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Rebar Calculator

Enter the slab size, bar spacing and cover, choose a bar size and stock length, and set the lap. The calculator counts bars each way, adds laps and waste and gives the total length, weight and number of stock bars. A 20 × 12 ft slab with #4 bars at 12 in each way needs about 487 ft of bar weighing 325 lb.

Estimates only. Structural elements such as beams, footings, stairs, retaining walls over 1 m (4 ft) and rebar design must be designed or approved by a licensed engineer and comply with your local building code and permits. Results are "within typical limits", never "approved" — verify with your building authority.

Measurements

Units

The length bars are sold in, e.g. 6 m or 20 ft.

Overlap where bars join. The required lap depends on bar size, concrete and code; use your engineer's figure.

Results

Total bar length

487.2 ft

Total weight

325.4 lb

Stock bars to buy
25
Bars along the length
12
Bars along the width
20

Bar size, spacing, cover and laps for structural slabs must come from an engineer's design.

Show the working
  1. Bars each way⌊(12 ft − 2 × 3 in) ÷ 12 in⌋ + 1 = 12; 20
  2. Total length incl. laps and waste487.2 ft
  3. Weight0.668 lb/ft × 487.2 ft = 325.4 lb

Material takeoff

ItemQuantityUnit priceCost
Rebar (stock lengths)25 bars–
Estimated total–

Enter your local prices to estimate the cost. Prices aren't stored anywhere.

Paper and orientation
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How to use the rebar calculator

  1. Choose Imperial (ASTM bar sizes #3–#11) or Metric (bar diameters in mm).
  2. Enter the slab length and width.
  3. Enter the bar spacing each way and the edge cover.
  4. Choose the bar size and the stock bar length your supplier sells.
  5. Enter the lap length from your design, and a waste allowance.

How the estimate works

Bars along the length = ⌊(width − 2 × cover) ÷ spacing⌋ + 1, and similarly for bars along the width. Each bar is the slab dimension minus twice the cover. If a bar is longer than the stock length, laps are added for each joint. The total length includes waste, the weight comes from the bar’s mass per unit length, and the stock bar count is a simple total length ÷ stock length, rounded up.

US bar weights follow ASTM A615 nominal values. Metric weights use the steel density formula 0.006165 × d² kg/m.

Worked examples

US patio slab. 20 × 12 ft, #4 at 12 in each way, 3 in cover, 20 ft stock, 24 in lap: 12 bars along the length and 20 across. The total with 5% waste is about 487 ft, weighing about 325 lb, or 25 stock bars.

Metric slab. 6 × 4 m, 12 mm bars at 200 mm, 50 mm cover, 6 m stock: 20 bars along the length and 30 across. Total about 247 m, weighing about 219 kg.

How to interpret the results

The weight helps with ordering and transport; the stock bar count is for cutting planning. Real schedules also include chairs, ties and starter bars, which this estimate leaves out.

Tips and common mistakes

  • Subtract cover from both edges before dividing by the spacing.
  • Stagger laps so that joints don’t all fall in one line.
  • Keep bars off the ground with chairs so the cover is maintained.
  • Order a little extra; short bars are hard to get later in the same batch.

Assumptions and limitations

This tool estimates quantities. It does not design reinforcement. Bar size, spacing, cover, laps and anchorage for any structural element must come from a licensed engineer and comply with your local code.

Frequently asked questions

How do I calculate how much rebar I need?

Count the bars in each direction: (slab dimension − 2 × cover) ÷ spacing, rounded down, plus one. Multiply each count by the bar length in the other direction, add laps where bars are longer than stock, then add waste.

How much does #4 rebar weigh?

ASTM A615 #4 bar weighs 0.668 lb per foot, so 100 ft weighs 66.8 lb. #3 is 0.376 lb/ft and #5 is 1.043 lb/ft.

How do I calculate metric rebar weight?

Steel weighs about 7,850 kg/m³, which gives kg per metre = 0.006165 × d², with d in millimetres. A 12 mm bar weighs 0.888 kg/m, so 10 m weighs 8.88 kg.

What lap length should I use?

The required lap depends on bar size, concrete strength, bar position and the design code. Use the figure on your engineer’s drawings; the calculator starts with an assumption you must replace.

What is concrete cover?

Cover is the distance between the bar and the concrete surface. It protects the steel from corrosion and fire. Cover requirements depend on exposure and code, so take them from the design.

Can I use this for footings or walls?

It handles a rectangular two-way grid. For footings and walls you can run it once per face or direction, but the design must still come from an engineer.

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