Rebar and wire mesh calculator
A 20 ft × 20 ft slab with #4 bar at 18 in on center each way and 3 in of edge cover needs 28 bars, 546 ft in all (about 364.73 lb): buy 31 bars of 20 ft with 10% extra.
Rebar to buy
31 bars of 20 ft
14 bars one way and 14 the other: 546 ft of #4 bar with laps, about 364.73 lb. The order adds 10% extra.
Rebar to buy. 31 bars of 20 ft. 14 bars one way and 14 the other: 546 ft of #4 bar with laps, about 364.73 lb. The order adds 10% extra.
| Bars running the length | 14 at 19.5 ft |
|---|---|
| Bars running the width | 14 at 19.5 ft |
| Lap length | 20 in (40 bar diameters) |
| Lap splices | 0 |
| Total bar length with laps | 546 ft |
| Steel weight | 364.73 lb |
| Stock bars to buy | 31 bars of 20 ft |
For estimating materials only. The engineer's drawings and your local building code govern bar size, spacing, cover and laps.
How it's calculated
Bars run both ways. The clear span is the slab's length or width less the edge cover at both ends. The bars across a span are the clear span divided by the spacing, rounded up, plus one: rounding up keeps the actual spacing at or under the spacing you enter. Each bar is as long as the clear span the other way.
When a bar is longer than a stock bar, stock bars are lapped. n stock bars joined by n − 1 laps reach lap + n × (stock − lap), so one bar takes (bar − lap) ÷ (stock − lap) stock bars, rounded up. The total length counts every lap.
Stock bars to buy are the total length plus the extra, divided by the stock length and rounded up. This assumes offcuts get reused; a cut list with short offcuts can need a few more.
Weight is the total length times the bar's nominal weight from ASTM A615: 0.376 lb per foot for #3, 0.668 for #4, 1.043 for #5 and 1.502 for #6 (Caltrans; Nucor). Steel at 490 lb per cubic foot gives the same weights to within 0.1%.
Cover: ACI 318-19 Table 20.5.1.3.1 calls for 3 in where concrete is cast against and permanently exposed to earth. ACI 318 does not govern most slabs on ground (§1.4.8, ACI FAQ 897); ACI 360R-10 §3.2.2, WRI TF 702-R-21 and IRC R506.2.4 put slab reinforcement in the upper third of the slab, so hold it up on chairs or supports.
Laps: 40 bar diameters is a common rule of thumb, not a code value. ACI 318-19 Table 25.5.2.1 makes a Class B lap the larger of 1.3 times the development length and 12 in. For Grade 60 bar in 3,000 psi concrete the development length is about 44 bar diameters (Maryland SHA), so a Class B lap is about 57.
Spacing: no authoritative source sets a typical residential spacing. For comparison, ACI 318-19 §24.4.3.3 limits shrinkage and temperature steel in structural slabs to the lesser of 5 times the slab thickness and 18 in.
Wire mesh: sheets and rolls overlap by one cross-wire spacing plus 2 in, 8 in for 6 × 6 mesh (ACI 318-19 §25.5.4.1; WRI WWR.7; ASCC). One piece then covers (width − overlap) × (length − overlap), and the pieces are the slab area plus the extra divided by that, rounded up. Weights are WRI's: about 21 lb per 100 sq ft for 6×6 W1.4×W1.4 and 42 lb for 6×6 W2.9×W2.9.
For estimating materials only. The engineer's drawings and your local building code govern bar size, spacing, cover and laps.
Sources
- Bridge Design Details 13.1: Reinforcement, Table 13.1.1 (bar diameters and weights), California Department of Transportation (Caltrans) (September 2025)
- Rebar: sizes, weights and stock lengths (ASTM A615/A615M-22), Nucor (2026)
- FAQ 903: concrete cover, 3 in when cast against earth (ACI 318-19 Table 20.5.1.3.1), American Concrete Institute (2026)
- FAQ 897: ACI 318 and slabs on ground (ACI 318-19 §1.4.8), American Concrete Institute (2026)
- Structural Detail Manual, Chapter 7: Reinforcing steel development and lap lengths, Maryland State Highway Administration (2017)
- TF 202-R-21: Welded wire reinforcement styles, weights, sheets and rolls, Wire Reinforcement Institute (2021)
- TF 702-R-21: Welded wire reinforcement in slabs on ground (placement), Wire Reinforcement Institute (2021)
- Drawing WWR.7: Lap splices for welded wire reinforcement, Wire Reinforcement Institute (2021)
- WWR lap splice requirements (ACI 318-19 §25.5.4.1), The Voice, American Society of Concrete Contractors (January 2021)
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement, National Institute of Standards and Technology (2026)
Last reviewed October 2026.