Rebar Calculator – Quantity, Spacing, Weight & Cost

Total grid bar lines is the number of longitudinal and transverse bar lines across all selected mats. Required rebar length includes the clear bar runs plus any entered splice laps. Calculated steel weight uses nominal circular cross-sectional area and a steel density of 7,850 kg/m³. Stock bars to order rounds the aggregate requirement up to whole stock lengths after the procurement allowance.

This is useful for early material planning, supplier enquiries and reasonableness checks. It does not select structural bar diameter, spacing, cover, lap length or reinforcement ratio for you.

1. Choose Metric or Imperial units and enter slab length and width.
2. Enter the edge cover/grid inset specified by the design.
3. Enter the maximum bar spacing for each direction and the number of reinforcement mats/layers.
4. Select the bar diameter and supplier stock length.
5. If long runs need splicing, enter the approved lap length. Do not guess structural lap lengths.
6. Add a procurement allowance and your supplier price basis.
7. Calculate, then review the schematic grid, actual spacing, direction table, weight and stock-bar estimate.

For a 6 m × 4 m slab, 50 mm edge inset, 200 mm maximum spacing both ways, one reinforcement mat and 12 mm bars:

• Clear grid length = 6.00 − 2 × 0.05 = 5.90 m.
• Clear grid width = 4.00 − 2 × 0.05 = 3.90 m.
• Bars running along the slab length are distributed across 3.90 m. The calculator chooses enough intervals so actual spacing does not exceed 200 mm.
• The opposite direction is calculated the same way across the 5.90 m clear span.
• Total length is multiplied by the nominal unit mass of the selected bar and then converted to whole stock-bar purchases after the selected allowance.

Unit mass = (π / 4 × d²) × 7,850 / 1,000,000 kg/m, where d is in mm.

• Using slab dimensions as bar run length without accounting for the specified cover/grid inset.
• Confusing the direction a bar runs with the direction in which its spacing is measured.
• Assuming the entered spacing will divide the clear span exactly. This calculator reports the resulting actual spacing.
• Adding an arbitrary lap length instead of the project-approved lap/development requirement.
• Treating the aggregate stock-bar estimate as a fabrication cutting plan. Use the dedicated optimizer for that.
• Mixing nominal bar diameter systems without checking project specifications.

• Rectangular, orthogonal reinforcement grids only.
• The tool estimates quantity from user-supplied spacing and does not design reinforcement for structural capacity.
• Cover/inset is treated as the location of the outermost bar centreline for this planning model. Confirm how your project defines cover.
• Lap length is user supplied. Development, anchorage, hooks, couplers and code-specific detailing are outside this calculator.
• Stock-bar quantity is aggregate length ÷ stock length, rounded up. It does not account for individual cut combinations or reusable offcuts.
• Nominal weight uses 7,850 kg/m³ and may differ slightly from manufacturer tables/tolerances.

How many rebars do I need for a slab?

Enter the slab dimensions, edge inset and bar spacing in both directions. The calculator determines the number of grid lines required so the maximum spacing is not exceeded.

Why is the actual spacing slightly smaller than what I entered?

The entered spacing is treated as a maximum. If the clear span is not an exact multiple of it, the calculator increases the number of intervals and distributes them evenly, which produces a slightly smaller actual spacing.

How is rebar weight calculated?

Nominal mass per metre is calculated from the circular bar area and a steel density of 7,850 kg/m³. The total mass is unit mass × total rebar length.

Does this calculator calculate lap length?

No. Lap/development length depends on structural design, bar conditions, concrete strength, code and project details. Enter the approved lap length from the project documentation.

Is the stock-bar count optimized?

No. It is a procurement estimate based on aggregate length. Use the Rebar Cutting Optimizer for stock-by-stock patterns, kerf/cutting loss and reusable remnants.

• Omni Calculator — Rebar Calculator — grid dimensions, total rebar length, stock pieces and cost workflow.
• Procore — Rebar Calculator — slab dimensions, edge clearance and rebar spacing workflow.
• Reuven Engineering Tools — Rebar Weight — nominal mass formula and diameter-based weight summaries.
• Concrete Reinforcing Steel Institute (CRSI) — reinforcement industry guidance and resources.

Project drawings, specifications and the governing design code take precedence over generic calculator assumptions.

1. Convert all entered dimensions to metres.
2. Subtract twice the entered edge inset from the slab length and width to obtain the clear bar-centre grid dimensions.
3. For each direction, calculate intervals = ceiling(clear span ÷ maximum spacing), then bar lines = intervals + 1. Actual spacing = clear span ÷ intervals.
4. Calculate the clear run length of each bar in the perpendicular dimension.
5. If a clear run exceeds the stock length, calculate the minimum number of stock segments using the user-entered lap length. Material length per run = clear run + lap × number of splices.
6. Multiply by bar lines and mats/layers, then sum both directions.
7. Calculate nominal unit mass from circular cross-sectional area and 7,850 kg/m³.
8. Apply the procurement allowance to total required length, divide by stock length and round up to whole stock bars.
9. Calculate purchase weight and cost according to the chosen price basis.

Intervals = ceil(clear span / maximum spacing)   ·   Actual spacing = clear span / intervals

Nominal mass (kg/m) = (π/4 × d²) × 7,850 / 1,000,000