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Rebar Calculator

Estimate a slab's rebar grid: the bars each way, the total length including lap splices, the stock bars to buy and the weight for US bar sizes #3 to #8.

Level: Grades 5–6

Printable worksheet

Rebar Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Units

The same spacing is used in both directions.

Distance from the slab edge to the end and first bar. 3 in is common for slabs on ground; check your plans.

20 ft is the usual stock length; 40 ft and 60 ft bars are also sold.

Typical value: about 40 bar diameters (20 in for #4). The required lap is set by your plans or code.

Offcuts that are too short to reuse. 5–10% is a common allowance.

Result

Total rebar length (with laps and waste)

1,268.75 ft

The grid needs about 1,268.8 ft of rebar: 64 stock bars weighing about 848 lb.

Stock bars (20 ft)
64 bars
Weight (#4)
847.53 lb
Bars running lengthwise
20 runs
Bars running widthwise
30 runs
Grid length before waste
1,208.33 ft
Lap splices
20

See the step-by-step solution ↓

Step-by-step solution

  1. Count the lengthwise bars: take the slab width minus both edge gaps, divide by the spacing and round down. Add 1, since a row has one more bar than gaps

    (20−2×0.25)1+1=rounded down:20

  2. Count the widthwise bars the same way, using the slab length minus both edge gaps

    (30−2×0.25)1+1=rounded down:30

  3. Each run lengthwise is longer than a stock bar, so it takes 2 pieces joined by 1 lap. Each overlap adds its length to the steel needed

    29.5+1×1.666667=31.166667 ft

  4. Each bar widthwise is 19.5 ft, which fits in one 20 ft stock bar with no splice

    20−2×0.25=19.5 ft

  5. Add up the steel in the grid: number of bars times length per bar, in each direction

    20×31.166667+30×19.5=1,208.333333 ft

  6. Add 5% for offcuts too short to reuse

    1,208.333333×1+0.05=1,268.75 ft

  7. Divide the total by the 20 ft stock length and round up, since you can only buy whole bars

    1,268.7520=63.4375=rounded up:64

  8. Multiply the total length by 0.668 lb/ft, the standard listed weight of #4 bar

    1,268.75×0.668=847.525 lb

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how many steel bars to buy for a concrete slab, and what they'll weigh. Slabs are often reinforced with a square grid of steel bars (rebar) running both ways, tied where they cross. To order the steel, you need four things. How many bars run each way? How long is each one? How many joints are needed where a run is longer than one bar? And what does the whole grid weigh?

Those joints are called lap splices: two bars overlap and are tied together. The calculator works it all out from the slab size, the grid spacing and the edge clearance. It also uses the length of the bars as sold (the stock length) and the lap length.

The weight uses the standard listed weight per foot for US bar sizes #3 through #8. The result is a material list for a simple grid, not an engineering design. The bar size, spacing, cover, lap length and position in the slab must come from your plans, your local building code or an engineer.

Where it's used

Why this math matters outside the classroom:

  • Slab grids. A 10 × 10 ft slab with 3 in edge clearance and 12 in spacing needs ⌊114 ÷ 12⌋ + 1 = 10 bars each way.
  • Bar lengths. Each of those bars is 120 − 6 = 114 in, or 9.5 ft, long.
  • Lap splices. When a run is longer than a 20 ft stock bar, the overlap at each splice is added to the total length to buy.
  • Weight for delivery. Number 4 bar weighs about 0.668 lb per foot, so 200 ft of it comes to about 134 lb.
  • Driveway slabs. Contractors total the rebar for a driveway slab before the pour so the right number of stock bars is on site.

How to use it

  1. Choose US units (feet, inches, pounds) or metric units (meters, centimeters, kilograms).
  2. Enter the slab length and width, the grid spacing and the clearance from the slab edges to the bars.
  3. Enter the stock bar length you'll buy and the lap splice length. Then pick the bar size.
  4. Read the bars in each direction, the total length with laps and waste, the stock bars to buy and the weight.

What the inputs mean

Grid spacing (center to center)
The distance between parallel bars, measured center to center. The same spacing is used in both directions. Grids of 12, 16 and 18 in are common on home slabs.
Edge clearance
How far the bars stop short of each edge of the slab. It sets both the length of each bar and where the first and last bars sit. The minimum is set by concrete cover rules, which say how much concrete must surround the steel.
Stock bar length
The length of the bars as sold. Runs shorter than this are cut from one bar. Longer runs are spliced.
Lap splice length
Where two bars join, they overlap by the lap length and are tied together. Laps are often given as a multiple of the bar's thickness (diameter), commonly around 40 diameters. The required figure comes from your plans or code.
Bar size (for weight)
Only the weight depends on the bar size. Standard listed weights: #3 0.376, #4 0.668, #5 1.043, #6 1.502, #7 2.044 and #8 2.670 lb per foot.
Extra for cutting waste
Offcuts that are too short to reuse. 5–10% is a common allowance.

