Rebar Calculator
This rebar calculator estimates the reinforcing steel for a concrete slab. Enter the slab length and width, the bar size and the on-center spacing, and it lays out a two-way rebar grid, then returns the number of bars each way, the cut length of every bar, the lap splices where bars are joined, the total steel weight, the number of stock bars to buy and a printable cut list. It is scoped to slabs only: this is a two-way slab grid estimator, so footing, wall, column and beam reinforcement are out of scope and need a separate design. Every figure is derived from the slab size, bar size and spacing you enter — no hidden assumptions — so each number can be checked by hand.
Bars to buy
43
20 ft stock bars · incl. 10% waste
- Total bars
- 40
- Total length
- 780.00 ft
- Weight
- 521.04 lb
- Grid
- 20 × 20
grid bars (before splicing)
includes lap splices
bars ∥ length × ∥ width
Dimension diagram
| Tag | Direction | Qty | Cut length | Notes |
|---|---|---|---|---|
| L | Bars parallel to length | 20 | 19.50 ft | 1 piece |
| W | Bars parallel to width | 20 | 19.50 ft | 1 piece |
| Buy | 43 bars | 20 ft each | Incl. 10% waste — rebar is spliceable, so offcuts are reusable | |
| Total steel length (reference — includes lap overlap) | 780.00 ft | Not a quantity to buy — see bars above | ||
About this rebar calculator
Use this calculator whenever you need to estimate the reinforcing steel for a concrete slab — a shed base, a garage floor, a driveway or a patio. Enter the slab size, bar size and on-center spacing and every figure is derived from what you enter, so it stays checkable by hand. Reinforcing a block wall on top of the slab instead? Size the masonry with the cinder block calculator, then come back here for the slab grid. For other layout jobs, see all construction calculators.
How to use the rebar calculator
- 1
Enter the slab length and width
Measure the slab in feet and inches and enter each side (for example 20 ft 6 in). The calculator lays a grid of bars running each way across these dimensions, so accurate slab dimensions drive every result.
- 2
Pick the bar size
Choose the bar size from #3 to #8 (the number is the diameter in eighths of an inch, so #4 is 4/8 = ½ inch). #4 is the default and a common choice for residential slabs; heavier slabs use #5 or larger. See the bar-size chart for the diameter and weight of each.
- 3
Set the on-center spacing
Enter the spacing between bars, measured center to center (on-center). Typical slab grids run 12 to 18 inches on-center each way; closer spacing adds steel. The calculator fits whole bars into the net width after cover and reports the count each way.
- 4
Set the clear cover to slab edges
Enter the clear cover — the gap kept between the bars and the slab edges — which defaults to 3 inches. This inset is applied to the slab edges to size the grid; it is not a claim about top or bottom cover, which depends on your slab thickness and design.
- 5
Set the stock bar length and waste
Enter the length of the stock bars you will buy (20 ft is the default) and a waste allowance (10% by default). Because rebar can be lap-spliced, the calculator adds the total steel length, adds waste and converts it to whole stock bars to buy.
- 6
Optionally enter a price
Enter a unit price — per foot or per stock bar — to add a material cost estimate. This is optional; leave it blank to get quantities and weight only.
- 7
Read the bars to buy, weight and cut list
The calculator reports the bars each way, the total number of stock bars to buy, the total steel weight, and a cut list of bar lengths with lap splices, alongside a dimensioned grid diagram.
Rebar size reference
Look up the nominal diameter and per-foot weight for each bar size, so you can match a bar size to your slab and check the steel weight at a glance.
| Bar size | Nominal diameter | Weight |
|---|---|---|
| #3 | 0.375 in | 0.376 lb/ft |
| #4 | 0.500 in | 0.668 lb/ft |
| #5 | 0.625 in | 1.043 lb/ft |
| #6 | 0.750 in | 1.502 lb/ft |
| #7 | 0.875 in | 2.044 lb/ft |
| #8 | 1.000 in | 2.67 lb/ft |
Frequently asked questions
How much rebar do I need for a concrete slab?
It depends on the slab size, the bar size and the spacing. As a two-way grid, the calculator fits bars across the width and across the length at your on-center spacing, adds one bar to close each run, and multiplies by the cut length to get the total steel length. Enter your slab dimensions, bar size and spacing and it returns the bar count each way, the total length and weight, and the number of stock bars to buy — every figure comes from the numbers you enter, so you can check it by hand.
How much does rebar weigh per foot?
Rebar weight per foot follows the ASTM A615 nominal values by bar size: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft, #7 = 2.044 lb/ft and #8 = 2.670 lb/ft. The calculator multiplies your total steel length (including lap splices) by the per-foot weight for the bar size you pick to get the total weight. The bar-size chart on this page lists the diameter and per-foot weight for each size.
What spacing should I use for rebar in a slab?
Slab rebar grids are commonly spaced 12 to 18 inches on-center (center to center) each way, with closer spacing for heavier loads. Spacing is a design decision that depends on the slab thickness, loads and your local code, so treat these as typical ranges, not a specification. The calculator takes whatever on-center spacing you enter and fits whole bars into the slab, reporting the resulting bar count each way.
What size rebar should I use for a slab?
#4 (½ inch) is a common choice for typical residential slabs and driveways, while thicker or more heavily loaded slabs use #5 (⅝ inch) or larger. The right size depends on the slab thickness, loads and spacing and must be confirmed against your design and local code — this tool does not select a bar size for you. Once you pick a size (#3 to #8), the calculator estimates the quantity, weight and cut list for it.
How are lap splices calculated?
When a run is longer than one stock bar, bars are overlapped (lap-spliced) so the steel is continuous. This calculator uses a lap of 40 times the bar diameter — for example 40 × ½ inch = 20 inches for #4 — which is a common rule of thumb, not an exact code value; the required lap depends on concrete strength, bar coating and your local code. Because rebar can be spliced, the bars to buy are figured from the total steel length: total length ÷ stock bar length, plus a waste allowance, rounded up to whole stock bars — rather than one full stock bar per run.
Rebar terms glossary
- Lap splice
- The overlap where two bars are joined end to end so the reinforcement runs continuously. This tool uses 40 × the bar diameter as a rule-of-thumb lap length; the code-required lap depends on concrete strength, bar coating and spacing.
- Clear cover
- The clear distance between the reinforcing steel and the concrete surface, which protects the steel from corrosion. Here it is entered as the cover to the slab edges (default 3 inches) and used to size the grid inside the slab.
- On-center spacing
- The distance from the center of one bar to the center of the next, abbreviated o.c. A "12 in on-center" grid places a bar every 12 inches measured center to center.
- Bar size (#N)
- The US rebar designation where the number is the nominal diameter in eighths of an inch: #4 = 4/8 = ½ inch, #5 = ⅝ inch, and so on up to #8 = 1 inch.
- Two-way / grid reinforcement
- A mat of bars running in both directions — across the length and across the width — that reinforces the slab in two directions. This calculator estimates a single two-way grid; it does not size top and bottom mats or edge bars.