Rebar Calculator
Estimate a rectangular rebar grid for a slab, wall or pad. Calculate bars in both directions, total length, order allowance, approximate weight, whole stock bars and cost. This is a quantity estimator, not structural reinforcement design.
Rebar calculator formulas
Bars parallel to length are spaced across the clear width. Bars parallel to width are spaced across the clear length.
Clear length = overall length − 2 × edge cover
Clear width = overall width − 2 × edge cover
Bars parallel to length = ceil(clear width ÷ spacing) + 1
Bars parallel to width = ceil(clear length ÷ spacing) + 1
Base length = layers × [(length-direction bars × clear length) + (width-direction bars × clear width)]
Order length = base length × (1 + allowance ÷ 100)
Stock bars = ceil(order length ÷ stock length)
Approximate order weight equals order length multiplied by the selected nominal mass per unit length. Actual mill tolerances, coatings and product specifications can change shipped weight.
Worked examples
| Rectangular grid | Inputs | Calculated quantity |
|---|---|---|
| 20 ft × 12 ft slab | 3 in cover, 12 in spacing, 1 layer, 10% allowance, 20 ft stock | 13 + 21 runs; 544.5 ft order length; 28 stock bars |
| 30 ft × 20 ft pad | 3 in cover, 18 in spacing, 1 layer, 5% allowance, 20 ft stock | 14 + 21 runs; 863.625 ft order length; 44 stock bars |
| 10 ft × 8 ft panel | 3 in cover, 12 in spacing, 2 layers, no allowance, 20 ft stock | 18 + 22 runs across two layers; 336 ft; 17 stock bars |
| 6 m × 4 m slab | 50 mm cover, 200 mm spacing, 1 layer, 10% allowance, 12 m stock | 21 + 31 runs; 269.28 m order length; 23 stock bars |
| 10 m × 8 m slab | 50 mm cover, 250 mm spacing, 2 layers, 5% allowance, 12 m stock | 66 + 82 runs across two layers; 1,366.26 m; 114 stock bars |
| 3 m × 2.5 m wall | 40 mm cover, 150 mm spacing, 1 layer, 7.5% allowance, 6 m stock | 18 + 21 runs; 111.134 m order length; 19 stock bars |
How to estimate a rectangular rebar grid
- Enter the overall slab, pad or wall dimensions in one consistent unit system.
- Copy cover, spacing, bar size and number of layers from approved project documents.
- Choose the nominal bar size so the calculator can estimate weight.
- Add a project-specific lap, cutting and waste allowance.
- Enter the stock length and current supplier price.
- Review both geometric length and whole-stock-bar quantity before ordering.
Important limitations
- The calculator estimates a uniform orthogonal rectangular grid only; it does not design reinforcement.
- It does not calculate circular layouts, beams, columns, footings, dowels, hooks, bends, chairs or localized strengthening.
- Edge cover is treated geometrically to the outer bar centerline and must match the intended estimating convention.
- The allowance is a percentage, not a code-compliant lap-splice or development-length calculation.
- Whole-stock-bar rounding assumes total length can be optimized across cuts; an actual cutting schedule may require more bars.
- Weight values are nominal approximations. Confirm the selected product, grade, coating, mill data and supplier units.
- Cost excludes fabrication, bends, couplers, supports, delivery, tax and installation labor.
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Frequently asked questions
How many rebar runs do I need for a slab?
The calculator divides each clear grid dimension by the selected maximum spacing, rounds upward and adds the opposite edge bar. It repeats this in both directions and for every layer.
Does this calculator design slab reinforcement?
No. It estimates quantities from bar size, spacing, cover and layers that must come from approved drawings or a qualified designer.
Can it estimate rebar weight?
Yes. It multiplies the order length by the nominal mass per unit length for the selected US or metric bar size.
Are laps and waste included?
Only as an editable percentage. The calculator does not determine required splice or development lengths.
Why can the purchased length exceed the order length?
Rebar is normally bought in whole stock lengths. The calculator rounds the number of stock bars upward.
Can I use it for a wall?
Yes for a uniform rectangular orthogonal grid when the entered width represents wall height. It does not design boundary steel, openings, dowels or other wall details.