Rebar Calculator

Estimate rebar quantity, length, weight, and cost for slabs, walls, and footings. Supports US and metric bar sizes, lap splices, and waste allowance.

Units:
in
ft
$

Per bar length (20 ft).

Slab / Mat Rebar Layout

ft
ft
in

Bottom layer

in

These bars run parallel to slab length.

in

These bars run parallel to slab width.

Results

Quantity & length

Bars: –

Total length: –

Standard bars: –

Weight & cost

Total weight: –

Estimated cost: –

Results include lap splices (if any) and waste/contingency.

How this rebar calculator works

This rebar calculator is designed for quick, practical estimating on site or in the office. It supports slabs/mats and walls/footings, US and metric bar sizes, lap splices, and waste allowance.

1. Geometry and spacing

The core idea is simple: number of bars × clear length × unit weight.

Number of bars in a direction:

\[ n = \left\lfloor \frac{L_\text{clear}}{s} \right\rfloor + 1 \]

where:

  • \(L_\text{clear}\) = slab or wall dimension minus cover on both ends
  • \(s\) = bar spacing

For slabs, the calculator computes bars in both directions for bottom and (optionally) top layers. For walls/footings, it computes vertical and horizontal bars, with an option for double mats (both faces).

2. Bar length and lap splices

Each bar’s clear length is the dimension between covers. If lap splices are enabled, the calculator adds lap length per splice:

Bar length with laps (per bar):

\[ L_\text{bar,total} = L_\text{clear} + n_\text{laps} \cdot L_\text{lap} \]

You can define lap length as a multiple of bar diameter (e.g. 40×Ø) or as a fixed length in your chosen units.

3. Weight calculation

Weight is based on bar cross-sectional area and steel density (default 7850 kg/m³, editable).

Unit weight of a bar:

\[ w_\text{unit} = \rho_\text{steel} \cdot A = \rho_\text{steel} \cdot \pi \left(\frac{d}{2}\right)^2 \]

where \(d\) is bar diameter and \(\rho_\text{steel}\) is steel density.

4. Waste and contingency

Finally, the calculator applies your waste percentage (cutting, offcuts, mistakes) to the total length and weight:

\[ L_\text{final} = L_\text{raw} \cdot \left(1 + \frac{p_\text{waste}}{100}\right) \]

5. Cost estimation

You can price rebar either per standard bar length (e.g. per 20 ft bar) or per weight (e.g. per kg or lb). The calculator converts total length and weight to match your chosen pricing basis.

Typical design assumptions (not a design tool)

This tool is for estimating only. It does not check structural code requirements such as minimum reinforcement ratios, development length, or bar spacing limits. Always have a licensed structural engineer design and review reinforcement layouts for safety and code compliance.

Practical tips for using the rebar calculator

  • Increase waste percentage for heavily cut or congested layouts (10–15% instead of 5%).
  • Round up the number of standard bars to the next whole bundle size required by your supplier.
  • For long walls, consider construction joints and lap locations when entering lap lengths.
  • Use the metric bar system if your project drawings specify diameters in millimeters.

FAQ

How accurate is this rebar calculator?

For straightforward slabs, mats, and walls with regular spacing, this calculator will typically get you within a few percent of the final rebar quantity. Complex shapes, openings, and special detailing will require manual adjustments.

Can I change the standard bar length?

Yes. Set the “Standard bar length” field to match what your supplier provides (e.g. 20 ft, 40 ft, 6 m, 12 m). The calculator will convert total length into an approximate number of stock bars.

What bar sizes are supported?

In US mode, the tool supports #3, #4, #5, #6, #7, #8, #9, #10, and #11 bars with standard diameters. In metric mode, it supports 6, 8, 10, 12, 16, 20, 25, and 32 mm bars.

Does this replace a structural engineer?

No. This calculator is an estimating aid. Final reinforcement design, detailing, and code compliance must be done and approved by a qualified structural engineer.