Concrete Calculator

Estimate the concrete volume and number of bags needed for a slab, footing, or any known volume.

🧮 Concrete Calculator
Volume (cubic yards)
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Volume (cubic feet)
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60 lb Bags
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80 lb Bags
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Step-by-step working

📖 What is the Concrete Calculator?

The concrete calculator is a general-purpose tool that finds the total volume of concrete you need for a slab, footing, or any known shape, and converts it into cubic yards (the standard unit ready-mix suppliers sell by) and into premixed bag counts. Enter your dimensions in feet and inches and get an instant materials estimate.

This tool covers the everyday concrete pours a contractor or DIY builder runs into: a patio or driveway slab, an isolated footing or pad, a strip footing under a wall, or any pour whose volume you have already worked out with another method. The Direct Volume mode lets you type in a known cubic foot or cubic yard figure, for example one computed from a round column or an odd-shaped pour, and get the bag count without re-deriving the geometry.

A common point of confusion is the difference between this calculator and the site's Concrete Mix Calculator. This calculator answers "how much concrete, in cubic yards or bags, do I need to buy" for someone using ready-mix trucks or premixed bags. The Concrete Mix Calculator instead answers "how much cement, sand, and aggregate do I need" for someone mixing concrete from raw materials on site at a chosen ratio like 1:2:4. They solve two different, complementary problems in the same pour.

Because concrete cannot easily be added mid-pour once it starts setting, and ready-mix trucks charge extra for small or partial loads, getting the volume estimate right before ordering is one of the highest-value calculations in a concrete project.

📐 Formula

Vft³  =  L × W × (T ÷ 12) × n
L, W = length and width in feet
T = thickness or depth in inches, divided by 12 to convert to feet
n = quantity of identical slabs or footings
Vyd³ = Vft³ ÷ 27
Bags = Vft³ ÷ bag yield (0.45 ft³ for a 60 lb bag, 0.6 ft³ for an 80 lb bag)
Example: A 10 x 10 ft slab, 4 in thick: 10 × 10 × (4/12) = 33.3 ft³ = 1.23 yd³ = 75 bags (60 lb) or 56 bags (80 lb).

📖 How to Use This Calculator

Steps

1
Choose the pour type: Slab, Footing, or Direct Volume using the tabs.
2
Enter the dimensions: length, width, and thickness (or depth), or a known volume.
3
Set the quantity: how many identical slabs or footings you are pouring.
4
Click Calculate: see the volume in cubic yards and the number of bags needed.

💡 Example Calculations

Example 1 - 10 x 10 ft patio slab, 4 in thick

1
Length = 10 ft, width = 10 ft, thickness = 4 in, quantity = 1
2
Volume = 10 × 10 × (4/12) = 33.33 ft³ (1.235 yd³)
3
60 lb bags = 33.33 ÷ 0.45 = 75 bags; 80 lb bags = 33.33 ÷ 0.6 = 56 bags
Volume = 1.235 yd³ (75 bags of 60 lb, or 56 bags of 80 lb)
Try this example →

Example 2 - 20 ft strip footing, 18 in wide, 12 in deep

1
Length = 20 ft, width = 1.5 ft (18 in), depth = 12 in, quantity = 1
2
Volume = 20 × 1.5 × (12/12) = 30 ft³ (1.111 yd³)
3
60 lb bags = 30 ÷ 0.45 = 67 bags; 80 lb bags = 30 ÷ 0.6 = 50 bags
Volume = 1.111 yd³ (67 bags of 60 lb, or 50 bags of 80 lb)
Try this example →

Example 3 - Known volume of 2 cubic yards

1
Direct volume = 2 yd³
2
2 × 27 = 54 ft³
3
60 lb bags = 54 ÷ 0.45 = 120 bags; 80 lb bags = 54 ÷ 0.6 = 90 bags
Volume = 2 yd³ (120 bags of 60 lb, or 90 bags of 80 lb)
Try this example →

