How much rebar do I need for a concrete slab?

It depends on slab dimensions, spacing, edge cover, grid layers, bar size, and lap lengths. The calculator counts both grid directions and adds the selected lap and waste allowance.

What rebar spacing should I use in a slab?

Spacing is a design decision based on loads, slab thickness, concrete strength, subgrade, and crack-control requirements. Residential layouts commonly use spacings such as 12, 16, 18, or 24 inches, but use the project drawings.

Does this calculator design the reinforcement?

No. It estimates material from a layout you specify. It does not determine structural capacity, splice development, supports, dowels, or reinforcement around openings and thickened sections.

Rebar Calculator for Concrete Slab

Estimate rebar for a rectangular concrete slab grid. Calculate bars in each direction, total linear feet, 20-foot stock lengths, tie intersections, steel weight, lap allowance, and material cost.

Slab Rebar Grid

Estimates a two-direction orthogonal grid with equal spacing.

Slab length

Slab width

Grid spacing

Edge cover

Grid layers

layers

Lap and waste

Rebar size

Rebar price

$/ft

Total rebar to order

296linear ft

20-foot stock equivalent

15bars minimum

Bars running lengthwise

7pieces

Bars running widthwise

14pieces

Grid intersections

98tie points

Estimated steel weight

198lb

Material estimate

$222before tax and delivery

The stock-bar figure is a linear-foot equivalent. Actual cutting, splice locations, required lap lengths, and offcuts can increase the purchase quantity.

How the Slab Rebar Calculation Works

Bars running in one direction are counted across the perpendicular slab dimension after subtracting edge cover. Total length includes both directions, all grid layers, and the selected lap and waste allowance.

Bars in a direction

bar count = floor[(perpendicular dimension − 2 × edge cover) ÷ spacing] + 1

Actual splice lengths, development, hooks, dowels, openings, beams, and supports must follow the structural drawings.

Common Rebar Sizes and Weights

Bar size	Nominal diameter	Weight
#3	0.375 in	0.376 lb/ft
#4	0.500 in	0.668 lb/ft
#5	0.625 in	1.043 lb/ft
#6	0.750 in	1.502 lb/ft

Rebar Layout Checks

Use the bar size, spacing, cover, layers, and lap lengths shown on the approved design.
Support rebar on chairs so it remains at the specified elevation during placement.
Add bars around openings, thickened edges, beams, and concentrated loads separately.
Do not place reinforcement directly on the ground and pull it upward during the pour.