Rebar Spacing Calculator: Bar Count, Length and Weight
Grid and purchase inputs
Material and layout result
Purchase summary
Longitudinal bars run along slab length; counted across width
Transverse bars run across slab width; counted along length
Grid geometry
Advisory checks
- Select the ACI advisory screen to compare this layout with limited reference checks.
Live plan-view layout
For legibility, very dense grids are sampled visually; the labels and material result retain the full counts.
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Exact formulas and layout convention
Let D be the slab dimension perpendicular to a bar direction, C the clear cover, d the selected bar diameter, and Smax the requested maximum center-to-center spacing. All arithmetic is performed in unrounded base units.
- Physical bar length: run dimension − 2C. Clear cover controls the cut length because cover ends at the bar surface.
- First and last bar centerlines: C + d/2 from each concrete face.
- Centerline span: D − 2C − d.
- Known-spacing mode: spaces = ceil(centerline span ÷ Smax), then bars = spaces + 1. Spaces round up so actual spacing cannot exceed the requested maximum.
- Known-count mode: actual spacing = centerline span ÷ (bars − 1). Bar count must be a whole number of at least two.
- Clear gap: actual center-to-center spacing − bar diameter.
- Base grid length: layers × [(longitudinal count × longitudinal length) + (transverse count × transverse length)].
- Pieces per run: max(1, ceil((run length − lap) ÷ (stock length − lap))). Required splices equal pieces per run minus one, multiplied by grid lines and layers.
- Adjusted purchase length: (base length + total splices × lap allowance) × (1 + waste%/100).
- Whole stock bars: ceil(adjusted purchase length ÷ stock length). This rounds up and assumes reusable offcuts; confirm a supplier cut list.
- Weight: sum each direction’s adjusted length multiplied by that direction’s ASTM nominal unit weight.
Far-edge convention: the first and last bars are anchored at equal surface cover, then every interval between them is equal. This differs from layouts that step the requested spacing from one edge and leave a larger remainder at the far edge.
Worked examples
20 ft × 20 ft slab, #4 bars at 16 in center-to-center
Using 1.5 in clear cover: centerline span in each direction = 240 − 2(1.5) − 0.5 = 236.5 in. Spaces = ceil(236.5 ÷ 16) = 15, so each direction has 16 bars and actual spacing is 236.5 ÷ 15 = 15.767 in. Each bar is 240 − 3 = 237 in = 19.75 ft. Total = 32 × 19.75 = 632 ft and 632 × 0.668 = 422.2 lb. With 20 ft stock, no laps and no waste, the length-based estimate is 32 stock bars.
24 ft × 12 ft driveway grid, #4 bars at 12 in center-to-center
Using 2 in clear cover: 13 longitudinal bars run 23.667 ft with 11.625 in actual spacing; 25 transverse bars run 11.667 ft with 11.813 in actual spacing. The single-layer grid totals 599.3 ft and about 400.4 lb. With 20 ft stock, no laps and no waste, aggregate length rounds to 30 stock bars; a real cut plan may require a different quantity.
6 m × 4 m metric slab, No. 13 bars at 200 mm center-to-center
Using 40 mm clear cover and a 12.7 mm bar diameter: longitudinal centerline span = 3,907.3 mm, giving 21 bars at 195.365 mm actual spacing and 5.92 m each. Transverse centerline span = 5,907.3 mm, giving 31 bars at 196.910 mm and 3.92 m each. Total length is 245.84 m and nominal weight is about 244.4 kg. At 12 m stock with no laps or waste, length rounds up to 21 stock bars.
A hand count may differ if it measures cover to the centerline, uses requested spacing all the way from one edge, omits edge bars, or treats laps and reusable offcuts differently. State the convention before comparing takeoffs.
Rebar properties and input definitions
| US / metric designation | Diameter | Area | Unit weight |
|---|---|---|---|
| #3 / No. 10 | 0.375 in / 9.5 mm | 0.11 in² / 71 mm² | 0.376 lb/ft / 0.560 kg/m |
| #4 / No. 13 | 0.500 in / 12.7 mm | 0.20 in² / 129 mm² | 0.668 lb/ft / 0.994 kg/m |
| #5 / No. 16 | 0.625 in / 15.9 mm | 0.31 in² / 199 mm² | 1.043 lb/ft / 1.552 kg/m |
| #6 / No. 19 | 0.750 in / 19.1 mm | 0.44 in² / 284 mm² | 1.502 lb/ft / 2.235 kg/m |
| #7 / No. 22 | 0.875 in / 22.2 mm | 0.60 in² / 387 mm² | 2.044 lb/ft / 3.042 kg/m |
| #8 / No. 25 | 1.000 in / 25.4 mm | 0.79 in² / 510 mm² | 2.670 lb/ft / 3.973 kg/m |
| #9 / No. 29 | 1.128 in / 28.7 mm | 1.00 in² / 645 mm² | 3.400 lb/ft / 5.060 kg/m |
| #10 / No. 32 | 1.270 in / 32.3 mm | 1.27 in² / 819 mm² | 4.303 lb/ft / 6.404 kg/m |
| #11 / No. 36 | 1.410 in / 35.8 mm | 1.56 in² / 1006 mm² | 5.313 lb/ft / 7.907 kg/m |
What the estimate includes
- Two orthogonal straight-bar grids with independent sizes and layouts.
