Calculate how much rebar you need for a slab, footing, or beam, including the number of bars, spacing, total length, weight, and cost.
Rebar Formula
The calculator uses a different formula depending on what you are reinforcing. For a slab with a grid of bars running both directions:
For a footing or beam with parallel bars:
Once the total length is known, weight and cost come from the bar size and price:
The variables are:
- Bars: number of bars needed in one direction
- Side: the slab dimension the bars are spaced across (length or width)
- Cover: clear distance from the concrete edge to the steel surface; bar centerlines are an additional half-diameter inward
- Spacing: center to center distance between bars
- Lengthclear / Widthclear: the slab dimension minus twice the cover, which is how long each bar is cut
- Run Length: the length of one footing or beam
- Runs: how many identical footings or beams you are building
- Unit Weight: the weight per foot for the chosen bar size
- Unit Price: the cost per foot, per piece, or per unit weight
In slab mode the calculator subtracts twice the clear cover and one nominal bar diameter to obtain the span between outer bar centerlines. It divides this span by the specified maximum center-to-center spacing, rounds up, and adds one bar. The bars are spaced evenly within that span, so actual spacing does not exceed the maximum. It does this in both directions and cuts each bar to the clear span, so the bars stop short of the concrete edge by the cover distance. In footing or beam mode it simply multiplies the bars in one run by the run length and the number of runs. The conversion mode skips the layout entirely and turns a length you already know into weight and cost. Weight always comes from the published unit weight of the selected bar size, and cost is applied on whatever basis you price your steel, whether that is per foot, per stock piece, or per pound. Stock-piece and piece-priced outputs based on total length are lower bounds, not a cutting plan; offcuts, cutting losses and individual run geometry can require more stock.
Rebar Sizes and Spacing
Bar weight is fixed by size, so the size you pick drives both the total weight and the cost. These are the US (imperial) bar designations the calculator uses.
| Bar size | Diameter | Weight (lb/ft) |
|---|---|---|
| #3 | 3/8 in | 0.376 |
| #4 | 1/2 in | 0.668 |
| #5 | 5/8 in | 1.043 |
| #6 | 3/4 in | 1.502 |
| #7 | 7/8 in | 2.044 |
| #8 | 1 in | 2.670 |
| #9 | 1.128 in | 3.400 |
| #10 | 1.270 in | 4.303 |
| #11 | 1.410 in | 5.313 |
The following are illustrative historical ranges, not reinforcement design recommendations. Required bar size, spacing and cover depend on the structure, exposure and governing design. Use approved project plans or a qualified designer.
| Application | Typical bar | Spacing (on center) |
|---|---|---|
| Patio, walkway | #3 or #4 | 12 to 18 in |
| Driveway, garage slab | #4 | 10 to 12 in |
| Footings | #4 or #5 | per design |
| Edge cover | any | about 3 in clear |
Example Problems
Example 1: Slab grid. A 20 ft by 10 ft slab uses #4 bars (0.500 in nominal diameter), a maximum center-to-center spacing of 12 in, and 3 in clear edge cover. Along the long direction: ceil((120 – 6 – 0.5) / 12) + 1 = 11 bars, each cut to 19.5 ft. Along the short direction: ceil((240 – 6 – 0.5) / 12) + 1 = 21 bars, each cut to 9.5 ft. Total length = 11 * 19.5 + 21 * 9.5 = 414 ft; nominal weight = 414 * 0.668 = 276.552 lb, before laps or extra material. Actual spacings are 11.35 in and 11.675 in. Although ceil(414/20) is 21 stock lengths, 21 pieces cannot supply these cuts: even with two 9.5 ft cuts per 20 ft stock and no cutting loss, this layout requires at least 22 pieces. A separate-stock-per-run estimate is 32 pieces.
Example 2: Footings. You are pouring 4 identical footings, each 24 ft long, with 3 #5 bars in each. Total length = 3 * 24 * 4 = 288 ft. Weight = 288 * 1.043 = about 300 lb.
Frequently Asked Questions
Why does the calculator add one bar to the spacing count? The centerline span divided by the maximum spacing gives the minimum number of gaps. Rounding this up and adding one gives the bars needed to include both outer grid edges without exceeding that spacing. Clear cover is measured to the steel surface, so one bar diameter is also removed from the transverse centerline span.
What edge cover should I use? Rebar is kept back from the concrete edge so it does not corrode and so the concrete can fully surround it. Three inches is a familiar requirement for reinforcement in concrete cast against and permanently in contact with ground; it is not a universal slab or footing rule. Use the cover required by the governing design and exposure. The calculator subtracts the cover from both ends of each bar before cutting it to length.
Does the calculator account for lap splices and waste? Yes, if you turn on the advanced options. Enter the specified lap length and any extra material percentage. For run length R, stock S and lap l, the minimum pieces per continuous run are max(1, ceil((R-l)/(S-l))), with 0 ≤ l < S. Material length includes one lap per splice before extra material is added. This is a quantity estimate, not splice design or a guaranteed stock cutting plan.
