Slabs, footings and walls are reinforced with a grid of steel bars tied at each crossing. The grid spacing (often 12–18 in for slabs on grade) and bar size come from the structural design or local code; this calculator turns them into a cut list and order. Bars are kept a few inches from the edge so they are fully covered by concrete.
How to use the Rebar Calculator
- Slab length
- Slab width
- Grid spacing
- Edge cover
- Bar size
- Stock bar length
- Price per stock bar
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the slab length and width, the grid spacing, the edge cover, the bar size and the stock bar length. The calculator counts the bars each way, the total length, how many stock bars to buy (each run is cut from whole bars; laps are not included) and the weight and cost.
Rebar weights
| Bar | kg/m | lb/ft |
|---|---|---|
| #3 (⅜ in, 10 mm) | 0.560 | 0.376 |
| #4 (½ in, 13 mm) | 0.994 | 0.668 |
| #5 (⅝ in, 16 mm) | 1.552 | 1.043 |
| #6 (¾ in, 19 mm) | 2.235 | 1.502 |
| #7 (⅞ in, 22 mm) | 3.042 | 2.044 |
| #8 (1 in, 25 mm) | 3.973 | 2.670 |
| 10M (Canada) | 0.785 | 0.527 |
| 15M (Canada) | 1.570 | 1.055 |
| 20M (Canada) | 2.355 | 1.582 |
| 8 mm | 0.395 | 0.265 |
| 10 mm | 0.617 | 0.415 |
| 12 mm | 0.888 | 0.597 |
| 16 mm | 1.578 | 1.060 |
| 20 mm | 2.466 | 1.657 |
Tips for an accurate estimate
- Sketch the job and split irregular areas into rectangles, circles and triangles before you measure.
- Add a waste allowance – typically 5–10% for simple layouts and 15–20% for diagonal, curved or heavily cut work.
- Check the coverage or yield printed on the product, because sizes and densities vary between manufacturers.
- Buy all units of a finished material (tile, brick, siding) from the same batch so colours match, and keep a few spares for repairs.
- Confirm local prices – quarry, lumber and steel prices vary widely by region and season.
Common mistakes to avoid
- Ordering exactly the calculated amount with no allowance for breakage, offcuts and settlement.
- Forgetting to subtract doors, windows and other openings – or subtracting small ones that still need cutting around.
- Confusing US short tons (2,000 lb) with metric tonnes (1,000 kg) when comparing quotes.
Inputs explained
| Field | What to enter |
|---|---|
| Slab length | Units: ft, m, in, yd, cm |
| Slab width | Units: ft, m, in, yd, cm |
| Grid spacing | Units: in, cm, mm, ft, m |
| Edge cover | Units: in, cm, mm, ft, m |
| Bar size | Choose one: #3 (⅜ in, 10 mm), #4 (½ in, 13 mm), #5 (⅝ in, 16 mm), #6 (¾ in, 19 mm) … |
| Stock bar length | Choose one: 10 ft, 20 ft, 40 ft, 6 m … |
| Price per stock bar | Units: /piece |
What each result means
- Bars running lengthwise = floor(([Slab width] – 2×[Edge cover])÷[Grid spacing]) + 1
- Bars running widthwise = floor(([Slab length] – 2×[Edge cover])÷[Grid spacing]) + 1
- Total length of rebar = [Bars running lengthwise]×([Slab length] – 2×[Edge cover]) + [Bars running widthwise]×([Slab width] – 2×[Edge cover])
- Stock bars needed = [Bars running lengthwise]×ceil(([Slab length] – 2×[Edge cover])÷[Stock bar length] – 0.000001) + [Bars running widthwise]×ceil(([Slab width] – 2×[Edge cover])÷[Stock bar length] – 0.000001)
- Total weight = [Total length of rebar]×[Mass per metre]
- Cost = [Stock bars needed]×[Price per stock bar]
Formula
Bars along length = floor((W − 2c) ÷ s) + 1, each L − 2c longBars along width = floor((L − 2c) ÷ s) + 1, each W − 2c longWeight = total length × mass per lengthWorked example
Inputs — Slab length: 20 ft; Slab width: 12 ft; Grid spacing: 12 in; Edge cover: 3 in; Bar size: #4 (½ in, 13 mm); Stock bar length: 20 ft; Price per stock bar: $12 /piece.
Results — Mass per metre = 0.668 lb/ft; Bars running lengthwise = 12; Bars running widthwise = 20; Total length of rebar = 464 ft; Stock bars needed = 32; Total weight = 309.9 lb; Cost = $384.00.
Key terms
- Rebar size number
- In U.S. sizes, the number is the diameter in eighths of an inch: #4 = 4/8 = ½ in.
Frequently asked questions
What size rebar for a slab?
#3 or #4 bars at 12–18 in for residential slabs on grade, #4 or #5 for driveways and footings – follow your engineer or local code.
How much does #4 rebar weigh?
0.668 lb per foot (0.994 kg/m); a 20 ft bar weighs about 13.4 lb.
How much should rebar overlap?
Typically 40–60 bar diameters (about 20–30 in for #4); add laps to the total if runs exceed one stock bar.
Where should rebar sit in a slab?
In the middle to upper third, on chairs or dobies – never on the ground.
What formula does the Rebar Calculator use?
Bars along length = floor((W − 2c) ÷ s) + 1, each L − 2c long.