Rebar Calculator

By Uzair Arshad , Senior Civil and Structural Engineer

Last updated: April 20, 2026

Use this steel rebar calculator to estimate how many bars you need before you call your supplier. Enter slab length, slab width, spacing, edge clearance, bar size, stock length, lap splice allowance, and waste factor. The tool returns stick count, total linear footage, weight in pounds and tons, and a 2026 material cost range for planning.

How to use this calculator

This rebar calculator is built for fast slab-grid takeoffs. It gives you ordering numbers you can actually use, including stick count, total linear feet, and total weight.

Enter slab length and slab width in feet. Use finished dimensions from the formwork layout, not rough excavation dimensions. If your slab has multiple sections, run each section separately and add the totals.
Set rebar spacing in inches on center. This rebar spacing calculator accepts 12 inches for heavier loading zones, 16 inches for common residential slabs, and 18 inches for lighter-duty areas where local code allows it.
Set edge clearance. Most slabs keep reinforcement a few inches from edges to maintain concrete cover and reduce edge cracking. If bars are too close to edges, corrosion risk and spalling risk increase over time.
Select bar size number from #3 to #8. #4 and #5 are common in residential slab and footing work, while larger bars appear in heavier structural applications.
Enter stock bar length from your supplier, then set lap splice allowance. If a bar run is longer than one stock piece, the calculator adds overlap length for each splice automatically.
Set waste factor and click Calculate rebar. Use 5% for simple rectangular slabs, 10% for typical field conditions, and 12% to 15% when many cuts or irregular sections are involved.

Pro tip: match your estimate to how your yard sells reinforcing steel. Some suppliers quote by stick, some by pound, and some by ton. This rebar grid calculator gives all three views so you can compare quotes quickly.

Common mistake: crews often forget to account for lap splices on long runs. Even a few splices per run can add enough length to require extra sticks on delivery day.

Rebar size and weight per foot reference

Use this table to match bar size with weight for takeoff conversion. Values are common US Grade 60 planning numbers.

Bar size	Nominal diameter	Weight per foot	Typical use
#3	0.375 in	0.376 lb/ft	Light residential flatwork
#4	0.500 in	0.668 lb/ft	Common slabs and small footings
#5	0.625 in	1.043 lb/ft	Heavier slabs and grade beams
#6	0.750 in	1.502 lb/ft	Structural footings and walls
#7	0.875 in	2.044 lb/ft	Heavy structural work
#8	1.000 in	2.670 lb/ft	High-load foundations

If your supplier quotes by ton, convert your linear-foot result to pounds first using bar weight per foot, then divide by 2,000 for tons.

How the calculation works

Geometry:
Effective Length (ft) = Slab Length - (2 × Edge Clearance in ft)
Effective Width (ft) = Slab Width - (2 × Edge Clearance in ft)

Bar Count:
Bars Along Length = floor(Effective Width / Spacing in ft) + 1
Bars Along Width = floor(Effective Length / Spacing in ft) + 1

Linear Footage:
Raw Linear Feet = (Bars Along Length × Effective Length) + (Bars Along Width × Effective Width)
Lap Splice Footage = Bars × Splices per Bar × Lap Length in ft
Total Linear Feet = (Raw Linear Feet + Lap Splice Footage) × (1 + Waste % / 100)

Weight and Cost:
Sticks Needed = ceil(Total Linear Feet / Stock Bar Length)
Total Weight (lb) = Total Linear Feet × Weight per Foot for bar size
Material Cost = Total Weight × price per lb ($0.45 to $0.90)

Slab Length: Total slab length in feet from formwork layout
Slab Width: Total slab width in feet from formwork layout
Spacing: Rebar spacing in inches on center (12 to 18 in is common)
Edge Clearance: Distance from slab edge to first bar in inches
Bar Size: US rebar number (#3 through #8), determines weight per foot
Stock Bar Length: Supplier stock bar length in feet (typically 20 ft)
Lap Splice: Overlap allowance in inches where bars connect end to end
Waste Factor: Extra percentage for cuts, bends, and site loss

This concrete rebar calculator uses a slab-grid method that mirrors common reinforcing steel takeoff workflows. It calculates effective slab dimensions after edge clearance, determines bars in each direction from spacing, and then adds lap splice and waste allowances before converting to sticks and weight.

The model is practical for material planning and bid checks. Use it as a reinforcement calculator for rectangular slabs only. It is not a replacement for engineered reinforcement design, but it helps you catch under-scoped material lists before ordering.

Assumptions and limitations

This rebar slab calculator assumes a flat rectangular slab with a uniform two-way grid pattern. Results may differ for L-shaped slabs, slabs with openings, or curved edges. Adjust by running each rectangular section separately.

Bar weight values use US Grade 60 deformed bar specifications.
Cost range of $0.45 to $0.90 per pound reflects 2026 US supplier averages. Your local price may vary by region and order volume.
Lap splice length uses your entered value uniformly. Actual required lap depends on bar size, concrete strength, and local code.
Waste factor covers cuts, bends, and site loss. Increase to 12% to 15% for slabs with many penetrations or irregular shapes.

