Rebar Calculator
Free rebar calculator for slabs and footings. Estimate how much rebar you need — bar count, linear feet, weight, and stock bars.
Last updated: October 2026 · Expanded bar sizes (#2–#18 + metric), dual unit weights, direction-wise stock bars, methodology sources (not a structural design or code takeoff)
Estimate rebar for rectangular slabs and footings with a regular grid both ways. Enter length, width, bar size (#2–#18 or metric), spacing (same or different each direction), cover, and stock length. This is a material-planning estimate only — not structural design, engineered drawings, or a code-compliant takeoff.
Quick Answer: How Much Rebar Do I Need?
To find how much rebar you need, count bars in each direction from clear span and spacing, multiply by the opposite clear span for linear feet, then add waste and convert to stock bars and weight.
Linear feet ≈ (bars running along length × clear length) + (bars running along width × clear width), then × (1 + waste%).
- Bars running along length ≈ floor(clear width ÷ spacing) + 1
- Bars running along width ≈ floor(clear length ÷ spacing) + 1
- Clear span = slab dimension − 2 × cover
- Most common residential bar: #4 @ 12–18" OC
- Always round up stock bars per direction (default 20 ft stock)
Example: 20 × 20 ft slab, #4 @ 12" both ways, 3" cover, 10% waste → about 858 linear feet, 44 pieces of 20-ft stock (~573 lb). Use the calculator above for your exact spacing and cover.
How to Use This Rebar Calculator
- Enter slab or footing length and width
- Choose bar size (#2–#18 or metric) and spacing on center (same both ways, or set each direction separately)
- Set clear cover (often 3" for slabs on grade — confirm with plans)
- Pick a waste factor (10% recommended), stock length (usually 20 ft), and optional price per foot
- Click Calculate Rebar for counts both ways, linear feet, weight, and stock bars
How Much Rebar for a Slab? (Examples)
Quick reference using #4 bar, 12" spacing both ways, 3" cover, 10% waste, and 20-ft stock. Stock counts use direction-wise cut lengths (not one global linear-feet pool). Re-run the calculator with your inputs.
| Slab Size | Linear Feet | Weight (lb) | 20-ft Stock |
|---|---|---|---|
| 10 × 10 ft | ~209 ft | ~140 lb | ~12 |
| 12 × 12 ft | ~304 ft | ~203 lb | ~16 |
| 16 × 20 ft | ~684 ft | ~457 lb | ~36 |
| 20 × 20 ft | ~858 ft | ~573 lb | ~44 |
| 24 × 24 ft | ~1,241 ft | ~829 lb | ~64 |
Figures are planning estimates. A driveway or garage slab with tighter spacing (or heavier bar) will need more steel — run the calculator with your exact inputs. Buy a few extra bars for cuts and damage.
Worked example (20 × 20 ft, #4 @ 12", 3" cover)
- Clear span each way ≈ 20 − 2 × (3/12) = 19.5 ft
- Bars running each way = floor(19.5 ÷ 1.0) + 1 = 20 → 40 pieces total
- Linear feet (no waste) = 20 × 19.5 × 2 = 780 ft
- With 10% waste ≈ 858 ft → weight ≈ 858 × 0.668 ≈ 573 lb
- Stock per direction ≈ ceil(20 × 19.5 × 1.10 ÷ 20) = 22 → total ≈ 44 pieces of 20-ft stock
Contrast example (same 20 × 20 ft pad, lighter patio spacing)
Same slab with #3 @ 18" both ways, 3" cover, 10% waste:
- Clear span = 19.5 ft
- Bars each way = floor(19.5 ÷ 1.5) + 1 = 14 → 28 pieces
- Linear feet (no waste) ≈ 14 × 19.5 × 2 = 546 ft → with 10% ≈ 601 ft
- Weight ≈ 601 × 0.376 ≈ 226 lb (much lighter than the #4 @ 12" grid)
- Stock per direction ≈ ceil(14 × 19.5 × 1.10 ÷ 20) = 16 → total ≈ 32 × 20-ft #3
Spacing and bar size change the order more than overall slab area alone. Always match the plan or local practice — these examples are planning illustrations only.
