Rebar Calculator
Use this rebar calculator to estimate how many bars, how much weight and how many stock lengths a slab or footing grid needs. Enter the length and width, the bar spacing, the edge offset, the bar size (#3 to #8) and the stock length. It counts bars in both directions, adds lap splices when a bar is longer than stock, and gives total length, weight in lb and kg, stock bars to buy and an optional cost. It is an estimate for one mat of rebar, so follow your structural drawings for size, spacing and laps.
Math runs in your browser. Nothing is uploaded.
A common rule of thumb is 40 bar diameters (20 in for #4). It is not a code value; use the laps on your drawings.
Bars along the length (count × length)
—
Bars along the width (count × length)
—
Total bar length (with laps)
—
Estimated weight
—
Stock bars to buy
—
Weight of stock bars
—
Lap splices
—
Lap length per splice
—
Estimated cost
—
Bars along the length = floor((width − 2 × offset) ÷ spacing) + 1, each (length − 2 × offset) long; bars along the width work the same way. Weights are ASTM A615 nominal values (#4 = 0.668 lb/ft). When a bar is longer than the stock length, the lap you set is added at each splice. This is one mat only: extra layers, dowels, bends, hooks, chairs and ties are not included. Bar size, spacing, cover and laps must come from your structural drawings or engineer and local code. 20 ft is a typical stock length, so enter the one your supplier sells.
Concrete Calculator Bags · Gravel Calculator · Square Footage Calculator · Retaining Wall Calculator · Concrete Block Calculator
More in this group
How it works
- How much rebar do I need for a slab?
- Count the bars each way, then multiply by the bar length. In a 10 ft × 10 ft slab with #4 bars at 12 in on center and a 3 in edge offset, each way has floor((120 − 6) ÷ 12) + 1 = 10 bars, each 9.5 ft long. That is 190 ft in total, about 127 lb at 0.668 lb/ft, and 10 stock bars of 20 ft, because each 20 ft bar yields two 9.5 ft bars.
- What rebar spacing should I use?
- Spacing comes from your structural drawings, engineer or local code, and depends on slab thickness, loads and soil, so a calculator cannot choose it for you. Enter the spacing from your plans, for example 12 in or 18 in on center, and the bar count updates. Closer spacing means more bars and more weight.
- How much does rebar weigh per foot?
- ASTM A615 nominal weights are #3 0.376 lb/ft, #4 0.668, #5 1.043, #6 1.502, #7 2.044 and #8 2.670 lb/ft (0.560, 0.994, 1.552, 2.235, 3.042 and 3.973 kg/m). The bar number is the diameter in eighths of an inch, so #4 is 1/2 in.
- How long should rebar laps be?
- A common field rule of thumb is 40 bar diameters, about 20 in for #4 and 25 in for #5, but it is not a code value. ACI 318 splice lengths depend on bar size, grade, concrete strength, cover and spacing, so use the lap on your structural drawings or ask your engineer. This calculator adds the lap you set only when a bar is longer than your stock length.
- How many 20 ft rebar bars do I need?
- Stock lengths vary by supplier, and 20 ft is a typical default you can change. Bars shorter than stock are cut from it with the offcut reused: 9.5 ft bars give two per 20 ft bar, so the 10 ft × 10 ft slab above needs 5 stock bars each way, 10 in total. Add some extra for cutting mistakes and bends.