How Many Bags of Concrete for a 10x10 Slab? 4-Inch Example

A 10 ft by 10 ft slab 4 inches thick needs 56 bags of 80 lb concrete mix, or 75 bags of 60 lb mix. Those counts assume 0 percent extra for spillage, and the bag count is always rounded up to a whole bag.

To get the count for your own slab, enter your measurements into the Concrete Calculator. It returns the bags to buy for the bag size you choose. The steps below show what the calculator does, so you can check the result by hand.

The worked example: 10 ft x 10 ft x 4 inches

This example uses a square slab 10 feet long and 10 feet wide, poured 4 inches thick. Those are the only inputs. The calculator works through four steps in order.

1. Find the volume in cubic feet: length x width x thickness (in inches) / 12. That is 10 x 10 x 4 / 12, which gives 33.33 cu ft.
2. Convert cubic feet to cubic yards by dividing by 27. That gives 1.23 cu yd, the unit a ready-mix supplier usually quotes.
3. Divide the cubic feet by the volume of one bag. An 80 lb bag yields 0.60 cu ft, so 33.33 / 0.60 gives an exact count of 55.6 bags.
4. Round up to a whole bag. 55.6 becomes 56 bags.

For 60 lb bags, the same 33.33 cu ft is divided by 0.45 cu ft per bag. That gives an exact count of 74.1 bags, which rounds up to 75 bags.

Bag count for 80 lb and 60 lb bags

The table below uses the same 10 x 10 x 4 inch slab. The volume is the same in both rows. Only the bag yield changes.

• 80 lb bag, 0.60 cu ft yield: exact count 55.6 bags, buy 56 bags.
• 60 lb bag, 0.45 cu ft yield: exact count 74.1 bags, buy 75 bags.
• Total volume for both rows: 33.33 cu ft, or 1.23 cu yd.

The smaller bag needs more bags to hold the same volume. That is why bag size matters. A larger bag means fewer bags to lift, fewer trips to the car or truck, and less chance of running short partway through a pour.

Why bag yield matters

Bag yield is the amount of finished concrete one bag makes when mixed with water. It is not the same as the weight of the bag, because the dry mix gains volume when water is added. A bag labeled 80 lb does not always yield exactly 0.60 cu ft, and a 60 lb bag does not always yield exactly 0.45 cu ft.

Always check the yield printed on the product label before you buy, because products differ. If your label shows a different yield, divide the 33.33 cu ft by that number instead and round up to a whole bag.

Thickness scales the volume in proportion, the same way length and width do. Doubling any one of the three dimensions doubles the volume. Use your real measurements in the calculator rather than reusing this example.

Adding extra for spillage

The calculator has an optional extra percentage for spillage, waste and uneven ground. This example uses 0 percent, so the counts above are the bare minimum. Many people add a margin so they do not run short mid-pour. When you add extra, the calculator applies it to the same volume and recalculates the bag count.

When ready-mix may be worth comparing

Bags make sense for small slabs, repairs and projects where you can mix in batches. For this 1.23 cu yd slab, the count is 56 bags of 80 lb mix, which is a lot of lifting and mixing for one pour. At some point, hauling and mixing that many bags takes more effort than ordering concrete by the cubic yard.

Compare the two options when:

• The slab must be poured continuously, and mixing bags by hand would take too long to keep the pour going.
• You have no clean, dry place to stack many bags out of the weather.
• The volume is large enough that one delivered load is simpler than many bag runs.
• A supplier can deliver to the pour area, and the truck can reach it.

Ask a local ready-mix supplier for a quote based on the cubic yard figure. The calculator gives you the 1.23 cu yd number to start that conversation. Pricing and delivery minimums vary by supplier, so get the quote from them directly.

Quick checklist before you buy

1. Measure the length and width of the slab in feet.
2. Choose the thickness in inches.
3. Pick the bag size you plan to buy and find its yield on the label.
4. Run the numbers in the Concrete Calculator and round up to whole bags.
5. Decide whether to add an extra percentage for spillage.
6. Compare bag counts with a ready-mix quote if the volume is large.

FAQ

Why does the calculator round up the bag count?

You cannot buy part of a bag, and rounding up makes sure you have enough mix. For this example, 55.6 bags becomes 56 bags of 80 lb mix.

Why is the 60 lb count higher than the 80 lb count?

In this example, the 60 lb bag yields 0.45 cu ft and the 80 lb bag yields 0.60 cu ft. The 60 lb yield is the smaller of the two, so each bag holds less concrete and more bags are needed. That is why 75 bags of 60 lb mix is more than 56 bags of 80 lb mix for the same 33.33 cu ft.

What if my slab is not square?

Enter your own length and width. Volume is the length times the width times the thickness, so a rectangle with different sides gets its own result. Use the Concrete Calculator with your measurements rather than the square example here.

Should I trust the yield on the bag or the calculator?

Trust the yield printed on your product label, because products differ. Use the calculator for the volume and the bag math, and replace its yield with the one on your bag if they do not match.