How much concrete do I need for a slab? Worked examples

Ordering concrete is one of those jobs where a small mistake costs real money. Order too little and you end up with a cold joint in the middle of a slab; order too much and you are paying for material you have to get rid of. The good news is that the arithmetic is simple. This article walks through it step by step, with examples you can check in the concrete calculator.

The formula

A slab is a box, so its volume is:

volume = length × width × thickness

The only thing that trips people up is units. All three measurements have to be in the same unit before you multiply. In the US that usually means converting the thickness from inches to feet; in metric countries, converting centimetres to metres.

• 4 inches = 4 ÷ 12 = 0.333 ft
• 10 cm = 0.10 m

Then convert the result into the unit your supplier sells:

• 1 cubic yard = 27 cubic feet
• 1 cubic metre = 1,000 litres

Example 1: a 10 × 10 ft patio, 4 inches thick

1. Thickness in feet: 4 ÷ 12 = 0.333 ft
2. Volume: 10 × 10 × 0.333 = 33.3 cubic feet
3. In cubic yards: 33.3 ÷ 27 = 1.23 cubic yards

That is the exact volume of the slab. Real slabs are never exactly that size: the ground is not perfectly flat, forms bow slightly, and some concrete stays in the mixer or the wheelbarrow. A waste allowance of 5–10% is a common starting point. With 10%:

1.23 × 1.10 = 1.36 cubic yards (about 36.7 cubic feet)

How many bags is that?

Bagged concrete mix is sold by weight, but what matters is the volume each bag makes once mixed. Manufacturers print this “yield” on the bag. Typical values for standard mixes are:

Bag size	Typical yield	Bags for 36.7 ft³
80 lb	about 0.60 ft³	62
60 lb	about 0.45 ft³	82
40 lb	about 0.30 ft³	123

Always round up to whole bags, and use the yield printed on the product you are actually buying. Yields differ between brands and mixes.

Sixty-two 80 lb bags weigh more than two tonnes, all of it lifted and mixed by hand. For slabs above roughly one cubic yard, many people find ready-mix delivery easier, even with a minimum-load charge. It is worth asking your local supplier for both prices.

Example 2: a metric slab, 3 × 4 m, 10 cm thick

1. Volume: 3 × 4 × 0.10 = 1.20 m³
2. With a 10% allowance: 1.20 × 1.10 = 1.32 m³

If you buy bagged dry mix, check the yield on the bag. A 25 kg bag of a standard mix typically makes only around 11–13 litres of concrete, so 1.32 m³ would be more than a hundred bags. At that volume ready-mixed concrete is usually the practical choice.

Example 3: round footings and post holes

For a cylinder, the volume is:

volume = π × radius² × depth

A hole 12 inches (1 ft) across and 3 ft deep has a radius of 0.5 ft:

π × 0.5² × 3 = 3.1416 × 0.25 × 3 = 2.36 cubic feet per hole

Eight holes for a deck therefore need about 18.8 ft³, or 0.70 cubic yards, before any allowance. If a post sits in the hole while you pour, its volume comes off: a 4×4 post (3.5 × 3.5 inches actual) takes up about 0.085 ft³ per foot of depth.

Things the formula does not cover

The arithmetic tells you how much concrete fills a shape. It does not tell you whether that shape is right for the job. Before you order, check:

• Thickness. Around 4 inches (10 cm) is common for patios and walkways; driveways and slabs that carry heavier loads are usually thicker. Local practice and codes vary.
• Sub-base. Gravel under the slab affects how much concrete you need if it is uneven. Measure thickness at several points and use the average, or level the base first.
• Reinforcement and joints. Mesh, rebar and control joints are structural decisions, not volume questions.
• Edges. Thickened edges or footings around a slab add volume. Calculate them separately and add them up.

When in doubt, especially for anything structural, ask a professional. A volume estimate is a planning number, not a design.

Quick checklist before you order

1. Measure length and width at the edges and the middle; use the largest values.
2. Measure the actual thickness of the forms, not the nominal size of the timber.
3. Calculate the volume, then add 5–10%.
4. Convert to the unit your supplier uses and round up.
5. Confirm the bag yield or minimum delivery with the supplier.

The BuildCalc concrete calculator does steps 3 and 4 for you, in imperial or metric units, and shows the bag counts for common sizes.