Rebar Calculator
Plan a reinforced slab grid — and estimate total rebar length, stock pieces, weight, and cost.
Enter the slab and grid dimensions
Use one unit system throughout. Edge clearance is measured from each slab edge to the nearest rebar line.
Total Grid Length
0 m
Installed straight rebar
Stock Order
0 pieces
0 m total length
Est. Weight & Cost
0 kg
$0.00
How a rebar estimate works
A rebar calculator determines the number, length, weight, and estimated cost of reinforcing steel required for a concrete project. Enter dimensions, bar size, spacing, and quantity to calculate rebar requirements. Results vary by slab, wall, footing, or beam dimensions and local structural design requirements.
A two-way slab grid has one set of parallel bars in each direction. The clear grid dimensions come from the slab dimensions minus edge clearance on both sides.
The calculator rounds the number of spaces up so the resulting spacing never exceeds the value entered. It then totals every grid segment, adds the selected allowance, and rounds the procurement quantity up to full stock lengths.
Subtract two edge clearances from both slab dimensions.
Round spaces up and add one bar to close each grid direction.
Add allowance, divide by stock length, and round up to whole bars.
The four core formulas
Grid dimensions
grid = slab − 2 × edge
Apply the equation independently to length and width.
Grid-line count
lines = ceil(grid ÷ spacing) + 1
The extra line closes the grid at the opposite edge.
Total length
L = nₗ × gridₗ + nᵥ × gridᵥ
Multiply each bar count by the segment length it runs.
Stock and cost
pieces = ceil(L ÷ stock)
Multiply whole pieces by the supplier price per bar.
Weight estimate: unit weight = πd²ρ/4, using nominal diameter d and steel density ρ = 7,850 kg/m³.
Bar-size reference
Common rebar sizes
Imperial bar numbers #2 through #8 correspond approximately to diameter in eighths of an inch. Soft metric labels are rounded nominal equivalents.
| Imperial size | Soft metric | Nominal diameter | Approx. theoretical kg/m |
|---|---|---|---|
| #3 | No. 10 | 9.525 mm | 0.559 kg/m |
| #4 | No. 13 | 12.700 mm | 0.994 kg/m |
| #5 | No. 16 | 15.875 mm | 1.554 kg/m |
| #6 | No. 19 | 19.050 mm | 2.237 kg/m |
| #7 | No. 22 | 22.225 mm | 3.045 kg/m |
| #8 | No. 25 | 25.400 mm | 3.978 kg/m |
| #9 | No. 29 | 28.650 mm | 5.061 kg/m |
Selection note: use the bar designation and mass stated by the supplier and project documents for purchasing; nominal conversions and theoretical weights are planning aids.
Worked example
Rebar quantity for a 6 m × 4 m slab
Suppose a slab is 6 m long and 4 m wide, with rebar at 400 mm centres, an 80 mm edge-to-bar-line clearance, 6 m stock bars, and no extra waste allowance.
-
1. Find the clear grid
5.84 m × 3.84 m
Subtract 2 × 0.08 m from each slab dimension. -
2. Count the grid lines
11 lengthwise + 16 crosswise
Round each number of spaces up, then add one line. -
3. Total the installed length
125.68 m
(11 × 5.84 m) + (16 × 3.84 m). -
4. Convert to stock bars
21 pieces of 6 m
ceil(125.68 ÷ 6) = 21, or 126 m ordered.
Actual maximum spacing
389 mm
The other direction is 384 mm.
Approx. #4 order weight
125.2 kg
Based on 126 m of nominal bar.
Example material cost
$252
At $2 per metre before add-ons.
Important: this example covers straight grid length only. Add drawing-specific laps, hooks, bends, supports, openings, fabrication, delivery, and site waste before ordering.
Project-type guide
Can this calculator estimate slabs, walls, footings, beams, or columns?