Rebar Size & Spacing Guide (Residential)
Common starting points for rectangular slabs on grade. These are planning ranges — not code minimums. Local practice, soil, load, and slab thickness change the answer.
| Project | Typical Bar | Typical Spacing | Notes |
|---|---|---|---|
| Patio / walkway | #3 or #4 | 18" OC | Light pedestrian loads; mesh is also common |
| Sidewalk | #3 or #4 | 18–24" OC | Often lighter than driveways |
| Driveway | #4 | 12–18" OC | Vehicles; tighter spacing on poor subgrade |
| Garage floor | #4 | 12–16" OC | Heavier point loads than a patio |
| Thick / heavy-use slab | #5 | 12–16" OC | Workshops, lifts, heavy equipment |
| Simple footing | #4 or #5 | Varies | Usually follows engineered detail |
Enter your chosen size and spacing in the calculator above. Use the same spacing both ways, or set different values if your plan calls for it.
Placement, Cover & Chairs
Quantity is only half the job. Placement controls whether the steel actually helps the slab.
- Clear cover: For slabs cast against soil, 3 inches is a common planning value from the bottom and edges. Formed edges and top cover can differ — follow your plans or local requirements.
- Middle third: In a typical slab on grade, bars should sit roughly in the middle third of the thickness (not on the ground and not at the very top).
- Chairs / dobies: Use plastic or concrete supports so bars stay at the correct height while you pour. Wire-tie intersections so the grid does not shift.
- Never on dirt: Bars resting on the subgrade lose cover and can rust or move during the pour.
This calculator uses cover only to locate the first and last bars for a planning count. It does not design chair height or check structural cover rules.
Standard Rebar Sizes & Weight
Use this chart for quick unit weights when estimating rebar weight. US sizes use ASTM A615 nominal weights (lb/ft). Metric sizes use common theoretical weights (kg/m). The calculator multiplies your total length by the selected unit weight and shows both lb and kg.
| Bar Size | Diameter | Weight | Common Use |
|---|---|---|---|
| #3 | 3/8" (9.5 mm) | 0.376 lb/ft | Light slabs, sidewalks, temperature steel |
| #4 | 1/2" (12.7 mm) | 0.668 lb/ft | Residential slabs, driveways, footings |
| #5 | 5/8" (15.9 mm) | 1.043 lb/ft | Thicker slabs, garage floors, heavier footings |
| #6 | 3/4" (19.1 mm) | 1.502 lb/ft | Heavier structural footings and grade beams |
| #7 | 7/8" (22.2 mm) | 2.044 lb/ft | Heavy structural, commercial |
| #8 | 1" (25.4 mm) | 2.670 lb/ft | Columns, heavy foundations |
| #9–#11 | 1.128"–1.410" | 3.400–5.313 lb/ft | Heavy structural members |
| #14 / #18 | 1.693" / 2.257" | 7.650 / 13.600 lb/ft | Very heavy structural / special applications |
| 10–12 mm | 10 / 12 mm | 0.617 / 0.888 kg/m | Light metric slabs and secondary bars |
| 16–20 mm | 16 / 20 mm | 1.578 / 2.466 kg/m | Common metric main bars |
| 25–32 mm | 25 / 32 mm | 3.853 / 6.313 kg/m | Heavy metric structural members |
Sources: US nominal weights from ASTM A615/A615M (Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement). Metric unit masses are theoretical values based on steel density ≈ 7850 kg/m³, consistent with common estimating practice. Actual mill weight can vary slightly within specification tolerances.
Formulas Used
Clear length = Length − 2 × (Cover ÷ 12)
Clear width = Width − 2 × (Cover ÷ 12)
Bars running along length = floor(Clear width ÷ Spacing along width) + 1
Bars running along width = floor(Clear length ÷ Spacing along length) + 1
Linear feet = (Bars running along length × Clear length) + (Bars running along width × Clear width)
With waste = Linear feet × (1 + Waste%)
Weight = Linear length with waste × unit weight of the selected bar (lb/ft for US sizes, kg/m for metric). Results always show both lb and kg.
Stock bars (per direction) = ceil(Number of bars × Cut length × (1 + Waste%) ÷ Stock length)
Total stock bars = Stock bars running along length + Stock bars running along width
Cut length for bars running along length = Clear length; cut length for bars running along width = Clear width. Each direction is pooled with waste, then converted to stock length — more realistic than one global linear-feet total, without forcing a near-full bar onto two sticks. Spacing is entered in inches on center (OC) and can differ in each direction. Cover is clear cover from the edge used to locate the first and last bars for planning.
Rebar Stock Length & Cutting
Rebar is usually sold in fixed stock lengths (often 20 ft in the US, or 6–12 m metric). You rarely buy “continuous” steel by the foot from a pool — you buy whole sticks and cut them to the lengths the grid needs.
