How Much Concrete For Footings

Figuring out how much concrete for footings starts with one honest admission: the answer is a range, not a single number. Concrete volume depends on the exact dimensions you measure, the depth you actually pour, and how much is lost to spillage and an uneven base — and the bag count depends on the bag size and the yield printed on the bag label. This page works through a 24 ft footing trench, 16 in wide and 10 in deep step by step, using the typical yields in our material data, and every result below is shown as a range. The product label governs: if the bag you buy yields a little less than the typical figure, the count goes up, and that is exactly why the range exists.

What makes a 24 ft footing trench, 16 in wide and 10 in deep different from a plain slab is the shape: a footing is a long, narrow trench, so the volume comes from length times width times depth, and the waste hides in the uneven trench bottom rather than in cuts. That shape decides which formula applies and where the waste hides, so the worked example below does the arithmetic for this exact shape rather than quoting a generic per-square-foot figure. If your dimensions differ, the same steps still work — substitute your own length, width and depth, keep the 10% waste allowance, and round up to whole bags.

How the calculation works

The calculator needs the shape of the pour, the depth, and the bag size you plan to buy, and each one changes the answer. For a rectangular pour the volume is length times width times depth, all in feet, divided by 27 to get cubic yards — one cubic yard is 27 cubic feet (0.76 cubic metres). Depth is where most mistakes happen: 4 in (10 cm) is 0.333 ft, 6 in (15 cm) is 0.5 ft, so always convert inches to feet before multiplying. A slab that is 10 ft by 10 ft at 4 in thick is 100 times 0.333, or 33.3 cubic feet, which is about 1.2 cubic yards before waste. The same slab at 6 in thick is half as much again, about 1.9 cubic yards, which is why the thickness in the keyword matters as much as the footprint.

Once the volume is known, it is converted to bags. Our material data gives typical yields of about 45 80 lb (36.3 kg) bags per cubic yard and about 60 60 lb (27.2 kg) bags per cubic yard, because an 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet — those are typical values, and the bag label on the exact product you buy governs. On top of that, add the typical waste allowance of about 10% for spillage, an uneven subgrade, and the concrete that stays in the mixer and the wheelbarrow. The worked example below shows the low end with no waste and the high end with the full 10% allowance, so you can see the range instead of a single number that pretends the pour will be perfect.

Footings also need steel: rebar runs the length of the trench, and the concrete has to flow around it, which is another reason to order the high end — a footing that is short by half a bag cannot be topped up later without a cold joint. Check local code for the minimum footing width and depth before you dig; the calculator estimates volume, it does not set the footing size.

Worked example: a 24 ft footing trench, 16 in wide and 10 in deep

Here is the full arithmetic for a 24 ft (7.3 m) footing trench, 16 in (41 cm) wide and 10 in (25 cm) deep.

Step 1 — convert to feet. Length 24 ft. Width 16 in ÷ 12 = 1.33 ft. Depth 10 in ÷ 12 = 0.83 ft.

Step 2 — volume in cubic feet. 24 × 1.33 × 0.83 = 26.5 cubic feet.

Step 3 — convert to cubic yards. 26.5 ÷ 27 = 0.99 cubic yards, or about 1.0 cubic yards.

Step 4 — bags, low end. At the typical yield of about 45 80 lb bags per cubic yard: 0.99 × 45 = 44.4 bags, rounded up to 45 bags. With 60 lb bags at about 60 per cubic yard: 0.99 × 60 = 59.3, rounded up to 60 bags.

Step 5 — bags, high end with the 10% waste allowance. 0.99 × 1.10 = 1.09 cubic yards. That is 49 80 lb bags or 66 60 lb bags. Order 45–49 80 lb bags (or 60–66 60 lb bags) and keep one unopened bag for patching. If the trench bottom is very uneven, plan on the high end.

Cost guidance

Costs on this page are ranges, labelled typical retail range — check the current shelf price before budgeting. No single national price is quoted, because bagged concrete, ready-mix and delivery charges vary by region, retailer and supplier, and a price that is right in one town can be wrong in the next. The way to budget is to multiply, not to guess: take the quantity from the worked example (1.0–1.1 cubic yards, or about 45–49 80 lb bags), multiply it by the shelf price per bag or the per-cubic-yard price your local supplier quotes, then add delivery, formwork timber, rebar or mesh, and a finishing tool if you do not own one. For a small pour like this one, bagged mix bought by the bag is usually the practical route; once the volume passes roughly 1–2 cubic yards, a ready-mix short load often works out cheaper per yard even after the delivery charge, so price both options against the same quantity.