The calculator is designed for a uniform, two-way rectangular grid. Other reinforced-concrete elements use different bar arrangements, so treat the guidance below as a suitability check—not structural design advice.
| Project type | When the calculator helps | What still needs separate detailing |
|---|---|---|
| Slab, pad, or driveway | Good fit for one uniform rectangular mat with bars in two directions. | Extra layers, thickened edges, openings, dowels, laps, and local reinforcement. |
| Wall | Useful only as a preliminary straight-grid estimate using wall length and height. | Each face, vertical and horizontal zones, openings, starter bars, laps, and boundary bars. |
| Isolated footing | Can estimate one rectangular bottom or top mat from plan dimensions. | Multiple footings, layers, column dowels, bends, anchorage, and pedestal reinforcement. |
| Beam | Not a direct fit because a beam is not a uniform two-way planar grid. | Longitudinal bars, curtailment, development, hooks, stirrups, and support zones. |
| Column | Not a direct fit; use the column schedule and bar bending details instead. | Vertical bars, ties or spirals, lap zones, couplers, starters, and confinement details. |
Drawing-to-order guide
Turn structural drawings into an orderable rebar quantity
A reliable estimate begins with the current structural drawings, reinforcement schedules, general notes, and revisions. Read each direction, face, layer, and bar mark separately before combining totals.
1. Confirm geometry and zones
Use finished concrete dimensions. Identify openings, recesses, edge strips, thickened areas, construction joints, and zones with different spacing.
2. Match every bar callout
Record the bar mark, size, grade, coating, direction, spacing, face, and layer. Do not combine callouts until their requirements match.
3. Convert cover to the bar line
Drawings usually state clear cover to the bar surface. For this calculator, edge-to-bar-centre distance equals clear cover plus half the nominal bar diameter.
4. Add shape and continuity length
Take lap splices, development, anchorage, hooks, bends, couplers, and dowels from the approved details—never from a generic rule of thumb.
5. Build a cutting and bar list
Group identical bars by mark, shape, diameter, and cut length. Check whether offcuts can be reused before applying a project waste allowance.
6. Reconcile the supplier quote
Confirm available stock lengths, certified mass, fabrication charges, minimum quantities, accessories, delivery, taxes, and price validity.
Included in this result
- One uniform two-way rectangular grid
- Straight installed length and selected waste percentage
- Rounding to full stock-bar lengths
- Theoretical steel weight and basic material cost
Add or verify separately
- Additional mats, faces, layers, and local zones
- Laps, development, hooks, bends, dowels, and couplers
- Ties, chairs, supports, fabrication, delivery, and tax
- Openings, cutoffs, damage, and jobsite constraints
Where to verify requirements: use the governing code and approved project documents. For U.S. projects, the ACI 318 Building Code Portal covers structural concrete requirements, while CRSI reinforcing terminology explains bar lists, placing drawings, schedules, and fabrication terms.
Frequently asked questions
How do I calculate the quantity of rebar for a concrete slab?
Subtract the edge clearance from both sides to get the grid dimensions. Divide each dimension by the maximum spacing, round up, and add one so the reinforcement mesh includes a bar at both grid edges.
Why does the calculator round the bar count up?
Rounding up keeps the actual spacing at or below the value you entered, giving consistent coverage across the layout. Rounding down could create a larger gap than intended.
Does the stock-bar count include cutting waste, overlap, and splice allowances?
The estimator converts the installed grid length plus your selected allowance into full stock lengths. This gives a practical construction and cost-planning figure, not an optimized cutting schedule.
How is rebar weight estimated?
The calculator treats each bar as a solid steel cylinder using its nominal diameter and a steel density of 7,850 kg/m³. Actual manufacturer weights can differ slightly, so use supplier data when precise procurement figures are required.
Does this calculator determine safe rebar spacing for a footing or foundation?
No. It estimates material from spacing, cover, and bar size already specified by the project drawings or a qualified structural professional. The same limitation applies to reinforcement in beams and columns.
Should rebar intersections be welded?
Do not assume welding is permitted. Typical slab grids are tied, but the approved drawings and bar specification must control the connection method.
Before you place the order
- Bar size matches the structural drawings
- Spacing and cover use the same units
- Laps, hooks, and bends are included separately
- Stock length matches the supplier catalogue
- Cutting and site waste are allowed for
- Bar supports, ties, and delivery are budgeted
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Disclaimer
This calculator is a material-estimating aid, not a structural design tool. It does not determine a safe bar size, spacing, concrete cover, development length, lap splice, anchorage, or reinforcement ratio.
Use the approved drawings, applicable building rules, and instructions from a qualified structural professional. Actual purchasing quantities may change with cutting plans, bends, hooks, laps, openings, chairs, damage, supplier tolerances, and delivery constraints.
The weight calculation is theoretical and based on nominal diameter. Confirm certified bar mass and pricing with the supplier before ordering.