Concrete Slab Gravel Calculator
Calculate the crushed stone base layer needed under a concrete slab. Enter your slab dimensions and desired base depth to get the exact volume and weight of compacted sub-base gravel required before pouring.
🏗️ Concrete Slab & Gravel Base Diagram
Sub-base LayerHow Much Gravel Do You Need Under a Concrete Slab?
The amount of gravel for a concrete sub-base depends on slab size, soil conditions, and local building codes. Most residential slabs require a minimum 4-inch compacted gravel base, while garage slabs and commercial pads need 6–8 inches.
Plan Your Slab Dimensions
Enter the length and width of your concrete slab. This determines the area of the gravel sub-base, which matches the slab footprint exactly.
Choose Sub-Base Depth
Set the depth based on your project: 4 inches for patios and sidewalks, 6 inches for garage floors and driveways, 8 inches for heavy load-bearing slabs.
Select #57 or Crushed Stone
Use #57 crushed stone (3/4" angular aggregate) or compactable crusher run. These materials compact firmly and provide the best drainage under concrete.
Get Volume & Weight
The calculator shows you how many cubic yards and tons of sub-base material to order. Add 10–15% for compaction factor.
Account for Compaction
Gravel compacts 10–15% during installation. If you need a 6-inch final depth, order for 6.6–6.9 inches to account for this settlement.
Concrete Slab Sub-Base Formula
The gravel sub-base under a concrete slab must support the entire load of the slab, structure, and traffic. Our calculator computes the volume of compacted base material needed based on your slab footprint and required base depth (typically 4–8 inches of compacted crushed stone).
Why Use Gravel Under a Concrete Slab?
Click any application card below to highlight specific specs and project guidelines.
Drainage & Moisture Control
A gravel sub-base allows water to drain away from beneath the slab, preventing hydrostatic pressure that cracks concrete from below.
Load Distribution
Compacted crushed stone spreads the load of the slab and anything on it across a wider area of subsoil, preventing differential settlement.
Frost Protection
In cold climates, a deep gravel base insulates the slab from freeze-thaw cycles that lift and crack concrete — especially important for unheated slabs.
Stable Work Surface
A firm, flat gravel sub-base makes it easier to set screeds, install vapor barriers, and pour concrete with consistent thickness across the entire slab.
Pro Tips for Concrete Slab Sub-Base Preparation
Interactive guidelines to ensure a flawless installation and avoid common planning mistakes.
Compact in Lifts
Add and compact gravel in 3–4 inch layers using a plate compactor. Never compact more than 4 inches at once or you'll get voids in the lower layers.
Add a Vapor Barrier
After compacting the gravel base, install a 6-mil polyethylene vapor barrier before pouring. This prevents moisture from wicking through the slab into your finished floors.
Check for Levelness
Use a long level or laser level after compacting. Variations greater than 1/4 inch per 10 feet must be corrected before pouring concrete.
Protect From Rain
Cover your compacted sub-base if rain is expected before your pour date. Water can wash fine particles from the surface and loosen the sub-base.
Concrete Slab Gravel Calculator — FAQs
Find answers to common questions regarding calculations, material selection, and installation.
Q1 How deep should the gravel base be under a concrete slab?
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The required depth depends on the application: Sidewalks and light-duty patios: 4 inches. Residential garage floors and driveways: 6 inches. Commercial driveways and heavy equipment pads: 8–12 inches. In areas with poor drainage or expansive clay soils, add 2 extra inches. Always check local building codes.
Q2 What type of gravel is best under a concrete slab?
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#57 crushed stone (3/4" clean crushed stone) is the most recommended material. It compacts well, drains freely, and doesn't contain fines that can shift over time. Crushed limestone and compactable gravel (crusher run / road base) are also excellent choices for load-bearing applications.
Q3 Do I need gravel under a concrete slab on stable soil?
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In most cases, yes. Even on stable, well-draining sandy soil, a gravel sub-base improves drainage, reduces moisture vapor transmission, and provides a clean, level working surface. The only exception might be compacted natural gravel subsoils in very arid climates, but local codes typically still require a prepared base.
Q4 How much does gravel for a concrete slab cost?
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For a 20 × 20 ft garage slab with a 6-inch gravel base, you need approximately 7.4 cubic yards or 10.3 tons of crushed stone. At $30–$45/ton plus delivery, that's $450–$660 for materials. Larger projects benefit from bulk pricing. Use our cost calculator for your specific dimensions.
Q5 How do I compact gravel under a concrete slab?
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Rent a gas-powered plate compactor from a tool rental shop. Make multiple overlapping passes in different directions over each lift of gravel. For small areas, a hand tamper can work. Aim for a firm, stable surface that doesn't shift when you walk on it — your boot heel should leave only a minimal impression.
Q6 Can I pour concrete directly on soil without gravel?
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Technically yes, but it's not recommended. Without a gravel sub-base, moisture can undermine the slab, tree roots or frost heaving can crack it, and it's harder to achieve a level pour. Most building codes and all reputable contractors require a proper gravel sub-base for any structural concrete slab.