How To Work Out Concrete For Footings

How to Work Out Concrete for Footings

You've got your plans in hand. This leads to the permit's approved. And then it hits you — you have no idea how much concrete you actually need for those footings.

This is one of the most common stumbling blocks for DIY builders and even some contractors who don't do this every day. Get it wrong and you're making emergency runs to the supply yard with a half-dug trench staring you in the face. Get it right and the rest of the project starts smooth.

So let's work through it. No fluff, no vague advice — just the math and the logic behind it.

What Are Footings, Anyway

Footings are the wide base sections of a foundation that spread the weight of your structure across the soil below. They're typically poured into trenches below the frost line, and they're what keep your building from settling unevenly or heaving when the ground freezes.

Most residential footings are rectangular in cross-section — think of them as thick, wide strips of concrete running under your walls. The dimensions depend on your soil conditions, the load you're carrying, and what your local building code requires.

Here's what matters: footings aren't a place to guess. Consider this: they're load-bearing elements, and undersizing them can cause real structural problems down the road. But sizing them correctly and ordering the right amount of concrete? That's a solvable problem, and you don't need to be an engineer to work it out.

The Difference Between Footings and Foundation Walls

People sometimes mix these up. Also, footings are the base — the part that sits on undisturbed soil. Foundation walls sit on top of the footings and rise to grade level. Some projects use block walls on footings, others use poured concrete walls, and some use a combination. The footings themselves are always poured concrete, regardless of what sits above them.

Why Getting the Calculation Right Actually Matters

Underordering concrete is the obvious problem. In real terms, you don't want to run short, so some people just order "a bit extra" without knowing why. That works until you're paying for wasted concrete or running into delivery scheduling issues on a busy job site.

Overordering has its own costs. Returned concrete isn't cheap, and excess concrete sitting in forms you haven't built yet creates scheduling headaches.

But here's the thing most people miss — getting the calculation right is also about getting the dimensions* right. If you don't know how to work out concrete quantities, you might not be checking your footing dimensions carefully enough. The math forces you to look at the numbers on your plans and understand what they mean. That's where a lot of problems get caught before they become problems.

How to Calculate Concrete for Footings

At its core, where we get into the actual math. It's not complicated, but it does require attention.

The Basic Formula

Concrete is measured in volume. For footings, you're almost always dealing with rectangular shapes, so the formula is straightforward:

Length × Width × Depth = Cubic Feet

That's it. Once you have the volume in cubic feet, you convert to cubic yards (the standard unit concrete is sold in), or to the number of bags you need if you're mixing by hand.

Step-by-Step

Let's say you're pouring a footing that's 2 feet wide, 1 foot deep, and 50 feet long.

First, multiply those three numbers together: 2 × 1 × 50 = 100 cubic feet.

Concrete is sold by the cubic yard. One cubic yard equals 27 cubic feet. So you divide: 100 ÷ 27 = 3.7 cubic yards.

You'd round up to 4 cubic yards for this pour, and you'd probably add a little extra for waste — typically 5 to 10 percent depending on your trench conditions. Uneven ground, overspill, and minor variances add up.

Working With Multiple Footings

Most projects have several footings running in different directions. Calculate each one separately, then add the volumes together.

Keep your units consistent. Practically speaking, if you're working in feet, stay in feet. In practice, if you need to check your plans and some dimensions are in inches, convert first — otherwise your math will be off. That's why one common mistake is mixing decimal feet with inch measurements. Just convert everything to feet before you start multiplying.

For more on this topic, read our article on how many ww points per day or check out how do i figure concrete yards.

Converting From Inches to Feet

If your footing depth is listed as 8 inches, that's 8 ÷ 12 = 0.83 feet. Think about it: 67 feet. Worth adding: same for width — 10 inches becomes about 0. It's not hard, but forgetting to do it will give you a wildly wrong number.

Hand Mixing vs. Ready Mix

For small projects — a single footing, a short section, a repair — you might be mixing bags on site. And an 80-pound bag yields about 0. 45 cubic feet of concrete. A 60-pound bag yields roughly 0.Think about it: most bagged concrete comes in 60-pound or 80-pound bags. 60 cubic feet.

Using those numbers, if you need 100 cubic feet, you're looking at around 222 sixty-pound bags or about 167 eighty-pound bags. That's a lot of mixing, and most people find that for anything over about 20 bags, ordering ready-mix makes more sense.

Ready-mix also gives you consistent quality. Plus, hand-mixed concrete, even with the best intentions, tends to vary in water content and consistency. For footings that need to carry structural loads, that consistency matters.

Common Mistakes People Make

Knowing what goes wrong helps you avoid it.

Forgetting to account for the actual trench shape. If your trench is wider at the top than at the bottom (common in undisturbed soil), you're not pouring a rectangle — you're pouring something closer to a trapezoid. This gets complicated to calculate exactly. Most pros estimate based on the average width and build in a healthy margin.

Ignoring the waste factor. Concrete doesn't pour perfectly. Some sticks to the forms, some spills, some settles into voids you didn't know were there. A 5 to 10 percent overage is standard. For rough ground or unfamiliar soil, lean toward the higher end.

Not verifying dimensions before ordering. Building plans sometimes have errors. A typo in a decimal point, a dimension shown in the wrong units — these things happen. Check your footing dimensions against the actual site conditions before you're standing there with a concrete truck on the way.

Skipping the soil check. Different soils have different bearing capacities. If you're building on soft or fill soil

, you may need wider or thicker footings than your plans show. A quick check with a local building inspector or geotechnical engineer can save you from a footing failure down the road.

Forgetting the weather. Concrete behaves differently in extreme temperatures. In hot weather, it sets faster, which can lead to cracking if you can't place and finish it in time. In cold weather, it may not cure properly without insulation or additives. Plan your pour around the forecast, and have blankets or heaters ready if temperatures are expected to drop.

Ordering the Concrete

Once you have your final volume, add your waste factor and round up to the nearest practical amount. That's why concrete companies typically sell by the cubic yard, and they often require minimum orders. One cubic yard equals 27 cubic feet.

If you calculate 1,200 cubic feet of needed concrete, that's roughly 44.4 cubic yards. Round up to 45 or even 46 cubic yards to be safe. Most dispatchers will appreciate a heads-up on the size of the pour so they can schedule the right number of trucks.

When ordering, also ask about:

• The maximum aggregate size available
• The slump (consistency) recommended for your type of pour
• Any air-entrainment requirements if you're in a freeze-thaw climate
• The delivery window and any fees for waiting time

Final Thoughts

Calculating concrete for a continuous wall footing isn't complicated, but it does require attention to detail. Convert your units, measure carefully, build in a waste factor, and double-check everything before you call the concrete plant. The time you spend on the math now will save you from a costly shortfall or an expensive overage later.

If you feel uncertain at any point, don't hesitate to consult with a contractor, engineer, or your local building department. Day to day, a few minutes of expert input can prevent a foundation that fails to perform as intended. Your footings carry everything above them, and getting them right starts with knowing exactly how much concrete you need.