Concrete Calculator: How Much Concrete Do I Need?

This concrete calculator works out the volume of concrete you need for a slab, footing, column, or other shape, and it can also estimate the number of bags of premix concrete required. You enter the dimensions of your project (length, width, depth, or diameter and height) and the calculator returns the volume in cubic feet, cubic yards, and cubic meters, plus the number of 60-lb or 80-lb bags. The first output below is the volume in cubic yards, which is the standard unit for ordering ready-mix concrete in the US.

Formula

The basic formula for a rectangular slab is:

Volume (cubic feet) = Length (ft) × Width (ft) × Depth (ft)

Since depth is often given in inches, convert it to feet first: Depth (ft) = Depth (in) ÷ 12. Then, to convert cubic feet to cubic yards, divide by 27 (because 1 cubic yard = 27 cubic feet).

For a round column or footing, use:

Volume (cubic feet) = π × (Radius in ft)² × Height (ft)

where radius = diameter ÷ 2. If you measure diameter in inches, convert to feet by dividing by 12.

For a triangular slab (right triangle), use:

Volume = (Base × Height × Depth) ÷ 2

All dimensions must be in the same unit before multiplying.

Worked Example

Using the calculator defaults: a slab 10 ft long, 10 ft wide, and 4 inches deep.

1. Convert depth to feet: 4 in ÷ 12 = 0.3333 ft
2. Calculate cubic feet: 10 × 10 × 0.3333 = 33.33 cu ft
3. Convert to cubic yards: 33.33 ÷ 27 = 1.23 cu yd
4. 60-lb bags: 33.33 ÷ 0.45 = 74.07 → round up to 75 bags
5. 80-lb bags: 33.33 ÷ 0.60 = 55.55 → round up to 56 bags

The calculator shows 1.23 cubic yards, 33.33 cubic feet, 75 bags of 60-lb mix, and 56 bags of 80-lb mix. Note that bag yields vary by brand and mix; 0.45 cu ft per 60-lb bag and 0.60 cu ft per 80-lb bag are common approximations.

Why You Need a Concrete Calculator

Estimating concrete volume by eye leads to two problems: ordering too little (which delays the pour while you run to the store) or ordering too much (which wastes money and leaves you with hardened concrete you cannot return). A concrete calculator helps you avoid both. It is especially useful for:

• Patios, walkways, and driveways
• Footings and foundations
• Fence posts and deck piers
• Steps and curbs
• Countertops and small DIY projects

For ready-mix concrete, you order by the cubic yard, so knowing your volume in cubic yards is essential. For bagged concrete, you need the number of bags, which depends on the bag size and the mix’s yield per bag.

How to Measure Your Project

Accurate measurements are the foundation of a good estimate. Use a tape measure and measure to the nearest inch or half-inch. For a slab, measure the length and width at the widest points, and measure depth at several spots because the ground may not be perfectly level. Use the average depth if it varies.

For round footings, measure the diameter (the widest distance across the circle). For irregular shapes, break the area into rectangles, triangles, or circles, calculate each volume, and add them together. For slopes or uneven ground, consider the average depth rather than the deepest point.

Always double-check your measurements. A 1-inch error in depth on a 10×10 slab changes the volume by about 0.31 cubic yards – that is roughly 17 bags of 80-lb mix.

Common Mistakes and How to Avoid Them

The most frequent error is mixing units. If you measure length in feet and depth in inches, you must convert one before multiplying. The calculator above does that for you, but if you do it by hand, keep everything in feet or everything in inches.

Another mistake is forgetting to account for the thickness of gravel or a base layer. The concrete depth is the thickness of the concrete itself, not the total excavation depth. Also, if you are filling a tube form for a post, use the inner diameter of the tube, not the outer.

Rounding errors can also add up. When you round the cubic yards to two decimals, you are fine for ordering, but do not round intermediate values too early. For bag counts, always round up to the next whole bag – you cannot buy a fraction of a bag.

Finally, remember that concrete shrinks slightly as it cures, and some mixes have different yields. The bag yield numbers are approximate; check the bag label for the exact yield.

