CONCRETE SLAB CALCULATOR

Concrete Slab Calculator — Volume for Any Slab or Patio

Calculate the volume of concrete needed for a slab, patio, driveway or floor. Enter the length, width and thickness to get the volume in cubic yards or cubic metres, plus an equivalent bag count if you plan to mix on-site.

Slab dimensions

Common thicknesses

Waste allowance

Concrete to order
—yd³

Enter slab dimensions to calculate.

Calculate Concrete for a Slab, Patio or Driveway in Seconds

The Interior Tailor Concrete Slab Calculator gives you the concrete volume for a slab, patio, driveway or floor from three simple measurements: length, width and thickness. Results come back in cubic yards or cubic metres, with an equivalent bag count if you plan to mix on site. Built-in thickness presets for a standard slab, a driveway and a heavy-load slab make it easy to start from a realistic number, and the waste allowance helps you order enough concrete for uneven sub-grades and small form variations.

Why Use This Slab Calculator

  • Simple inputs: length, width and thickness are all you need.
  • Units that match how you measure: imperial with feet and inches, or metric with metres and millimetres, so there is no manual conversion.
  • Thickness presets: standard slab, driveway and heavy-load options in one tap.
  • Waste allowance: a percentage field for spillage, uneven sub-grade and form variation.
  • Volume and bag estimate: see the amount to order and the equivalent number of bags for on-site mixing.

Concrete Slab Volume Formula

Volume (ft³) = Length × Width × Thickness

Volume (yd³) = Volume (ft³) ÷ 27

Add 5–10% waste allowance for uneven sub-grades, spillage and minor form variations. For large pours over 1 yd³, ready-mix truck delivery is typically more cost-effective than bag mixing and produces a more consistent result.

Standard Slab Thicknesses by Use

  • 3.5–4 in (90–100 mm): Residential floor slabs, patios, sidewalks
  • 4–5 in (100–125 mm): Driveways and garage floors — handle vehicle loads
  • 5–6 in (125–150 mm): Heavy equipment pads, commercial floors
  • 8 in (200 mm): Foundation walls and load-bearing structural slabs

Residential driveways should be at least 4 inches thick and reinforced with rebar or wire mesh to resist cracking under vehicle weight and frost heave.

Sub-Base Preparation

A properly prepared sub-base is as important as the concrete itself. Poor sub-base preparation is the leading cause of slab cracking and settlement:

  • Remove all organic material (topsoil, roots, vegetation) — minimum 6 inches
  • Compact sub-grade soil to at least 95% Proctor density
  • Add 4–6 inches of compacted crushed stone base (gravel)
  • Install a 6-mil polyethylene vapor barrier for interior slabs
  • Place rebar chairs to hold reinforcement at the correct height (mid-slab)

Slab Projects You Can Plan

The calculator suits any flat, uniform-thickness pour:

  • Patios, sidewalks and walkways
  • Driveways and garage floors
  • Shed, workshop and utility room floors
  • Equipment pads and light commercial floors
  • Basement and house floor slabs

Example Slab Estimates

Each example adds a 10% waste allowance. The metric example shows how the same method works in metres and millimetres.

Example Slab Estimates
ProjectInputsVolumeWith 10% waste
Patio12 ft x 10 ft x 4 in1.48 cu yd1.63 cu yd
Driveway12 ft x 24 ft x 5 in4.44 cu yd4.89 cu yd
Metric slab4 m x 3 m x 100 mm1.20 cu m1.32 cu m

Round up to the next quantity your supplier delivers. Always confirm thickness for your load and local code.

Tips for an Accurate Slab Estimate

  • Measure the inside of your forms and check the depth in several places across the sub-base.
  • Use the same thickness across the slab, or calculate thicker edges and footings separately and add them.
  • Add 5 to 10% waste for a typical slab, and more if the sub-grade is uneven.
  • Build in the gravel base and any vapour barrier when you set your finished thickness.
  • Reinforcement such as rebar or wire mesh does not change the volume, so use the Rebar Calculator for that.

Frequently Asked Questions

How much concrete does a 20×20 ft slab at 4 inches require?

20 × 20 × 0.333 = 133.2 cu ft ÷ 27 = 4.93 cubic yards. Order 5.5 yards with a 10% waste allowance. That is approximately 247 bags of 80 lb pre-mix if mixing by hand.

When should I use control joints in a slab?

Control joints should be cut into the slab within 4–12 hours of pouring, at intervals no more than 2–3 times the slab thickness in feet (e.g., every 8–12 ft for a 4-inch slab). They guide cracking to controlled locations.

How long before I can drive on a new concrete slab?

Passenger vehicles can typically use a residential driveway slab after 7 days. Heavy vehicles and loaded trucks should wait 28 days for full design strength.

Do I need to convert inches to feet before I enter my slab size?

No. In imperial mode you enter length and width in feet and thickness in inches, and the calculator handles the conversion. In metric mode, use metres for length and width and millimetres for thickness.

How thick should a concrete driveway slab be?

A residential driveway is commonly at least 4 inches thick, and 5 inches gives extra strength for heavier vehicles. Use the driveway preset as a starting point, then confirm with a local contractor.

Can I order less than one cubic yard of ready-mix?

Minimum loads are usually about one cubic yard. Some suppliers offer smaller short loads at a premium, so for small slabs compare that cost with mixing bags.