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Rebar Quantity Calculator
Estimate the total length, weight, and number of rebar pieces for your concrete slab.
Slab & Rebar Details
Total Rebar Weight
0 lbs
Total Length
0 ft
# of Pieces (20ft)
0
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What is a Rebar Quantity Calculator?
A Rebar Quantity Calculator is a tool used to estimate the total amount of reinforcing steel bar (rebar) needed for a concrete slab. By inputting the dimensions of the slab and the desired spacing of the rebar grid, it calculates the total length, number of standard pieces, and total weight of the rebar required.
How to Calculate Rebar for a Slab
The 3 Core Steps
Follow these steps for an accurate rebar estimate.
1
Determine Number of Pieces
For each direction (length and width), divide the slab dimension by the rebar spacing, then add one.
2
Calculate Total Length
Multiply the number of pieces in one direction by the length of the other direction, and add both results together.
3
Calculate Total Weight
Multiply the total length of rebar by its specific weight per foot, which varies by rebar size.
How to Use the Rebar Quantity Calculator
1. Input Dimensions
Enter your slab's length and width in feet. Accuracy is key for a good estimate.
2. Specify Rebar Details
Select the rebar size (#3, #4, etc.) and the desired center-to-center spacing in inches.
3. Get Instant Results
Receive an instant estimate of the total rebar weight, length, and number of pieces needed.
Key Rebar Calculation Formulas
These are the fundamental formulas our calculator uses to estimate the amount of rebar for your slab.
Pieces (Lengthwise) = (Slab Width (in) / Spacing) + 1
Pieces (Widthwise) = (Slab Length (in) / Spacing) + 1
Total Length = (Pieces L × Slab Length) + (Pieces W × Slab Width)
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Why Calculating Rebar Quantity Matters
Reinforcing steel (rebar) gives concrete tensile strength, drastically reducing the risk of cracking under tension. Accurately calculating the required amount of rebar ensures your slab has the necessary structural integrity. It prevents over-ordering, which saves money, and under-ordering, which can compromise safety and lead to project delays.
Calculate Your RebarFrequently Asked Questions
The number corresponds to the diameter of the bar in eighths of an inch. For example, a #4 rebar is 4/8" (or 1/2") in diameter. A #5 rebar is 5/8" in diameter, and so on. Larger numbers indicate thicker, stronger (and heavier) rebar.
For residential slabs like patios and driveways, a spacing of 18 inches on-center is very common. For footings or slabs that will bear heavier loads, a tighter spacing of 12 inches is often specified. Always check local building codes and your project's engineering plans for required spacing.
Yes, if your slab's dimensions are longer than a single piece of rebar (which are typically 20 feet long). A lap splice is the length by which two pieces of rebar must overlap to safely transfer tension. A common rule is to overlap by at least 40 times the bar's diameter. This calculator automatically accounts for lap splices on dimensions over 20 feet.
A wythe is a continuous vertical section of masonry one unit in thickness. A "single wythe" wall is one brick thick (typically used for veneers or facades). A "double wythe" wall is two bricks thick, often with an air gap or insulation in between, and is used for structural, load-bearing walls.
Rebar chairs (or supports) are small plastic or metal devices used to hold the rebar grid at the correct height within the concrete slab. For effective reinforcement, the rebar must be located in the middle or upper-third of the slab's thickness, not lying on the ground. Chairs ensure it stays in place during the pour.
No. This calculator provides an estimate for material quantity only. It does not perform structural analysis. The appropriate rebar size and spacing for your project depend on the load it will bear and must be determined by a qualified structural engineer or architect in accordance with local building codes.
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Contact UsTrusted by Masons and DIYers
See why our calculator is a go-to tool for brick projects.
Alistair Finch
Master Mason
"I use this for every job. It's spot-on for material orders and saves me from wasting money on extra pallets."
Bronwyn Rhys
DIY Homeowner
"I used this for my back patio slab and it was a lifesaver. The bag estimate was perfect for my trip to the hardware store."
Cassian Valerius
General Contractor
"Quickly gives me the numbers my crew needs on site. Reduces errors and saves time. A must-have."
Elara Vance
Landscape Architect
"Great for planning out backyard patios and walkways for clients. Helps me quickly create a budget."
Peregrine Thorne
Property Manager
"I manage several rental units, and this calculator is essential for estimating flooring costs for turnover. Simple and very accurate."
Isolde Beaumont
Retail Store Owner
"Used this for budgeting the new flooring for my boutique. The estimate was incredibly close to my final cost."
Theron Petrov
Construction Manager
"Great for quickly estimating material needs for bidding on jobs. Simple, fast, and reliable."
Seraphina Dubois
Remodeler
"This calculator is so simple, but it saved me a couple hundred dollars by making sure I included a waste factor. Highly recommend."
Leander Croft
Handyman
"Does exactly what it says. Would be cool to see a feature that helps calculate costs for removing old flooring."
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