How it works
The calculator lays bars in both directions: each run starts and ends one edge cover in from the slab edges, and the number of bars is rounded up so the gaps are never wider than your spacing. Then it works out how the bars are cut from stock lengths.
Bars along the length = ⌈ (Width − 2 × Cover) ÷ Spacing ⌉ + 1, each Length − 2 × Cover long
Bars along the width = ⌈ (Length − 2 × Cover) ÷ Spacing ⌉ + 1, each Width − 2 × Cover long
Bar ≤ stock: Stock bars = ⌈ Bars ÷ ⌊ Stock ÷ Bar length ⌋ ⌉
Bar > stock: Splices per bar m = ⌈ (Bar length − Stock) ÷ (Stock − Lap) ⌉
Total length = Σ Bars × (Bar length + m × Lap)
Weight = Total length × Weight per length × (1 + Extra ÷ 100)
⌈ ⌉ means rounding up and ⌊ ⌋ rounding down. A spliced bar uses m full stock bars plus one last piece of Bar length + m × Lap − m × Stock, and those last pieces are cut from stock bars as many as fit. The extra allowance is applied to the stock bar count and the weight.
US bar weights are the ASTM nominal values listed by Nucor and in Caltrans’ bar data table; metric masses are Celsa Steel UK’s BS 4449 figures. The 20 ft stock length is the shortest of Nucor’s standard cut lengths. This tool counts material only; bar size, spacing, cover and lap length are design decisions for your plans, local code or an engineer.
Worked example
Inputs
- Slab length
- 20 ft
- Slab width
- 10 ft
- Bar spacing (on center)
- 12 in
- Edge cover
- 3 in
- Bar size
- #4 (1/2 in) – 0.668 lb/ft
- Stock bar length
- 20 ft
- Extra bars
- 0 %
Result
- Stock bars to buy
- 22
- Total bar length
- 414 ft
- Weight
- 276.6 lb (0.138 short tons)
- Bars along the length
- 11
- Bars along the width
- 21
- Lap splices
- 0
Common mistakes
- Dividing the slab size by the spacing and stopping there. A run with bars at both ends needs one more bar than the number of spaces.
- Forgetting the edge cover. Bars stop short of the slab edges, so each bar is shorter than the slab by twice the cover.
- Assuming offcuts can be used anywhere. A 12 ft bar cut from a 20 ft stock bar leaves 8 ft that only fits a shorter bar or a lap.
- Leaving out lap splices on long runs. Every splice adds one lap length of steel.
- Taking bar size, spacing, cover or lap length from a calculator. Those are design values from your plans, local code or an engineer.
Assumptions and limits
- One layer of straight bars over a rectangular slab, the same spacing both ways. Bends, hooks, dowels, chairs, tie wire and a second mat are not included.
- The number of bars is rounded up so the spacing between them is never larger than the spacing you enter.
- A bar that fits the stock length is cut from it, as many as fit per stock bar. A longer bar is made of full stock bars plus one shorter piece, joined with the lap length you enter.
- Weights are nominal (ASTM bar weights from Nucor and Caltrans; BS 4449 masses from Celsa Steel UK). Actual weight varies slightly by mill.
- This is a material quantity only, not a structural design.
Frequently asked questions
How do I calculate how much rebar I need for a slab?
Subtract the edge cover from both ends, divide by the spacing, round up and add one – that gives the number of bars running each way. For a 20 × 10 ft slab with a 12 in grid and 3 in cover, that is 11 bars 19.5 ft long and 21 bars 9.5 ft long, 414 ft in total.
How many 20 ft bars is that?
Each 19.5 ft bar needs its own 20 ft stock bar (11), and two 9.5 ft bars come from one (21 ÷ 2 → 11), so 22 stock bars. Adding up the feet and dividing by 20 would give 21, which is not enough because the 0.5 ft offcuts are too short to use.
How much does rebar weigh?
Multiply the total length by the weight per foot. Nucor lists #4 bar at 0.668 lb/ft, so 414 ft of #4 weighs about 277 lb. For metric bars, Celsa Steel UK lists 12 mm bar at 0.888 kg/m.
What spacing and bar size should I use?
Those depend on the slab, its load and your local code. This calculator doesn’t recommend them – use your plans or ask a qualified professional.
Sources
- Steel Rebar – Rebar Sizes — Nucor Corporation. Rebar in cut lengths of 20, 40 and 60 ft (stock-length default 20 ft); weight per foot: #3 0.376 lb, #4 0.668 lb, #5 1.043 lb, #6 1.502 lb.
- Bridge Design Details 13.1 Reinforcement, Table 13.1.1 ASTM Standard Reinforcing Bar Data (September 2025) — California Department of Transportation (Caltrans). Nominal diameter and weight of ASTM bars: #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.
- Reinforcing bar – Grade B500C to BS 4449:2005 + A3:2016 — Celsa Steel UK (now 7 Steel UK). Mass per metre: 10 mm 0.616 kg, 12 mm 0.888 kg, 16 mm 1.579 kg, 20 mm 2.466 kg; standard lengths 6, 12, 14 or 15 m depending on the diameter, so no metric stock-length default.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 4, 2026