About the Rebar Calculator
This rebar calculator lays out a two-way reinforcing grid for a rectangular slab, patio, driveway or footing pad. Enter the slab size, bar spacing and concrete cover at the edges, choose a bar size, and it counts the bars running each way, adds lap splices where a bar is longer than your stock length, and converts the total into sticks to buy, pounds of steel and cost.
It is built for DIYers and contractors pricing a pour, and for checking a supplier’s quote. A 12-inch or 16-inch grid of #3 or #4 bar is common for residential slabs, while driveways and structural slabs follow whatever spacing the engineer or local code specifies — this tool does not choose the design for you.
Bar weights use ASTM A615 nominal values (#4 = 0.668 lb/ft). Lap splices are entered as a multiple of bar diameter; 40 diameters is a common rule of thumb, but structural drawings take precedence.
With the default inputs, the sticks to buy is 41. Change any value above to recalculate instantly.
How to use the rebar calculator
- 1Enter the slab length and width in feet.
- 2Set the bar spacing and the edge cover from your plans (12 in and 3 in are common).
- 3Choose the bar size and the stock length your supplier sells.
- 4Adjust the lap splice multiple and waste if your spec differs.
- 5Read the sticks to buy, total weight and cost.
Formula and method
Edge cover is subtracted from both ends of each direction, giving the inside grid dimensions. The number of bars running lengthwise is the inside width divided by the spacing, rounded down, plus one for the starting bar; bars running widthwise are counted the same way across the inside length.
Any run longer than a stock bar is made of several pieces overlapped by the lap splice length (bar diameter × lap multiple), which adds steel. The total linear footage plus a waste allowance is divided by the stock length and rounded up to whole sticks, and multiplied by the ASTM A615 unit weight to give pounds.
- span
- Inside slab dimension after edge cover (in)
- lap
- Splice overlap = lap multiple × bar diameter
- stock
- Length of bars sold (ft)
Worked examples
20 × 20 ft slab, #4 at 12 in, 3 in cover
Inside the 3-inch cover the grid is 19.5 × 19.5 ft, so each direction has floor(234 ÷ 12) + 1 = 20 bars of 19.5 ft: 780 ft total. Adding 5% gives 819 ft, or 41 twenty-foot sticks weighing about 547 lb.
30 × 12 ft driveway, #4 at 18 in
The 29.5 ft lengthwise bars exceed a 20 ft stick, so each needs one 20-inch (40 × 1/2 in) lap: 8 bars × 31.17 ft. Plus 20 bars × 11.5 ft widthwise gives 479 ft, or 26 sticks with waste.
24 × 24 ft garage slab, #3 at 16 in, 10% waste
The inside grid is 23.5 ft square, so floor(282 ÷ 16) + 1 = 18 bars run each way. Each 23.5 ft run is longer than a 20 ft stick, so it needs one 15-inch lap (40 × 3/8 in): 36 × 24.75 = 891 ft. With 10% waste that is 980 ft — 50 sticks of #3 weighing about 369 lb.
Frequently asked questions
How much rebar do I need for a slab?+
Count bars each way: inside dimension ÷ spacing + 1. A 20 × 20 ft slab with #4 bar at 12 inches on center needs about 40 bars and 780 linear feet — roughly 41 twenty-foot sticks with waste.
What is standard rebar spacing for a residential slab?+
Residential slabs and patios commonly use #3 or #4 bar at 12–18 inches on center each way. Driveways and structural slabs should follow local code or engineered drawings.
How long should a rebar lap splice be?+
A common rule of thumb is 40 bar diameters — 15 inches for #3 and 20 inches for #4 — but required lap lengths depend on concrete strength and bar grade, so follow your plans or ACI 318 where specified.
How much does rebar weigh per foot?+
Nominal ASTM A615 weights are 0.376 lb/ft for #3, 0.668 lb/ft for #4, 1.043 lb/ft for #5 and 1.502 lb/ft for #6. A 20 ft stick of #4 weighs about 13.4 lb.
How far from the edge should rebar stop?+
Bars typically stop about 2–3 inches from slab edges so they are fully covered by concrete, which protects them from moisture and corrosion. Cover requirements vary with exposure.