Rebar Calculator
Lay out the reinforcing bar for a rectangular slab as a grid. From the slab size, the on-center spacing and an edge clearance, get the bars each way, the total length and weight of steel by bar size, how many stock lengths to buy with a lap and waste allowance, and an optional cost.
Formula
Worked example
A 20 ft x 12 ft slab, 12 in (1 ft) spacing, 3 in (0.25 ft) edge clearance: grid is 19.5 x 11.5 ft. Lengthwise bars = floor(11.5 / 1) + 1 = 12, widthwise bars = floor(19.5 / 1) + 1 = 20. Net length = 12 x 19.5 + 20 x 11.5 = 234 + 230 = 464 ft; plus 10% = 510 ft. At #4 rebar (0.668 lb/ft) that is about 341 lb of steel.
How the rebar grid is counted
A slab is reinforced with two layers of parallel bars that cross at right angles to form a grid. The grid does not run to the very edge of the slab: the outer bars are held in by the concrete cover, also called the edge clearance, so the slab is first shrunk by that clearance on all four sides. To count the bars in one direction you divide the grid dimension perpendicular to those bars by the on-center spacing, round down to the last full gap, and add one so a bar sits at both inner edges. Doing this each way gives the full grid, because a bar running the length of the slab is spaced across its width, and vice versa.
Length, weight and bars to buy
Once you know how many bars run each way, the net length is each count multiplied by the grid span that bar covers, then added together. A lap and waste allowance, typically about ten percent, is added on top because bars overlap where they join and leave offcuts when cut. The weight comes from the bar size: each US bar number has a fixed mass per length under ASTM A615 (a #4 bar is 0.668 pounds per foot, a #5 is 1.043, and so on), so the total length times that mass gives the steel weight. Dividing the total length by the stock bar length your supplier sells, rounded up, gives the number of full bars to order.
Spacing, cover, laps and cost
Spacing, bar size and cover all come from an engineered slab design, never guesswork: tighter spacing and larger bars add steel and cost, while too little cover lets the bars corrode. Lap splices where bars join are typically 40 to 50 bar diameters, which is why the waste allowance matters on larger slabs. Turn on the cost estimate to price the order three ways, by length, by weight, or per stock bar, whichever matches how your supplier quotes. The total is a planning figure since local steel prices and delivery vary, so confirm the spacing, bar size, cover and lap lengths against your structural drawings before ordering.
Rebar size and weight (ASTM A615)
| Bar size | Diameter | Weight (lb/ft) | Mass (kg/m) |
|---|---|---|---|
| #3 | 10 mm (0.375 in) | 0.376 | 0.560 |
| #4 | 13 mm (0.500 in) | 0.668 | 0.994 |
| #5 | 16 mm (0.625 in) | 1.043 | 1.552 |
| #6 | 19 mm (0.750 in) | 1.502 | 2.235 |
| #7 | 22 mm (0.875 in) | 2.044 | 3.042 |
| #8 | 25 mm (1.000 in) | 2.670 | 3.973 |
Nominal bar diameters and weights per unit length. Common slab choices are #3 to #5.
Frequently asked questions
Why subtract the edge clearance before counting bars?
Rebar must sit inside the concrete by the specified cover so it does not corrode or break out of the surface. The grid is therefore pulled in from every slab edge by that clearance (commonly 3 inches or 7.5 cm for slabs on grade), so the calculator shrinks the slab by twice the clearance in each direction before working out spacing and bar lengths.
How is the weight of rebar calculated?
Each US bar size has a fixed mass per length set by ASTM A615. A #3 bar is 0.376 lb/ft, a #4 is 0.668 lb/ft, a #5 is 1.043 lb/ft, and so on. The calculator multiplies your total bar length (including the lap and waste allowance) by the mass for the bar size you pick, then shows it in pounds or kilograms.
Does this include the overlap where bars join?
Yes, through the lap and waste allowance. The net length is the bare sum of every bar at its grid span; the total length then adds a percentage (10% by default) for lap splices, which run roughly 40 to 50 bar diameters, plus cutting offcuts. Increase the allowance for many short bars or long laps.
How many stock bars should I buy?
Rebar is sold in fixed lengths, commonly 20 ft or 40 ft (6 m or 12 m). The calculator divides your total length by the stock length you choose and rounds up to whole bars, so you can place an order. Bars can be cut to fit the slab, with offcuts covered by the waste allowance.