Rebar Calculator
Count both directions before you order steel.
Rebar Calculator
Bars each way = floor(dimension in inches / spacing) + 1; total length = bars x other dimension, both directions.
Calculated instantly
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What This Calculator Does
The Rebar Calculator estimates the linear feet of reinforcing bar in a two-way slab grid, then converts it to stock bars and weight. It counts bars in both directions from the slab dimensions and spacing, because a one-direction estimate is a common and expensive mistake.
How It Works
Bars running one way are spaced across the other dimension. The width in inches is divided by spacing, rounded down, and one is added for the starting bar. That count is multiplied by the slab length. The same is done in the other direction, and the two lengths are added. Lap and waste percentage increases the total, which is then divided by stock bar length and multiplied by bar weight.
Example
A 20 ft by 10 ft slab at 12 in spacing has floor(120/12) + 1 = 11 bars running the 20 ft length, or 220 ft, and floor(240/12) + 1 = 21 bars running the 10 ft width, or 210 ft. Total is 430 ft. With 5 percent laps and waste that is 451.5 ft, 23 stock bars at 20 ft, and about 301.6 lb of number 4 bar.
Formula
floor(other dimension in inches / spacing) + 1bars along length x length + bars along width x widthceiling(total length x (1 + waste) / stock length)total length x bar weightReading the Results
Total rebar length
The grid length before laps. It includes the extra bar at each edge, which is why the count is spacing plus one rather than area divided by spacing.
Stock bars
Whole bars to buy at your stock length. Cutting and lapping means the usable length per bar is less than the nominal length.
Estimated weight
Length times unit weight. Use it for transport and handling planning, not as a substitute for a mill certificate or a supplier quote.
Sources and References
ACI 318 - Building Code Requirements for Structural Concrete
Reinforcement detailing and lap splices
The design standard behind spacing, cover, and splice requirements for structural concrete.
CRSI - Placing Reinforcing Bars
Field placing, supports, and laps
Practical guidance on chairs, cover, and splice placement.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
How much does rebar weigh per foot?
Number 3 is about 0.376 lb/ft, number 4 about 0.668, number 5 about 1.043, number 6 about 1.502, and number 8 about 2.670. The calculator takes the unit weight as an input so you can match the bar you are using.
What spacing should I use for a slab?
Residential slabs often use a 12 in to 24 in grid, but the correct spacing depends on the slab thickness, loads, subgrade, and whether it is structural. Follow the engineered design or the prescriptive code for your situation; this tool only measures the grid you specify.
Does the waste percentage cover lap splices?
It can, if you set it high enough, but laps are better calculated explicitly. A lap of 40 bar diameters on number 4 bar is about 20 in, and a 20 ft bar cannot span a 20 ft slab without a lap. Add the lap length or order longer bars.
Can I use welded wire mesh instead?
Sometimes, for crack control in a lightly loaded slab. Mesh is easier to place but must be supported at the correct height and lapped properly. Structural reinforcement should follow the design, not a substitution based on convenience.
Do I need rebar in a driveway slab?
Most driveway slabs benefit from reinforcement or mesh, but thickness, base preparation, jointing, and drainage matter as much as the steel. A 4 in slab on poor subgrade will crack regardless of a light grid. Ask for a local design rather than scaling up a patio detail.