How to use the rebar calculator
- Enter the slab. Type the length and the width of the slab and choose the unit for each.
- Enter the grid. Type the spacing between bars and the clearance from the edge of the slab. Choose the bar size, the length of the bars you will buy and, if bars must be joined, the lap length.
- Read the bars and the weight. The result shows the stock bars to buy, the total length, the weight, the number of bars in each direction and the crossing points.
What is a rebar calculator?
A rebar calculator is a tool that estimates the steel reinforcing bars for a concrete slab. Rebar in a slab is laid as a grid: one set of bars runs along the length, and a second set crosses it along the width. The calculator counts the bars in each direction, adds their lengths and converts the total into stock bars and weight.
It answers how much rebar to buy. It does not design the slab: the bar size and the spacing must come from your plan.
Features
- The number of bars in each direction for a rectangular slab
- The total bar length in feet and metres
- The number of stock bars to buy, for any stock length
- The weight of the steel for bar sizes #3 to #8
- Lap splices added where a bar is longer than the stock length
- The number of crossing points, where bars are tied
- A live top view of the grid that redraws as you type
- A cost from your own price per bar, per foot or per pound
- A PDF of the estimate and a WhatsApp share button
What you can use it for
- Ordering bars for a patio, a shed base or a garage slab
- Checking a supplier's quote by length or by weight
- Comparing bar sizes and spacings before you buy
- Counting the crossing points that need a tie
How the rebar grid is calculated
- Length of a bar along the length = slab length − 2 × edge clearance.
- Number of bars along the length = (slab width − 2 × edge clearance) ÷ spacing, rounded down, plus 1.
- The bars along the width are worked out the same way, with length and width swapped.
- Total length = the bars in each direction × their length, plus the laps.
- Weight = total length × the weight of the bar per foot.
Worked example: a 20 ft by 10 ft slab, #4 bars at 12 inches
- Edge clearance: 3 in = 0.25 ft on each side.
- Bars along the length: each is 20 − 0.5 = 19.5 ft. Number: (10 − 0.5) ÷ 1 = 9.5, rounded down to 9, plus 1 = 10 bars.
- Bars along the width: each is 10 − 0.5 = 9.5 ft. Number: (20 − 0.5) ÷ 1 = 19.5, rounded down to 19, plus 1 = 20 bars.
- Total length: 10 × 19.5 + 20 × 9.5 = 385 ft. With 10% waste: 423.5 ft.
- Stock bars of 20 ft: 423.5 ÷ 20 = 21.2, so 22 bars.
- Weight of #4 bar at 0.668 lb per foot: 423.5 × 0.668 = 283 lb.
- Crossing points: 10 × 20 = 200.
These are the numbers the calculator shows when the page opens.
Lap splices
When a bar is longer than the stock length, it is made from two or more pieces that overlap. The overlap is the lap splice. Enter the lap length and the calculator adds one lap for every joint. For a 30 ft slab with 20 ft stock bars, each 29.5 ft bar is two pieces with one lap.
The lap length is a design value. It depends on the bar size, the concrete and the building code, so this calculator has no built-in figure for it.
Rebar sizes and weights
The bar number is the diameter in eighths of an inch, so a #4 bar is 4/8 inch, which is half an inch. The weights below are from the California Department of Transportation Bridge Design Details 13.1 (September 2025), Table 13.1.1, ASTM standard reinforcing bar data.
| Bar size | Diameter | Weight per foot | Weight of a 20 ft bar |
|---|---|---|---|
| #3 | 0.375 in | 0.376 lb | 7.5 lb |
| #4 | 0.500 in | 0.668 lb | 13.4 lb |
| #5 | 0.625 in | 1.043 lb | 20.9 lb |
| #6 | 0.750 in | 1.502 lb | 30.0 lb |
| #7 | 0.875 in | 2.044 lb | 40.9 lb |
| #8 | 1.000 in | 2.670 lb | 53.4 lb |
What the calculator includes
- The slab is a rectangle with one layer of bars in each direction. Two layers, footings, walls, columns and stirrups are not covered.
- The stock bar count divides the total length by the stock length. It does not plan how each bar is cut, so offcuts can change the real number.
- Tie wire, bar chairs and wire mesh are not included.
- Each dimension can be up to 10,000 in its unit, waste 0 to 50%, and a grid can have up to 10,000 bars in one direction.
Is it private?
Yes. The calculation and the PDF are made in your browser. The numbers you type are not sent to our server and are not saved.
Tips for an accurate estimate
- Use the bar size and spacing from your plan; the calculator counts what you enter.
- Set the edge clearance so no bar sits at the very edge of the slab.
- Choose the stock length your yard sells, so the number of bars to buy matches the store.
- If bars must be joined, enter the lap length from your plan to add it to the total.
- Keep the waste allowance for offcuts, because cut ends are rarely reused.
Related calculators
Frequently asked questions
- How much rebar do I need for a slab?
- Count the bars in each direction and add their lengths. A 20 ft by 10 ft slab with bars every 12 inches and 3 inches of edge clearance needs 10 bars of 19.5 ft and 20 bars of 9.5 ft, which is 385 ft of rebar before waste.
- How do I calculate the number of rebar bars?
- Take the slab width, subtract the edge clearance on both sides, divide by the spacing, round down and add 1. That is the number of bars running along the length. Do the same with the length for the bars running along the width.
- How much does rebar weigh per foot?
- A #3 bar weighs 0.376 lb per foot, a #4 bar 0.668 lb and a #5 bar 1.043 lb. These figures are from the Caltrans Bridge Design Details table of ASTM standard reinforcing bars.
- How much does a 20 ft piece of #4 rebar weigh?
- About 13.4 pounds: 20 ft × 0.668 lb per foot.
- What spacing should I use for rebar in a slab?
- This calculator does not choose the spacing. It depends on the load, the slab thickness and the building code, so take it from your plan or your engineer. The 12 inches shown when the page opens is only a starting value.
- What is a lap splice?
- A lap splice is the overlap where two bars are joined to make a longer one. The length of the lap is set by the design, so enter the value from your plan and the calculator adds it for every joint.
Sources
Every figure this calculator uses comes from the published sources below. The formulas are on this page, so any result can be checked by hand.
- California Department of Transportation (Caltrans), Bridge Design Details 13.1 (September 2025), ASTM reinforcing bar table. Diameter and weight per foot of bar sizes #3 to #8.