- Cut length for bars running along length ≈ clear length of the slab (after cover).
- Cut length for bars running along width ≈ clear width.
- For each direction, stock pieces ≈ ceil(number of bars × cut length × (1 + waste%) ÷ stock length).
- Total stock bars = both directions added together.
Example: 20 bars × 19.5 ft with 10% waste → ceil(20 × 19.5 × 1.10 ÷ 20) ≈ 22 stock pieces for that direction (not 40). Short 9.5 ft cuts pool more efficiently on each stick than near-full-length cuts.
Simply dividing total linear feet by stock length ignores that each direction has its own cut length. This calculator counts stock per direction so the purchase number tracks how bars are cut on site, without over-ordering when waste is only a small trim on a full stick.
What it does not do: It does not optimize mixing different cut lengths on one stick, plan lap splice locations, or produce a shop cutting list. Fabricators and detailers may improve yield further from the structural drawings.
Calculation Methodology
This calculator is a material-planning tool for rectangular rebar grids. The methods and reference values are:
- US bar unit weights use the nominal weights published in ASTM A615 / A615M (Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement). These are the industry-standard values used for estimating and ordering.
- Metric bar unit weights use commonly accepted theoretical masses derived from steel density ≈ 7850 kg/m³ (for example 12 mm ≈ 0.888 kg/m, 16 mm ≈ 1.578 kg/m), consistent with standard estimating practice.
- Bar count uses the standard grid formula: bars = floor(clear span ÷ on-center spacing) + 1, applied in each direction from the clear span after subtracting cover on both sides.
- Stock bars are calculated per direction from the actual cut length of each bar (plus the selected waste factor), not by simply dividing total linear feet by stock length. This better reflects how bars are purchased and cut.
- Waste factor is a practical allowance for cuts, short laps, and handling loss. It is not a substitute for a detailed lap or splice schedule from the structural drawings.
Scope and limits: Results are for planning quantities only. The calculator does not check structural capacity, development length, code minimums, or engineered splice requirements. Bar size, spacing, cover, and detailing must follow your project plans, engineer, or local building department. For the official ASTM standard text, see the ASTM website (ASTM A615/A615M).
When Not to Use This Calculator
Use this tool for a rectangular slab or footing grid when you need a planning estimate of bar count, linear feet, weight, and stock bars. Do not rely on it for:
- Non-rectangular layouts (L-shaped, circular, or irregular pads) unless you split them into simple rectangles and run the calculator per area
- Walls, beams, columns, or multi-layer / multi-mat reinforcement
- Structural design — bar size and spacing from loads, code minimums, or an engineer’s judgment
- Lap splice schedules, development length, or shop fabrication cutting lists
- Special systems (for example epoxy-coated detailing rules) where project specs override simple grid math
- Any job where stamped drawings or local code specify different steel than a simple both-ways grid
If your project is structural or non-standard, follow the drawings and a licensed professional. This calculator is for material planning only.
Rebar Planning Tips
- Place bars in a grid both ways for typical residential slabs unless your plan says otherwise. Spacing can be the same each way or different if the detail requires it.
- Keep bars up on chairs or dobies so they sit in the middle third of the slab thickness. Steel on the ground loses cover and does little to control cracking.
- Lap splices add length where bars meet end-to-end. A 5–10% waste factor covers simple cuts and short laps; long development lengths or engineered splice schedules need plan review.
- Driveways and garage slabs often use tighter spacing or heavier bar than patio slabs because of vehicle and point loads.
- Wire mesh vs rebar: Many light patios and sidewalks use welded wire mesh or fiber for crack control. Rebar is more common on driveways, garage floors, and thicker or heavier-use slabs.
- After you know concrete volume, pair this tool with the Concrete Calculator for yards and bags. Confirm area with the Square Footage Calculator if you measured rooms or pads separately.
Important: This calculator provides a material-planning estimate only for a rectangular rebar grid. It is not a structural design tool, engineered drawing, lap schedule, or code-compliant takeoff. It does not check loads, capacity, development length, or minimum code requirements. Bar size, spacing, cover, and detailing must follow your project plans, a licensed engineer, or local building department. Using these results for construction without professional review is at your own risk.
Frequently Asked Questions
Count bars each way from clear span and spacing, multiply by the opposite clear span for linear feet, add waste, then convert to stock bars and weight. A 20×20 ft slab with #4 at 12" both ways, 3" cover, and 10% waste needs about 858 linear feet (44 pieces of 20-ft stock, ~573 lb). This free rebar calculator does the math from your dimensions.