- Equal clear cover on all four slab edges.
- Entered layers, lap allowance, waste and optional price.
- Length-based stock count with reusable offcuts assumed.
What it omits
- Required steel area, capacity, loads and crack-control design.
- Development length, hooks, bends, dowels and engineered splice length.
- Openings, thickened edges, beams, chairs, tie wire and supports.
- Cut optimization, mill tolerances, taxes, delivery and supplier minimums.
Common counting mistakes
- Measuring cover to a centerline instead of the bar surface.
- Counting lengthwise bars along the length instead of across the width.
- Using requested spacing as actual after intervals round up.
- Forgetting extra layers, lap steel or stock-length constraints.
Purchase checklist
- Match bar grade, coating and size to project drawings.
- Confirm stock lengths and whether the supplier provides cuts.
- Obtain the engineer-specified lap and development details.
- Check openings, edge bars, supports and local inspection requirements.
Terminology: center-to-center spacing is the distance between bar axes; clear spacing is the open surface-to-surface gap. Concrete cover is the clear distance from a concrete face to a bar surface, while edge-center distance is cover plus half the bar diameter.
Methodology, safety limits and sources
Methodology last reviewed: 17 July 2026. No qualified technical reviewer is claimed. The arithmetic and reference mapping should be checked against the project documents before purchase or placement.
The ACI option provides three limited advisory comparisons: a conservative maximum-spacing screen of the lesser of 2 × slab thickness and 18 in (457.2 mm), minimum clear spacing of the greatest of one bar diameter, 1 in (25.4 mm), and 4/3 maximum aggregate size, and example cover values selected by exposure and bar size. Applicability varies by member, reinforcement purpose and adopted edition. In particular, ACI 318 does not generally apply to an ordinary soil-supported slab unless it transfers structural loads or forces to the soil.
- ACI CODE-318-25, Building Code for Structural Concrete—Code Requirements and Commentary: current reference edition for the optional screening controls. Locally adopted editions and amendments may differ.
- American Concrete Institute technical FAQ: Slabs-on-ground: explains the scope limitation for soil-supported slabs and points to ACI 360R guidance.
- American Concrete Institute technical FAQ: cover against ground: identifies the 3 in (76 mm) example requirement for concrete cast against and permanently in contact with ground in ACI 318-19. The calculator treats this as an advisory reference, not a universal slab rule.
- American Concrete Institute technical FAQ: nominal maximum aggregate size: explains the 3/4-of-clear-spacing aggregate limitation and its placement rationale.
- ASTM A615/A615M-26, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement, Table 1: nominal bar designations, dimensions, cross-sectional areas and masses used by the calculator.
Structural-design disclaimer: This calculator is a quantity and field-layout aid. It does not select the required reinforcement, verify strength or serviceability, design development or lap length, check fire resistance, or replace stamped drawings. Use the engineer of record’s details and the locally adopted code. Do not describe a result as “ACI compliant” based on this screen.
Frequently asked questions
How many rebar bars do I need?
Choose Known spacing to round the number of centerline intervals up in each direction, then add one bar. The calculator anchors the first and last bar centerlines at clear cover plus half the selected bar diameter.
How does center-to-center spacing differ from clear spacing?
Center-to-center spacing is measured between bar centerlines. Clear spacing is the open gap between adjacent bar surfaces, so it equals center-to-center spacing minus the bar diameter.
Is concrete cover measured to the bar surface or centerline?
Concrete cover is the clear distance from the concrete face to the nearest bar surface. The first bar centerline is therefore one-half bar diameter farther inward than the specified cover.
Why do bar counts round up?
When a maximum spacing is entered, the required number of spaces rounds up so the actual evenly distributed spacing does not exceed the request. Stock bars also round up because suppliers sell whole lengths.
What stock rebar lengths are common?
Common availability varies by market and supplier; 20 ft and 40 ft lengths are often stocked in the United States, while 6 m and 12 m lengths are common in metric markets. Enter the length your supplier actually sells.
Are lap splices and waste included?
They are included only when you enter a lap-splice allowance and waste percentage. The tool estimates the number of splices from stock length but does not design the required lap or optimize a cutting schedule.
Does the calculator support metric units?
Yes. The unit switch converts dimensions between feet and metres and smaller measurements between inches and millimetres, while metric bar labels show diameter, area and kilograms per metre.
Does this tool design a safe reinforced concrete slab?
No. It is a quantity and layout estimator. It does not select required reinforcement, verify structural capacity, design development or lap length, or replace stamped drawings and locally adopted requirements.