Free Rebar Calculator — How Much Rebar Do I Need?

Rebar — short for reinforcing bar — is deformed steel cast into concrete to handle tensile stress. Plain concrete carries compression well but cracks under tension; rebar supplies the tensile strength that keeps slabs, footings, and walls intact under load. Most residential projects use #3 to #5 Grade 60 bars manufactured to ASTM A615.

The deformations on the surface — those raised ribs and lugs — give the bar its grip on the surrounding concrete. That mechanical bond transfers force from the slab into the steel, converting an inert mass into a composite structural element. Without rebar, a 4-inch driveway slab cracks under the first wheel load; with rebar, the same slab carries vehicle traffic for decades.

Rebar is the second step in any structural concrete project. You confirm your layout, then calculate rebar quantity (this page), then pour the concrete. The calculator above produces the bar count, total linear feet, weight, and cost so you can hand the order to a steel supplier before the concrete truck arrives.

Bar size designations follow a simple rule: the number equals the nominal diameter in eighths of an inch. A #4 bar is 4/8 = 1/2-inch in diameter. Residential slabs typically use #3 or #4; footings and foundations commonly use #4 or #5; commercial and structural work runs from #6 through #11.

ASTM A615 Bar Size Chart (#3 Through #11)

Picking a Size for Your Project

#3 rebar covers patios, sidewalks, and lightly loaded slabs where temperature and shrinkage are the only design driver. #4 rebar is the residential workhorse for driveways, garage floors, and continuous footings. #5 rebar handles foundation walls, retaining walls, and structural slabs that carry vehicle or equipment loads. Anything #6 and larger is engineered work — beam, column, and heavy structural reinforcement that an engineer should specify by analysis, not by rule of thumb.

The CalcSummit calculator carries the full ASTM A615 range because the bar size you ultimately order is rarely the bar size you first picked. A driveway pour next to a retaining wall often shares one crew and one rebar order. Consolidating to a single larger size sometimes cuts the total ton count and the supplier delivery fee.

Pick a project type, enter dimensions, choose bar size and on-center spacing, and read your results. The calculator returns bar count by direction, total linear feet, total weight in pounds and kilograms, and an estimated cost when you supply a local price per ton.

Input Fields Explained

Project type selects the formula. Slab uses a two-way grid. Footing uses a single direction with cover on both faces. Wall and column use vertical bars on a single pitch. Length is the longest dimension; width hides on wall mode and is replaced by height. Spacing is on-center inches; a value over 18 inches triggers an ACI 318-19 §24.4.3.3 warning. Bar size picks the #3 through #11 entry that drives the weight conversion. Edge clearance holds the first and last bars off the slab perimeter at the ACI cover distance. Waste factor defaults to 10% and absorbs cutoffs and lap splices; the slider runs to 20% for projects with many openings.

Reading Your Results

The results card displays bar count broken out by direction for slabs. It also shows total linear feet before and after the waste factor, total weight in pounds and kilograms, and an approximate cost when price per ton is supplied. The 20-foot stick count rounds up to whole bars — yards sell rebar by the stick, not by the foot. A 40-foot length toggle covers commercial deliveries.

Changing Units (Imperial / Metric)

The unit toggle swaps every input and output between imperial (ft, in, lbs) and metric (m, mm, kg). The URL state persists across the toggle so a metric link shared from a Canadian project keeps its values when re-opened. Imperial is the default for the U.S. market.

For a rectangular slab, bars in one direction equal the slab width minus twice the edge clearance, divided by the spacing, plus 1 for the closing bar. Total linear feet equals bars per direction times the run length, summed across both directions. Weight equals total linear feet times the ASTM A615 weight-per-foot for the chosen bar size.

Slab Formula (Two-Way Grid)

Footing Formula (Single-Direction)

A continuous footing runs bars along its length only. The count equals horizontal bars per row times the number of rows. A typical 12-inch-wide footing uses 2 #4 bars top and bottom, for 4 bars at full footing length. Linear feet equal the bar count times footing length. A lap splice is added when the footing exceeds the 40-foot commercial bar length. The calculator's footing mode handles the lap splice automatically using the ACI 318-19 §25.5.2 Class B tension splice formula.

Wall Formula (Vertical and Horizontal Reinforcement)

Concrete walls carry two layers of reinforcement: vertical bars on a length-based pitch and horizontal bars on a height-based pitch. The wall mode counts both. Bars horizontally = floor(H ÷ S) + 1 and bars vertically = floor(L ÷ S) + 1, with linear feet summed across both. A 6-foot tall retaining wall at 12-inch on-center with #5 bars in both directions converts to roughly 28 bars and 196 linear feet of #5. The calculator above produces the exact number from your dimensions.

