Concrete Footing Calculator: How Much Concrete You Need

Ever finished digging your trenches, bags stacked and mixer primed, only to realise you’ve under-ordered concrete? That’s the moment you find out panic costs twice as much as planning. In this guide, you’ll learn exactly how to size up your footings, avoid classic blunders, and use a concrete footing calculator to get your numbers bang on—no guesswork, no waste, no red faces at the builders’ merchant.

What are Concrete Footings—and Why Getting the Volume Right Matters

Let’s get one thing straight: footings (or "concrete foundations" if you’re feeling posh) are the bedrock of any build—from garden sheds and porches to decks and full-blown extensions. Get the concrete quantity wrong and you’re either lugging excess back out (nobody’s idea of fun) or doing the ‘half-pour, half-prayer’ routine as the last drop comes out. Ordering concrete without a proper take-off is like doing your weekly shop with no list and a blindfold—you’ll always forget the milk (or in this case, run short halfway down the trench).

The Formula for Calculating Concrete for Footings: Simple and Foolproof

Here’s the bit most people get wrong: you need to work out the volume (not just the length) of concrete required. The formula is dead simple:

Volume = Length × Width × Depth

Length: How long are your footings? (metres / feet)
Width: How wide? (metres / feet)
Depth: How deep? (metres / feet)

Just remember: keep all your measurements in the same units, or you’ll end up ordering concrete like you’re measuring the room in feet and the timber in metres and hoping for the best.

Worked Example: Sizing Up a Footing for a Small Extension

Say you’re building a 3m x 4m (10ft x 13ft) conservatory. Your foundation trench runs around the perimeter—so your total length is:

3m + 4m + 3m + 4m = 14m (46ft)

Let’s say your local regs (or the frost line if you’re in North America) call for a footing 0.45m (18in) wide and 0.3m (12in) deep.

Metric:

Volume = 14m × 0.45m × 0.3m = 1.89 cubic metres

Imperial:

Volume = 46ft × 1.5ft × 1ft = 69 cubic feet
1 cubic yard = 27 cubic feet, so 69 ÷ 27 ≈ 2.56 cubic yards

Don’t forget wastage! Add 10% for spillage, overdigging, and the fact that no trench is ever perfectly straight (unless you live in a geometry textbook):

Metric: 1.89 × 1.1 ≈ 2.08 cubic metres
Imperial: 2.56 × 1.1 ≈ 2.82 cubic yards

Want the easy button? Plug your numbers into our concrete footing calculator and let it do the heavy lifting.

Why Not Just Overorder? (And Other Expensive Myths)

Bet the merchant never told you this: concrete isn’t cheap, and it’s not like you can return the leftovers. Overordering "just to be safe" is the concrete equivalent of making 40 sandwiches for a five-person picnic—messy, costly, and you’ll still end up with a pile of waste no one wants to deal with. Underordering, on the other hand, means a cold, hard stop halfway through your pour (and a frantic trip back to the supplier).

A little precision pays off. That’s where our concrete footing calculator comes in—type in your trench dimensions, and it’ll spit out exactly how much you need in both metric and imperial units. No maths headache, no excess, no shortage.

Decks, Sheds, and More: Footing Numbers for Common Builds

Here’s a quick run-down of typical footing sizes you might encounter:

Project Type	Typical Footing Width (mm/in)	Typical Depth (mm/in)
Deck Post	300mm (12in)	600mm (24in)
Shed Base	450mm (18in)	300mm (12in)
House Extension	600mm (24in)	300mm (12in)

For decks, you’ll want one footing per post—space them every 1.8m to 2.4m (6ft to 8ft) along beams and joists.
For sheds, continuous strip footings or individual pads at the corners and midpoints are common.

Every project is different, so check your local building code (or council rules) before you dig. And when you’re ready to order, let our concrete footing calculator take care of the numbers.

Don’t Forget the Extras: Reinforcement and Site Realities

Here’s the kicker: concrete alone isn’t always enough. Most footings need a bit of steel to keep them honest—rebar (reinforcing bar) or mesh, depending on your region. The amount of rebar depends on the footing size and local code, but a typical setup for a 0.6m (2ft) wide footing is two or three 12mm (½in) bars laid horizontally, spaced evenly.

And as for site realities? Allow for a bit of extra volume if your trench sides are crumbly, or if the bottom isn’t billiard-table flat. Remember, it’s easier to pour a bit extra than pray for a miracle with a half-mixed wheelbarrow at sunset.

Questions people are actually asking

How much concrete footing do I need?

Work out the volume by multiplying the length, width, and depth of your trench—all in the same units (metres or feet). Add about 10% extra for wastage, overdigging, and the occasional wonky trench. If you want the quick route, put your numbers into our concrete footing calculator for an instant answer.

How many concrete footings are needed for a deck?

It depends on your deck’s size, the spacing of your beams/joists, and the load. As a rule of thumb, you’ll need a footing under every support post, spaced every 1.8m to 2.4m (6ft to 8ft). For a small 3m x 4m (10ft x 13ft) deck, expect at least 6 to 8 footings, one at each corner and spaced along the beams.

How much does a concrete foundation cost?

Costs vary by country, concrete prices, and site complexity. As a ballpark, multiply your required volume by the local price per cubic metre (or cubic yard). Don’t forget to factor in delivery, barrowing, and any reinforcement. For a real figure, work out your volume first with a concrete footing calculator, then price it up locally.

How much concrete base do I need for a shed?

Measure the length, width, and depth of your planned base. For a typical 2.4m x 3m (8ft x 10ft) shed, with a base 100mm (4in) thick, you’ll need about 0.72 cubic metres (25.4 cubic feet) of concrete. Always add a bit extra—no one ever complained about a stronger base.

How much rebar is needed in a concrete footing?

Most domestic footings use two or three 12mm (½in) rebar bars, spaced evenly and supported on chairs to keep them off the bottom. For wider footings, or heavier loads, add an extra bar or up the size. Always check local code—the inspector’s word is law!

How much does it cost to pour concrete footings?

Add up your total concrete volume, multiply by local cost per cubic metre (or cubic yard), and don’t forget delivery and reinforcement costs. Pricing fluctuates, so get a quote after you’ve used a concrete footing calculator to nail down the quantity.

Wrapping Up: Get Your Footings Right, First Time

Getting your concrete footings right is the difference between a build that lasts and one that sags, cracks or (worst of all) has to be redone. Don’t wing it—use the formula, double-check your measurements, and let the concrete footing calculator take care of the heavy maths. Save cash, time, and your back. Now go build something solid.

Free tools to put this into practice

Reading is one thing — working out your own numbers is what saves time and money. Here are the free HowMuchDoINeed calculators that pair best with this guide:

Concrete Footing Calculator — work out exactly how much you need and what it will cost.
Concrete Cube Calculator Guide — get an instant materials estimate for your project.
Concrete Post Calculator — size up quantities and costs before you head to the merchant.
Concrete Weight Calculator — plan your order so you avoid over- or under-buying.

Explore every free calculator on HowMuchDoINeed — no sign-up required.