Rebar Calculator

How This Calculator Works

Rebar needed for a slab is the total length of bar in a two-way grid, spaced a fixed distance apart in each direction, plus a margin for lap splices and offcuts. Enter the slab’s length and width, pick a bar spacing, and this calculator works out how many bars run each way, the total linear footage, how many standard-length bars to buy, and roughly how many wire ties the grid needs to hold together during the pour.

A bar running the full length of the slab is placed at intervals across the width — so how many of those bars fit depends on the width, not the length (and vice versa for bars running the width). That’s the one part of this calculation that trips people up: each direction’s bar count depends on the other dimension, even though each bar’s own length matches its own direction.

The Formula

Bars running the length, spaced across the width:

$$n_L = \left\lfloor \frac{\vB{\text{Width}}}{\vC{\text{Spacing}}} \right\rfloor + 1$$

Bars running the width, spaced across the length:

$$n_W = \left\lfloor \frac{\vA{\text{Length}}}{\vC{\text{Spacing}}} \right\rfloor + 1$$

Total linear footage:

$$\vD{\text{Total}} = n_L \times \vA{\text{Length}} + n_W \times \vB{\text{Width}}$$

Recommended order: the total plus a waste margin (10% by default, adjustable above) for lap splices — where two bars overlap at a joint — and offcuts.

Standard bars to buy: the recommended footage divided by 20 ft (the commonly-cited standard retail rebar length in the US) and rounded up, since you buy whole bars.

Wire ties: roughly one tie per crossing point, $n_L \times n_W$.

Worked Example

A 20 ft × 12 ft patio slab, rebar spaced 18 inches on center (1.5 ft):

1. Bars running the length (spaced across the 12 ft width): $\left\lfloor \vB{12} \div \vC{1.5} \right\rfloor + 1 = 9$
2. Bars running the width (spaced across the 20 ft length): $\left\lfloor \vA{20} \div \vC{1.5} \right\rfloor + 1 = 14$
3. Total linear footage: $9 \times \vA{20} + 14 \times \vB{12} = \vD{348}$ ft.
4. Recommended order: $\vD{348} \times 1.1 \approx \vE{383}$ ft.
5. Standard bars to buy: $\vE{383} \div 20 \approx 20$ bars.
6. Wire ties: roughly $9 \times 14 = 126$.

That’s within the commonly-cited 12-18 inch spacing range for a typical residential slab — always verify the exact spacing and bar size against your local building code before pouring, since the real requirement depends on the slab’s load, not just its size.