Rebar Calculator

Estimate bar count, total linear length, weight and number of standard bars to order for a rectangular slab — ASTM A615 (#3–#8) in imperial or metric.

Slab & grid

ft
ft
in
ACI 318-19: 3 in (75 mm) for concrete cast against earth.
0.668 lb/ft · 0.994 kg/m
in
in
Pick the length your supplier actually stocks.
%
10 % covers cuts, lap splices and offcuts.
$

Calculation results

Standard bars to order

— bars

Total weight

— lb

Bars in grid (no waste)

— bars

Bar count uses the standard fence-post rule: bars are placed at the cover line and at every spacing interval. Order rounds up to whole bars including the waste factor.

Informational only. These results are estimates for planning and ordering. Reinforcement design for structural slabs, footings or beams must be sized by a licensed engineer to ACI 318-19 or local code.

Understanding the formula

Bar counts use the fence-post rule: bars start at the cover line on each edge and repeat at every spacing interval. Linear length is the count times the slab dimension in each direction. Standard bars are the linear length divided by your stock length, rounded up after the waste factor.

n_long = ⌊(W − 2c) / s_long⌋ + 1 · n_trans = ⌊(L − 2c) / s_trans⌋ + 1
linear = n_long × L + n_trans × W · ordered = ⌈ linear × (1 + waste %) / stock ⌉

Weight is the ordered length times the ASTM A615 unit weight per foot: 0.376 (#3), 0.668 (#4), 1.043 (#5), 1.502 (#6), 2.044 (#7), 2.670 (#8).

Worked example

A 20 × 10 ft slab with #4 bars at 12″ o.c. each way, 3″ cover, 20-ft stock and 10 % waste:

  • nlong = ⌊(10 − 0.5) / 1⌋ + 1 = 10 bars (running long way)
  • ntrans = ⌊(20 − 0.5) / 1⌋ + 1 = 20 bars (running short way)
  • Linear = 10 × 20 + 20 × 10 = 400 ft
  • Ordered = ⌈ 400 × 1.10 / 20 ⌉ = 22 bars
  • Weight = 22 × 20 × 0.668 = ~294 lb

When to use this calculator

It’s built for the most common reinforcement layout in residential and light-commercial concrete: a single mat in a rectangular slab on grade.

  • Patios and driveways: #3 or #4 at 18″–24″ o.c. each way, 3″ cover.
  • Garage and shop slabs: #4 at 12″–16″ o.c. each way over compacted base.
  • Footings and grade beams: top and bottom mats — run the calculator twice and add the results.
  • Wall reinforcement: treat the wall like a slab with vertical and horizontal bars.

The calculator does not size reinforcement for load — that’s the job of a structural engineer. It also does not account for chairs, dowels, hooks or splice details; add those by hand for a complete material take-off.

Common mistakes & tips

  • Don’t skip the cover. Cover protects the steel from corrosion. ACI 318-19 mandates 3″ (75 mm) for concrete cast against and permanently in contact with the ground; ¾″ for slabs not exposed to earth or weather.
  • Use the right stock length. Big-box retailers sell 20 ft or 6 m; commercial fabricators stock 40 ft or 12 m. Using a longer stock length reduces splice count but increases delivery cost on small jobs.
  • Bump the waste factor on complex jobs. 10 % covers cuts and minor offcuts; for jobs with many cut-outs (drains, plumbing penetrations) push it to 15 %.
  • Account for lap splices. ACI 318-19 § 25.5 — typical Class B lap is 40d for #4, 50d for #5. The 10 % waste factor covers most splice overlap on simple slabs; long runs need explicit splice math.
  • Don’t scale spacing past code maximum. ACI 318-19 § 24.4: maximum spacing is 3 × slab thickness or 18″ (450 mm), whichever is smaller.
  • Watch unit mismatches. Slab dimensions are in feet/metres but cover and spacing are in inches/millimetres — the calculator handles the conversion, but double-check your inputs against the suffix labels.

Frequently asked questions

What is the standard length of a rebar stick?
In the United States, retail rebar comes in 20-foot sticks; commercial fabricators stock 40-foot sticks. Internationally the equivalent metric standards are 6 m (residential) and 12 m (commercial). 20 ft and 12 m are not equivalent — 20 ft is about 6.10 m, and 12 m is about 39.4 ft.
How is rebar weight calculated?
ASTM A615 publishes a nominal unit weight per foot for each bar size: #3 = 0.376 lb/ft, #4 = 0.668, #5 = 1.043, #6 = 1.502, #7 = 2.044, #8 = 2.670. Total weight is total linear length × unit weight.
What does “#4 rebar” mean?
The number is the bar diameter in eighths of an inch. #4 = 4/8 = ½″ diameter (about 13 mm). #5 = 5/8 = ⅝″ (16 mm). It’s an ASTM A615 designation used across North America.
How much cover should I use under a slab on grade?
ACI 318-19 Table 20.6.1.3.1: 3 inches (75 mm) of cover for concrete cast against and permanently in contact with the ground. For slabs not in contact with earth or weather (like a top mat under a finished floor), the minimum drops to ¾″.
Is 10 % waste factor enough?
For straight, clean slabs with simple lap splices, yes. For pours with many penetrations, irregular shapes, or long laps, push it to 12–15 %. Industry consensus is 5–10 % for simple work.
Can I use this calculator for two-mat reinforcement?
Yes — run it twice (once for the bottom mat, once for the top mat) and add the totals. Most slabs on grade only need one mat; structural slabs and elevated decks usually have two.
Does this calculator include lap splices?
Indirectly — the 10 % waste factor covers typical splice overlap on slabs where bar length matches stock length. For long runs that need multiple splices, add the explicit lap length (typically 40d–50d for Class B splices) to the linear total before applying the waste factor.
How do I convert inches per foot of rebar to kg per metre?
1 lb/ft = 1.488 kg/m. So #4 rebar at 0.668 lb/ft is about 0.994 kg/m. The calculator shows both units in the result.