Rebar Calculator
Find how many bars of rebar you need, and what they weigh and cost.
Measure your job
The IRC asks for 3 in of concrete over bars cast against the ground. So we stop the bars that far from each edge. We use the IRC's laps for footings and walls with Grade 60 bars (#4: 30 in). Grade 40 needs less (#4: 20 in). For #3 we use Texas DOT's 16 in.
Your answer
#4 rebar. A grid every 12 in, both ways.
We cut short pieces from whole bars. We don't join cut-off ends again, so you may have a few spare ends.
This answer is a guide. Bars, spacing and laps for anything that holds weight must follow your plans. Check your local building rules or ask a builder or engineer.
What it costs
Prices are the middle of 3 to 5 US sellers per size (Oct 2026), before tax.
Just a slab? Our rebar for slab calculator adds wire mesh.
Quick answers
- A 10 × 10 ft slab, bars 12 in apart
- 10 bars of 20 ft (#4).
- #4 bar weight
- 0.668 lb per foot, so 13.36 lb for a 20 ft bar (Nucor).
- #4 bars per ton
- About 150 bars of 20 ft. Bundles vary.
- Lap for #4
- For walls and footings, the IRC (US home code) gives 30 in with Grade 60 bars. Grade 40 needs 20 in.
- Price of a #4 bar
- $12.51 for 20 ft, the middle of 5 sellers (Oct 2026).
How to work it out
- Measure the slab or footing in feet.
- Pick the bar size. For a slab, pick the space between bars too.
- Count the bars. Where a run is longer than one bar, add a lap.
- Cut short pieces from whole bars. Count the whole bars to buy.
Footing: 2 runs of 40 ft, #4 bars sold 20 ft long
Each run: 3 bars, 2 laps of 30 in = 45 ft
That is 2 full bars + a 5 ft piece
One 20 ft bar gives 4 of those pieces
2 × 2 + 2 ÷ 4, rounded up = 5 bars
5 × 20 ft × 0.668 lb = 67 lb
Bar sizes, weights and bars per ton
Rebar is sold by number. Nucor lists 11 sizes, from #3 to #18. This calculator covers #3 to #6. For those, the number is the thickness in eighths of an inch.
| Bar | Thick | lb per ft | lb per 20 ft bar | Bars per ton |
|---|---|---|---|---|
| #3 | ⅜ in | 0.376 | 7.52 | 266 |
| #4 | ½ in | 0.668 | 13.36 | 150 |
| #5 | ⅝ in | 1.043 | 20.86 | 96 |
| #6 | ¾ in | 1.502 | 30.04 | 67 |
Weights from Nucor. Bars per ton match Sonoma Reinforcing's chart for #3 to #5.
Nucor makes bars 20, 40 and 60 ft long. The stores we checked sell 10 and 20 ft bars.
A bundle is not one fixed size. Sonoma Reinforcing ships #4 bars 150 or 300 to a bundle, about 1 or 2 tons. So ask for bars, or for weight, not "a bundle".
Laps: how far bars overlap
When one bar is too short, two bars overlap. This is a lap. Longer laps use more steel.
| Bar | IRC, Grade 40 | IRC, Grade 60 | Texas DOT |
|---|---|---|---|
| #3 | not listed | not listed | 16 in |
| #4 | 20 in | 30 in | 21 in |
| #5 | 25 in | 38 in | 26 in |
| #6 | 30 in | 45 in | 31 in |
IRC is the US home building code (Table R608.5.4(1)). Texas DOT is a state highway spec.
The two don't agree. For #4, the IRC asks 20 in for Grade 40 steel. Texas DOT asks 21 in.
The IRC gives laps for concrete walls, and for footings in Seismic Design Categories D0, D1 and D2. It gives none for slabs, so we use the same numbers there.
We start with the Grade 60 laps, which are longer. The IRC table has no #3, so for #3 we use Texas DOT's 16 in. You can type your own lap in step 1.
Concrete cover, and footings
Cover is the concrete between the bar and the outside. For footings and concrete walls, the IRC asks for at least:
- 3 in where concrete is cast against the ground.
- In forms open to soil or weather: 1½ in for #5 and smaller, 2 in for #6 and larger.
- ¾ in in forms that are not open to soil or weather.
Texas DOT allows 1 in, but that is for highway work. It is less than the IRC, so don't use it for a house.
Rebar in house footings
Footing bars are an IRC rule only in Seismic Design Categories D0, D1 and D2. For the full rule, see our concrete footing calculator.
Footings hold up the house. Check your local building rules or ask a builder or engineer.
How the calculator counts bars
It cuts short pieces from whole bars. Take a 10 × 10 ft slab with bars 12 in apart. It needs 10 bars each way, each 9.5 ft long.
One 20 ft bar gives 2 pieces that long. So 10 pieces take 5 bars each way, or 10 in all.
Cut-off ends are not joined again. So you may have a few spare ends.
| Bar | Length | Middle price | Sellers |
|---|---|---|---|
| #3 | 20 ft | $7.27 | 4 |
| #4 | 10 ft | $7.98 | 3 (rough) |
| #4 | 20 ft | $12.51 | 5 |
| #5 | 20 ft | $18.05 | 4 |
US sellers, Oct 2026, before tax. Prices mix Grade 40 and Grade 60 bars. One seller's may include delivery. With 3 sellers or fewer, treat the price as rough.
Need rebar or mesh in a slab? See the rebar for slab calculator.
Questions people ask
How much rebar do I need?
For a footing 40 ft long with 2 #4 bars side by side, buy 5 bars of 20 ft. That counts the laps. Type your own sizes above.
Do sidewalks need rebar?
It depends on your city. Sugar City, Idaho, does not require rebar in a 4 in walk. Warren, Pennsylvania, requires welded wire mesh in a 4 in walk. Sanford, North Carolina, lets the city engineer ask for mesh where traffic is heavy. Palatine, Illinois, does not allow wire mesh in the apron or public walk.
How many #4 rebar are in a bundle?
It varies. Sonoma Reinforcing sells #4 bars, 20 ft long, 150 or 300 to a bundle. Ask your seller.
How many #4 rebar are in a ton?
About 150 bars of 20 ft. Each one weighs 13.36 lb.
How many sizes of rebar are there?
Nucor lists 11, from #3 to #18. For #3 to #6, the number is the thickness in eighths of an inch. So #4 is 4/8, or ½ in.
What size rebar goes in a footing?
Where the IRC asks for footing bars, it names #4 bars. The calculator joins #4 bars with 30 in laps (Grade 60). Follow your plans.
How long should a rebar lap be?
For #4, the IRC's wall and footing table says 30 in with Grade 60 bars. With Grade 40, it says 20 in. It gives no laps for slabs.
Where our numbers come from
- Rebar sizes and weights: Nucor
- Rebar bars per ton: Sonoma Reinforcing chart
- Rebar laps and wall weights: US home building code (IRC R608)
- Footings, frost and footing rebar: US home building code (IRC R403)
- Rebar and mesh laps: Texas DOT Item 440
- Rebar spacing in a driveway: Concrete Network (a third-party guide)
- Rebar spacing guide: MyConcreteCalc (a third-party guide)
- Sidewalk rule, Sugar City, Idaho (no rebar needed)
- Sidewalk rule, Warren, Pennsylvania (mesh needed)
- Sidewalk rule, Sanford, North Carolina (engineer decides)
- Rebar prices: 3 to 5 US sellers per size (Oct 2026)
Made by BuildersCount. Last checked 3 Oct 2026.
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