Concrete Calculator For A Footing

Concrete is the one material where guessing wrong is expensive in both directions: order too little and the pour stops halfway through a slab that cannot wait, order too much and you pay for a cubic yard you have to dispose of. This page works out the numbers for a 20 ft long strip footing, 12 in wide and 24 in deep, and it starts with the warning that every figure here is an estimate. Volumes, bag yields and prices vary by brand, product line and region, and the label on the bag — or the ticket from the ready-mix plant — governs. The worked example below gives a range, not a single number, so you can order the high end and finish the pour in one go.

The strip footing in the worked example is a typical residential job: flat ground, a compacted base, and no structural loading beyond its normal use. If your ground is soft, your slab carries a heavy load, or local code asks for a thicker slab or reinforcement, those requirements govern — the calculator gives you the volume, and the code and the ground give you the specification.

How the calculation works

The BuildCalc concrete calculator needs three inputs, and each one changes the answer. First, the length and width in feet — measure the longest and widest points, and measure each section separately if the shape is not a rectangle. Second, the depth in inches, converted to feet before multiplying: a 4 in slab is 4 ÷ 12 = 0.333 ft deep, and a 6 in slab is 0.5 ft deep. Depth is the input people most often guess, and it is the one that most changes the answer — a slab poured 5 in thick instead of 4 in needs a quarter more concrete. Third, the waste allowance: about 10% extra for waste, spillage and uneven subgrade is the typical allowance in our material data.

The formula is short: cubic yards = length (ft) × width (ft) × depth (ft) ÷ 27, because 1 cubic yard = 27 cubic ft. For bagged concrete, the conversion is about 45 80 lb bags per cubic yard (a 60 lb bag yields about 0.45 cu ft, an 80 lb bag about 0.6 cu ft, so a cubic yard takes about 60 60 lb bags or about 45 80 lb bags — typical figures from our material data, and the bag label governs). Always round up, and always add the waste allowance before you order: a slab that runs short halfway through cannot be finished later with a cold joint you did not plan.

Worked example: a 20 ft long strip footing, 12 in wide and 24 in deep

Here is the full arithmetic for a 20 ft long strip footing, 12 in wide and 24 in deep.

Step 1 — volume in cubic feet. The footing is 20 ft long, 1 ft wide and 24 in deep: 20 × 1 × (24 ÷ 12) = 40.0 cu ft.

Step 2 — convert to cubic yards. 40.0 ÷ 27 = 1.48 cubic yards.

Step 3 — add waste. Add about 10% for spillage and uneven trench bottoms: 1.48 × 1.10 = 1.63 cubic yards to order.

Step 4 — bags if you mix by hand. At about 45 80 lb bags per cubic yard, 1.48 cubic yards is about 67 bags, and 1.63 cubic yards is about 74 bags — so plan on 67–74 80 lb bags, and keep a few extra dry bags for the last barrow-load.

If your slab is bigger, the same steps scale: measure again, convert the depth to feet, divide by 27, add about 10% waste, and round up. A slab twice this size needs roughly twice the concrete, but the waste percentage stays the same — it is tied to the subgrade and the spillage, not the size of the job.

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 concrete prices vary by brand, product line, region and retailer, and a price that is right in one store can be wrong in the next. The way to budget is to multiply, not to guess: take the quantity from the worked example (about 1.48 cubic yards (67–74 80 lb bags)), multiply it by the shelf price per bag — or the ready-mix price per cubic yard quoted by your local plant — on the exact product you have chosen, then add reinforcement mesh or rebar, formwork timber, a vapour barrier if the slab needs one, and the cost of hiring a mixer or a pump if the pour is large.

Two cost traps catch first-time concrete buyers. The first is ordering the low end of the range and then paying a second delivery charge — or a higher per-bag price — for the few bags that finish the pour. The second is forgetting that ready-mix plants charge a minimum load and a waiting-time fee: a small order can cost more per cubic yard than a large one, and a truck kept waiting while you finish the formwork is charged by the minute. Compare the total, not the per-bag price, and get the quote against the same volume you used for the material count. If you are hiring the work out, labour is usually quoted separately and varies by region and by the complexity of the formwork, so get the quote against the same dimensions.

Step by step: measure, calculate, order

1. Measure twice. Measure the strip footing in feet at the longest and widest points, and measure the depth you intend to pour — not the depth of the hole you have already dug, which is usually deeper. Mark the finished level with string lines and pegs before you order.

2. Prepare the base. Excavate, compact the subgrade, and lay a compacted gravel base. A slab poured over soft or uncompacted ground settles and cracks, whatever the mix. Check the fall: a patio or driveway should fall away from the house so water does not pool against the wall.

3. Use the calculator on this page. Enter the length, width and depth for your strip footing. The tool returns a range — about 1.48 cubic yards (67–74 80 lb bags) for the example above — and you should plan on the high end. Check the bag label against the calculator's assumed yield; if your bag yields less, the bag count goes up.

4. Order and check the delivery. Order the high end of the range for the example job, all from the same batch, and confirm the delivery time against the weather — concrete poured in heavy rain or in a heatwave needs different handling. Have the formwork, the reinforcement and the tools ready before the truck arrives.

5. Before you pour. Wet the base lightly so it does not suck water out of the mix, have a screed board, a float and a spare pair of hands ready, and plan the finishing: screed, float, then broom or trowel to the finish you want, and keep the slab damp while it cures for the first few days.

Frequently asked questions

How much concrete do I need for a 20 ft long strip footing, 12 in wide and 24 in deep?

Use length × width × depth ÷ 27, then add about 10% waste. For the worked example on this page the answer is about 1.48 cubic yards (67–74 80 lb bags). Your job will differ if the dimensions or the depth differ, so run your own measurements through the calculator.

How many bags of concrete do I need per cubic yard?

About 45 80 lb bags per cubic yard, or about 60 60 lb bags per cubic yard — typical figures from our material data, and the bag label governs. Add about 10% extra for waste and spillage, so a 1 cubic yard job is about 49–50 80 lb bags or about 66 60 lb bags.

Should I use bagged concrete or ready-mix for this job?

Bagged concrete suits jobs up to about 1 cubic yard, where a truck's minimum load costs more than the bags. Above that, ready-mix is usually cheaper per cubic yard and gives a more consistent mix, because the first barrow-load of a hand-mixed pour starts setting before the last one is mixed.

How thick should a strip footing slab be?

A 4 in slab is the typical figure for a light-duty patio or path, and a 6 in slab is the typical figure where vehicles cross it — but thickness is a specification, not a guess, and local practice and the load govern. Measure the depth you actually intend to pour and run it through the calculator; a 1 in error in depth changes the volume by a quarter on a 4 in slab.

What happens if I order too little concrete?

The pour stops halfway and the slab gets a cold joint where the new concrete meets concrete that has already started to set. That joint is a weak line and a visible line, and it cannot be fixed by pouring more on top later. That is why every page on this site tells you to order the high end of the range and keep a few spare bags dry.

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.