Rebar Calculator

Enter your slab, bar spacing and edge clearance to get bars in each direction, feet of rebar, stock bars and weight.

Slab length

Slab width

Bar spacing, center to center

Take it from your plans. It is treated as the maximum gap. The 18 shown is only an example.

Edge clearance

Distance from the slab edge to the first bar. Take it from your plans. Empty means 0. The 3 shown is only an example.

Bar size

Sets the weight per foot. Take the size from your plans. #4 is preselected only as an example.

Layers of bars

Stock bar length (ft)

Optional. Ask your supplier. The 20 shown is only an example. Empty skips the count.

Extra to add (%)

Optional. For laps, cuts and waste. Empty means 0.

How to calculate how much rebar you need

Rebar for a slab is laid as a grid. Some bars run along the length and some run along the width. The calculator counts both sets, adds up the feet and turns them into stock bars and weight.

Step	Formula
1. Clear size	slab length − 2 × clearance, and slab width − 2 × clearance
2. Bars along the length	(clear width ÷ spacing, rounded up) + 1, each as long as the clear length
3. Bars along the width	(clear length ÷ spacing, rounded up) + 1, each as long as the clear width
4. Feet per layer	bars along the length × clear length + bars along the width × clear width
5. Total feet	feet per layer × layers × (1 + extra ÷ 100)
6. Stock bars	total feet ÷ stock bar length, rounded up
7. Weight	total feet × pounds per foot of the bar size

The spacing is treated as a maximum. The bars are spread evenly, so the real gap is equal to or a little smaller than the number you type. That is why the count is rounded up and one bar is added. The calculator reads measurements typed like 10'6" or 10 ft 6 in, and it accepts inches, feet, yards, centimeters and meters.

Example: a 16 ft by 12 ft slab

A slab measures 16 ft by 12 ft. The plans call for #4 bars at 18 inches with 3 inches of clearance at every edge, and you want 10% extra. The supplier stocks 20 ft bars.

Step	Operation	Result
Clear size	16 − 2 × 0.25 and 12 − 2 × 0.25	15.5 ft × 11.5 ft
Bars along the length	(11.5 ÷ 1.5 rounded up) + 1	9 bars of 15.5 ft
Bars along the width	(15.5 ÷ 1.5 rounded up) + 1	12 bars of 11.5 ft
Feet per layer	9 × 15.5 + 12 × 11.5	277.5 ft
With 10% extra	277.5 × 1.10	305.25 ft
Stock bars	305.25 ÷ 20 rounded up	16 bars
Weight	305.25 × 0.668 lb/ft	203.9 lb

Weight per foot of each bar size

The weights come from Table 1 of Item 440 in the 2014 Standard Specifications of the Texas Department of Transportation, checked in October 2026. The document says the table shows the nominal size, area and weight of the reinforcing bars the item covers, which are bars made to ASTM A615, A996 or A706. These are nominal figures, so ask your supplier if you need the exact weight of a specific product.

Bar size	Nominal diameter (in)	Weight (lb per ft)
#3	0.375	0.376
#4	0.500	0.668
#5	0.625	1.043
#6	0.750	1.502
#7	0.875	2.044
#8	1.000	2.670
#9	1.128	3.400
#10	1.270	4.303
#11	1.410	5.313

Feet of rebar per layer for common slabs

Total feet of bars in one layer, with 3 inches of clearance at every edge. Multiply by the number of layers and add your extra. Read the bar counts and the weight in the calculator above.

Slab	12 in spacing	16 in spacing	18 in spacing	24 in spacing
10 ft × 10 ft	209 ft	171 ft	152 ft	114 ft
12 ft × 12 ft	299 ft	230 ft	207 ft	161 ft
10 ft × 20 ft	414 ft	327.5 ft	289 ft	221.5 ft
20 ft × 20 ft	819 ft	624 ft	546 ft	429 ft

Tips for ordering

Take the bar size, the spacing and the edge clearance from your plans. This page counts material and does not choose them.
Ask the supplier which stock lengths they sell, and type that length in the calculator to get the number of bars.
The count does not add laps, bends or the supports that hold the grid off the ground. Use the extra box if you know an allowance.
For a slab with two layers, pick “2 layers”. The result doubles the feet, the bars and the weight.
To find the concrete for the same slab, use the concrete calculator. For the area alone, use the square footage calculator.

Frequently asked questions

How do I calculate how much rebar I need for a slab?

Subtract the edge clearance from both sides of each dimension. Divide each clear dimension by the bar spacing, round up and add one. That gives the number of bars in each direction. Multiply each count by the length of its bars and add the two. A 16 ft by 12 ft slab with 18 inch spacing and 3 inches of clearance needs 9 bars of 15.5 ft and 12 bars of 11.5 ft, which is 277.5 ft.

How many bars do I need for a 10 by 10 slab?

It depends on the spacing and the clearance. With 3 inches of clearance, one layer takes 22 bars at 12 inch spacing, 18 bars at 16 inch spacing, 16 bars at 18 inch spacing and 12 bars at 24 inch spacing. Each bar is 9.5 ft long. Type your own spacing in the calculator for the exact count.

How much does rebar weigh per foot?

It depends on the bar size. The TxDOT table checked in October 2026 gives 0.376 lb per foot for a #3 bar, 0.668 for #4, 1.043 for #5, 1.502 for #6, 2.044 for #7 and 2.670 for #8. The calculator multiplies your total feet by the figure for the bar you pick.

What size and spacing of rebar should I use?

This is a structural decision, so it comes from your plans, your engineer or your local building code. This calculator only counts the material for the size and spacing you give it. It does not suggest either one.

How many sticks of rebar do I need?

Divide the total feet by the length of the bars your supplier stocks and round up. Enter that length in the calculator and it does this for you. The count assumes that offcuts are reused, so it does not add bars for laps. Ask your supplier which lengths they sell.

Do I need extra rebar for overlaps and waste?

Where bars must overlap and by how much is set by your plans or your local code. The calculator does not add laps on its own. If you know the allowance, type it as a percentage in the extra box and it raises the feet, the bars and the weight together. Leave it empty to add nothing.

How much does rebar cost?

Prices change by region, bar size and length, so this calculator does not show them. Take the total feet, the stock bars or the weight from the result and ask local suppliers for a quote in the same unit.