Rebar Calculator

How to measure

Spacing is measured center to center of the bars, in both directions. The first bar is usually a few inches from the edge, so the count is one more than the number of gaps: a 20 ft span at 16 in spacing has 15 gaps, so 16 bars. Measure the slab as the poured dimensions, not the dimensions of the excavation.

Bar size is the “#” number. It is the diameter in eighths of an inch: #3 is 3/8 in, #4 is 1/2 in, #5 is 5/8 in and #6 is 3/4 in. Reinforcing design, including bar size, spacing, cover and laps, comes from the plans or an engineer. This tool only turns your chosen layout into quantities.

The rebar grid formula

For a rectangle of length L and width W with spacing S, the bars running along the length number floor(W ÷ S) + 1, because they are spread across the width. The bars running along the width number floor(L ÷ S) + 1. The total length is the sum of all bars. Multiply by the weight per foot for the weight and divide by 20 ft per stick for the number of pieces.

The calculator adds an allowance for laps and cuts. Splices are normally lapped 30 to 40 bar diameters or as the plans specify, so a long slab needs more than the straight-line total. Cover at the slab edge, chairs and tie wire are not counted.

Formula: bars = (floor(W ÷ S) + 1) × L + (floor(L ÷ S) + 1) × W. Weight = length × lb per foot.

Worked example: a 20 × 20 ft slab with #3 bars at 16 in

1. Convert to base units: Slab length 20 ft; Slab width 20 ft; Bar spacing (on center) 16 in = 1.3333 ft
2. Bars running along the length = floor(20 ÷ 1.3333) + 1 = 16 bars × 20 ft
3. Bars running along the width = floor(20 ÷ 1.3333) + 1 = 16 bars × 20 ft
4. Total length = 640 ft; with 10% for laps and cuts = 704 ft
5. Weight = 704 ft × 0.376 lb/ft (#3, ASTM A615) = 264.7 lb
6. Sticks = 704 ÷ 20 ft = 35.2 → 36

That is 36 × 20 ft bars, 704 ft total · 265 lb (120 kg). Without the allowance the net length is lower; the allowance covers the laps at the splice and cuts at the edges.

Weight per foot of reinforcing bar

What spacing should I use?

There is no universal answer. Residential slabs on grade often use #3 or #4 bars at 12 to 24 in in both directions, or welded wire mesh. Thicker slabs, heavier loads and poor soil call for a designed layout. Reinforcement in a slab on grade controls crack width but does not prevent cracking. Joint spacing is separate and covered by the control joints guide. The rebar spacing guide describes typical layouts.

Bars should sit in the middle or upper third of the slab on supports (chairs), not on the ground. Bars that are left on the base provide no benefit.

Mesh or rebar?

Welded wire mesh is cheap and fast, but it tends to sink to the bottom during the pour unless it is held up, which defeats its purpose. Rebar on chairs is easier to place correctly and gives you a stiffer grid. For driveways and garage floors, rebar is more common. Whichever you use, put it where the plans say.

Mistakes to avoid

• Counting gaps instead of bars. The first bar adds one.
• Forgetting the second direction.
• Not allowing for laps.
• Letting the grid rest on the base without chairs.
• Using the wrong bar size: a #4 weighs 78% more per foot than a #3.

Next steps

Add the concrete with the slab calculator and the base with the gravel base calculator. The Project Kit passes your slab size to this page.

Frequently asked questions