Estimate rebar quantity, spacing, total length, weight and cost for concrete slabs, footings and walls — metric or US bar sizes.
Direction 1 — bars running the full length
Direction 2 — bars running the full width
Stock bar length (optional)
Cost (optional)
| Item | Value |
|---|
Written and reviewed by Mohsin Iqbal · Last reviewed August 2026
This rebar calculator estimates the quantity, spacing, total length and weight of reinforcing bar needed for a concrete slab, footing or wall. Enter your slab dimensions, bar spacing and edge cover, and it works out how many bars you need in one or two directions, plus the total weight for whatever bar size you choose. It's an estimating and planning tool, not a structural design calculator — see Limitations below.
Concrete slabs are usually reinforced with a two-way grid: one set of bars running the length of the slab, spaced across the width, and a second set running the width, spaced across the length. This calculator handles exactly that, or a single-direction layout if you untick Direction 2 — useful for footings, strip foundations or one-way slabs where only one direction of steel is specified.
It depends entirely on your slab's dimensions, your bar spacing, and your edge cover — there's no fixed answer that applies to every slab. A slab with bars every 150mm uses roughly a third more rebar than the same slab at 200mm spacing. The rebar calculator above works out the bar count, total length and weight once you enter your slab dimensions, spacing and edge cover.
Getting bar count right means handling the first and last bar correctly, not just dividing distance by spacing. The formula this calculator uses:
Usable Width = Slab Dimension − (2 × Cover)
Number of Spaces = ROUND UP( Usable Width ÷ Spacing )
Number of Bars = Number of Spaces + 1
Rounding the number of spaces up (not to the nearest whole number, and never down) guarantees the actual spacing between bars never exceeds what you entered — it can only be equal to or slightly tighter. The "+1" accounts for the fact that N gaps between bars need N+1 bars: a single gap has a bar at each end, so 1 space is 2 bars, not 1.
Switch to the Rebar Weight tab above for a standalone weight calculation: pick a bar size, enter a length and quantity, and get weight per metre, per foot, and totals in kg, tonnes and lb. It uses the same bar-size data as the slab calculator, so results stay consistent between both tools.
Steel bar weight follows directly from its cross-sectional area and length:
Cross-sectional Area = π × d² ÷ 4
Weight = Area × Length × Steel Density (7,850 kg/m³)
For a quick mental estimate, metric reinforcement is commonly approximated with the shortcut kg/m ≈ d² ÷ 162, where d is the diameter in millimetres. For a 12mm bar: 12² ÷ 162 = 0.889 kg/m, very close to the exact-formula result of 0.888 kg/m this calculator uses. The shortcut is handy for a rough check; the calculator itself always uses the exact formula.
| Metric bar diameter | kg per metre |
|---|---|
| 6 mm | 0.222 |
| 8 mm | 0.395 |
| 10 mm | 0.617 |
| 12 mm | 0.888 |
| 16 mm | 1.578 |
| 20 mm | 2.466 |
| 25 mm | 3.853 |
| 32 mm | 6.313 |
| US bar size | Nominal diameter | lb per foot |
|---|---|---|
| #3 | 0.375 in (3/8in) | 0.376 |
| #4 | 0.500 in (1/2in) | 0.668 |
| #5 | 0.625 in (5/8in) | 1.043 |
| #6 | 0.750 in (3/4in) | 1.502 |
| #7 | 0.875 in (7/8in) | 2.044 |
| #8 | 1.000 in | 2.670 |
| #9 | 1.128 in | 3.400 |
| #10 | 1.270 in | 4.303 |
| #11 | 1.410 in | 5.313 |
Metric values are calculated from the exact steel-density formula above. US bar weights are the published standard nominal values used industry-wide (ASTM A615 / CRSI reference figures), not values inferred from diameter alone.
Lap (splice) allowance and waste allowance are kept separate in this calculator because they cover different things. Lap allowance is extra length for overlapping bars where two pieces join end to end — actual lap length depends on bar diameter, concrete strength, steel grade, whether the bar is in tension or compression, and local code detailing requirements, so this calculator uses whatever percentage you enter rather than calculating a structural lap length. Waste allowance covers offcuts, bends and general site wastage. Both appear as separate line items in the results so you can see exactly what each one added.
Rebar is sold in fixed stock lengths — commonly 6m, 9m or 12m in metric markets, and 20ft, 30ft or 40ft in the US. Enter a stock length above (optional) and the calculator divides your total required length, after lap and waste, by the stock length and rounds up to give a theoretical minimum stock bar count. This is a simple division, not a cutting-layout optimisation, so the actual number of stock bars you need to buy may be higher once individual piece lengths, offcuts and splice locations are accounted for. If any single required bar is longer than your chosen stock length, the calculator will flag that an approved splice/lap arrangement may be needed — it does not calculate splice length or structural splice requirements.
