How do I calculate rebar for a concrete slab?

Divide the slab length and width by the rebar spacing (in feet). Add 1 bar to each direction for the perimeter. Bar count = (L / spacing) + 1 in each direction. Multiply by bar length for total linear feet, then by weight per foot for total weight.

What is rebar lap splice length?

Lap splice length is how far two rebar pieces must overlap when joining. For #4 rebar in 3000 psi concrete, the standard Class B tension splice is 40 × bar diameter = 40 × 0.5" = 20 inches minimum per ACI 318.

How much does rebar cost per linear foot?

Rebar typically costs $0.35–0.70 per linear foot for #3–#5 sizes, and $0.80–2.00 for #6–#11. Prices vary by region and market conditions. Always add 10% waste when estimating material quantities.

Rebar Calculator

Calculate rebar quantities, weight, lap splice lengths, and cost for any concrete project — per ACI 318

Slab / Wall Dimensions

Length (ft)

Width (ft)

Bar Size

Spacing (in)

Bar Length (stock) (ft)

Waste Factor

10%

ACI Cover Requirement

Total Rebar 0 LF

Bars (L dir)0

Bars (W dir)0

Total Bars0

Net Linear Ft 0

With Waste 0 LF

Weight 0 lbs

Stock Bars Needed 0

Cover Depth 1.5"

Bar Spacing 12"

Grid Layout Preview

Weight & Cost Estimator

Calculate total weight and material cost by entering your rebar quantities.

Bar #QtyLength (ft)LFLbs

Price per lb ($)

Total LF0

Total Weight0lbs

Tons0.00

Material Cost$0.00

With Tax (8%)$0.00

Cost/LF$0.00

Weight by Bar Size

Lap Splice Calculator

Calculate required lap splice length per ACI 318. Class B tension splice = 1.3 × development length. Standard for most residential applications.

Bar Size

Concrete Strength (f'c)

Splice Location

Number of Splices

Required Splice Length 0"

Bar Diameter—

Ld (dev. length)—

Splice length—

Extra LF (all splices)—

Extra weight—

ACI ReferenceACI 318

Class B Splice Formula (ACI 318-19 §25.5.2)

Ls = 1.3 × Ld ≥ 12"

where Ld = development length = (fy / (20√f'c)) × db

Rebar Bar Properties (ASTM A615)

Bar #	Diameter (in)	Area (in²)	Weight (lb/ft)	Common Use
#3	0.375"	0.11	0.376	Light slabs, footings, small walls
#4	0.500"	0.20	0.668	Residential slabs, foundations (most common)
#5	0.625"	0.31	1.043	Structural slabs, walls, columns
#6	0.750"	0.44	1.502	Columns, beams, heavy foundations
#7	0.875"	0.60	2.044	Large columns, transfer beams
#8	1.000"	0.79	2.670	Heavy columns, seismic applications
#9	1.128"	1.00	3.400	Heavy structural members
#10	1.270"	1.27	4.303	High-rise, bridges
#11	1.410"	1.56	5.313	Heavy industrial, bridges

ACI Concrete Cover Requirements

Condition	Min Cover
Cast against/in contact with ground (e.g., footings)	3"
Exposed to weather (#6–#18 bar)	2"
Exposed to weather (#3–#5 bar)	1.5"
Interior slabs, beams, columns (not exposed)	1.5"
Interior thin slabs (walls, joists)	0.75"

Formulas

Bar Count (per direction) N = (Length ÷ Spacing) + 1 Length and spacing in same units

Total Weight W = LF × lbs/ft From bar property table above

Development Length (simplified) Ld = (fy / 20√f'c) × db ACI 318 §25.4, fy = 60,000 psi Grade 60

Class B Lap Splice Ls = 1.3 × Ld ≥ 12" ACI 318-19 §25.5.2.1

How to Calculate Rebar

Measure your slab or wall — Enter the length and width in feet (or meters). For walls, enter height instead of width.

Choose bar size and spacing — #4 at 12" OC is the most common for residential slabs. Structural slabs often use #5 at 6" or 8".

Calculate bar count — Bars per direction = (dimension ÷ spacing) + 1. Both directions are calculated separately and added together.

Add overlap for splices — Whenever stock bars need to be joined, add the lap splice length (minimum 20" for #4 in 3000 psi concrete).

Add waste factor — Standard 10% waste covers cut-offs and setup losses. Add more for irregular shapes.

Rebar Glossary

Bar Number (#) — Indicates diameter in eighths of an inch. #4 = 4/8" = 0.5" diameter.

OC (On Center) — Spacing measured from center of one bar to center of the next.

Lap Splice — Overlap length required when two bar pieces are joined. Minimum per ACI 318.

Development Length (Ld) — Minimum embedment needed for a bar to develop its full yield strength.

Cover — Minimum concrete thickness between the rebar and the concrete surface.

ASTM A615 — Standard for deformed steel bars for concrete reinforcement. Grade 40 (40 ksi) or Grade 60 (60 ksi).

Frequently Asked Questions

What size rebar do I need for a residential slab?

#4 rebar (0.5" diameter) at 12" on center in both directions is the standard for residential concrete slabs 4–5 inches thick. For 6-inch structural slabs or slabs subject to vehicle loads, use #4 at 6" OC or #5 at 12" OC.

How much does rebar weigh per linear foot?

#3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft. The formula is approximately 2.67 × d² where d is diameter in inches. This matches the ASTM A615 standard weight table.

Do I always need rebar in concrete?

Not always. Small decorative pads, garden borders, and some lightweight slabs can use fiber reinforcement or welded wire mesh (WWM) instead. Structural slabs, footings, walls, and any concrete that will carry loads should always have rebar per ACI 318 requirements.

What is the minimum lap splice for #4 rebar?

For #4 rebar (Grade 60) in 3000 psi concrete, the Class B tension splice is approximately 20–24 inches. Always use the longer of the calculated lap splice length or 12 inches per ACI 318 §25.5.2. Your structural engineer may specify a longer lap for seismic or other conditions.