How much concrete do I need for a 10 × 10 slab?

At the standard 4 in thickness: 10 × 10 × (4 ÷ 12) = 33.3 ft³, which is 1.23 yd³ or 56 eighty-pound bags. Thicken the slab to 6 in and it becomes 1.85 yd³.

How many 80 lb bags of concrete make a cubic yard?

About 45 bags. An 80 lb bag yields roughly 0.60 ft³ of mixed concrete, and a yard is 27 ft³: 27 ÷ 0.60 = 45.

What about 60 lb bags?

A 60 lb bag yields about 0.45 ft³, so it takes around 60 bags per cubic yard. The calculator’s bag rows assume 80 lb bags — multiply the 80 lb count by 4/3 for 60 lb bags.

How much extra concrete should I order?

Add 10% for slabs and columns with straightforward formwork, and 15% or more for footings in loose soil or odd shapes. A short load fee or a cold joint costs far more than the extra quarter yard.

How thick should a concrete slab be?

4 in handles patios, walkways, and shed floors. Driveways that carry vehicles want 5–6 in over compacted base. Thickness is the single most volume-sensitive input — going from 4 to 6 in adds 50% more concrete.

Does a standard truck carry my whole order?

Most ready-mix trucks hold 8–10 yd³. The truck row in the results divides your waste-adjusted volume by 10 so you can see whether one delivery covers it.

Concrete Calculator | Online Calculators Free

What the concrete calculator does

It converts the three shapes DIY projects actually pour into a buyable quantity: slabs (patios, shed floors), footings (frost walls, deck trench footings), and round columns (deck piers, post holes). You get cubic feet, cubic yards, and the 80 lb bag count with a 10% cushion already separated out.

The volume formulas

Slab and footing: volume = length × width × (thickness ÷ 12), with thickness in inches. Round column: volume = π × (diameter ÷ 24)² × height × count — dividing the inch diameter by 24 gives the radius in feet. Divide by 27 to move from cubic feet to cubic yards, because one yard is 3 ft × 3 ft × 3 ft.

Bags or a truck?

One cubic yard equals 45 eighty-pound bags (27 ÷ 0.60), so a two-car driveway quickly becomes absurd in bags. Rule of thumb: under about 1 yd³, bags mixed in a rented drum are fine; above that, order ready-mix and pour it all at once. Most ready-mix trucks carry 8–10 yd³.

Worked example

A patio slab measures 10 ft × 10 ft at 4 in thick.
Volume: 10 × 10 × (4 ÷ 12) = 33.33 ft³.
In yards: 33.33 ÷ 27 = 1.23 yd³ — order 10% more, 1.36 yd³.
Bag route: 33.33 ÷ 0.60 = 55.6 → 56 bags, or 62 with the waste cushion.

How to use it

Pick the structure: slab, footing, or round column.
Enter dimensions — thickness and diameter go in inches, everything else in feet.
Read the cubic yards line for a ready-mix order, or the bag rows for bagged mix.
Round up to include the 10% waste cushion before you buy.