Reinforcing steel is laid in a grid: one set of bars runs the length of the slab and another runs across it, tied where they cross. To estimate how much rebar you need, count how many bars fit across each direction at the chosen spacing, multiply by the length of each run, then convert the total into stock bars and weight. This rebar calculator does that for a rectangular slab or footing pad. It allows for edge clearance (so the steel stays inside the concrete), lap splices on long runs and offcut waste. It gives the number of stock bars to buy, the total linear feet and the weight from the standard bar tables.
For the concrete itself, use the concrete slab calculator. For posts and piers, use the concrete column & post hole calculator.
How to use the rebar calculator
- Enter the slab length and width.
- Enter the grid spacing from your plans, such as 12, 16, 18 or 24 inches on center.
- Enter the clearance from the slab edge to the end of each bar. Concrete cover protects the steel from corrosion; follow your plans.
- Choose the bar size and the stock length you will buy (20 ft and 40 ft are common).
- If a run is longer than one bar, enter the lap splice length your plans specify.
Formula
Unit weights for ASTM A615 deformed bars: #3 0.376 lb/ft, #4 0.668, #5 1.043, #6 1.502. Bar numbers are the nominal diameter in eighths of an inch, so a #4 bar is ½ inch.
Worked example
A 20 × 12 ft slab, #4 bar at 18 in on center, 3 in edge clearance, 20 ft stock, 10% waste.
- Runs: 20 − 0.5 = 19.5 ft lengthwise; 12 − 0.5 = 11.5 ft crosswise.
- Bars running lengthwise: ⌈11.5 ÷ 1.5⌉ + 1 = 8 + 1 = 9 bars of 19.5 ft = 175.5 ft.
- Bars running crosswise: ⌈19.5 ÷ 1.5⌉ + 1 = 13 + 1 = 14 bars of 11.5 ft = 161 ft.
- Total: 336.5 ft, or 370.2 ft with 10% waste. That is ⌈370.2 ÷ 20⌉ = 19 bars of 20 ft.
- Weight: 370.2 × 0.668 = about 247 lb (112 kg).
Rebar sizes and weights
| Bar | Diameter | Metric | lb/ft | kg/m |
|---|---|---|---|---|
| #3 | 0.375 in | #10 (9.5 mm) | 0.376 | 0.560 |
| #4 | 0.500 in | #13 (12.7 mm) | 0.668 | 0.994 |
| #5 | 0.625 in | #16 (15.9 mm) | 1.043 | 1.552 |
| #6 | 0.750 in | #19 (19.1 mm) | 1.502 | 2.235 |
| #7 | 0.875 in | #22 (22.2 mm) | 2.044 | 3.042 |
| #8 | 1.000 in | #25 (25.4 mm) | 2.670 | 3.973 |
Weights are the same in the 1957 CRSI Design Handbook and current ASTM A615 bar charts. The "soft metric" designations (#10, #13…) are the same physical bars labelled in millimetres.
Placement matters as much as quantity
Rebar only helps if it ends up in the right place in the concrete, usually supported on chairs or concrete blocks rather than lying on the ground. Edge clearance keeps the bar ends covered. For concrete cast against soil, building codes based on ACI 318 typically require more cover than for formed surfaces. Your plans, local code and engineer set the bar size, spacing, cover and lap lengths. This calculator estimates material for the layout you are given; it does not design reinforcement.
Count tie wire too: each intersection is tied, and a 20 × 12 ft grid at 18 in has about 126 intersections. Many crews tie every other intersection in the field of a slab and every one at the edges.
Caveats and assumptions
- Rectangular slabs only. For L-shapes, split the slab into rectangles and add the results.
- Does not design reinforcement. Bar size, spacing, cover and splice lengths must come from your plans, the local code or an engineer.
- Assumes a single mat (one layer in each direction). Double the result for top and bottom mats.
- Weights are nominal; actual mill weights vary slightly within ASTM tolerances.
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
At 18 in on center with 3 in edge clearance, you need 14 bars of 19.5 ft each way: about 546 ft, or about 601 ft with 10% waste. That is 31 bars of 20 ft (#4 weighs about 401 lb).
What is the weight of #4 rebar per foot?
0.668 lb per foot (0.994 kg/m). A 20 ft #4 bar weighs about 13.4 lb.
What spacing should rebar be in a slab?
Your plans or engineer specify it. Residential slabs commonly use #3 or #4 bars at 12–24 inches on center, but requirements depend on loads, soil and local code.
How do I calculate the number of rebar bars?
Divide the run across the slab (minus edge clearance) by the spacing, round up and add one. Do this for both directions.
What does #4 rebar mean?
The number is the nominal diameter in eighths of an inch, so #4 is 4/8 = ½ inch. Its soft-metric name is #13 (about 13 mm).
Sources
- Concrete Reinforcing Steel Institute — CRSI Design Handbook (1957), bar weights table (Internet Archive) — weight, area and diameter of standard bars
- InterMetal — ASTM A615 Standard Reinforcing Bars chart — imperial and metric bar sizes, lb/ft and kg/m
- American Concrete Institute (ACI) — ACI 318 building code requirements for structural concrete
- Concrete Reinforcing Steel Institute (CRSI) — reinforcing steel placement guidance
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.