Rebar Calculator

Created by Filip Derma
Reviewed by Bogna Szyk
Last updated: Aug 15, 2022

Rebar calculator helps to find out how many rebars you need to create a reinforcement in a concrete slab and how much it will cost you. Moreover, it can estimate the dimensions of the grid that should be used. In the article below, you will find what is rebar, check the rebar sizes and find the step-by-step tutorial on how to use the rebar calculator.

What is rebar?

Rebar (reinforcement steel/reinforcing steel) is a building material used to improve properties of concrete blocks. Those wires are made of patterned steel (improves adhesion), and they are usually placed in a way to create a mesh. Thanks to the steel characteristics (thermal expansion coefficient very similar to the concrete's), reinforcement compensates for low tensional strength of concrete. Rebar may boost the resistance to failure of concrete even several times.

Civil engineers apply the reinforcing steel while designing buildings and poured concrete driveways. Besides the increased tension resistance, rebar also improves concrete cracking resilience and allows for reducing the thickness of concrete blocks. Applying the reinforcement is for sure a more expensive building solution. However, contractors tend to use it in practically all construction projects - it's uncommon to use non-reinforced concrete. After a few years, you will notice that placing rebar was, in fact, a money-saving decision. Why? Because reinforced concrete slabs, blocks, driveways and buildings will last much, much longer.

Rebar sizes

For the imperial-unit countries, bar sizes give the diameter in units of the ⅛ inch for bar sizes #2 through #8. For example, 8⁄8 = #8 = 1 inch diameter. Equivalent metric size is typically specified as the nominal diameter rounded to the nearest millimeter. These sizes are not considered as standard metric sizes - they are considered soft conversion or the soft metric size. Imperial sizes system recognizes true metric bar sizes (No. 10, 12, 16, 20, 25, 28, 32, 36, 40, 50 and 60). That indicates the nominal bar diameter in millimeters.

Imperial bar size Metric bar size (soft) Nominal diameter (in) Nominal diameter (mm)
#2 No.6 0.250 = 1⁄4 6.35
#3 No.10 0.375 = ​3⁄8 9.525
#4 No.13 0.500 = ​1⁄2 12.7
#5 No.16 0.625 = ​5⁄8 15.875
#6 No.19 0.750 = ​3⁄4 19.05
#7 No.22 0.875 = ​7⁄8 22.225
#8 No.25 1.000 = ​8⁄8 25.4
#9 No.29 1.128 ≈ ​9⁄8 28.65
#10 No.32 1.270 ≈ ​10⁄8 32.26
#11 No.36 1.410 ≈ ​11⁄8 35.81
#14 No.43 1.693 ≈ ​14⁄8 43
#18 No.57 2.257 ≈ ​18⁄8 57.3

How to use the rebar calculator?

Don't worry about your rebar calculations. All that math behind the construction planning might be quite confusing, but the rebar calculator takes this upon itself. It calculates the following parameters:

  1. Rebar grid dimensions (length & width). They are calculated by subtracting the edge-rebar (grid) spacing from the sizes of the slab.

grid length = slab_length - (2 * edge_rebar_spacing)

  1. Total length of needed rebars. To calculate this, we need to know how many vertical and horizontal lines of rebars will be placed. E.g., the number of rows is calculated by dividing the grid length by the rebar-rebar spacing. To obtain the legth of rebars, multiply this number by grid width.

total rebars length = (rebar_columns * rebar_length) + (rebar_rows * rebar_width)

  1. Number of rebar pieces. To estimate this, divide total rebars length by the length of single rebar. This value should be rounded up to the nearest whole number (because we cannot buy, e.g., 0.4 rebar piece - only standard lengths).

rebar pieces = total_rebars_length / single_rebar_length

  1. Total cost of rebars. Multiply the number of rebar pieces by the price of a single rebar piece.

cost of rebars = rebar_pieces * single_rebar_price

Rebar cost - an example of calculation

Check out how to use the rebar calculator properly. Below is a step-by-step example of calculations. For its purposes, we have made some assumptions regarding the input data:

  1. First, enter the concrete slab sizes: length = 6m, width = 4m.
  2. Specify the spacings: rebar-rebar spacing = 40cm, edge-grid spacing = 8cm.
  3. Fill in the price and single rebar length that you will buy at your supplier:

rebar price = €2/m, single rebar length = 6m.
4. Finally, take a look at the calculation results:
grid length = 5.84m, grid width = 3.84m, total rebars length = 112.13m, rebar pieces = 19, rebars cost = €228.


How to calculate the rebar cost for my project?

  1. Define the total length of rebar you will use in your slab.
  2. Multiply it by the price per meter of your chosen rebar diameter — now you have the total rebar cost.
  3. You can also use the rebar calculator on the Omnicalculator website to verify your findings.

How to calculate a rebar weight?

  1. If it is standardized rebar, find the manufacturing code and check its density.
  2. With the length () and diameter (d) of the rebar, calculate the rebar volume:
    (3.14 × d² / 4) × ℓ.
  3. Multiply the volume per the material density. Now, you have the rebar weight.

Do I need rebar for my patio?

Not necessarily. When you expect heavy loads on the surface, you will require rebar because the weight will cause concrete to experiment tension stress. However, we always recommend you to check with a specialist.

Should I weld rebar?

You should avoid welding rebar. Instead, you should use a steel wire of one-sixteenth of an inch (1.6 mm) to tie the rebar. Rebar steel material typically has high carbon content, which makes them fragile after the thermal stress of uncontrolled welding. As a consequence, a structure with welded rebar may fail at a lower tension than what it was designed for.

Filip Derma
Slab dimensions
Rebar-rebar (grid)
At the supplier
Wire price
$ /
Length of a single rebar
Price of a single rebar
Grid length
Grid width
Total rebars length
# of rebar pieces
Total cost of rebars
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