Practical note:

Always confirm lap splice length and bar spacing with local building codes or structural drawings.
Structural engineers may require different layouts depending on load conditions.

Step by step breakdown

Here is how the calculation works in plain terms:

Convert inches to feet. Divide edge clearance, spacing, and lap splice by 12.
Find effective slab size. Subtract edge clearance from both ends of the slab length and width.
Count bars in each direction. Divide the effective width by spacing to get bars running along the length, then add 1 for the starting bar. Repeat for the other direction.
Calculate raw linear feet. Multiply bar count by run length in each direction and add both totals.
Add lap splice footage. If a bar run exceeds stock bar length, each splice adds extra footage based on your overlap allowance.
Apply waste factor. Multiply total linear feet by (1 + waste percentage / 100).
Convert to sticks. Divide total linear feet by stock bar length and round up.
Calculate weight. Multiply total linear feet by the weight per foot for your bar size.

Example: 20 ft × 12 ft slab with 16 in spacing and #4 rebar

For a 20 by 12 foot slab at 16 inch spacing, 3 inch edge clearance, #4 bar, 20 foot stock, 24 inch overlap, and 10% waste:

Given:

Slab = 20 ft × 12 ft
Spacing = 16 in (1.333 ft)
Edge Clearance = 3 in (0.25 ft)
Bar size = #4 (0.668 lb/ft)
Stock bar = 20 ft, Lap splice = 24 in (2 ft), Waste = 10%

Calculations:

Effective Length = 20 - (2 × 0.25) = 19.5 ft
Effective Width = 12 - (2 × 0.25) = 11.5 ft
Bars Along Length = floor(11.5 / 1.333) + 1 = 9 bars
Bars Along Width = floor(19.5 / 1.333) + 1 = 15 bars
Raw Linear Feet = (9 × 19.5) + (15 × 11.5) = 175.5 + 172.5 = 348 ft
Lap splices = 0 (all runs fit within one 20 ft stock bar)
Total with 10% Waste = 348 × 1.10 = 382.8 ft
Sticks Needed = ceil(382.8 / 20) = 20 pieces
Total Weight = 382.8 × 0.668 = 255.7 lb (0.13 tons)
Material Cost = $115 to $230 at $0.45 to $0.90 per lb

For a deeper look at how spacing and edge clearance affect your grid layout, see our rebar spacing guide for concrete slabs.

For a full walkthrough with a worked patio slab example and stock bar ordering tips, see our guide on how to calculate rebar for a slab.

Typical slab spacing guide

Spacing needs vary by loading, thickness, and design criteria. This table is a planning reference only.

Project type	Typical spacing	Common bar size	Planning note
Walkway slab	16 to 18 in	#3 to #4	Light foot traffic, low point loads
Patio slab	16 in	#3 to #4	Common residential baseline spacing
Driveway slab	12 to 16 in	#4 to #5	Vehicle loading usually needs tighter grid
Garage slab	12 in	#4 to #5	Frequent wheel loads and concentrated weight
Footing mat	6 to 12 in	#5 to #6	Confirm engineered design before placing steel

Another common mistake is skipping chair supports and tie-wire planning. If bars sag during placement, spacing and cover can drift out of tolerance. Include supports and ties in your field material list before pour day.

Use this rebar calculator to set your baseline, then verify spacing, splice, and cover with your local structural requirements before final install.

Frequently Asked Questions

How do you calculate rebar for a slab?

Measure slab length and width, subtract edge clearance, and divide each direction by spacing to get bar count. Multiply by run length for linear feet, then add lap splice and waste. This rebar calculator handles those steps automatically and converts totals to stick count and estimated weight.

How much rebar do I need for a 20x20 slab?

A 20 by 20 slab with 16 inch spacing and 3 inch edge clearance usually lands near 30 bars in a two-way grid. With lap splice allowance and 10% waste, many takeoffs fall around 640 to 720 linear feet. Final quantity depends on stock length and overlap assumptions.

What spacing should rebar use in a concrete slab?

Many residential slabs use 12 to 18 inch spacing, and 16 inch spacing is a common planning baseline. Heavier loads often require tighter spacing, larger bar sizes, or both. Always verify spacing and cover requirements with local code and the slab design before installation.

How do you calculate the amount of rebar needed for a concrete slab?

Multiply bar counts in each direction by the run length, add lap splice footage, and apply a waste factor. For a 30 by 20 foot slab at 12 inch spacing with #4 bar and 10% waste, expect roughly 50 to 55 sticks of 20 foot bar. Run the full takeoff before ordering because small spacing changes shift stick count.

How much overlap should I include for rebar splices?

A quick planning allowance of 24 inches per splice is common for estimates, but required lap splice length depends on bar size, concrete strength, and code criteria. Confirm final splice length with your engineer. Underestimating overlap is one of the most common causes of short orders.