Rebar

Sizing a Two-Way Rebar Grid for a Slab

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.

Key Factors to Consider

• Rebar’s job is to resist tension, complementing concrete’s own natural strength in compression. Plain concrete is strong when squeezed but weak when pulled apart or flexed — embedding steel rebar (which handles tension well) inside the slab is what lets the combined material resist both types of stress, which is the whole reason reinforced concrete exists as a building technique.
• Rebar size (diameter) and grid spacing both scale with the load a slab needs to bear, and they trade off against each other. A slab expecting heavier loads (like a driveway rather than a patio) may need thicker bars, tighter spacing, or both — always check local building code or an engineer’s specification for the actual load case rather than assuming a residential-patio spacing applies universally.
• Rebar needs to sit properly positioned within the concrete, not resting on the ground beneath it. Plastic or concrete “chairs” are commonly used to hold the rebar grid at the correct height above the base before the pour, ensuring it ends up properly embedded within the slab rather than sitting at the very bottom where it wouldn’t provide the intended reinforcement.
• Rebar corrosion is a real long-term durability concern in some environments. In particularly harsh conditions (like coastal areas with salt exposure), epoxy-coated or stainless rebar is sometimes used instead of standard steel rebar specifically to resist corrosion over the structure’s lifetime — a detail worth discussing with a contractor for a project in a corrosive environment.

Common Mistakes

• Confusing which dimension sets a direction’s bar count. As covered above, the number of bars running the length is set by the width (and vice versa) — swapping the two dimensions in this calculation gives a bar count for the wrong direction.
• Ignoring the waste margin and ordering exactly the calculated linear footage. Lap splices and offcuts consume real material — ordering with no margin at all commonly leaves a project short by the time the last few bars are cut and joined.
• Assuming one spacing works for every slab. A driveway or heavily loaded slab typically needs tighter spacing or thicker bar than a simple patio — always confirm the actual spacing and bar size against local building code or an engineer’s specification rather than reusing a residential default.

Useful to Know

• Need the concrete volume for this same slab, not just its reinforcement? Concrete Calculator calculates concrete volume, bag count, and cost from slab dimensions.
• Sizing the gravel base layer that goes under the slab? Gravel Calculator estimates gravel volume and weight from area and depth.
• Working out a footing rather than a flat slab? Foundation Concrete Calculator calculates concrete volume for footings and foundation walls.