Formula and method

bars running lengthwise = ⌊(width − 2 × clearance) ÷ spacing⌋ + 1, each (length − 2 × clearance) long
bars running widthwise = ⌊(length − 2 × clearance) ÷ spacing⌋ + 1, each (width − 2 × clearance) long
pieces per run = 1 if run ≤ stock length, else ⌈(run − lap) ÷ (stock length − lap)⌉
length per run = run + (pieces − 1) × lap
total = (sum of all runs) × (1 + waste % ÷ 100);  stock bars = ⌈total ÷ stock length⌉
weight = total × nominal weight per foot  (kg/m = lb/ft × 0.45359237 ÷ 0.3048)

A run is one continuous line of steel across the slab, which may be several bars joined. The symbols ⌊ ⌋ mean round down to a whole number, and ⌈ ⌉ mean round up. The weight per foot in the last line is the bar's standard listed weight.

The bars that run along the length are spread across the width, so their count comes from the width. It works the same way the other way round. The calculator divides the space between the first and last bars by the spacing and rounds down. Then it adds one, because a row of bars has one more bar than gaps.

When a run is longer than a stock bar, the first bar covers a full stock length. Every extra bar adds its length minus one lap. So each lap adds its length to the steel you need. The stock-bar count assumes you reuse offcuts where they fit, so a cutting list may need a few more bars.

Worked example

A 30 ft × 20 ft slab has #4 bars at 12 in each way, 3 in from the edges, using 20 ft stock bars, 20 in laps and a 5% waste allowance.

  1. Lengthwise bars: (20 − 0.5) ÷ 1 = 19.5, rounded down to 19, + 1 = 20 bars, each run 29.5 ft.
  2. Widthwise bars: (30 − 0.5) ÷ 1 = 29.5, rounded down to 29, + 1 = 30 bars, each 19.5 ft.
  3. Each 29.5 ft run needs 2 pieces with one 1.667 ft lap: 29.5 + 1.667 = 31.167 ft.
  4. Grid length: 20 × 31.167 + 30 × 19.5 = 623.33 + 585 = 1,208.33 ft.
  5. With 5% waste: 1,208.33 × 1.05 = 1,268.75 ft, or 1,268.75 ÷ 20 = 63.4, so 64 stock bars.
  6. Weight: 1,268.75 × 0.668 = 847.5 lb.

Result: About 1,268.75 ft of #4 rebar: 64 twenty-foot bars weighing roughly 848 lb, with 20 lap splices.

Understanding the result

The stock-bar count is the number to order. The total length and weight help when you compare quotes, since steel is sometimes priced by weight.

Laps on neighboring bars are usually staggered rather than lined up across the slab. That doesn't change the amount of steel, only where the joints fall.

Assumptions and limitations

  • The slab is a rectangle with one layer of bars in a square grid, at the same spacing both ways.
  • Every bar stops the same distance from the edges, and every splice uses the same lap length.
  • Offcuts can be reused, so the stock-bar count comes from the total length.
  • This is a material estimate, not a structural design. Bar size, spacing, cover, laps and chairs (the stands that hold bars off the ground) come from your plans, local code or an engineer. So does whether you need reinforcement at all.
  • Dowels, edge bars, corner bars, hooks, thickened edges and a second mat (layer of bars) aren't included.
  • Chairs, tie wire and mesh aren't counted.

Frequently asked questions

How much does rebar weigh per foot?

The standard listed weights are 0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5, 1.502 for #6, 2.044 for #7 and 2.670 for #8. For metric, multiply by about 1.488 to get kilograms per meter.

What does the bar number mean?

In the US system, the number is the bar's diameter in eighths of an inch. A #4 bar is 4/8 = 1/2 in thick, and a #8 bar is 1 in.

How long should a rebar lap splice be?

It's often given as a multiple of the bar diameter. Around 40 diameters is a common figure, which is 20 in for a #4 bar. The exact length depends on the bar, the concrete and the code, so use the figure on your plans.

Why is there one more bar than the slab width divided by the spacing?

A row of bars has a bar at each end, so it always has one more bar than gaps. Twelve feet of space at a 12 in spacing holds 13 bars.

How this calculator is checked

Method: Grid layout from spacing and clearance; lap splices from stock length; nominal unit weights of US reinforcing bars #3–#8 as tabulated in ASTM A615/A706. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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