Rebar Calculator

Estimate Slab Steel Without the Guesswork

Ordering rebar is one of the easiest things to get wrong on a concrete job. Buy too little and the pour stalls while you chase more steel; buy too much and the surplus sits rusting in the yard. This rebar calculator turns four simple measurements — slab length, slab width, grid spacing, and edge clearance — into the three numbers that actually go on your materials list: how many bars, how many linear feet of steel, and how many tie-wire points you'll knot together.

How the Grid Is Counted

A reinforced slab uses two layers of bar laid at right angles to form a mat. The tool counts each direction on its own:

Bars lengthwise = floor((width × 12 − 2 × clearance) ÷ spacing) + 1 Bars widthwise = floor((length × 12 − 2 × clearance) ÷ spacing) + 1

The "+1" is the fence-post rule: bars sit at both edges plus every interval between, so you always have one more bar than there are gaps. Dimensions are entered in feet and converted to inches internally so the spacing and clearance (also in inches) line up cleanly.

A Worked Example: A 20 × 12 ft Garage Slab

Say you're pouring a 20 ft long by 12 ft wide garage slab on a 12 in grid with 3 in of edge clearance.

• Usable width: 12 × 12 − 2 × 3 = 138 in → 138 ÷ 12 = 11.5 → floor is 11 → 12 lengthwise bars
• Usable length: 20 × 12 − 2 × 3 = 234 in → 234 ÷ 12 = 19.5 → floor is 19 → 20 widthwise bars
• Total bars: 12 + 20 = 32 bars
• Linear feet: 12 bars × 20 ft + 20 bars × 12 ft = 240 + 240 = 480 linear feet, or about 504 ft once the 5% lap-and-cut allowance is added.
• Tie points: 12 × 20 = 240 intersections to wire together.

That single screen tells you to order roughly 504 linear feet of bar (about 25 lengths of 20 ft stock) and to bring enough tie wire for 240-plus crossings.

Why the 5% Lap Allowance Matters

Rebar ships in fixed lengths — 20 ft is common — so any bar longer than a stock length must be lapped, overlapping two pieces by 30–40 bar diameters and tying them. Offcuts at the slab edges add more waste. The calculator's net linear-foot figure is the bare geometry; the "with lap" figure adds 5% so the number you order survives real-world cutting and splicing. On a large pour you may bump this higher.

Practical Tips for a Clean Pour

• Keep the mat up off the ground. Use chairs or dobies so the steel sits in the middle third of the slab thickness, not on the dirt where it can't do its job.
• Stagger your laps. Don't line up every splice in the same plane; offset them so no single section is weakened.
• Match spacing to load. Tighter spacing (closer to 12 in) carries more load than a wide 18 in grid — follow your engineer's drawings rather than guessing.
• Round up on ties. Plan for about 1.5 ties per crossing; re-tying after a bar shifts during the pour is normal.

Pairing With Your Other Estimates

Rebar is only half the slab. Once you know your steel, size the pour itself with the concrete calculator to get cubic yards, and if you're building walls on top, the brick calculator will scope your masonry. Together they give you a complete, defensible materials list before you ever place an order.