation
You need this calculation before you place an order, to know how much steel your slab, footing or beam needs and in what form: how many 6 m bars, how many mesh sheets and how many kilograms in total. Rebar is priced per kilogram but delivered to site as bars or mesh, so you need both – the weight for the price and the pieces for ordering and transport.
For each element the calculator works out the number of bars from the spacing, the length of the longitudinal bars and the number and length of stirrups or ties, adds a percentage for waste and overlaps and converts the length into weight according to the diameter. It shows the result per element and in total.
Who the calculator is for
- Homeowners and DIYers reinforcing a garage slab, patio or footing strip themselves who want to know how many bars or mesh sheets to buy.
- Contractors who need a quick steel quantity for a quote, without counting bars on the drawing by hand.
- Checking the rebar schedule from a structural design or a supplier's quote.
- Cost planning: with the price per kilogram you immediately see a rough cost of the rebar.
How to use the rebar calculator – step by step
- 1. Add an element: slab, beam, column, footing slab or footing strip. One calculation holds up to 30 elements.
- 2. For a slab or footing slab enter the length and width and choose Use Mesh or Use Bars.
- 3. For mesh choose the mesh type, the overlap (default 40 cm) and the number of layers; for bars choose the bar spacing (5–30 cm), the diameter and the number of layers.
- 4. For a beam or footing strip enter the length, the cross-section in centimetres, the top, middle and bottom bars and the stirrup spacing and diameter. For a column the same, with ties.
- 5. Check Waste/Overlap (default 10 %, maximum 30 %) and click Calculate.
- 6. Save the calculation to a project or export a PDF for the supplier.
Formula: how rebar is calculated
For a slab with bars: number of bars in one direction = ⌈width ÷ spacing⌉ + 1, length = number of bars × slab length, and the same for the other direction. The total is multiplied by the number of layers and by (1 + waste). Weight = length × weight per metre, and number of 6 m bars = ⌈length ÷ 6⌉.
For mesh: effective sheet area = (sheet width − overlap) × (sheet length − overlap), number of sheets = ⌈area ÷ effective area × (1 + waste)⌉ × number of layers.
For a beam, footing strip and column: number of stirrups or ties = length in cm ÷ spacing, rounded down, and the length of one stirrup = 2 × (cross-section width + height) + 0.2 m for hooks.
Calculation examples
Slab 6 × 4 m, Ø10 at 15 cm, 1 layer, 10 % waste: ⌈4 ÷ 0.15⌉ + 1 = 28 bars of 6 m = 168 m and ⌈6 ÷ 0.15⌉ + 1 = 41 bars of 4 m = 164 m. Total 332 m × 1.10 = 365.2 m, weight 365.2 × 0.617 = 225.3 kg, 6 m bars: 365.2 ÷ 6 = 60.9 → 61.
The same slab with R 166 mesh (sheet 6.00 × 2.20 m, 24.5 kg), overlap 40 cm: effective area 1.80 × 5.60 = 10.08 m². 24 ÷ 10.08 × 1.10 = 2.62 → 3 sheets, 73.5 kg.
Footing strip 30 m, cross-section 40 × 50 cm, 2 top and 3 bottom bars Ø12, stirrups Ø8 at 25 cm, 10 % waste: longitudinal bars 150 m and 146.5 kg, 120 stirrups of 2.0 m = 240 m and 104.3 kg. Total 250.8 kg and 28 bars of 6 m for the longitudinal reinforcement.
Rebar weight by diameter
The calculator uses standard weights per metre for diameters from Ø6 to Ø40, and offers Ø6, Ø8 and Ø10 for stirrups and ties.
| Diameter | Weight per m | Weight of a 6 m bar | Default in the calculator for |
|---|---|---|---|
| Ø6 | 0.222 kg | 1.33 kg | – |
| Ø8 | 0.395 kg | 2.37 kg | stirrups and ties |
| Ø10 | 0.617 kg | 3.70 kg | slab with bars |
| Ø12 | 0.888 kg | 5.33 kg | footing slab, beam, footing strip |
| Ø14 | 1.208 kg | 7.25 kg | column |
| Ø16 | 1.578 kg | 9.47 kg | – |
Mesh or individual bars?
Mesh is faster to lay and needs less tying. The calculator includes R meshes (rectangular spacing) and Q meshes (square spacing), mostly in 6.00 × 2.20 m sheets, and for each type shows the wire diameter, spacing and weight per m² – e.g. R 166 weighs 24.5 kg per sheet and Q 188 40.4 kg. The overlap reduces the effective area of a sheet, so you need more mesh than the bare area suggests.
Bars make sense when the design calls for a diameter or spacing that meshes do not offer. For a footing slab the default is 2 layers, so the quantity doubles. The type and layout of reinforcement for load-bearing elements are determined by the structural design.
The most common mistakes when calculating rebar
- Forgetting the edge bar – the number of bars is ⌈dimension ÷ spacing⌉ + 1.
- Bar laps and anchorage are not calculated separately but through the Waste/Overlap percentage.
- The wrong number of layers: a second layer doubles the weight and the number of mesh sheets.
- For beams, columns and strips the number of 6 m bars refers to the longitudinal reinforcement only; stirrups and ties are shown as weight.
- Cross-section entered in metres – the element length is entered in metres, but the cross-section in centimetres.
- Treating the quantity as a structural calculation – the calculator only counts rebar according to the layout you enter.
What else to calculate along with rebar
Rebar always needs concrete too: the concrete calculator works out the cubic metres for a slab, footing strip, column or beam. For digging foundations use the excavation and fill calculator, and stair reinforcement is handled by the Staircase Calculator. All calculations can be saved to the same project and exported as a single PDF materials list.
Frequently asked questions
How much rebar do I need for a 6 × 4 m slab?
With Ø10 at 15 cm in one layer and 10 % waste: 365.2 m, which is 225.3 kg and 61 bars of 6 m. With R 166 mesh and a 40 cm overlap you need 3 sheets, 73.5 kg.
How much rebar do I need for a 5 m beam?
With the default 25 × 40 cm cross-section, Ø12 bars (2 + 3), Ø8 stirrups at 20 cm and 10 % waste, about 40.7 kg, of which 16.3 kg is for 25 stirrups.
How many stirrups do I need for a footing strip?
The length in centimetres divided by the stirrup spacing, rounded down. For a 30 m strip and 25 cm spacing that is 120 stirrups.
How many mesh sheets do I need for an 8 × 6 m footing slab?
With R 166 mesh, a 40 cm overlap and 10 % waste, one layer needs 6 sheets; with the default 2 layers that is 12 sheets, 294 kg.
Is the rebar calculation free?
Yes. The calculator is free on the web and in the Android app, where calculations are saved to projects and exported to PDF.
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