Construction

Concrete Calculator

Enter the dimensions of each pour and get the volume in the units suppliers actually use, with an allowance for spillage and uneven ground built in.

This calculator needs JavaScript to run. The formula, the worked examples and the explanations below all work without it.

Pour 1

Project type

How many

Length

Width

Thickness

Waste allowance

10% is the usual allowance for spillage and uneven subgrade

An estimate for planning. Confirm quantities with your supplier before ordering.

The formulas

Project	Formula	Typical use
Slab	`volume = length × width × thickness`	Driveways, patios, shed and garage floors.
Footing / wall	`volume = length × width × depth`	Continuous strip footings and low walls.
Square column	`volume = width × depth × height`	Rectangular posts and piers.
Round footing / column	`volume = π × (diameter ÷ 2)² × height`	Sonotube piers, deck footings, fence posts.

Every dimension is converted to feet first, so you can enter a slab as feet by feet by inches — which is how anyone actually measures one — without doing the conversion yourself.

Formulacubic feet ÷ 27 = cubic yards · cubic feet × 0.0283168 = cubic meters

The 27 is the one number worth memorising: a cubic yard is 3 × 3 × 3 feet.

Why concrete is ordered in cubic yards

In the United States ready-mix concrete is sold by the cubic yard, and a standard mixer truck carries about 10 yards. Suppliers usually have a minimum order of 1 yard and charge a "short load" fee below 3 or 4 — so for anything under about a yard, bags from a hardware store are almost always cheaper.

Outside the US the unit is the cubic meter, and one cubic yard is 0.7646 m³. Both figures are shown for every result.

Bags or ready-mix?

The bag counts are based on the yields printed on standard pre-mixed bags:

Bag size	Yield per bag	Bags per cubic yard
40 lb	0.3 ft³	90
50 lb	0.375 ft³	72
60 lb	0.45 ft³	60
80 lb	0.6 ft³	45

It takes roughly 45 eighty-pound bags to make a single cubic yard, which is about 3,600 lb of material to move and mix. That is the practical crossover: below half a yard, bags are fine; above one yard, order ready-mix.

This is an estimate, not an engineering specification. Real pours vary with subgrade compaction, over-excavation, form deflection, slump and how level the ground actually is. For anything structural — a foundation, a retaining wall, anything carrying a building — have the quantities and the mix specified by a qualified engineer or your concrete supplier.

Why the waste allowance is not optional in practice

Ordering the exact calculated volume is how people end up half a wheelbarrow short with a truck already leaving. Concrete is unforgiving that way — you cannot pause a pour, and a cold joint from a second delivery is a permanent weak line.

5–10% — a formed slab on well-prepared, level subgrade.
10% — the standard default, and what this calculator starts with.
15% — footings dug by hand or an excavator, where the trench is never exactly the width you dug for.
20% — sloping or soft ground, or a first pour.

Over-ordering costs a few dollars. Under-ordering costs the pour.

Worked examples

A 10 ft × 10 ft shed slab, 4 inches thick

10 × 10 × 0.333 = 33.33 ft³ = 1.235 cubic yards. With a 10% allowance, order 1.36 yards — in practice, 1.5.

Six deck footings in 12-inch tubes, 4 ft deep

π × 0.5² × 4 = 3.14 ft³ each, so 18.85 ft³ for six = 0.70 cubic yards. That is bag territory: about 35 eighty-pound bags with the allowance.

A 20 ft footing, 16 in wide and 8 in deep

20 × 1.333 × 0.667 = 17.78 ft³ = 0.66 cubic yards.

Common uses

Slabs: sheds, garages, patios and driveways.
Footings: strip footings for walls and extensions.
Deck and fence posts: round tubes and post holes.
Piers and columns: square and round supports.
Budgeting: volume × your supplier's yard price.

Work out the slab area first with the square footage calculator if the shape is complicated.

Frequently asked questions

How do I calculate cubic yards of concrete?

Multiply length × width × thickness with every dimension in feet, then divide by 27. A 10 by 10 foot slab 4 inches thick is 10 × 10 × 0.333 = 33.33 cubic feet, which is 1.24 cubic yards.

How many bags of concrete are in a cubic yard?

About 45 eighty-pound bags, 60 sixty-pound bags or 90 forty-pound bags, based on the yields printed on standard pre-mixed bags. That is a lot of mixing — above roughly one cubic yard, ready-mix is usually cheaper and always easier.

How thick should a concrete slab be?

4 inches is standard for patios, walkways and shed floors. 5 to 6 inches is common for driveways and anything carrying vehicles. Local codes and soil conditions govern this, so check before you pour — and get structural work specified by an engineer.

How much extra concrete should I order?

10% for a formed slab on level ground, 15% or more for hand-dug footings where the trench width varies. Running short mid-pour creates a cold joint and a permanent weak line, so the extra is cheap insurance.

Can I mix feet, inches and meters?

Yes. Every dimension has its own unit selector, so a slab can be entered as feet by feet by inches. Everything is converted to feet before the volume is worked out.

Is this accurate enough to order from?

It is accurate enough to plan and budget with, and to have a sensible conversation with a supplier. It cannot know your subgrade, your over-excavation or your form deflection, so confirm the final quantity with your supplier and get anything structural specified professionally.