Ordering Ready-Mix vs. Bags

For small projects (under about 1 cubic yard), bagged concrete is usually more practical because ready-mix companies have a minimum order charge that makes small deliveries expensive. For larger projects, ready-mix is cheaper per cubic yard and saves labor.

Here is a rough comparison:

• 1 cubic yard = 27 cubic feet = about 60 60-lb bags or 45 80-lb bags (using the yields above).
• Ready-mix is delivered by truck and you need to be ready to pour immediately.
• Bagged concrete requires mixing, which is labor-intensive for large volumes.

If you are ordering ready-mix, add 10% extra for waste, spillage, and uneven subgrade. For bags, you might add 5-10% as well, but because bags are easy to return unopened, you can be more precise.

Cost Considerations (Without Specific Prices)

The total cost includes the material (ready-mix per cubic yard or bags per bag), delivery fees, and any rental tools (mixer, wheelbarrow). Ready-mix prices vary by region and by the mix design (e.g., higher strength costs more). Bagged concrete is generally more expensive per cubic foot than ready-mix, but you only pay for what you use.

To estimate your cost, multiply the volume in cubic yards by the local ready-mix price, or multiply the number of bags by the per-bag price. Don’t forget to include a waste factor of 5-10%.

Safety and Accuracy Tips

Concrete work involves heavy materials and chemicals. Always wear gloves and eye protection when handling dry mix or wet concrete. Wet concrete can cause skin burns, so avoid prolonged contact. When mixing, follow the manufacturer’s instructions for water content – too much water weakens the concrete.

For structural work (foundations, footings), consult a structural engineer or local building codes. The calculator gives volume, not structural specifications. The thickness and strength of concrete depend on the load and soil conditions.

When pouring, have a plan for finishing and curing. Concrete needs to stay moist for at least a few days to cure properly. Use a trowel to smooth the surface and cover it with plastic or wet burlap to prevent rapid drying.

Frequently Asked Questions

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

Using the calculator, a 10×10 slab at 4 inches thick is 33.33 cubic feet. An 80-lb bag yields about 0.60 cu ft, so you need 33.33 ÷ 0.60 = 55.6, so round up to 56 bags. This assumes the bag yield is 0.60 cu ft, which is typical but varies by brand.

What is the difference between a concrete calculator and a mortar calculator?

Concrete is made of cement, sand, and gravel (aggregate) and is used for structural elements. Mortar is a mix of cement, sand, and water (no gravel) and is used to bond bricks or blocks. A concrete calculator computes the volume of concrete mix, while a mortar calculator would estimate the amount of mortar for masonry – they use different mix ratios and yields.

How do I calculate concrete for a round tube form?

For a cylinder, use the formula: Volume = π × (diameter/2)² × height. Convert all measurements to feet first. For example, a 12-inch diameter tube (1 ft) that is 3 feet tall: π × (0.5)² × 3 = 2.36 cu ft. Then divide by 27 to get cubic yards, and divide by bag yield to get bags.

Can I use the calculator for a triangular slab?

Yes, but you need to adjust the formula. For a right triangle, the area is (base × height) ÷ 2. Multiply that area by the depth to get volume. The calculator above is set for rectangles, so for a triangle you would manually compute the volume and then use the bag count formulas.

Why does the calculator show cubic yards as the main result?

In the US, ready-mix concrete is ordered by the cubic yard. Contractors and suppliers quote prices per cubic yard, so that is the most useful unit for planning a pour. The calculator also shows cubic feet and bag counts for smaller projects.

How accurate is the bag yield estimate?

Bag yields vary by manufacturer, mix design, and how much water you add. The values 0.45 cu ft per 60-lb bag and 0.60 cu ft per 80-lb bag are common averages. Always check the bag label for the exact yield, and consider buying one extra bag to cover spillage or slight miscalculations.

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in l | Length | ft | 10 | 0
in w | Width | ft | 10 | 0
in d | Depth | in | 4 | 0
out cy | Concrete volume | cu yd | (l * w * (d / 12)) / 27 | 2
out cf | Concrete volume | cu ft | l * w * (d / 12) | 2
out bags60 | 60-lb bags | bags | ceil(((l * w * (d / 12)) / 0.45)) | 0
out bags80 | 80-lb bags | bags | ceil(((l * w * (d / 12)) / 0.60)) | 0