Rebar Calculator: Concrete Slab Grid & Weight Estimator
Calculate rebar grid length, bar quantities, and steel weight for concrete.
Privacy First: 100% Client-Side Processing
Processed in your browser. Files are never uploaded.
How to Use the Rebar Calculator: Concrete Slab Grid & Weight Estimator
- 1Enter slab length and width in feet.
- 2Select rebar size (#3, #4, #5, etc.) and grid spacing (12", 16", 18", or 24").
- 3Specify edge clearance (usually 2 to 3 inches) and lap splice length.
- 4Review total linear feet, number of 20-foot bars, and steel weight.
Formula & How It Works
Rebar Grid Quantity Formula
Runs along length + runs along width adjusted for edge clearance and lap splice.
Worked Examples
24x24 Garage Slab
Input: 24x24 ft, #4 rebar, 18" grid
Explanation: Grid requires 768 linear feet of #4 rebar (0.668 lbs/ft).
Driveway 12x40
Input: 12x40 ft, #4 rebar, 12" grid
Explanation: 960 linear feet of rebar with 10% lap allowance.
Small Patio 10x10
Input: 10x10 ft, #3 rebar, 18" grid
Explanation: 160 linear feet of #3 rebar (0.376 lbs/ft).
Frequently Asked Questions
How do you calculate rebar for a concrete slab?▼
Divide slab dimensions by grid spacing to count runs in both directions, add lap splice allowance, and divide by 20 ft stock length.
What is the standard rebar size for a residential slab?▼
#4 rebar (1/2 inch diameter) spaced 12 to 18 inches on center is the most common residential standard.
How long is a standard rebar stick?▼
Standard rebar is sold in 20-foot lengths at building supply yards.
What is the weight of #4 rebar?▼
#4 rebar weighs 0.668 pounds per linear foot (a 20-foot bar weighs ~13.36 lbs).
What is a lap splice?▼
A lap splice is the required overlap when joining two pieces of rebar end-to-end, typically 30 to 40 bar diameters.
Is this tool free?▼
Yes, Toolzy construction calculators are completely free and private.
Quick Reference & Conversion Table
| Rebar Size | Diameter (in) | Weight per Foot (lbs) | Lap Splice (in) |
|---|---|---|---|
| #3 | 0.375 in (3/8") | 0.376 lbs/ft | 15 in |
| #4 | 0.500 in (1/2") | 0.668 lbs/ft | 20 in |
| #5 | 0.625 in (5/8") | 1.043 lbs/ft | 25 in |
| #6 | 0.750 in (3/4") | 1.502 lbs/ft | 30 in |
| #8 | 1.000 in (1") | 2.670 lbs/ft | 40 in |
⚠️ Common Pitfalls to Avoid
- •Forgetting lap splice overlap (bars must overlap by 30 to 40 bar diameters when spliced together).
- •Neglecting edge clearance (rebar must stay 2-3 inches inside the concrete edges to prevent rust).
💡 Practical Real-World Uses
- •Pouring garage slabs, patios, and residential concrete foundations.
- •Civil construction footing and grade beam reinforcement sizing.
- •Ordering steel bundle stock from local masonry and lumber supply yards.
Related Online Tools
Explore complementary calculators and converters with instant client-side execution
Paver Calculator: Patio Pavers, Sand & Gravel Base
Calculate patio pavers, bedding sand, and gravel base requirements.
Siding Calculator: Estimate Siding Squares & Costs
Calculate house exterior siding squares, gable triangles, and material costs.
Flooring Calculator: Hardwood, Vinyl Plank & Tile
Calculate room square footage, flooring boxes, and installation waste.
Insulation Calculator: Batt, Roll & Blown-In R-Value
Calculate batt rolls, blown-in bags, and target R-values for insulation.
Explore Other Tool Categories
Browse our full collection of 50 free web utilities
Unit Converters
Exact metric & imperial conversions for weight, length, temperature & volume.
Math & Education
Percentage changes, fractions, square roots, geometry & statistics.
Date & Time
Calendar arithmetic, business day intervals, stopwatch & multi-counter.
Home & Construction
Paint coverage, pool volume, topsoil, roof pitch, square footage & materials.