Cubic Yards Of Concrete For A Slab

Before you order a truck or stack bags in the driveway, you need a realistic quantity for cubic yards of concrete for a slab. Too little concrete means a cold joint halfway through the pour, a second trip, and a weak seam; too much means bags or a short-load fee you did not need. This guide walks through a 16 ft by 12 ft slab, 4 in thick with the volume formula, the typical bag yields from our material data, and a 10% waste allowance, and it ends with a range you can actually order against. Everything here is an estimate — measure twice, check the bag label, and buy a little extra for the parts of the job you cannot see until the forms come off.

The calculator on this page asks for the same inputs the worked example uses: the dimensions of a 16 ft by 12 ft slab, 4 in thick, the depth, and the bag size. It returns a range, because the honest answer is a range — the low end assumes a flat base and no spillage, and the high end assumes the usual 10% waste. Plan your order on the high end. A spare bag costs little; a short pour costs a second delivery, a cold joint, and a visible repair.

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.

A slab is the simplest concrete shape: one box, three dimensions. The only trap is the depth — a base that is not flat means the slab is thicker in some places than others, so measure the depth in at least four places and plan on the deepest. If the slab has a thickened edge or a footing around the perimeter, that edge is extra volume on top of the slab itself and must be added separately.

Worked example: a 16 ft by 12 ft slab, 4 in thick

Here is the full arithmetic for a 16 ft by 12 ft slab, 4 in thick.

Step 1 — convert to feet. Length 16 ft. Width 12 ft. Depth 4 in ÷ 12 = 0.333 ft.

Step 2 — volume in cubic feet. 16 × 12 × 0.333 = 64.0 cubic feet.

Step 3 — convert to cubic yards. 64.0 ÷ 27 = 2.37 cubic yards, or about 2.4 cubic yards.

Step 4 — bags, low end. At the typical yield of about 45 80 lb bags per cubic yard: 2.37 × 45 = 106.7 bags, rounded up to 107 bags. With 60 lb bags: 2.37 × 60 = 142.2, rounded up to 143 bags.

Step 5 — bags, high end with the 10% waste allowance. 2.37 × 1.10 = 2.61 cubic yards, or 118 80 lb bags (157 60 lb bags). Order 107–118 80 lb bags and keep one or two spare; a slab that runs short at the far corner leaves a patch that will always show.

Cost guidance

Budgeting for cubic yards of concrete for a slab means pricing the range, not a single number. Take the worked-example quantity (2.4–2.6 cubic yards, or about 107–118 80 lb bags) and price it twice: once at the low end of the range and once at the high end, using the current shelf price per bag at the store where you will actually buy, or the per-cubic-yard ready-mix quote from a local supplier. The difference between the two totals is your contingency, and it is usually smaller than the cost of running short mid-pour. Two cost traps catch first-timers: ordering the low end and then paying a second delivery charge for the few bags that finish the job, and forgetting the extras — formwork, stakes, rebar or mesh, a float, and curing compound or plastic sheeting. Compare the total delivered cost, not the per-bag price, and keep one or two unopened bags for patching; an opened bag that gets damp is wasted money.

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 slab length and width in feet, and the depth in inches at four or more places across the base. 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 cubic yards of concrete for a slab 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 cubic yards of concrete for a 16x12 slab?

Multiply length times width times depth and divide by 27. A 16 ft by 12 ft slab at 4 in thick is 64.0 cubic feet, or about 2.4 cubic yards before waste — 2.4–2.6 cubic yards with the typical 10% allowance. That is 107–118 80 lb bags.

How do I convert cubic feet to cubic yards?

Divide by 27, because one cubic yard is 27 cubic feet. The worked example is 192 square feet at a third of a foot deep, which is 64 cubic feet, and 64 divided by 27 is about 2.4 cubic yards. Always do the conversion before you order, because suppliers sell by the cubic yard and bags are rated by the cubic yard too.

Should I order ready-mix or bags for a slab this size?

At about 2.4 cubic yards, both are practical. Bagged mix means about 107–118 80 lb bags to mix by hand or in a rented mixer, which is a long day for one person. A ready-mix short load delivers the same volume in one truck, usually with a short-load charge — price both against the same volume including waste and compare the delivered totals.

How thick should a concrete slab be?

About 4 in is common for a patio or a shed slab that carries foot traffic and light loads. A driveway slab or a slab that will carry vehicles is usually 5–6 in thick with reinforcement. Check local code for the minimum thickness in your area; the code governs, and a thin slab cracks under loads it was not designed for.

What happens if I order too little concrete for a slab?

The pour stops short and the rest has to be placed later, which leaves a cold joint — a visible line and a weak seam across the slab. That is why this guide tells you to order the high end of the range and keep the receipt for unopened bags. A spare bag costs little; a cold joint across a new slab costs the look of the whole job.

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.