| Step | Working | Result |
|---|---|---|
| Slab size | 6m × 4m, 40mm cover, 200mm spacing both directions | — |
| Direction 1 (along length) | (4 − 0.08) ÷ 0.2 = 19.6 → 20, +1 | 21 bars, 5.92m each |
| Direction 2 (along width) | (6 − 0.08) ÷ 0.2 = 29.6 → 30, +1 | 31 bars, 3.92m each |
| Total bars / raw length | 21 + 31 bars | 52 bars, 245.84m |
| +5% waste (0% lap) | 245.84 × 1.05 | 258.13m |
| Weight (12mm, 0.888 kg/m) | 258.13 × 0.888 | ~229.2 kg |
| Step | Working | Result |
|---|---|---|
| Slab size | 20ft × 15ft, 1.5in cover, 12in spacing both directions | — |
| Direction 1 | (15 − 0.25) ÷ 1 = 14.75 → 15, +1 | 16 bars, 19.75ft each |
| Direction 2 | (20 − 0.25) ÷ 1 = 19.75 → 20, +1 | 21 bars, 14.75ft each |
| Total bars / raw length | 16 + 21 bars | 37 bars, 625.75 ft |
| +10% waste (0% lap) | 625.75 × 1.10 | 688.33 ft |
| Weight (#4, 0.668 lb/ft) | 688.33 × 0.668 | ~459.8 lb (~208.6 kg) |
| Step | Working | Result |
|---|---|---|
| Bars | 50 pieces of #5 rebar, 20ft each | 1,000 ft total |
| Weight (#5, 1.043 lb/ft) | 1,000 × 1.043 | 1,043 lb |
| In kg | 1,043 ÷ 2.20462 | ~473.1 kg |
Single-direction mode can be used when your engineering drawings or project specification call for reinforcement in one direction — untick Direction 2 above for a footing or wall with one row of longitudinal bars. Use two-direction mode when reinforcement is specified in both directions, leaving Direction 2 ticked for a two-way mat. For a strip footing, use the footing's length as the slab length and its width as the slab width, with your specified bar spacing across the width. This calculator estimates quantity the same way regardless of whether the concrete element is a slab, footing or wall — it's the geometry and spacing you enter that changes, not the underlying formula.
| Region | Common term(s) |
|---|---|
| US / Canada | Rebar, reinforcing bar, reinforcement steel |
| UK | Reinforcement bar, reinforcing bar, rebar, steel reinforcement |
| Australia / New Zealand | Reinforcing bar, reinforcement, rebar, and informally "reo" or "reo bar/reo steel" |
All of these refer to the same product and are calculated identically — this calculator doesn't need to know which term you use, only the bar size, spacing and dimensions you enter.
This calculator estimates rebar quantity, length, weight and cost from the dimensions, spacing, cover, bar size, and lap/waste percentages you enter. It does not design reinforcement, determine structurally required bar size or spacing, check cover requirements against exposure or fire rating, or verify building-code compliance. Bar size, spacing and cover are all determined by structural design, loading, soil conditions, concrete grade and local code — not by this calculator. Lap length and development length are not calculated; the lap allowance here is a simple user-defined percentage, not a code-derived splice length. Always confirm reinforcement details with a qualified structural engineer and your local building authority before construction.
Enter your slab's length and width, your chosen bar spacing and edge cover, and a bar size into the calculator above. It works out how many bars fit in each direction, their total length, and their total weight. There's no single answer that applies to every slab - it depends entirely on the spacing and cover your design (or your engineer) specifies.
Divide the usable width by your bar spacing and round up, then add one bar for the starting edge - that's your bar count in one direction. Repeat for the other direction if you have a two-way grid. Multiply each bar's count by its length, then add your lap and waste allowances. This calculator does all of that automatically.
Multiply the total bar length by the weight-per-metre (or weight-per-foot) of your chosen bar size. For metric bars, weight per metre is approximately diameter-in-mm squared, divided by 162. This calculator does the calculation using the exact steel-density formula rather than the shortcut.
Approximately 0.888 kg per metre, calculated from the cross-sectional area of a 12mm bar and a standard steel density of 7,850 kg per cubic metre. This is a widely used theoretical value, consistent with the common d-squared-over-162 shortcut.
This depends on your structural design, slab thickness, loading, soil conditions and local building code - it is not something a general calculator can safely tell you. This tool calculates the bar count and length from whatever spacing you enter; it does not determine what spacing is structurally required. Confirm spacing with an engineer or your local code for anything structural.
Subtract twice the edge cover from the slab dimension being spaced across, divide by the bar spacing, round up, then add one. For example, a 4m-wide slab with 40mm cover and 200mm spacing gives (4 - 0.08) / 0.2 = 19.6, rounded up to 20, plus 1 = 21 bars in that direction.
This varies with bar size and spacing, so there's no fixed figure - a slab with bars every 12 inches uses roughly twice as much rebar as the same slab with bars every 24 inches. Enter your slab's dimensions and spacing above for an exact linear-feet and weight figure.
Bar size selection depends on structural design, loading, slab thickness and local building code - this calculator does not recommend a bar size. It estimates quantity and weight once you've chosen a size, whether from an engineer's drawing, a code table, or your own project requirements.
Only if you enter a lap/splice percentage - it's an optional, user-defined allowance shown separately from waste, not a calculated structural lap length. Actual lap length depends on bar diameter, concrete strength, steel grade and local code; confirm the required lap with your engineer or code reference.
Enter 20 ft as your stock length in the calculator above. It divides your total required length (after waste and lap) by 20 ft and rounds up to give a theoretical minimum stock bar count, not a cutting-optimised purchase quantity - actual bars needed may be higher once individual piece lengths, offcuts and splice locations are accounted for.
They're the same product. Rebar and reinforcing bar are the common US, UK and Canadian terms; reo (short for reinforcement) is common Australian and New Zealand trade slang. All are calculated identically - this calculator doesn't need to know which term you use.
No. It estimates the quantity, length, weight and cost of rebar from the bar size, spacing and cover you enter. It does not check whether that spacing, bar size or cover is structurally adequate, code-compliant, or suitable for your loading and soil conditions - that requires a qualified structural engineer.
Rebar pricing varies significantly by region, bar size and market steel prices, so this calculator doesn't assume a figure - enter your local supplier's price per kg, lb, metre, foot or bar for a material cost estimate. Labor, fixing wire, chairs/spacers, fabrication and delivery are not included unless you factor them in separately.