How to use the rebar calculator
Reinforcing steel is ordered by weight but placed by count, so a quick takeoff needs both numbers. This calculator works out the bar layout for a rectangular slab, pad footing or strip footing from the size, bar diameter, spacing and cover you enter, adds lap splices where a bar run is longer than a stock bar, and converts the total length to weight and stock bars to order.
- Choose US or metric units and whether you are reinforcing a slab or pad, with bars running both ways, or a strip footing with a few long bars and cross bars.
- Enter the outside dimensions of the concrete element.
- Pick the bar size, spacing and cover from your drawings. For a strip footing, also enter how many bars run along it.
- Set the stock bar length, lap length and waste. The defaults are 20 ft or 12 m bars, 40 diameters and 5%.
- Read the results: bars each way, cut lengths, total length, weight and stock bars.
Rebar takeoff formulas
Bars across a width = floor((width − 2 × cover) ÷ spacing) + 1
Cut length = length − 2 × cover
Pieces per run (if longer than stock) = ceil((run − lap) ÷ (stock − lap)), lap = 40 × bar diameter
Total length = Σ bars × (cut length + laps) × layers
Weight = total length × weight per foot or metre × (1 + waste)
| US bar | Diameter | Weight | Metric bar | Weight |
|---|---|---|---|---|
| #3 | 0.375 in (9.5 mm) | 0.376 lb/ft | 8 mm | 0.395 kg/m |
| #4 | 0.500 in (12.7 mm) | 0.668 lb/ft | 10 mm | 0.617 kg/m |
| #5 | 0.625 in (15.9 mm) | 1.043 lb/ft | 12 mm | 0.888 kg/m |
| #6 | 0.750 in (19.1 mm) | 1.502 lb/ft | 16 mm | 1.578 kg/m |
| #7 | 0.875 in (22.2 mm) | 2.044 lb/ft | 20 mm | 2.466 kg/m |
| #8 | 1.000 in (25.4 mm) | 2.670 lb/ft | 25 mm | 3.853 kg/m |
Worked examples
A garage slab with #4 bars
A 20 × 12 ft slab with #4 bars at 12 inches each way and 3 inches of cover. Across the 12 ft width, (144 − 6) ÷ 12 = 11.5, so 11 spaces and 12 bars, each 19.5 ft long. Across the 20 ft length, (240 − 6) ÷ 12 = 19.5, so 20 bars, each 11.5 ft long. That is 234 + 230 = 464 ft of bar, or 310 lb, and about 325 lb with 5% waste. No laps are needed, and you would order 32 bars of 20 ft.
A square pad footing in metric
A 2.4 × 2.4 m pad with 16 mm bars at 200 mm each way and 75 mm cover gets (2,400 − 150) ÷ 200 = 11.25, so 12 bars each way, each 2.25 m long. That is 54 m of bar, or 85 kg before waste. Five pieces come out of each 12 m stock bar, so 6 stock bars cover the job.
A large slab with laps and two mats
A 40 × 30 ft slab with #5 bars at 12 inches each way, 2 inches of cover and top and bottom mats. The long bars run 39.7 ft, so with 20 ft stock and a 25 inch lap (40 × 0.625 in) each run needs 3 pieces and two laps. The calculator shows about 5,170 ft of bar and 5,390 lb of steel before waste, with 340 stock bars.
Who this calculator helps
- Architects and designers estimating reinforcement for early cost plans and feasibility studies.
- Contractors and concrete crews ordering bar and chairs for slabs, driveways and footings.
- Quantity surveyors and estimators making a fast check against a full takeoff.
- Engineers and students turning a bar size and spacing into weight per square metre or foot.
- Self-builders pricing a garage, shed base or patio slab.
Practical tips
- Pair the steel with the concrete volume from the concrete calculator and roll both into the construction cost calculator.
- Stagger lap splices so they don't all fall on one line, and keep laps out of the zone of highest stress where the engineer says so.
- Order bar chairs or spacers to hold the cover; roughly one per 3–4 ft (1 m) of bar each way is common for slabs.
- Add trimmer bars around openings and at re-entrant corners, which this simple count doesn't include.
- Check slab falls and drainage with the slope calculator before the steel goes down.
Want the client to see the finished space before the pour? Turn your plan into a render with Sketch to Render.
