Rebar Calculator
Count the rebar and total linear feet needed for a slab grid.
What you'll need to order
- rebar
- rebar chairs
- tie wire
- bolt cutters or rebar bender
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
total ft = ((width × 12 ÷ spacing) + 1) × length + ((length × 12 ÷ spacing) + 1) × width
The "+1" on each direction accounts for the bar at both edges. This gives grid length only — add 10% if your slab is long enough to need lap splices, which must overlap 40 bar diameters.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ACI 318 (Building Code Requirements for Structural Concrete) 2019, § 25.5 — lap splice length in bar diameters
Cited to a specific edition on purpose — table numbers and values move between cycles. Confirm against whichever edition is enforced locally.
Worked example
With slab length 20 ft, slab width 10 ft, bar spacing 18 in, bar weight 0.668 lb, this calculator returns 296.67 ft.
Rebar quantity by grid spacing
Figures below assume slab length 20 ft × slab width 10 ft × bar weight 0.668 lb. Every row here opens the calculator already filled in. Copy that link and the numbers travel with it.
| Bar spacing | Total rebar length (ft) | Number of bars | Approximate weight (lb) |
|---|---|---|---|
| 6 in | 830.0 | 62.00 | 554.4 |
| 8 in | 630.0 | 47.00 | 420.8 |
| 12 in | 430.0 | 32.00 | 287.2 |
| 16 in | 330.0 | 24.50 | 220.4 |
| 18 in | 296.7 | 22.00 | 198.2 |
| 24 in | 230.0 | 17.00 | 153.6 |
A 20 by 10 ft slab in #4 bar. Halving the spacing roughly doubles both the steel and its cost.
Rebar for common slabs
Each row is a link that fills in the calculator above. Send one and the other end sees the same figures you do.
| Job | Size | Total rebar length |
|---|---|---|
| Shed base, #3 at 18 in | 12 ft × 10 ft × 18 in × 0.376 lb | 182.0 ft |
| Patio, #4 at 18 in | 16 ft × 12 ft × 18 in × 0.668 lb | 284.0 ft |
| Garage floor, #4 at 16 in | 24 ft × 24 ft × 16 in × 0.668 lb | 912.0 ft |
| Driveway, #4 at 12 in | 40 ft × 12 ft × 12 in × 0.668 lb | 1,012 ft |
| Workshop floor, #5 at 12 in | 30 ft × 40 ft × 12 in × 1.043 lb | 2,470 ft |
Figures are for a single mat both ways. Heavy structural slabs use two mats, top and bottom, which doubles the quantity.
What does rebar cost?
Material typically runs $1 to $1 per pound. Material only, at typical yard prices. Placing and tying by a contractor adds roughly $0.30 to $0.60 per pound.
What moves the price:
- Steel commodity price, which moves more than most construction materials
- Bar size — smaller bars cost more per pound
- Epoxy-coated or galvanised bar for corrosive exposure, at a substantial premium
- Whether you buy full 20 ft lengths or cut-to-size
Prices are US averages and move with fuel, season and haul distance. Always get two local quotes before budgeting.
Rebar sizes and weights
| Type | Weight (lb per ft) | Notes |
|---|---|---|
| #3 (3/8 in) | 0.38 | Light slabs, footpaths |
| #4 (1/2 in) | 0.67 | The residential default |
| #5 (5/8 in) | 1.04 | Driveways, heavy slabs |
| #6 (3/4 in) | 1.50 | Footings, retaining walls |
| #7 (7/8 in) | 2.04 | Structural, engineered only |
| #8 (1 in) | 2.67 | Columns and heavy structure |
Bar number is the diameter in eighths of an inch: #4 is 4/8, or half an inch. Add 10% to your total for lap splices, which must be at least 40 bar diameters.
Frequently asked questions
How much rebar do I need for a slab?
For a 20 by 10 ft slab at 18 inch spacing you need about 220 linear feet of #4 bar, roughly 147 pounds. Add 10% for lap splices.
Do I need rebar or is wire mesh enough?
Mesh is adequate for patios, footpaths and shed floors. Use rebar on driveways, garages and anywhere a vehicle parks — mesh is too easily trodden to the bottom of the pour to do its job.
How much does rebar weigh?
A #4 bar weighs 0.668 lb per foot, so a standard 20 ft length is about 13.4 lb. A #5 is 1.043 lb per foot and a #3 is 0.376.
Can I use rebar and fibre together?
Yes, and they do different jobs. Fibre controls the fine shrinkage cracking that happens as concrete cures; rebar carries structural load and holds a crack closed once it forms.
What size rebar should I use in a slab?
Number 4 bar, 1/2 inch, is standard for residential slabs, driveways and patios. Number 3 suits light-duty pours and number 5 goes into footings and anywhere heavy loads bear.
How far apart should rebar be spaced?
Twelve to 18 inches on centre in both directions for most slabs. Tighter spacing at 12 inches is worth it in driveways or over questionable subgrade; 18 inches is fine for a patio.
How much should rebar overlap at a splice?
Forty bar diameters is the usual rule — 20 inches for #4 bar, 25 inches for #5. Tie the lap securely; a splice that shifts during the pour is no splice at all.
Should rebar sit on the ground or be raised?
Raised, always. Rebar belongs in the middle third of the slab thickness, held there by chairs. Steel lying on the subgrade does nothing structurally and will rust from below.