# Concrete Slab Calculator

Calculate concrete for a slab with rebar grid estimation. Enter length, width, and thickness to get cubic yards, bag counts, and reinforcement details. Includes waste factor.

## What this calculates

Whether you are pouring a patio, garage floor, or driveway, this calculator gives you the exact concrete volume and rebar grid layout for a rectangular slab. Enter your dimensions and thickness, choose a rebar spacing, and you get cubic yards for ordering ready-mix, bag counts for smaller pours, and a complete rebar bill of materials.

## Inputs

- **Slab Length** (ft) — min 0
- **Slab Width** (ft) — min 0
- **Slab Thickness** (in) — min 2 — Standard: 4" for patios, 4-6" for driveways
- **Waste Factor** (%) — min 0, max 30 — 10% is standard; 5% for well-prepared sites
- **Rebar Grid Spacing** — options: No rebar (wire mesh only), 12" on center (#3 or #4), 18" on center, 24" on center — Grid spacing in both directions
- **Rebar Size** — options: #3 (3/8", 0.376 lb/ft), #4 (1/2", 0.668 lb/ft), #5 (5/8", 1.043 lb/ft)

## Outputs

- **Volume (no waste)** (ft³)
- **Volume (no waste)** (yd³)
- **Volume (with waste)** (yd³) — Order this amount from the ready-mix supplier
- **80-lb Bags** — Each fills 0.60 ft³ of mixed concrete
- **60-lb Bags** — Each fills 0.45 ft³ of mixed concrete
- **Concrete Weight** (lbs) — At 150 lbs per cubic foot
- **Rebar (Lengthwise)** — formatted as text — Bars running the length of the slab
- **Rebar (Widthwise)** — formatted as text — Bars running the width of the slab
- **Total Rebar Needed** (ft) — Total linear feet of rebar for the grid
- **Rebar Weight** (lbs)

## Details

## Slab Volume Formula

**Volume = Length x Width x Thickness**

Thickness is entered in inches and converted to feet. Results include both the raw volume and the volume with your chosen waste factor applied.

## When to Use Bags vs Ready-Mix

| Volume | Best Option |
|--------|------------|
| Under 0.5 yd³ | 80-lb bags (hand mixing) |
| 0.5-1.0 yd³ | 60-lb or 80-lb bags + mixer |
| 1.0-2.0 yd³ | Consider ready-mix delivery |
| Over 2.0 yd³ | Ready-mix truck (saves hours) |

## Rebar Grid Basics

A rebar grid adds tensile strength to prevent cracking. The grid sits in the lower third of the slab:

- **12" on center** -- Standard for driveways and structural slabs. Each bar is 12 inches from the next in both directions.
- **18" on center** -- Lighter duty patios and walkways.
- **24" on center** -- Minimum reinforcement for basic slabs.

Rebar should have 3 inches of concrete cover from the bottom and edges. Use rebar chairs or dobies to hold the grid at the right height during the pour.

## Thickness Recommendations

- **Sidewalks:** 4 inches
- **Patios:** 4 inches
- **Driveways (cars):** 4-5 inches
- **Driveways (trucks):** 5-6 inches
- **Garage floors:** 4-6 inches (thicker at the apron)
- **Workshops with heavy equipment:** 6+ inches

## Frequently Asked Questions

**Q: How much concrete for a 20x20 slab at 4 inches thick?**

A: A 20x20-foot slab at 4 inches needs 133.3 ft³ or 4.94 yd³. With 10% waste, order 5.4 yd³. At this volume, ready-mix delivery is the clear choice -- that would be about 225 bags of 80-lb mix, which is impractical to mix by hand.

**Q: Do I need rebar or wire mesh for a patio slab?**

A: For a simple 4" patio on well-compacted soil, welded wire mesh (6x6 W1.4/W1.4) is usually sufficient. Rebar grid is better for driveways, garage floors, or any slab that will carry significant weight. In areas with expansive clay soil, rebar is recommended for all slabs.

**Q: What is the waste factor and how much should I add?**

A: The waste factor accounts for over-excavation, uneven subgrade, spillage, and forms that are not perfectly straight. Use 5% for a well-prepared, level site with straight forms. Use 10% as the standard. Use 15% for your first pour or irregular shapes.

**Q: Can I pour a concrete slab in sections?**

A: Yes, but you need proper control joints or cold joints. Plan section boundaries along natural break points. For cold joints (poured on different days), the second pour bonds poorly to the first without special preparation. Control joints (cut within 24 hours) guide where the slab cracks naturally.

**Q: How long does concrete take to cure?**

A: Concrete reaches about 70% of its rated strength in 7 days and full strength (typically 3,000-4,000 PSI) in 28 days. You can walk on it after 24-48 hours. Light vehicle traffic after 7 days. Heavy loads after 28 days. Keep the surface moist for the first 7 days for best results.

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Source: https://vastcalc.com/calculators/construction/concrete-slab
Category: Construction
Last updated: 2026-04-08