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Rebar Calculator

Use this rebar calculator to find how much rebar a slab takes, as total length and weight, from the slab size, the bar size and the spacing you enter.

  • Instant answer as you type
  • Nothing you type is sent anywhere
  • Worked example with clear steps

Your measurements

ft
The limit of 10,000 is ours.
ft
The limit of 10,000 is ours.
in
Centre to centre. 12 is an example value, not a recommendation: enter the spacing on your plan. The limits are ours.

#4 is an example value, not a recommendation: pick the size on your plan.

Stock length and price (optional)

The length of one bar as your supplier sells it, and the price per pound. We give no default for either, and we store and show no prices.

ft
$ per lb

Result

Calculated live
Total length of bar 11 + 11 bars
220 ft
Weight of the bar #4 at 0.668 lb per ft
146.96 lb
Bars each way 11 run 10 ft, 11 run 10 ft
11 and 11

Based on NIST SP 811 Caltrans TxDOT Item 440 Full list of sources

Assumptions and constants

You can change

Fixed values (exact definitions and references)

These are exact definitions and cannot be edited.

Inches in a foot Reference [1]

12 in per ft

The foot is 0.3048 m and the inch is 0.0254 m, both exact, so 12 inches make a foot.

How to use the rebar calculator

  1. Enter the Slab length and Slab width, in feet.
  2. Enter the Spacing of the bars, in inches. We give no recommended spacing, so use the one on your plan.
  3. Pick a Bar size.
  4. Read Total length of bar and Weight of the bar.
  5. For the number of bars to buy, open Stock length and price (optional) and enter the Length of one bar.
  6. Use Copy result or Print, and check the count against your plan and your supplier.

How much rebar for a concrete slab?

Bars each way = the slab dimension in inches divided by the spacing, rounded down, plus 1, counting the first bar at one edge. Total length = the bars that run the width times the width, plus the bars that run the length times the length. [1] Weight = total length x the weight of the bar per foot.

The last bar can fall short of the far edge. When it does, the calculator shows the gap that is left, so you can decide what to do about it.

Worked example

A slab 10 ft x 10 ft with bars 12 in apart. Bars each way = 10 x 12 / 12 = 10, plus 1 = 11. 11 bars of 10 ft each way is 11 x 10 + 11 x 10 = 220 ft of bar. A #4 bar weighs 0.668 pounds per foot, so 220 x 0.668 = 146.96 pounds.

Rebar grid for a slab: length 10 ft, width 10 ft, bars 12 in apart The slab seen from above, a square, with a grid of bars across it: eleven bars each way. The length, 10 ft, is marked under the slab and the width, 10 ft, along the right side. The gap between two neighbouring bars, 12 in, is marked at the top. From above Length 10 ft Width 10 ft 12 in apart
Drawn to scale for a 10 ft slab with bars 12 in apart: eleven bars each way, with the first bar at one edge. A real slab has the bars the distance apart that you enter.

What does rebar weigh per foot?

The California Department of Transportation lists the nominal diameter, area and weight per foot of each bar size in its reinforcement table for ASTM A706 Grade 60 bars, and the Texas Department of Transportation lists the same figures for the bars its Item 440 covers. [2] [3] The weights agree. The table gives the six sizes this calculator offers.

Rebar bar sizes, diameter, area and weight per foot
Bar size Nominal diameter (in) Area (in²) Weight (lb per ft)
#3 0.3750.110.376
#4 0.5000.200.668
#5 0.6250.311.043
#6 0.7500.441.502
#7 0.8750.602.044
#8 1.0000.792.670

What spacing for rebar in a slab?

We give no recommended spacing, because we found no source we can cite for one. Enter the spacing shown on your plan, or ask your local building authority or an engineer.

What size rebar for a concrete slab?

We found no source we can cite for the bar size a slab needs. This is not engineering advice: ask your local building authority or an engineer.

Rebar for common slabs

Four example slabs with #4 bars 12 in apart. The spacing and bar size are examples to show the sum, not recommendations.

Rebar for common square slabs, #4 bars 12 in apart
Slab Bars each way Total length (ft) Weight (lb)
10 x 10 ft 11 and 11220146.96
12 x 12 ft 13 and 13312208.42
16 x 16 ft 17 and 17544363.39
20 x 20 ft 21 and 21840561.12

Rebar chairs

We found no source we can cite for rebar chairs or for how many a slab needs. Ask your supplier or your local building authority.

How much does rebar cost?

We state no price. Ask local suppliers for a price per pound, then enter it under Stock length and price (optional) above to see the total. The site does not store the prices you type.

Common mistakes and limits

  • Using our spacing as a recommendation. The 12 in is an example. Enter the spacing from your plan.
  • Counting one bar too few. A 10 ft slab with bars 12 in apart has 11 bars each way, not 10, because the first bar sits at the edge.
  • Mixing up bar sizes. Read the weight of your own bar from the table: Caltrans lists a #5 bar at 1.043 pounds per foot, against 0.668 for a #4. [2]
  • What this tool does not cover: lap length, cover, bends, chairs and the design of the reinforcement.

Frequently asked questions

How many rebar do I need?

Bars each way = the slab dimension in inches divided by the spacing, rounded down, plus 1. A 10 ft slab with bars 12 in apart has 11 bars each way. Then add the lengths and divide by the length of one bar.

How much does a #4 bar weigh per foot?

Caltrans and TxDOT both list a #4 bar at 0.668 pounds per foot, with a nominal diameter of 0.500 inch. [2][3]

Is rebar needed in a concrete slab?

We found no source we can cite for when a slab needs reinforcing bar. This is not engineering advice: ask your local building authority or an engineer.

How long does rebar come in?

We found no source we can cite for stock lengths. Ask your supplier, and type the length into Length of one bar to count the bars.

What about lap length and cover?

We give no lap length and no cover. Your plan, your local code or an engineer sets them, and this calculator adds nothing for them.

Sources and references

  1. National Institute of Standards and Technology (NIST). NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically (SP 811, accessed November 2026). Used for: the inch and foot relationship (12 inches in a foot).
  2. California Department of Transportation (Caltrans). Bridge Design Details 13.1, Reinforcement, Table 13.1.1, ASTM Standard Reinforcing Bar Data (September 2025, accessed November 2026). Used for: the diameter, area and weight per foot of each bar size.
  3. Texas Department of Transportation (TxDOT). Item 440, Reinforcement for Concrete (2014 Standard Specifications, accessed November 2026). Used for: the same table read a second time, for the weight per foot.

How we use sources: methodology.

Estimates only. Confirm quantities with your supplier. See the disclaimer. Found a wrong result? Report an error.