Concrete Quantity Calculator: How Much Concrete Do I Need?

This concrete quantity calculator works out the volume of concrete you need for a slab, footing, column, or other shaped pour. You enter the dimensions of the project (length, width, and depth, or radius and depth for a round column) and the calculator returns the volume in cubic yards, cubic feet, and cubic meters, plus an estimate of the number of 60-lb and 80-lb bags required. It uses the standard formula: Volume = Length × Width × Depth (or π × Radius² × Depth for cylinders).

Formula

The basic formula for a rectangular slab is:

Volume = Length × Width × Depth

Where:

• Length (L) = length of the slab in feet (or meters)
• Width (W) = width of the slab in feet (or meters)
• Depth (D) = thickness of the slab in inches (or centimeters) – convert to feet by dividing by 12

For a round column or footing, the formula is:

Volume = π × Radius² × Depth

Where Radius = diameter ÷ 2. The result is in cubic feet if you use feet for all dimensions. To convert to cubic yards, divide by 27 (since 1 yard = 3 feet, so 1 cubic yard = 27 cubic feet). To convert to cubic meters, multiply cubic feet by 0.0283168.

Worked Example

Using the default values from the calculator above: a slab that is 10 feet long, 4 feet wide, and 4 inches deep.

1. Convert depth to feet: 4 in ÷ 12 = 0.3333 ft
2. Calculate volume in cubic feet: 10 ft × 4 ft × 0.3333 ft = 13.33 ft³
3. Convert to cubic yards: 13.33 ft³ ÷ 27 = 0.49 yd³ (about half a yard)
4. Convert to cubic meters: 13.33 ft³ × 0.0283168 = 0.377 m³
5. Estimate 60-lb bags: each 60-lb bag yields about 0.45 ft³ of concrete, so 13.33 ÷ 0.45 = 29.6 → round up to 30 bags

The calculator shows the same: 13.33 ft³, 0.49 yd³, 0.38 m³, and 30 bags (60-lb).

Why You Need an Accurate Concrete Estimate

Concrete is usually ordered by the cubic yard from a ready-mix truck, and you pay for every yard delivered – even if you don’t use it all. Overestimating wastes money; underestimating means a second, smaller delivery with extra delivery fees and a cold joint in your pour. For bagged concrete, buying too few means a trip back to the store mid-project, and too many leaves you with heavy bags you can’t return. A quick volume calculation prevents both.

How to Use the Calculator Correctly

Measure every dimension in the same unit. If you mix feet and inches, convert first. For slabs, measure the actual thickness you plan to pour – not the form height if you’re using a gravel base. For footings, measure the width at the bottom of the trench, not the top, because trenches often widen. For columns, measure the diameter of the form tube, not the hole you dug. Enter the depth in inches (the calculator converts it), and remember to account for multiple identical sections by multiplying the result by the number of sections.

Common Mistakes and How to Avoid Them

• Unit mix-ups: The biggest error is mixing feet and inches. Always convert depth to feet (divide inches by 12) before multiplying.
• Forgetting to round up: Concrete is sold in whole cubic yards for ready-mix. Round up to the next full yard – you can’t order 0.3 of a yard from most suppliers.
• Ignoring waste: Add 5-10% extra for spillage, uneven subgrades, or slight measurement errors. For a 10×4×4″ slab, 5% extra means about 0.7 ft³ more.
• Using the wrong bag yield: A 60-lb bag yields about 0.45 ft³, an 80-lb bag yields about 0.6 ft³. Check the bag – yields vary slightly by mix.
• Not accounting for rebar or gravel: The calculator gives the volume of the finished concrete, not the volume of the hole. If you have a gravel base, subtract its thickness from the depth.

Ready-Mix vs. Bagged Concrete

For small projects (under 1 cubic yard), bagged concrete is usually more practical. You mix it in a wheelbarrow or mixer and pour it yourself. For larger pours, ready-mix is more cost-effective and ensures a consistent mix. A standard concrete truck holds about 10 cubic yards, but you can order less – many suppliers have a minimum order of 1-2 yards. Bagged concrete is more expensive per cubic foot but avoids the delivery fee and the rush to place the concrete before it sets. Use this calculator to compare: if your project needs more than 20 bags, ready-mix might be cheaper.

Safety and Accuracy Tips

• Always wear gloves and eye protection when handling concrete – the cement powder is caustic and can burn skin.
• Mix only as much as you can place in 30 minutes, especially in hot weather. Concrete sets faster when it’s warm.
• Use a straightedge and level to check the form depth before pouring – a 1-inch error in depth on a 10×4 slab changes the volume by 3.3 ft³ (about 7 bags).
• For structural footings or load-bearing walls, consult a structural engineer – this calculator is for estimating volume, not for design.
• If you’re ordering ready-mix, have the truck’s chute reach the pour area or plan a wheelbarrow path. Concrete hardens quickly, and delays cause issues.

Frequently Asked Questions

How many 80-lb bags of concrete do I need for a 10×10 slab at 4 inches thick?

Volume = 10×10×(4/12) = 33.33 ft³. Each 80-lb bag yields about 0.6 ft³, so 33.33 ÷ 0.6 = 55.6, so you need 56 bags. That’s about 4,480 lbs of concrete – a ready-mix delivery of 1.25 yards might be easier.

What is a “yard� of concrete?

A yard of concrete means one cubic yard – a volume of 3 feet × 3 feet × 3 feet, or 27 cubic feet. It’s the standard unit for ordering ready-mix. One cubic yard weighs roughly 4,000 lbs (about 2 tons).

How do I calculate concrete for a round column?

Use the formula π × r² × h, where r is the radius (half the diameter) and h is the height. For a 12-inch diameter column that is 8 feet tall: radius = 6 in = 0.5 ft, so volume = 3.1416 × 0.5² × 8 = 6.28 ft³. That’s about 0.23 yd³, or 14 bags of 60-lb mix.

Should I order extra concrete?

Yes, add 5-10% to your calculated volume to cover waste, uneven subgrades, and spillage. For a 2-yard pour, that means ordering 2.1 to 2.2 yards. Most suppliers expect you to order slightly over – they’ll charge for what you use, but you can’t order a fraction of a yard.

Can I use this calculator for concrete footings?

Yes, as long as you measure the actual dimensions of the footing trench. Footings are usually rectangular, so use the length, width, and depth. For stepped footings, calculate each section separately and add them together. Remember that the width at the bottom of the trench may be wider than at the top due to slumping – measure at the bottom.

What if my project is not a simple rectangle?

Break the shape into rectangles, triangles, or cylinders, calculate each volume, and add them. For a triangle, use (base × height × depth) / 2. For an L-shaped slab, split it into two rectangles. This calculator handles one shape at a time, so do each section separately and sum the results.

Related

• Board and Batten Wall Calculator: How to Estimate
• Board Feet Calculator Formula & How to Use It
• Board Feet Calculator: Lumber Volume Explained

in l | Length | ft | 10 | 0
in w | Width | ft | 4 | 0
in d | Depth | in | 4 | 0
out vol_ft3 | Volume | ft³ | l * w * (d / 12) | 2
out vol_yd3 | Volume | yd³ | (l * w * (d / 12)) / 27 | 2
out vol_m3 | Volume | m³ | (l * w * (d / 12)) * 0.0283168 | 2
out bags60 | 60-lb bags | bags | ceil((l * w * (d / 12)) / 0.45) | 0