Use this rebar calculator for a rectangular slab with a two-way grid. Enter slab length, slab width, and bar spacing in inches on center. The calculator counts the bars that run the length of the slab and the bars that run the width, adds their linear feet, then shows a second result with 10 percent added for laps and waste. It is a quantity calculator, not an engineering design. Bar size, cover, slab thickness, soil, loads, footings, and code requirements still come from the plan, the spec, or the engineer.
Rebar Calculator: Linear Feet for a Slab Grid
Estimate rebar linear feet for a slab grid from length, width, and spacing. Includes a 10 percent laps and waste result.
Rebar Linear Feet Calculator
Enter slab length, width, and grid spacing. The calculator returns base linear feet and a 10 percent laps/waste total.
Rebar length392.0 linear ft
With 10% laps/waste431.2 linear ft
Estimates for planning only. Check the result against your plans and a supplier quote before you order.
How to Use the Calculator
Enter the outside slab dimensions in feet. Then enter the grid spacing in inches. Sixteen inches on center is the default because it is a common slab planning spacing, but your plan may call for 12 inches, 18 inches, or a different layout. The calculator assumes a full rectangular grid with bars running both directions. It does not deduct for edge cover, blockouts, thickened edges, dowels, chairs, or bent bars. Use the result as the takeoff base, then adjust for the exact reinforcing detail on the drawings.
The Formula
The count has two directions. Bars running the slab length are spaced across the width, so bars along length = floor(width feet x 12 / spacing inches) + 1. Each of those bars is the slab length. Bars running the slab width are spaced across the length, so bars along width = floor(length feet x 12 / spacing inches) + 1. Each of those bars is the slab width. Add those two linear-foot totals. The plus one matters because a grid has a bar at the starting edge and then bars at each spacing line across the slab.
Worked Example
For a 20 foot by 12 foot slab at 16 inches on center, the bars running length are spaced across the 12 foot width. Twelve feet is 144 inches. Divide by 16 and get 9, then add 1, so there are 10 bars running the 20 foot direction. That is 200 linear feet. Now count the other way. Twenty feet is 240 inches. Divide by 16 and get 15, then add 1, so there are 16 bars running the 12 foot direction. That is 192 linear feet. The base grid is 392 linear feet. Add 10 percent for laps and waste and the planning total is 431.2 linear feet.
Ordering Notes
The calculator returns linear feet, but the yard or supplier sells stock lengths by bar size. Convert the linear feet into the lengths you will buy, then account for lap length, cuts, splices, and the stock lengths available. Ten percent is a planning add-on for laps and waste, not a substitute for reading the detail. If the slab has thickened edges, dowels, grade beams, openings, or re-entrant corners, count those separately. For bar selection, use the rebar size chart and rebar grades and placement. For price context, use rebar prices.
Cover, Stock Lengths, and Details
A slab grid is not the same thing as an engineered reinforcing schedule. The calculator assumes bars run the full slab length and width, which is useful for a fast takeoff, but real bars need concrete cover at the edges and may need laps where stock lengths stop. A 20 foot slab can use 20 foot stock cleanly. A 34 foot run forces a splice or a different stock-length plan. Lap length depends on bar size, grade, concrete strength, and the detail, so do not turn the 10 percent add-on into a design rule. Also separate support materials. Chairs, dobies, tie wire, stakes, dowels, and vapor barrier repairs are not linear feet of rebar, but they decide whether the steel lands at the right height when the concrete shows up. On a small slab, those accessories can be the difference between steel in the middle third of the pour and steel sitting uselessly on the ground.
Rebar Grid Reference
| Reference | Planning value | Field note |
|---|---|---|
| Default spacing | 16 inches on center | Change to match the plan or engineer detail |
| Bars each direction | floor(span / spacing) + 1 | The plus one counts the starting edge bar |
| Waste result | Base x 1.10 | Planning allowance for laps and cuts |
| Output unit | Linear feet | Convert to stock lengths before ordering |
| Not included | Dowels, bends, chairs, openings | Count project details separately |
FAQ
Why does the formula add one bar? A grid has a bar at the starting edge and then bars at each spacing line. Dividing the span by spacing counts the intervals, so adding one converts intervals into bars.
Does this pick the rebar size? No. The calculator estimates linear feet only. Bar size, grade, cover, and slab reinforcement requirements come from the plan, spec, or engineer.
What spacing should I enter? Enter the spacing shown on the drawing. If you are only planning a rough material count, 16 inches on center is the default in this calculator.
Does the 10 percent add-on cover all laps? It is a planning allowance for ordinary laps, cuts, and waste. Long lap requirements, stock length limits, dowels, bends, and openings need a separate count.