How to Calculate Rebar for Concrete (Chart & Example)

Calculating the amount of steel rebar for a concrete project is essential if you want a strong slab or footing without overbuying material. This guide walks you through simple formulas, a quick weight chart and a full example so you can estimate both the length and the weight of rebar with confidence.

If you are still choosing materials, you can compare our wholesale steel rebar and durable fiberglass rebar options to match your budget and project needs.

Quick Answer: #4 Rebar Weight per Foot

Many people search for “#4 rebar weight per foot” because they need to estimate the total weight for delivery or lifting. In the US bar size system, standard carbon steel rebar has the following approximate weights per foot:

Once you know the total linear feet of rebar, you can multiply by the value in the table to get total weight.

Key Factors That Affect Rebar Quantity

Before you start calculating the amount of steel rebar in concrete, make sure you know the following project details:

• Slab or footing size: length, width and thickness of the concrete element.
• Rebar size: for example #3, #4 or #5 depending on the structural design.
• Bar spacing: how far apart the bars will be placed, such as 8 in, 12 in or 18 in on center.
• Layout: single direction bars, or a grid with bars in both directions.
• Lap splices and hooks: extra length where bars overlap or bend, plus a waste allowance.

Exact spacing and bar size should always follow your engineered drawings and local building codes. The methods below help you estimate material quantity for ordering and budgeting.

Basic Formula: How to Calculate Rebar in a Concrete Slab

For a typical rectangular slab with rebar in two directions, you can use this simple approach:

1. Convert dimensions and spacing into the same units. Work in feet, and remember that 12 inches equals 1 foot.
2. Calculate the number of bars in each direction. Divide the slab width by the spacing and round up, then add one more bar at the edge. Repeat for the other direction.
   Bars running along the length: number of bars = (slab width ÷ spacing) + 1.
   Bars running along the width: number of bars = (slab length ÷ spacing) + 1.
3. Estimate the length of each bar. For a quick estimate, you can use the full slab length or width. For a more precise number, subtract cover at both ends (for example 3 in of cover on each side).
4. Multiply bars by length to get total linear feet. Total length in each direction = number of bars × bar length. Then add the two directions together.
5. Convert linear feet to weight. Use the weight per foot from the rebar chart above. Total weight = total linear feet × weight per foot.
6. Add a waste factor. Add about 10% to cover lap splices, cutting and offcuts.

This method works for most slabs on grade, driveways, patios and interior floors where rebar is placed in a simple grid.

Worked Example: 10 ft × 20 ft Concrete Slab with #4 Rebar

Let’s walk through a complete example so you can see how the math works in practice.

Project: 10 ft by 20 ft driveway slab, 4 in thick, using #4 rebar at 12 in on center in both directions.

Step 1: Dimensions and spacing

• Slab length = 20 ft
• Slab width = 10 ft
• Spacing = 12 in = 1 ft
• Rebar size = #4 (0.668 lb/ft)

Step 2: Number of bars in each direction

• Bars running the 20 ft length (spaced across the 10 ft width): number of bars = (10 ÷ 1) + 1 = 11 bars.
• Bars running the 10 ft width (spaced along the 20 ft length): number of bars = (20 ÷ 1) + 1 = 21 bars.

Step 3: Total linear feet of rebar

• Length-direction bars: 11 bars × 20 ft = 220 ft.
• Width-direction bars: 21 bars × 10 ft = 210 ft.
• Total linear feet = 220 ft + 210 ft = 430 ft.

Step 4: Calculate total weight

• #4 rebar weight per foot = 0.668 lb/ft.
• Total weight = 430 ft × 0.668 lb/ft ≈ 287 lb.
• With 10% waste and lap splices: 287 lb × 1.10 ≈ 316 lb (you can round up to about 320 lb).

So for this 10 ft × 20 ft slab, you would order around 430 linear feet of #4 rebar, or about 320 pounds including a reasonable waste allowance.

Estimating Rebar for Footings and Beams

Footings and beams usually use bars running in one main direction with stirrups or ties around them. The basic idea is similar, but the geometry changes slightly.

• Continuous strip footing: calculate the total footing length, multiply by the number of longitudinal bars (for example 2, 3 or 4), then add extra for lap splices and hooks at corners.
• Isolated pad footing: treat it like a small slab and use the same grid method as above if it has two-way reinforcement.
• Beams: multiply the clear span length by the number of top and bottom longitudinal bars, then add length for hooks and development length at supports. Stirrups are counted separately based on spacing along the beam.

Because beams and load-bearing footings are critical structural elements, always follow the bar sizes, numbers and spacing specified by the structural engineer.

Typical Rebar Spacing for Concrete Slabs

Rebar spacing affects both strength and crack control. Exact spacing must follow design codes, but these ranges are common in practice:

• Driveways and garage slabs: #4 bars at 10 in to 12 in on center in both directions.
• Patios and walkways: #3 or #4 bars at 12 in to 18 in on center, depending on load and soil conditions.
• Heavy-duty slabs (equipment pads, industrial floors): closer spacing such as 8 in or 10 in may be required.

Closer spacing generally improves crack control but increases material cost. When in doubt, follow the engineer’s drawings rather than guessing.

FAQs About Calculating the Amount of Steel Rebar in Concrete

How do I quickly calculate rebar for a concrete slab?

Measure the slab length and width, choose your bar spacing (for example 12 in), then divide each dimension by the spacing and add one extra bar in each direction. Multiply bar count by bar length to get total linear feet, then use a weight-per-foot chart to convert to pounds.

How much overlap should I allow when splicing rebar?

Lap splice length depends on bar size, concrete strength and code requirements, but a common rule of thumb is around 40 times the bar diameter. For #4 bar (1/2 in diameter), that is about 20 in of overlap. Always check your local code and project drawings for the exact value.

What waste factor should I add when ordering steel rebar?

For most small to medium projects, adding around 10% on top of your calculated total is enough to cover cutting waste, lap splices and small changes on site. Complex shapes or heavily reinforced elements may require a higher allowance.

Can I use fiberglass rebar instead of steel?

Fiberglass rebar is lighter, corrosion-resistant and easier to handle than steel, which makes it attractive for coastal or high-moisture environments. However, its design rules are different, so it should only be used when the structure is specifically engineered for fiberglass reinforcement. If you are interested, you can explore our fiberglass rebar options.

When should I talk to an engineer instead of estimating on my own?

Any time you are working on structural elements such as foundations, retaining walls, multi-story buildings or heavily loaded industrial slabs, you should work from drawings prepared by a licensed engineer. The methods in this guide are meant for estimating material quantities, not for replacing structural design.

By using these formulas, charts and examples, you can estimate how much steel rebar your concrete project will need, compare options between different bar sizes and place more accurate orders from suppliers like Wellco Wholesale.