ACI 318-19 §24.4.3.3 caps temperature and shrinkage rebar spacing at the lesser of 5 times the slab thickness or 18 inches. For a 4-inch slab, the maximum spacing is 18 inches (5 × 4 = 20, so 18 governs). Flexural reinforcement spacing is limited to the lesser of 3 times slab thickness or 18 inches.

ACI 318-19 Spacing by Application

Lap Splice Length by Bar Size — Class B Tension, ACI 318-19 §25.5.2

A lap splice transfers tensile force between two bars that overlap rather than join end to end. ACI 318-19 §25.5.2 requires a Class B tension splice of 1.3 times the development length for most field conditions. The table below assumes Grade 60 (ASTM A615) bottom bars in normal-weight concrete with clear spacing of at least 2 bar diameters.

Concrete Cover Requirements — ACI 318-19 §20.6.1

Field Note — Why Cover Matters More Than Spacing

Measure the area, pick a bar size and spacing, and divide each dimension by the spacing. Add 1 bar per direction, multiply by the run length, add a 10% waste factor, then convert to weight using ASTM A615 lb/ft values. The six-step worked example below uses a 10-foot by 10-foot slab.

Step 1 — Measure Your Area and Choose Project Type

Confirm length and width in feet. A 10 ft × 10 ft slab covers 100 square feet — a typical garden shed or AC pad. Project type is "slab," which selects the two-way grid formula. For a continuous footing or single-direction wall, the project type changes and width drops out.

Step 2 — Select Bar Size and Spacing

Pick #4 rebar at 12-inch on-center for a residential slab that will carry foot traffic and light equipment. ACI 318-19 §24.4.3.3 allows up to 18 inches for temperature and shrinkage; 12 inches is the field default that gives a serviceable grid and matches drywall-style layout.

Step 3 — Calculate Bar Count

Step 4 — Calculate Total Linear Feet

Step 5 — Add Waste Factor (10% Standard)

Multiply by 1.10 to absorb cutoffs and lap splices: 200 × 1.10 = 220 linear feet. Round up to the next whole 20-foot stick: 11 sticks. Most yards sell rebar in 20-foot and 40-foot lengths; the calculator above shows both options on the result card.

Step 6 — Convert to Weight and Cost

A 10 ft × 10 ft slab at #4, 12-inch on-center needs about 11 sticks of 20-foot rebar. The pour weighs around 147 lbs and costs in the $70 range at 2026 mid-grade pricing. The calculator at the top of this page does the same math in real time.

In 2026, #4 rebar runs $0.45 to $0.70 per linear foot or $850 to $1,000 per ton at most suppliers. Prices vary by region, order volume, and bar size. Add 10–15% for delivery and 5% for waste, and always pull a current quote from your local supplier before placing the order.

Rebar Prices by Bar Size — 2026 National Average

How to Estimate Your Total Rebar Cost

Start with total linear feet from the calculator above. Multiply by the ASTM A615 weight per foot to get pounds, then divide by 2,000 for tons. Multiply tons by your local price per ton — typically $850–$1,000 for #4 in 2026 — to get material cost. Add 10–15% for delivery on small orders, 5% for waste if your project has many openings or cuts, and request a written quote before ordering anything over one ton.

Rebar is step 2 in any structural concrete project. After confirming the layout, calculate rebar (this calculator), then concrete volume, then the gravel base under the slab. Three CalcSummit calculators map directly to the next three steps:

• Concrete volume. Once you have your rebar estimate, your next step is calculating the concrete volume for the same project — use the concrete calculator for an instant cubic-yard result that ties to the same slab dimensions.
• Total project cost. Combine rebar tons, concrete cubic yards, and formwork into a single project estimate using the concrete cost calculator — it pulls 2026 regional pricing for both materials and labor.
• Gravel sub-base. Every slab needs 4 to 6 inches of compacted gravel beneath it. The gravel calculator sizes the base layer in cubic yards and tons for the same slab footprint you just reinforced.
• Retaining wall projects. For walls 6 feet and taller, the retaining wall calculator handles the lateral-earth-pressure analysis that ACI 318 §11.6 expects on engineered designs, then hands the reinforcement layout back to this calculator for the bar order.