Free tool
Rebar Calculator
Enter your slab length, width, grid spacing, and edge clearance — get the bar count in each direction, total bars, total linear feet, and bars to buy. Free, no signup for the estimate.
Rebar takeoff
0 lin ft
- Bars (length-run)0
- Bars (width-run)0
- Total bars0
- Bars to buy—
- Estimated cost—
Turn takeoffs into paid jobs
Get the free Job Pricing Kit — a printable pricing worksheet, the markup & margin cheat-sheet, and clean invoice + quote templates so the job you just measured becomes a professional quote. We’ll email it now.
Free PDF. No spam. Unsubscribe anytime.
✓ Check your inbox — the kit is on its way.
Or download it now (PDF)How to calculate rebar for a slab
Rebar in a flat slab is a simple orthogonal grid: bars running each direction, spaced evenly on-center, held back a little from every edge so the concrete can encase them. Get the grid right on paper and the bar count, linear feet, and the number of full-length bars you actually buy all fall out of it — no guessing at the supply yard.
1. Mark the grid off the edge clearance
Rebar shouldn’t run all the way to the edge — it needs cover so it doesn’t rust or spall the concrete. Pull your edge clearance in from each side first; the usable width and length inside those clearances is the field your grid actually fills. The calculator subtracts the clearance from both sides of each dimension before it lays anything out.
2. Count the bars each direction from on-center spacing
Divide the usable dimension by your on-center spacing and add one — the “plus one” is the starting bar at the first line. Do it once across the width for the bars that run the length, and once along the length for the bars that run the width. That gives you the two bar counts that define the mat.
3. Sum the linear feet
Each length-running bar spans the slab length minus the clearance at both ends; each width-running bar spans the width minus its clearances. Multiply each bar count by the length that bar spans, add the two totals, and you have the total linear feet of rebar — the number your order is really based on.
4. Add laps and convert to stock lengths
Rebar comes in fixed stock lengths (20 ft is common). Divide your linear feet by the stock length and round up to get bars to buy — then add extra for lap splices wherever bars overlap, because every lap consumes length this geometry doesn’t include.
Measuring the job is step one. Turning it into a priced, professional quote is faster inside CrewRivet. See the concrete calculator, the excavation calculator, or the job pricing calculator.
Rebar calculator FAQ
- How much rebar do I need for a slab?
- Lay out an orthogonal grid: bars run each direction, spaced on-center, starting one edge-clearance in from every side. Count the bars in each direction, multiply each count by the length that bar spans, and add the two totals for the linear feet of rebar. Enter your slab length, width, spacing, and edge clearance and the calculator returns the bar counts and total linear feet instantly.
- What spacing should rebar be?
- A common residential slab uses #3 or #4 bar on a 16 in or 18 in on-center grid, but the right spacing depends on the load, slab thickness, and soil — 12 in for heavier loads is not unusual. Spacing drives the bar count directly: tighter spacing means more bars and more linear feet. Always build to the spacing on your engineered plan.
- How much overlap do I need for rebar laps?
- Where bars are too short to span in one piece you overlap (lap-splice) them. A common rule of thumb is 40 bar diameters — roughly 20 in for #4 bar — but the required lap length is set by code and the engineer. This calculator sizes the grid itself; add your lap allowance on top before you order, because every splice adds length.
- What size rebar for a slab?
- #3 (3/8 in) and #4 (1/2 in) are the everyday sizes for residential slabs and footings, with #4 the common default. Heavier structural slabs step up to #5 and beyond. Bar size does not change the grid geometry — the counts and linear feet here hold for any size — so pick the bar your engineer specifies and use these totals to order it.
- Is the calculator free?
- Yes — the live estimate is free with no email required. Enter your email only if you want the free Job Pricing Kit (pricing worksheet, markup cheat-sheet, and invoice + quote templates) sent to your inbox.