Rebar Calculator
Use this rebar calculator to work out how many lengths a slab grid or footing needs, how much lap steel the splices add, and what the whole lot weighs and costs.
Slab Grid
Footing or Beam
Continuous bars running the length of a footing, with laps at the splices.
Rebar Cost
How to use the Rebar Calculator
For a slab, enter the dimensions, the bar spacing and the edge cover. A grid is two separate runs of bar, one set the long way, one set the short way, and each run is counted on its own, because the number of bars in one direction is set by the slab’s dimension in the other.
For a footing, enter the run length and how many continuous bars sit side by side in the section. The splice laps are added automatically.
Bar comes in 20 ft lengths as standard, so anything longer needs a lap, and lap steel is real steel you have to buy.
Rebar Cost prices by the piece, the linear foot or the pound, which covers how the three kinds of supplier quote. Tie wire is in there too, because a grid needs a tie at every intersection and the count climbs faster than people expect.
Rebar formula
usable span = dimension − 2 × coverbars in a run = floor(usable span ÷ spacing) + 1bar length = the other dimension − 2 × coverlap length = lap diameters × bar diameterweight = total linear feet × lb per foot
The + 1 is the same off-by-one that catches stud and fence-post layouts. A grid has it twice.
| Bar | Diameter | lb per ft | 40d lap | lb per 20 ft length |
|---|---|---|---|---|
| #3 | ⅜ in | 0.376 | 15 in | 7.5 |
| #4 | ½ in | 0.668 | 20 in | 13.4 |
| #5 | ⅝ in | 1.043 | 25 in | 20.9 |
| #6 | ¾ in | 1.502 | 30 in | 30.0 |
| #7 | ⅞ in | 2.044 | 35 in | 40.9 |
| #8 | 1 in | 2.670 | 40 in | 53.4 |
The bar number is the diameter in eighths of an inch, which is why #4 is a half inch and #8 is one inch.
Worked example
A 30 × 20 ft slab, #4 bar at 18 in centres, 3 in cover, 20 ft stock:
- Usable: 29.5 ft × 19.5 ft after cover both sides
- Bars running the 30 ft way: floor(19.5 × 12 ÷ 18) + 1 = 14 bars, each 29.5 ft long
- Bars running the 20 ft way: floor(29.5 × 12 ÷ 18) + 1 = 20 bars, each 19.5 ft long
- Steel: 14 × 29.5 + 20 × 19.5 = 413 + 390 = 803 linear feet
- Laps: the 29.5 ft bars exceed 20 ft stock, so each needs one splice
- Weight: 803 × 0.668 = 536 lb before laps
Note the crossing count: 14 × 20 = 280 intersections to tie.
What the result means
A grid is two runs, and each is counted off the opposite dimension. The bars running lengthwise are spaced across the width, so it is the width that decides how many there are. Getting that backwards is the most common error on a slab take-off.
Cover is structural, not cosmetic. Three inches where concrete is cast against ground, less where it is formed. Rebar sitting too close to the surface rusts, and rust expands and spalls the slab off.
Rebar has to be up in the slab, not on the ground. Chairs or dobies hold it at mid-depth or in the upper third. Bar lying on the subgrade does nothing at all, a very common and completely wasted expense.
Common mistakes
- Counting one run and doubling it. The two directions have different bar counts and different bar lengths unless the slab is square.
- Forgetting the last bar in each run. Spaces plus one, in both directions.
- Ignoring laps. Anything longer than the stock length needs a splice, and 40 diameters on #4 is 20 inches of extra steel per joint.
- Laying bar on the ground. It must sit in the concrete, on chairs. On the subgrade it is decoration.
- Treating this as a design. Bar size, spacing and cover come from an engineer or from code for anything structural.
Frequently asked questions
How much rebar do I need for a 30×20 slab?
At 18 inch spacing with 3 inch cover, 14 bars running the long way at 29.5 feet each and 20 bars running the short way at 19.5 feet each, about 803 linear feet, or roughly 536 pounds in number 4 bar, before splice laps.
What spacing should rebar be in a slab?
Twelve to eighteen inches on centre each way is typical for residential slabs, with 18 inches common for a simple patio or garage floor and 12 inches where loads are higher. Anything structural should follow an engineer or your local code rather than a rule of thumb.
How long should a rebar lap be?
Forty bar diameters is the common rule of thumb, so 20 inches for number 4 bar and 25 inches for number 5. The actual requirement depends on concrete strength, bar grade and whether the splice is in tension, and is set by code or by your engineer.
How much does rebar weigh per foot?
Number 3 is 0.376 pounds per foot, number 4 is 0.668, number 5 is 1.043 and number 6 is 1.502. A standard 20 foot length of number 4 therefore weighs about 13.4 pounds, which matters when you are pricing by weight or carrying it.
What does the number on rebar mean?
The diameter in eighths of an inch. Number 4 bar is four eighths, or half an inch; number 8 is eight eighths, or one inch. It is the simplest thing about rebar sizing and it catches people out constantly.
How much concrete cover does rebar need?
Three inches where concrete is cast directly against the ground, one and a half inches for formed surfaces exposed to weather, and three quarters of an inch for interior slabs. Cover protects the steel from moisture, too little and the bar rusts and spalls the concrete off.
Sources and assumptions
Bar diameters and unit weights are the ASTM A615 standard sizes used in the United States. The bar number is the nominal diameter in eighths of an inch.
Lap length defaults to 40 bar diameters, a widely used rule of thumb. The governing figure comes from ACI 318 and depends on concrete strength, bar grade, coating and whether the splice is in tension or compression.
Cover figures quoted above follow common ACI minimums. Your local code may differ.
This tool estimates quantities only. Bar size, spacing, cover and lap length for any structural element are a design decision for an engineer, not a calculator.
Bar sizes are the standard designations in ASTM A615. Lap lengths and cover are governed by ACI 318 and vary with concrete strength, so treat the defaults here as a starting point rather than a specification.