Rebar Calculator
Estimate the reinforcing steel for a rectangular concrete slab, patio or footing pad. Enter the slab length and width, bar spacing, edge cover, number of layers, bar size, stock bar length and lap length, and the calculator works out the bars in each direction, the net length of steel, the number of stock bars including laps, their weight and the number of tie points.
Rebar calculator
How to use the rebar calculator
- Enter slab size and cover. Enter the slab length and width and the edge cover (the concrete between the bar ends and the slab edge).
- Enter spacing and layers. Enter the bar spacing centre to centre and choose one or two layers (mats).
- Choose bar size. Pick #3–#6 (US), 10–20 mm (UK/AU) or 10M–20M (Canada).
- Enter stock length and lap. Enter the length bars are sold in and the lap splice length where bars join. Use the lap your engineer or code specifies.
Reinforcement is ordered in bars of fixed length, so the useful numbers are how many bars to buy, what they weigh, and how many intersections need tying. This calculator lays out a simple rectangular grid for a slab and turns it into those numbers.
Design note. This tool estimates quantities only. Bar size, spacing, cover, laps and whether reinforcement is needed at all should come from an engineer, your local code or the drawings.
The formula explained
Reinforced length Lr = Slab length − 2 × Cover; Wr = Slab width − 2 × Cover
Bars along length = ⌊ Wr ÷ Spacing ⌋ + 1 (each Lr long)
Bars along width = ⌊ Lr ÷ Spacing ⌋ + 1 (each Wr long)
Stock bars per run = ⌈ (Run length + laps) ÷ Stock length ⌉
Weight = Stock bars × Stock length × Mass per length
Laps. When a run is longer than one stock bar, bars are overlapped by the lap length. The calculator adds one lap for each joint before dividing by the stock length.
Tie points are the intersections of the two directions: bars along length × bars along width, per layer. In practice not every intersection is always tied; your contractor or spec decides.
Worked examples
Example 1: US garage slab
A 20 × 20 ft slab, #4 bars at 12 in centres, 3 in cover, 20 ft stock, 20 in lap, one layer:
- Reinforced area: 19.5 × 19.5 ft.
- Bars each way: 19.5 ÷ 1 = 19.5 → 19 + 1 = 20, each 19.5 ft long.
- Net length: 40 × 19.5 = 780 ft.
- Each bar fits in one 20 ft stock bar: 40 stock bars.
- Weight: 40 × 20 ft × 0.668 lb/ft = 534 lb; ties: 400.
Example 2: UK patio base
A 6 × 4 m slab, 12 mm bars at 200 mm centres, 50 mm cover, 6 m stock:
- Reinforced: 5.9 × 3.9 m.
- Bars along length: 3.9 ÷ 0.2 + 1 = 20 × 5.9 m; bars along width: 5.9 ÷ 0.2 = 29.5 → 29 + 1 = 30 × 3.9 m.
- Net length: 118 + 117 = 235 m.
- Stock bars: 50; weight: 50 × 6 m × 0.888 = 266 kg.
Example 3: Canadian shed slab
An 8 × 6 m slab, 10M at 300 mm, 75 mm cover, 6 m stock with 450 mm laps: 67 stock bars, about 316 kg.
Bar sizes and weights
| Bar | Diameter | Weight |
|---|---|---|
| #3 (US) | 3/8 in | 0.376 lb/ft |
| #4 (US) | 1/2 in | 0.668 lb/ft |
| #5 (US) | 5/8 in | 1.043 lb/ft |
| #6 (US) | 3/4 in | 1.502 lb/ft |
| 10 mm | 10 mm | 0.617 kg/m |
| 12 mm | 12 mm | 0.888 kg/m |
| 16 mm | 16 mm | 1.58 kg/m |
| 10M (CA) | 11.3 mm | 0.785 kg/m |
| 15M (CA) | 16.0 mm | 1.570 kg/m |
US weights are ASTM A615 nominal values; metric weights are based on a steel density of 7,850 kg/m³; Canadian weights are CSA G30.18 nominal masses.
Regional notes: US, UK, Australia and Canada
United States. Bars are designated by number (#4 = 4/8 in). Stock lengths of 20 ft and 40 ft are common. ACI 318 governs structural concrete; residential slabs on ground often follow the IRC or local practice. Lap lengths are often specified in bar diameters.
United Kingdom. High-yield bars are designated H10, H12, H16 and so on, and supplied in 6 m and 12 m lengths. Mesh (A142, A193, A252) is very common for ground slabs.
Australia. Deformed bars are N10, N12, N16 etc., and slab mesh (SL72, SL82 and others) is widely used. AS 3600 covers concrete structures; residential slabs often follow AS 2870.
Canada. Bars are 10M, 15M, 20M etc. under CSA G30.18, and design follows CSA A23.3 and the National Building Code.
Tips and common mistakes
- Cover matters for durability; use chairs or spacers.
- Do not forget edge and trimmer bars around openings, which this grid does not include.
- Order extra for cutting waste, especially if runs are just over a stock length.
- Tie wire is ordered by the coil; count ties to estimate.
- Concrete volume for the slab comes from the concrete calculator.
Related tools
Ordering ready-mix? Use the cubic yard calculator. Building a block wall on the slab? Try the concrete block calculator.
Frequently asked questions
How do I calculate how much rebar I need for a slab?
Subtract cover from each side, divide each reinforced dimension by the spacing and add one to get bars in each direction. Multiply each count by the bar length in the other direction, add laps, and divide by the stock length.
How much rebar for a 20 × 20 ft slab?
With #4 bars at 12 in centres and 3 in cover, you need 20 bars each way, each 19.5 ft long, for 780 ft of bar. Using 20 ft stock that is 40 bars weighing about 534 lb, with 400 tie points.
What spacing should rebar be?
It depends on slab thickness, loads and design. Residential slabs often use 12–18 in (300–450 mm) centres; structural slabs follow an engineer's drawings.
How much does rebar weigh?
Per ASTM A615 nominal weights, #3 is 0.376 lb/ft, #4 is 0.668 lb/ft and #5 is 1.043 lb/ft. Metric bars weigh about 0.617 kg/m (10 mm), 0.888 kg/m (12 mm) and 1.58 kg/m (16 mm).
What is concrete cover?
The thickness of concrete between the steel and the surface. It protects bars from corrosion and fire. Typical values are 2–3 in (50–75 mm) for slabs on ground; codes specify minimums.
What is a lap splice?
Where two bars overlap so that load transfers between them. Lap length depends on bar size, concrete strength and code; it is often expressed as a multiple of bar diameter.
Should I use mesh instead of rebar?
Welded wire mesh is common for light residential slabs and is sold in sheets. Rebar is used where design requires heavier reinforcement. Follow the design for your slab.
How do I keep rebar at the right height?
Use bar chairs, spacers or concrete blocks so the steel sits at the correct depth. Rebar lying on the ground provides little benefit.
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Sources and references
About this calculator
Written and maintained by Rashid Mehmood, founder of CalcTen. The formulas are unit-tested against hand-worked examples, and the content is reviewed whenever product sizes or guidance change. Last updated 1 November 2026. See our editorial policy.
Results are estimates for planning and ordering. Always check product labels, supplier advice and local building codes, and consult a qualified professional for structural work. Read the disclaimer.