# Rebar Calculator

Calculate rebar needed for concrete slabs and footings. Get bar count, linear feet, stock bars to order, and total weight. Supports #3 through #6 rebar.

## What this calculates

Reinforcement steel (rebar) strengthens concrete and prevents cracking. This calculator determines how many rebar bars you need for a slab or footing, based on dimensions and spacing. It calculates both directions of the grid, accounts for lap splices where bars must overlap, and provides the total weight for ordering.

## Inputs

- **Slab Length** (ft) — min 0
- **Slab Width** (ft) — min 0
- **Bar Spacing** (in) — min 4, max 24 — Center-to-center spacing. Typical: 12" for slabs, 6-8" for footings
- **Rebar Size** — options: #3 (3/8", 0.376 lb/ft), #4 (1/2", 0.668 lb/ft), #5 (5/8", 1.043 lb/ft), #6 (3/4", 1.502 lb/ft)
- **Edge Offset** (in) — min 1, max 12 — Distance from slab edge to first bar (typically 2-3")
- **Stock Rebar Length** (ft) — min 10, max 60 — Standard stock lengths: 20 or 40 feet
- **Lap Splice Length** (in) — min 12, max 48 — Overlap at bar splices (typically 30x bar diameter)

## Outputs

- **Bars (lengthwise)** — Running along the length
- **Bars (widthwise)** — Running along the width
- **Total Bars**
- **Total Linear Feet** (ft)
- **Stock Bars to Order** — Standard length bars to purchase
- **Total Weight** (lbs)

## Frequently Asked Questions

**Q: What size rebar for a concrete slab?**

A: For residential slabs 4 inches thick, #3 or #4 rebar at 12-18 inch spacing is common. For driveways and garage floors, use #4 rebar at 12-inch spacing. For structural footings, #5 or #6 rebar may be required. Always follow structural engineer specifications or local building codes.

**Q: How far apart should rebar be spaced?**

A: Typical spacing is 12 inches on center for slabs, 6-8 inches for footings, and per engineering for structural elements. The maximum spacing is generally the lesser of 3 times the slab thickness or 18 inches. Closer spacing provides more reinforcement.

**Q: What is a lap splice and how long should it be?**

A: A lap splice is where two rebar bars overlap. The minimum splice length is typically 30 to 40 times the bar diameter. For #4 rebar (1/2-inch diameter), that is 15-20 inches. For #5 rebar, 19-25 inches. In practice, 24 inches is a safe minimum for most residential work.

**Q: How much does rebar weigh?**

A: #3 rebar weighs 0.376 lb/ft, #4 weighs 0.668 lb/ft, #5 weighs 1.043 lb/ft, and #6 weighs 1.502 lb/ft. Standard 20-foot bars weigh 7.5 lbs (#3), 13.4 lbs (#4), 20.9 lbs (#5), and 30 lbs (#6).

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Source: https://vastcalc.com/calculators/construction/rebar
Category: Construction
Last updated: 2026-04-21