If the job grows while you measure — a footing that turns out longer than the plan, a sidewalk that needs an extra section — redo the volume from the new dimensions rather than guessing a percentage on top of the old total. Concrete quantities scale with volume, not with length alone, so a small change in depth changes the total more than most people expect. And if you are comparing bagged mix with ready-mix, remember the comparison only works when both quotes cover the same volume including waste, the same delivery distance, and the same extras; a cheap per-bag price with an expensive delivery is not a cheap job.

Step by step: measure, calculate, order

1. Measure twice. Measure the trench in feet at the longest run, the width in inches at three places, and the depth in inches at three places along the run. Measure each run separately — a footing that turns a corner is two runs, not one — and write every dimension down in feet and inches before you convert anything.

2. Convert and calculate. Convert inches to feet (divide by 12), then apply the volume formula for your shape: length times width times depth divided by 27 for a rectangle, or pi times radius squared times depth for a round tube or post hole. Use the calculator on this page to check your arithmetic, and run it twice with the low and high depth if your excavation is uneven.

3. Add waste and round up. Add the typical 10% waste allowance for spillage and an uneven subgrade, then round up to whole bags or to the nearest quarter cubic yard for a truck. A bag cannot be bought in fractions, and a pour that stops short is far more expensive than one spare bag.

4. Order and check the delivery. Order the high end of the bag range below, all the same mix, all the same product and mix, and check the bag labels against the yields the calculator assumed. If you are ordering ready-mix, confirm the slump, the mix strength and the delivery window, and make sure the truck can reach the pour.

5. Before you pour. Set and brace the forms, check the depth with a straightedge in several places, wet the subgrade lightly so it does not steal water from the mix, and have your screed, float and curing cover within reach. Concrete waits for nobody once it is mixed.

Common mistakes to avoid

The most common mistake with how much concrete for footings is measuring the top of the excavation and pouring to a different depth at the bottom. Footings and slabs are dug by hand or by machine, and the base is rarely flat — a trench that is 10 in deep at one end and 13 in at the other needs noticeably more concrete than the drawing says. Measure the depth in at least three places and plan on the deepest. The second mistake is forgetting the 10% waste allowance: spillage, the mix left in the barrow, and the uneven subgrade all eat volume. The third is ordering the low end of the range and then trying to stretch the last half-bag across a corner that still needs filling — that patch will always show. Order the high end, keep the receipt, and return unopened bags.

Frequently asked questions

How many bags of concrete do I need for footings?

Multiply the trench volume by the bag yield and add the 10% waste allowance. For the worked example — a 24 ft trench, 16 in wide, 10 in deep, about 1.0 cubic yards — the answer is 45–49 80 lb bags or 60–66 60 lb bags. Measure the depth in several places, because a trench that deepens halfway along needs more than the drawing says.

How deep should footings be?

Footing depth is set by local building code and by the frost line in your area, not by a calculator — the calculator only estimates the volume once the size is decided. Typical residential footings run about 12 in deep or more below grade in cold climates, and the trench must sit on undisturbed soil. Check the code for your area before you dig, and call the utility-locating service before any excavation.

Should I use 60 lb or 80 lb bags for footings?

Both work; the difference is handling and count. An 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet (typical), so the same pour needs fewer 80 lb bags — about 45 per cubic yard against about 60. Heavier bags are harder to carry and mix, so many DIY pours use 60 lb bags even though the count is higher. Pick one size and do all the arithmetic in that size.

Do footings need rebar?

Usually yes — most codes require at least one or two runs of rebar in a footing, and the concrete has to flow around the steel. Rebar does not change the volume estimate in any meaningful way, but it does change the order: buy the steel, the chairs that hold it off the trench bottom, and the tie wire at the same time as the concrete so the pour is not delayed.

Can I pour footings in sections?

You can, but each joint between pours is a cold joint and a weak point, so a single continuous pour is better for a footing. If the trench is long, plan the labour and the mixing so the whole run is placed before the first part starts to set. That is another reason to order the high end of the range — running short halfway along a footing is the worst place to run short.

Related calculators

Results are estimates — verify with the product label. Coverage, pack sizes, yields and prices vary by brand, product line, surface and region; the label on the product you buy governs. Cost figures are typical retail ranges — check the current shelf price before budgeting.