❓ Frequently Asked Questions

How do I calculate how much concrete I need?+
Multiply the length, width, and thickness of the pour (all in the same unit, then converted to feet) to get the volume in cubic feet, then divide by 27 to get cubic yards. A 10 by 10 foot slab poured 4 inches thick needs 10 x 10 x 0.333 = 33.3 cubic feet, or about 1.23 cubic yards.
How many bags of concrete do I need per cubic yard?+
One cubic yard equals 27 cubic feet. A 60 lb bag yields about 0.45 cubic feet, so it takes about 60 bags to make one cubic yard. An 80 lb bag yields about 0.6 cubic feet, so it takes about 45 bags per cubic yard.
What is the difference between this calculator and the Concrete Mix Calculator?+
This calculator finds the total volume of concrete you need and how many premixed bags (or cubic yards of ready-mix) that equals. The Concrete Mix Calculator instead works out the cement, sand, and aggregate proportions if you are mixing your own concrete from raw materials rather than buying premixed bags or ready-mix.
How thick should a concrete slab be?+
A typical residential patio or walkway slab is 4 inches thick. Driveways for passenger vehicles are usually 4 to 5 inches, while areas carrying trucks or heavy equipment often use 6 inches or more. Footings and structural slabs should follow your local building code or an engineer's specification.
Should I order ready-mix or use bagged concrete?+
For pours over roughly 1 cubic yard (about 60 to 80 bags), ready-mix delivered by truck is usually cheaper and faster than bagged concrete. Smaller pours, like fence post footings or a small patch, are typically more practical with premixed bags.
How much extra concrete should I order for waste?+
Add 5 to 10 percent to your calculated volume for spillage, an uneven subgrade, and minor formwork overage. For irregular or hand-dug excavations, use the higher end of that range.
How is a concrete footing different from a slab in this calculator?+
Mathematically, a rectangular footing and a slab are calculated the same way: length times width times depth. The Footing mode is separated because footings are commonly poured in multiple identical units (for example, several isolated pad footings), so it includes a quantity field to multiply by the number of footings.
How many 80 lb bags of concrete are in a cubic yard?+
An 80 lb bag yields approximately 0.6 cubic feet of mixed concrete. Since 1 cubic yard equals 27 cubic feet, it takes about 45 bags (27 divided by 0.6) to make one cubic yard.
Can I use this calculator for a round or non-rectangular pour?+
For round columns or Sonotube footings, use the dedicated Concrete Column Calculator or Concrete Estimator - Tube, which use the pi r squared h formula for a cylinder. This general calculator's slab and footing modes are for rectangular pours; use the Direct Volume mode if you have already computed a volume from another shape.
How do I convert a known cubic foot volume to bags and cubic yards?+
Switch to Direct Volume mode, select cubic feet as the unit, and enter your volume. The calculator divides by 27 for cubic yards and by the bag yield (0.45 or 0.6 cubic feet) for the 60 lb and 80 lb bag counts.
Tags: Concrete Calculator Concrete Volume Calculator Cubic Yards of Concrete Concrete Bags Calculator Calculator Formula Free Online

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How do I calculate how much concrete I need?

Multiply the length, width, and thickness of the pour (all in the same unit, then converted to feet) to get the volume in cubic feet, then divide by 27 to get cubic yards. A 10 by 10 foot slab poured 4 inches thick needs 10 x 10 x 0.333 = 33.3 cubic feet, or about 1.23 cubic yards.

How many bags of concrete do I need per cubic yard?

One cubic yard equals 27 cubic feet. A 60 lb bag yields about 0.45 cubic feet, so it takes about 60 bags to make one cubic yard. An 80 lb bag yields about 0.6 cubic feet, so it takes about 45 bags per cubic yard.

What is the difference between this calculator and the Concrete Mix Calculator?

This calculator finds the total volume of concrete you need and how many premixed bags (or cubic yards of ready-mix) that equals. The Concrete Mix Calculator instead works out the cement, sand, and aggregate proportions if you are mixing your own concrete from raw materials rather than buying premixed bags or ready-mix.

How thick should a concrete slab be?

A typical residential patio or walkway slab is 4 inches thick. Driveways for passenger vehicles are usually 4 to 5 inches, while areas carrying trucks or heavy equipment often use 6 inches or more. Footings and structural slabs should follow your local building code or an engineer's specification.

Should I order ready-mix or use bagged concrete?

For pours over roughly 1 cubic yard (about 60 to 80 bags), ready-mix delivered by truck is usually cheaper and faster than bagged concrete. Smaller pours, like fence post footings or a small patch, are typically more practical with premixed bags.

How much extra concrete should I order for waste?

Add 5 to 10 percent to your calculated volume for spillage, an uneven subgrade, and minor formwork overage. For irregular or hand-dug excavations, use the higher end of that range.

How is a concrete footing different from a slab in this calculator?

Mathematically, a rectangular footing and a slab are calculated the same way: length times width times depth. The Footing mode is separated because footings are commonly poured in multiple identical units (for example, several isolated pad footings), so it includes a quantity field to multiply by the number of footings.

How many 80 lb bags of concrete are in a cubic yard?

An 80 lb bag yields approximately 0.6 cubic feet of mixed concrete. Since 1 cubic yard equals 27 cubic feet, it takes about 45 bags (27 divided by 0.6) to make one cubic yard.

Can I use this calculator for a round or non-rectangular pour?

For round columns or Sonotube footings, use the dedicated Concrete Column Calculator or Concrete Estimator - Tube, which use the pi r squared h formula for a cylinder. This general calculator's slab and footing modes are for rectangular pours; use the Direct Volume mode if you have already computed a volume from another shape.

How do I convert a known cubic foot volume to bags and cubic yards?

Switch to Direct Volume mode, select cubic feet as the unit, and enter your volume. The calculator divides by 27 for cubic yards and by the bag yield (0.45 or 0.6 cubic feet) for the 60 lb and 80 lb bag counts.

📌 Quick Tips

💡Ready-mix concrete is ordered by the cubic yard. Always round your order up, since a truck cannot deliver a partial load and running short mid-pour is costly.
💡A 60 lb bag of concrete yields about 0.45 cubic feet; an 80 lb bag yields about 0.6 cubic feet, once mixed with water.
💡Add 5 to 10 percent extra to any volume estimate for spillage, uneven subgrade, and formwork overage.
💡For a wall built from hollow blocks instead of a poured slab, use the Concrete Block Calculator instead.
💡If you need cement, sand, and aggregate proportions for mixing your own concrete, use the Concrete Mix Calculator.