For a 20×20 ft slab with #4 bar at 12-inch spacing, 3-inch cover, and 10% waste, plan about 858 linear feet (44 pieces of 20-ft stock), or about 573 lb. Always re-run the calculator with your exact spacing and cover.
For a 10×10 ft slab with #4 bar at 12-inch spacing, 3-inch cover, and 10% waste, plan about 209 linear feet (about 12 pieces of 20-ft stock with direction-wise cuts), or roughly 140 lb. Re-run the calculator with your exact spacing, cover, and bar size.
There is no fixed number per square foot — it depends on spacing and whether the grid is both ways. At 12-inch spacing both ways you get roughly 2 linear feet of bar per square foot before waste (one foot each direction). At 18-inch spacing both ways it is closer to 1.33 linear feet per square foot. Convert linear feet to pieces using your stock length.
Clear span = dimension − 2 × cover. Bars running along length = floor(clear width ÷ spacing) + 1; bars running along width = floor(clear length ÷ spacing) + 1. Linear feet = (bars along length × clear length) + (bars along width × clear width). Add waste, then convert to stock bars and weight. This calculator does those steps from your inputs.
There is no single rule of thumb that fits every slab. Rebar quantity depends on bar size, spacing, and whether steel is required both ways — not only on concrete volume. A thin patio with light spacing needs far less steel per yard than a thick driveway grid. Estimate bars from the grid (this calculator), estimate concrete volume separately with the Concrete Calculator, then compare if needed.
A rebar calculator estimates bar count both ways, linear feet, weight, and stock bars from slab size, bar size, spacing, cover, and waste. This tool is for material planning on rectangular grids — not structural design or a code takeoff.
#4 (1/2 inch) is the most common for residential slabs and driveways. #3 is lighter (sidewalks, thin slabs); #5 is heavier for thicker slabs or higher loads. Confirm size with your plans or local practice.
Residential driveways commonly use #4 bar at 12 to 18 inches on center both ways. Heavier vehicles or poor subgrade often push toward 12-inch spacing. Always confirm with local practice or your plans.
Many 4-inch patio and sidewalk slabs use welded wire mesh or fiber instead of rebar for crack control. Rebar becomes more common on thicker slabs, driveways, garage floors, and any slab with heavier loads. Local code and soil conditions matter.
For slabs cast against soil, 3 inches of clear cover is a common planning value. Formed edges and top cover can differ. Place bars on chairs or dobies so steel sits in the middle third of the slab thickness — never on the ground.
Spacing is the center-to-center distance between parallel bars (often 12, 16, or 18 inches for residential slabs). Count = floor(clear span ÷ spacing) + 1. Clear span is the slab dimension minus cover on both ends. This calculator supports different spacing in each direction.
US bar weights (ASTM A615): #3 ≈ 0.376, #4 ≈ 0.668, #5 ≈ 1.043, #6 ≈ 1.502, #7 ≈ 2.044, #8 ≈ 2.670 lb/ft. Metric sizes use kg/m (e.g. 12 mm ≈ 0.888, 16 mm ≈ 1.578). This calculator multiplies total length by the selected bar unit weight and shows both lb and kg.
A single 20-ft #4 bar weighs about 13.4 lb (0.668 lb/ft × 20). Ten pieces ≈ 134 lb; a full grid order is usually hundreds of pounds — use the calculator above for your exact linear feet and total weight.
About 1.578 kg per meter (theoretical unit mass). A 12 m stock length of 16 mm bar is roughly 18.9 kg. Select 16 mm in the calculator to get total weight from your grid length.
Total weight = total linear length × unit weight of the bar size. For US bars use lb/ft from ASTM A615; for metric use kg/m. Example: 858 linear feet of #4 (0.668 lb/ft) ≈ 573 lb. This calculator applies waste to length first, then multiplies by the selected unit weight and shows both lb and kg.
Yes. 5–10% is a practical default for cuts, laps, and damaged bars. Complex layouts or many openings may need more. This tool applies the waste factor to linear feet before weight and stock counts.
No. This is a material-planning estimate for rectangular grids only. It is not structural design, engineered drawings, or a code-compliant takeoff. It does not check loads, capacity, development length, or code minimums. Confirm bar size, spacing, cover, and detailing with your plans, a licensed engineer, or local building department before construction.