Concrete Calculator

Planning a slab? Enter length, width and thickness to get the concrete volume in cubic feet and cubic yards, plus how many 80 lb and 60 lb bags you would need — with the usual advice to order a little extra.

How it works: Volume is length × width × thickness, with the thickness converted from inches to feet; one cubic yard is 27 cubic feet. Bag counts are whole bags rounded up. As a rule of thumb, order about 5% extra for spillage, uneven ground and settling.

How the Concrete Calculator Works

The Concrete Calculator estimates how much concrete you need for a rectangular slab, footing, or column, and how many bags of ready-mix that volume equals. You enter the dimensions in feet and inches, the calculator works out the volume in cubic feet and cubic yards, then divides it by the volume of one bag of your chosen size to give the number of bags. It is used for patios, driveways, paths, footings, and any small project where ordering the right amount avoids a second delivery or a pile of leftover mix.

The core method is volume from the three dimensions. For a slab, the volume in cubic feet is length in feet times width in feet times thickness in inches divided by 12, because the thickness must be in feet to match the other two sides. The result is then converted to cubic yards by dividing by 27, since one cubic yard holds 27 cubic feet. Bags are then found by dividing the cubic feet by the bag’s yield in cubic feet, and the answer is always rounded up to a whole number of bags, because you cannot buy part of a bag and running short mid-pour is far worse than having one spare.

This calculator implements the same steps for a footing or a column by treating them as a rectangular block with length, width, and depth, with the same division by 12 for inches and by 27 for yards. Bag sizes are 40, 60, and 80 pounds, each with its own yield in cubic feet. The calculator does not add a waste allowance, so add 5 to 10 percent for spillage, uneven ground, and formwork bulge. The result is an estimate, because real sub-bases are rarely perfectly flat and a thin slab over soft ground will take more mix than the plan suggests.

How to Use This Calculator

Select the shape or structure you are pouring: slab, footing, or column.
Enter the length, width, and thickness or depth in feet and inches, as the fields ask.
Choose the bag size you plan to buy: 40, 60, or 80 pounds.
Press Calculate to work out the volume and bags needed.
Read the volume in cubic feet and cubic yards, and note the number of bags rounded up.
Add 5 to 10 percent extra for waste before you order.

Example

For a tested example, enter a slab that is 10 feet long, 10 feet wide, and 4 inches thick, with 80-pound bags. The volume is 10 × 10 × (4 ÷ 12) = 33.33 cubic feet. Converted to cubic yards, that is 33.33 ÷ 27 = 1.23 cubic yards. The bags needed are 33.33 ÷ 0.6 = 55.6, which rounds up to 56 bags of 80 pounds. So the tested result is 33.33 cubic feet, or 1.23 cubic yards, needing 56 × 80 lb bags.

As a second example, enter a slab that is 12 feet long, 8 feet wide, and 6 inches thick, with 60-pound bags. The volume is 12 × 8 × (6 ÷ 12) = 48 cubic feet. Converted to cubic yards, that is 48 ÷ 27 = 1.78 cubic yards. The bags needed are 48 ÷ 0.45 = 106.7, which rounds up to 107 bags of 60 pounds. So the result is 48 cubic feet, or 1.78 cubic yards, needing 107 × 60 lb bags.

Understanding Your Result

Cubic feet and cubic yards are the same volume in different units. Suppliers usually quote concrete by the cubic yard for ready-mix deliveries and bags by the cubic feet they cover, so having both figures lets you compare the two buying options directly. The bag count is always rounded up, because a part bag cannot be bought and a shortage in the middle of a pour can ruin the finish as the first section starts to set before the second arrives.

A few practical points will help you order with confidence:

Add 5 to 10 percent extra for waste, spillage, and uneven ground, and order that allowance on top of the calculated bags.
Check your thickness carefully, because a slab poured an inch thinner than planned uses far less mix, and an inch thicker uses far more.
If the job is large, compare the bag total with a ready-mix delivery, which is often cheaper per cubic yard once the bag count passes about 40.
Remember that the calculator treats the shape as a perfect rectangle, so irregular edges and slopes need a separate allowance.
Confirm the bag yield with your supplier, because different brands of the same weight bag can cover slightly different volumes.

Concrete Calculator FAQs

How do I convert cubic feet to cubic yards?

Divide by 27, because one cubic yard contains 27 cubic feet. A slab of 33.33 cubic feet is therefore 1.23 cubic yards.

Why are the bags rounded up?

Because you cannot buy part of a bag, and running short during a pour is worse than having a spare bag. The calculator always rounds up to the next whole bag.

What bag sizes can I choose from?

The calculator offers 40, 60, and 80-pound bags, each with a different yield in cubic feet. Larger bags cover more volume each, so the total count is lower.

Should I add extra for waste?

Yes, add 5 to 10 percent for spillage, formwork bulge, and uneven ground. The calculator’s figure is the exact volume of the perfect shape, not the amount you should order.

Can I use this for a circular slab or a footing?

The calculator is built for rectangular slabs, footings, and columns. For a circular slab, calculate the area from the radius first, then multiply by the thickness, or use a specialist circular slab tool.