Rebar Calculator
How many bars, how much bar and how much it weighs, for a slab grid or straight runs — from the spacing, cover and bar size on your drawings.
The working
Bars each way = floor((Dimension − 2 × Cover) ÷ Spacing) + 1
Weight
pounds
Waiting on you
Choose the bar size from your drawings. Nothing here assumes one: the weight per foot depends on it.
Enter the size of the slab, the bar size, the spacing and the cover from your drawings, and the calculator counts the bars in each direction, adds up the length, weighs it from the tabulated weight per foot, and, if you give a stock length, counts the sticks to buy. It suggests no spacing, no cover and no lap length, because those are design decisions and it cannot know which kind of slab you are pouring.
How this calculator works
Only your inputs and exact unit conversions. Where a figure comes from anywhere else, it is recorded below with its source.
- Formula version
- 1.0.0
- Last reviewed
- 2026-09-29
- External data
- 1 source
- Category
- Materials
What this calculates
The bar in a slab or a run of footings: how many bars, how long each is, the total length, its weight, and how many sticks that is at the length your supplier stocks. It is a quantity take-off from a layout someone else has designed.
The count
For a slab grid the bars run both ways. The number that run along the length is spaced across the width: floor((width − 2 × cover) ÷ spacing) + 1. The number that run along the width is floor((length − 2 × cover) ÷ spacing) + 1. Each bar is its own dimension less the cover at both ends. The “+ 1” is there because n spaces have n + 1 bars, and the floor is there because a partial space cannot hold a bar.
Two layers double both counts. For straight runs, footings and beams, you enter the length of each run and how many bars there are.
The weight
Total length times the weight per foot of the bar size. The weights are the nominal figures in the Texas Department of Transportation’s 2024 Standard Specifications, Item 440, Table 1 (bars #3 to #18), used exactly as printed and recorded with their source on this page. They are nominal values for ASTM A615 bars: what arrives on the truck varies within the standard’s tolerance, so treat the weight as an estimate for ordering and hauling, not a scale reading.
Sticks to buy, and splices
Enter the stock length and the calculator counts whole sticks: how many bars of each length fit in a stick, rounded up, with no reuse of offcuts. If a bar is longer than the stock length it has to be spliced, and the lap length depends on the bar size, the concrete and the code, so it is yours to enter. With a lap the calculator counts the pieces each bar needs, adds a lap of extra length at every splice, and includes that in the weight.
What is deliberately not here
Spacing, cover, lap length, bar size and number of layers are design decisions made by whoever designed the element. This calculator does not suggest them, does not fill them in from a project type, and does not check them against a code: a spacing that is right for a patio is wrong for a load-bearing slab, and the tool cannot tell which it has been asked about. It also publishes no price. Bar size, spacing and layout for any load-bearing element must come from the stamped drawings, the local code or a licensed engineer.
A 20 ft × 12 ft slab, one layer
The spacing, cover and bar size below are made up to show the arithmetic. They are not a recommendation for any slab: yours come from your drawings.
- Slab
- 20 ft × 12 ft
- Bar size
- #4 (0.668 lb per foot)
- Spacing
- 12 in
- Cover
- 3 in (0.25 ft)
- 01Bars along the length= floor((12 − 0.5) ÷ 1) + 1= 12 bars, each 19.5 ft
- 02Bars along the width= floor((20 − 0.5) ÷ 1) + 1= 20 bars, each 11.5 ft
- 03Total length= (12 × 19.5) + (20 × 11.5)= 464 ft
- 04Weight= 464 × 0.668= 309.95 lb
About 310 lb, 32 bars
What it assumes
- 01The layout is a regular grid with the same spacing both ways, or straight runs of equal length.
- 02Each bar stops one cover short of the edge at both ends.
- 03Weight is the nominal weight per foot for the bar size chosen.
- 04Sticks are counted whole, with no reuse of offcuts.
What it does not do
- It does not design or check reinforcement. It counts a layout it is given.
- It does not model bends, hooks, stirrups, ties, chairs, dowels or corners.
- It covers imperial bar sizes #3 to #18 only: no #2, metric bars, or coated or stainless bar.
- It does not know your supplier’s stock lengths, minimum orders or tolerances.
Common questions.
- How do I calculate how much rebar I need for a slab?
- Count the bars each way, multiply by their lengths, add them up, and multiply the total length by the weight per foot of the bar size. Bars in one direction number floor((other dimension − 2 × cover) ÷ spacing) + 1, and each is its own dimension less the cover at both ends. The calculator does each step and shows the working. The spacing, cover and bar size come from your drawings.
- How much does #4 rebar weigh per foot?
- 0.668 pounds per foot, nominal, in the Texas Department of Transportation’s 2024 specification table. The weights of all eleven sizes, #3 to #18, with their metric sizes, are in the rebar size chart guide linked on this page.
- What size rebar and spacing should I use?
- That is a design question, and it depends on what the slab or footing carries, the soil, the concrete and the local code, so this calculator does not answer it. Use the bar size and spacing on your drawings, or ask the engineer or building department. If you enter them here, the calculator will count exactly that layout.
- Why does it not fill in a spacing or a cover for me?
- Because a value it picked would look like advice. A spacing that is fine for a patio can be wrong for a load-bearing slab, and the tool cannot tell which it has been given. The spacing and cover fields stay empty until you enter them.
- What is a lap splice, and why does it ask for one?
- A splice is where one bar overlaps the next to make a longer run, and the lap is the length of the overlap. It matters when a bar is longer than the stock length you buy. The required lap depends on the bar size, the concrete and the code, so the calculator asks for it and adds that much extra bar at each splice.
- How many sticks of rebar do I need?
- Enter the stock length your supplier delivers. The calculator counts how many bars of each length fit in a stick, rounds up, and does not reuse offcuts. If the layout needs bars longer than the stock, it asks for the lap length.
Behind the numbers
The methods and definitions this calculator applies, explained in full and with their sources.
Where people go next
Ordered by what usually comes next in the same job, not by keyword.