How the slab rebar count is calculated
The calculator models a rectangular, orthogonal grid of straight bars. Clear cover is measured from the concrete edge to the nearest bar surface. Half the selected nominal bar diameter is added to obtain the first and last centreline positions. The remaining centreline span is divided by the entered maximum spacing.
bars across a centreline span = ceiling(span ÷ maximum spacing) + 1
actual spacing = centreline span ÷ (bar count − 1)
Rounding the number of intervals upward makes the calculated actual spacing no greater than the entered maximum. Bars running along the slab length are counted across the width; bars running along the width are counted across the length. The straight length of each bar is the relevant slab dimension minus clear cover at both ends.
Worked 6 m × 4 m metric example
For a 6 m by 4 m slab with 12 mm bars, 200 mm maximum centres, 50 mm clear cover, one orthogonal layer and 5% allowance, the calculator places 21 bars running the slab length and 31 bars running the slab width. Their individual straight lengths are 5.9 m and 3.9 m. Scheduled straight length is 244.8 m; after the separate 5% allowance it is 257.04 m. At the nominal 0.888 kg/m mass, the estimate is 228.3 kg. Actual calculated centres are 194.4 mm across the width and 196.27 mm across the length.
That example intentionally excludes laps, anchorage, bends, supports and cutting optimisation. Those items depend on the approved detailing and cannot be inferred from plan dimensions alone.
Metric nominal bar mass reference
The metric presets use conventional theoretical nominal mass per metre for the stated diameter. Verify the required designation, grade, tolerances and product information before procurement.
| Diameter | Nominal mass | 100 m mass |
|---|---|---|
| 6 mm | 0.222 kg/m | 22.2 kg |
| 8 mm | 0.395 kg/m | 39.5 kg |
| 10 mm | 0.617 kg/m | 61.7 kg |
| 12 mm | 0.888 kg/m | 88.8 kg |
| 16 mm | 1.580 kg/m | 158.0 kg |
| 20 mm | 2.470 kg/m | 247.0 kg |
| 25 mm | 3.850 kg/m | 385.0 kg |
| 32 mm | 6.310 kg/m | 631.0 kg |
| 40 mm | 9.860 kg/m | 986.0 kg |
| 50 mm | 15.400 kg/m | 1,540.0 kg |
US reinforcing bar size and weight reference
The US presets use nominal ASTM bar dimensions and weights listed in the US Federal Highway Administration’s Field Formulas reference. The table supports quantity checking only; it does not select a bar or demonstrate compliance.
| Bar | Nominal diameter | Nominal weight | 100 ft weight |
|---|---|---|---|
| #3 | 0.375 in | 0.376 lb/ft | 37.6 lb |
| #4 | 0.500 in | 0.668 lb/ft | 66.8 lb |
| #5 | 0.625 in | 1.043 lb/ft | 104.3 lb |
| #6 | 0.750 in | 1.502 lb/ft | 150.2 lb |
| #7 | 0.875 in | 2.044 lb/ft | 204.4 lb |
| #8 | 1.000 in | 2.670 lb/ft | 267.0 lb |
| #9 | 1.128 in | 3.400 lb/ft | 340.0 lb |
| #10 | 1.270 in | 4.303 lb/ft | 430.3 lb |
| #11 | 1.410 in | 5.313 lb/ft | 531.3 lb |
What the estimate deliberately excludes
A real reinforcement schedule may contain different bar marks, zones, top and bottom mats, trimmers, starters, chairs, links, couplers, openings, edge details, cranks, hooks, curtailment, laps and anchorage. Fabrication and cutting plans can also reuse offcuts. A single percentage allowance cannot reproduce those details, so the result should be reconciled with the current bar bending schedule and supplier process.
Frequently asked questions
How many rebar bars fit across a slab?
The calculator establishes the centreline span between the first and last bars after clear cover and half a bar diameter are allowed for. It divides that span by the entered maximum spacing, rounds up, and adds the end position so the actual spacing does not exceed the entered value.
Does the rebar calculator design the reinforcement?
No. Bar size, spacing, cover, layers, laps, anchorage and detailing must come from current approved structural information. The calculator only estimates a simple straight orthogonal grid from those inputs.
Are rebar weights exact?
The displayed masses use nominal theoretical mass per unit length. Product tolerances, coatings, fabrication, bends, couplers, laps, chairs, supports and cutting waste can change an order or delivered weight.