Rebar Calculator for Concrete

How to Use the Rebar Calculator

Select the application type (slab, wall, or footing), enter dimensions and bar spacing, and choose your bar size. The calculator computes total linear footage and weight in pounds using ASTM A615 unit weights. Choose a pricing mode — per lb, per lf, per bar, or per ton — to get material cost.

How It Calculates

For a slab (bidirectional mat):

Runs Along Length = ceil(Width / Spacing) + 1
Runs Along Width = ceil(Length / Spacing) + 1
Total LF = (Runs Along Length × Length) + (Runs Along Width × Width)
Weight (lb) = Total LF × Unit Weight (lb/lf)

ASTM A615 unit weights: #3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502, #7 = 2.044, #8 = 2.670 lb/lf.

What to Know Before You Calculate

Clear cover: Rebar must maintain minimum concrete cover from the surface. This affects placement but not the quantity calculation.
Lap splices: Where bars overlap, you need extra length. Add 10–15% to total calculated length to cover laps.
Bar size vs. spacing: The structural engineer specifies both. Do not substitute a different bar size or spacing without engineering approval.
Pricing mode: Rebar is typically sold by weight (ton or lb) but field labour is often estimated by linear foot or bar count. Match your pricing mode to the quote you are comparing.

Common Mistakes

Forgetting lap splice allowance. Structural drawings specify minimum lap lengths. Skipping this always means ordering short.
Using the wrong unit weight. Rebar weight is precise. Using an approximate weight leads to significant error on large projects.
Not separating top mat from bottom mat. Two-layer mats require calculating both layers separately — total LF is roughly double a single mat.
Ignoring special zones. Edge reinforcement, column strips, and openings often require extra bars not covered by the standard grid spacing.

Frequently Asked Questions

How do I calculate how much rebar I need for a concrete slab?

For a bidirectional mat, divide slab length by bar spacing to get runs in one direction, then add slab width for each run. Do the same for the other direction. Total linear feet = both directions combined. Add 10–15% for lap splices and waste.

What is the most common rebar size for a residential slab?

#4 rebar (1/2 inch diameter) at 12-inch spacing is the most common choice for residential slabs. #3 (3/8 inch) is used for lighter slabs and small footings. #5 and larger are used for structural slabs, walls, and footings under significant load.

How much does rebar weigh per foot?

ASTM A615 unit weights: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft, #7 = 2.044 lb/ft, #8 = 2.670 lb/ft. Weight increases roughly as the square of the bar number.

What rebar spacing should I use for a concrete slab?

Residential slabs typically use 12-inch on-center spacing. Driveways and structural slabs often use 6–8 inch spacing. Heavily loaded slabs may require closer spacing specified by an engineer. Always follow the structural drawings — spacing is load-dependent.

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Disclaimer: Rebar quantities are estimates based on standard spacing formulas. Reinforcement requirements must be designed and verified by a licensed structural engineer. Do not substitute bar sizes or spacing without engineering approval.