Concrete Yard Calculator: How Much Concrete Do I Need?

This concrete yard calculator works out the volume of concrete you need for a driveway or slab in cubic yards. You enter the length, width, and thickness of the pour, and it returns the cubic yards, plus the number of 60-lb and 80-lb bags required. The formula is straightforward, but the real challenge is getting the units right and accounting for waste.

Formula

The volume of concrete in cubic yards is calculated as:

Volume (cu yd) = (Length × Width × Thickness) / 27

Where:

• Length and Width are in feet (ft).
• Thickness is in feet (ft) – convert inches to feet by dividing by 12.
• 27 is the number of cubic feet in one cubic yard (3 ft × 3 ft × 3 ft).

To account for waste, add a percentage: multiply the result by (1 + waste/100). For bagged concrete, a 60-lb bag yields about 0.45 cubic feet, and an 80-lb bag yields about 0.60 cubic feet. The calculator uses these standard yields.

Worked Example

Using the default values: a 20 ft long, 10 ft wide driveway with a 4-inch thickness and 10% waste allowance.

1. Convert thickness to feet: 4 in ÷ 12 = 0.333 ft
2. Calculate volume in cubic feet: 20 × 10 × 0.333 = 66.7 cu ft
3. Convert to cubic yards: 66.7 ÷ 27 = 2.47 cu yd
4. Add 10% waste: 2.47 × 1.10 = 2.72 cu yd
5. For 60-lb bags: 2.72 × 27 = 73.4 cu ft ÷ 0.45 = 163 bags (rounded up)
6. For 80-lb bags: 73.4 cu ft ÷ 0.60 = 122 bags (rounded up)

The calculator shows 2.72 cubic yards, 163 sixty-pound bags, or 122 eighty-pound bags.

How to Measure Your Driveway

Accurate measurements are the foundation of a correct estimate. Here’s how to get them:

• Length and width: Use a tape measure or a measuring wheel. For a rectangular driveway, measure the longest and widest points. If your driveway is irregular (curved, angled, or has a widening near the garage), break it into rectangles and triangles, calculate each area, and add them together.
• Thickness: Standard driveway thickness is 4 inches for passenger cars, 5–6 inches for heavier vehicles like trucks or RVs. If you’re not sure, check local building codes – many require 4 inches minimum. The calculator uses inches, so you don’t need to convert manually.
• Waste factor: Even with careful planning, you’ll spill some, the ground may be uneven, or the forms may bulge. A 10% waste factor is common for slabs; for footings or complex shapes, use 15%.

Ready-Mix vs. Bagged Concrete

For a driveway, you have two main options: ordering ready-mix from a truck or mixing bags yourself.

• Ready-mix: Ordered by the cubic yard, delivered by a mixer truck. This is the standard for any pour larger than about 1 cubic yard. The minimum order is often 1–2 yards, and you’ll pay a delivery fee. You need to have the site prepared and a crew ready to spread and finish the concrete quickly.
• Bagged concrete: Each bag is mixed with water on site. A 60-lb bag yields about 0.45 cu ft; an 80-lb bag yields 0.60 cu ft. For a 10×10 slab at 4 inches thick (1.23 cu yd), you’d need about 74 eighty-pound bags – that’s a lot of mixing. Bagged concrete is practical for small patios, walkways, or repairs, but for a full driveway it’s labor-intensive and often more expensive per cubic yard.

Most contractors use ready-mix for driveways. The calculator gives you both so you can compare costs.

Common Mistakes and How to Avoid Them

• Mixing units: The most common error is entering thickness in inches while length and width are in feet. The calculator handles this correctly, but if you do it manually, always convert to feet first.
• Forgetting waste: If you order exactly the calculated amount and spill a little, you’ll be short. Add 10% – it’s cheaper than ordering a second truck or running to the store for extra bags.
• Rounding too early: If you round the volume to 2.5 yards before adding waste, you might end up short. Keep full precision until the end.
• Ignoring the subgrade: The volume of concrete is based on the dimensions of the pour, but if the ground is uneven, you’ll need more concrete to fill low spots. Level the subgrade before you measure.

How to Read the Result

The main output is cubic yards. This is the number you’ll give to a ready-mix company when ordering. For example, “I need 2.7 yards” – they’ll round up to the nearest quarter or half yard.

The bag counts are based on standard yields. If you use a different brand or mix, check the bag – some mixes yield slightly more or less. The calculator rounds up to the nearest whole bag because you can’t buy a partial bag.

Remember: the calculator gives the volume of concrete, not the number of truckloads. A typical mixer truck holds 9–10 cubic yards, but for a small driveway you might only need 2–3 yards.

Frequently Asked Questions

How many cubic yards do I need for a 20×20 driveway at 4 inches thick?

Using the formula: 20 × 20 × (4/12) = 133.3 cu ft, divided by 27 = 4.94 cu yd. Add 10% waste = 5.43 cu yd. So you’d order about 5.5 yards.

What is the difference between a cubic yard and a square yard?

A square yard is a measure of area (length × width) used for flat surfaces like carpet. A cubic yard is a measure of volume (length × width × height) used for concrete, soil, or gravel. Concrete is sold by volume, so you need cubic yards.

Can I use the calculator for a patio or sidewalk?

Yes, the same formula works for any rectangular slab. Just enter the appropriate length, width, and thickness. For a sidewalk, thickness is often 4 inches; for a patio, 4–6 inches depending on use.

How accurate is the waste factor?

10% is a good rule of thumb for a simple slab. If your driveway has curves, slopes, or you’re using forms that might bulge, use 15%. If you’re pouring a perfect rectangle on level ground, 5% might be enough. It’s better to overestimate slightly – leftover concrete is easier to deal with than a shortage.

What if my driveway isn’t a perfect rectangle?

Break it into smaller rectangles and triangles. Calculate each part’s volume separately, then add them together. For a triangle, the area is (base × height) / 2. Then multiply by thickness and divide by 27. Add the volumes before applying waste.

How much does a yard of concrete weigh?

Standard concrete weighs about 4,000 pounds per cubic yard (about 2,400 kg per cubic meter). This is useful for estimating load capacity or if you’re mixing bags – an 80-lb bag weighs 80 lbs, so 60 bags would be 4,800 lbs.

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in l | Length | ft | 20 | 0
in w | Width | ft | 10 | 0
in d | Thickness | in | 4 | 0
in waste | Waste | % | 10 | 0
out yards | Concrete needed | cu yd | (l * w * (d / 12) / 27) * (1 + waste / 100) | 2
out bags60 | 60-lb bags | bags | ceil((l * w * (d / 12) / 27) * (1 + waste / 100) * 27 / 0.45) | 0
out bags80 | 80-lb bags | bags | ceil((l * w * (d / 12) / 27) * (1 + waste / 100) * 27 / 0.6) | 0