Concrete Calculator
A 10 ft by 10 ft slab 4 in thick needs 1.23 cubic yards (33.3 cubic feet). With 10% extra that is 1.36 yd³, or 62 bags of 80 lb premix (0.60 ft³ each).
How to use
- Choose US units (feet and inches) or metric (meters and centimeters).
- Pick Slab, Column (round) or Stairs and enter the size. Thickness, diameter, rise and run are in inches or centimeters.
- Set the extra percentage for spills and uneven ground. 10% is a common allowance.
- Choose a premix bag size. Read the volume in cubic yards, cubic meters and cubic feet, and the number of bags to buy.
How it works
Slab = length x width x thickness. Round column = π x (diameter / 2)² x height x number of columns. Stairs = width x run x rise x n(n + 1) / 2, where n is the number of steps. The last formula treats the stairs as a solid stack sitting on the ground, so step 1 is one rise high, step 2 is two rises, and so on.
Total = volume x (1 + extra % / 100). For a 10 x 10 ft slab 4 in thick, volume = 10 x 10 x 4/12 = 33.33 ft³ = 1.23 yd³. With 10% extra that is 36.67 ft³ = 1.36 yd³.
Bags = total volume / yield per bag, rounded up. Yields of 0.30 ft³ for a 40 lb bag, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb are the figures printed on Quikrete concrete mix bags. Brands differ by a few percent, so check the label on the bag you buy.
Assumptions: 20 kg and 25 kg yields are not a single published figure. The tool scales the 80 lb yield by weight, which gives about 0.33 ft³ (9.4 L) per 20 kg bag and 0.41 ft³ (11.7 L) per 25 kg bag. The stairs formula ignores landings, a sloped underside and a thicker base. Forms, subbase, rebar and uneven ground are not modeled, so the extra percentage covers them.
Premix bag yield
| Bag | Yield | Basis |
|---|---|---|
| 80 lb (36.3 kg) | 0.60 ft³ (0.0170 m³) | Label value |
| 60 lb (27.2 kg) | 0.45 ft³ (0.0127 m³) | Label value |
| 40 lb (18.1 kg) | 0.30 ft³ (0.0085 m³) | Label value |
| 25 kg | 0.41 ft³ (0.0117 m³) | Scaled from 80 lb |
| 20 kg | 0.33 ft³ (0.0094 m³) | Scaled from 80 lb |
Unit conversions
- 1 yd³ = 27 ft³ = 0.764555 m³
- 1 m³ = 35.3147 ft³ = 1.30795 yd³
- 1 in = 2.54 cm, 1 ft = 0.3048 m
FAQ
How much concrete do I need for a 10x10 slab?
At 4 inches thick a 10 x 10 ft slab needs 33.3 cubic feet, which is 1.23 cubic yards. Add about 10% for spills and uneven ground and order 1.36 cubic yards, or about 62 bags of 80 lb premix.
How many 80 lb bags make a cubic yard?
One cubic yard is 27 cubic feet. An 80 lb bag yields about 0.60 cubic feet, so a yard takes 45 bags. A 60 lb bag takes 60 bags and a 40 lb bag takes 90.
How much extra concrete should I order?
Most guides suggest 5% to 10% extra. Use 10% or more for hand-dug footings, uneven subgrade or thin slabs, because a small error in thickness changes the volume a lot.
How is the volume of a round column calculated?
Volume = π x radius² x height. A 12 in diameter column 4 ft high holds π x 0.5² x 4 = 3.14 cubic feet. Enter the number of columns to multiply it.
How are stairs calculated here?
As a solid block for each step down to the ground: width x run x rise x n(n + 1) / 2. Four steps with a 7 in rise, 11 in run and 3 ft width come to 16.0 cubic feet. Landings are extra, so add them as a slab.
Should I buy bags or order ready-mix?
Bags are practical up to about half a cubic yard (roughly 22 bags of 80 lb). Above that, mixing by hand is slow and ready-mix delivery is usually easier, often with a minimum load.