Cubic Yards Of Concrete For A Driveway

Figuring out cubic yards of concrete for a driveway 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 20 ft by 12 ft driveway section, 4 in thick 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 20 ft by 12 ft driveway section, 4 in thick different from a plain slab is the shape: a driveway is a flat rectangular slab, so the volume is length times width times depth — but a driveway is usually poured thicker than a patio, so check the depth before you calculate. 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.

A driveway is a slab, so the formula is the same: length times width times depth divided by 27. The difference is the thickness and the reinforcement — a driveway that carries cars is usually poured at 5–6 in rather than 4 in, and it usually has rebar or mesh, so if your driveway is thicker than the worked example, redo the arithmetic with your own depth rather than scaling the answer. A driveway is also often poured in sections, so work out the volume per section and order for the sections you pour that day.

Worked example: a 20 ft by 12 ft driveway section, 4 in thick

Here is the full arithmetic for a 20 ft by 12 ft driveway section, 4 in thick.

Step 1 — convert to feet. Length 20 ft. Width 12 ft. Depth 4 in ÷ 12 = 0.333 ft.

Step 2 — volume in cubic feet. 20 × 12 × 0.333 = 80.0 cubic feet.

Step 3 — convert to cubic yards. 80.0 ÷ 27 = 2.96 cubic yards, or about 3.0 cubic yards.

Step 4 — bags, low end. At the typical yield of about 45 80 lb bags per cubic yard: 2.96 × 45 = 133.3 bags, rounded up to 134 bags. With 60 lb bags: 2.96 × 60 = 177.8, rounded up to 178 bags.

Step 5 — bags, high end with the 10% waste allowance. 2.96 × 1.10 = 3.26 cubic yards, or 147 80 lb bags (196 60 lb bags). Order 134–147 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

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 (3.0–3.3 cubic yards, or about 134–147 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 driveway length and width in feet, and the depth in inches at several 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 cubic yards of concrete for a driveway 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 driveway?

Multiply length times width times depth and divide by 27. A 20 ft by 12 ft driveway section at 4 in thick is about 3.0 cubic yards before waste — 3.0–3.3 cubic yards with the typical 10% allowance, or 134–147 80 lb bags. A full driveway is usually several such sections, so add the sections together.

How thick should a concrete driveway be?

About 4 in is the minimum for light use, but a driveway that carries cars every day is usually poured 5–6 in thick with reinforcement. The worked example uses 4 in; if your driveway is 6 in thick, the same 20 ft by 12 ft section needs about half as much again. Check local code for the minimum thickness in your area.

Do driveways need rebar?

Usually yes — a driveway slab benefits from rebar or welded wire mesh to control cracking, especially at 5–6 in thick. The steel does not change the volume estimate, but it changes the order and the pour: the mesh has to be held up in the middle of the slab, not left lying on the base, and the concrete has to flow around it.

Can I pour a driveway in sections?

Yes, and most driveways are poured that way, with a joint between sections. Work out the volume per section and order for the sections you plan to pour that day, so each section is a single continuous pour with no cold joint inside it. Plan the joints to fall where they look deliberate — usually at regular intervals along the run.

How long before I can drive on a new driveway?

In normal conditions, about a week for a car, and longer for heavy vehicles — concrete keeps gaining strength for about 28 days. Keep foot traffic off for a day or two, keep it damp or covered for the first few days, and do not let de-icing salts sit on a new driveway in its first winter.

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.