🧱 Concrete Calculator
Work out how much concrete you need for a slab, footing, post hole, column or stairs — in cubic yards, cubic feet and 40, 60 or 80 lb bags.
Add prices for a cost estimate
| Premixed bag | Yield each | Bags needed | Total weight |
|---|---|---|---|
| 80 lb | 0.6 ft³ | 62 | 4,960 lb |
| 60 lb | 0.45 ft³ | 82 | 4,920 lb |
| 40 lb | 0.3 ft³ | 123 | 4,920 lb |
Over about 1 cubic yard, ready-mix delivery is usually more practical than mixing bags by hand.
What Concrete Calculator Does
This concrete calculator works out the volume of a slab, footing, wall, round column, post hole or set of stairs, adds an allowance for waste, and converts the result into cubic yards for ready-mix and into 40, 60 and 80 lb premixed bags — with the total weight so you know what you are hauling.
Bag counts use the yields manufacturers print on their technical data sheets: an 80 lb bag of standard concrete mix makes 0.60 cubic feet of concrete, a 60 lb bag 0.45 cubic feet and a 40 lb bag 0.30 cubic feet. That works out to 45, 60 or 90 bags per cubic yard. Ready-mix is sold by the cubic yard, which is 27 cubic feet.
How to Use Concrete Calculator
- Choose the shape: slab/footing/wall, round column or post hole, or stairs
- Pick feet and inches or meters and centimeters
- Enter the dimensions — thickness is in inches (or centimeters)
- Set the extra percentage for waste (10% is typical)
- Read the cubic yards and the number of 40, 60 or 80 lb bags; add prices for a cost estimate
Formula Used by Concrete Calculator
Slab, footing or wall
Cubic yards = length (ft) × width (ft) × thickness (in ÷ 12) ÷ 27
Worked example
A 10 × 10 ft patio, 4 inches thick, with 10% extra.
- 10 × 10 × (4 ÷ 12) = 33.33 ft³
- 33.33 × 1.10 = 36.67 ft³
- 36.67 ÷ 27 = 1.36 yd³
- 36.67 ÷ 0.60 = 61.1, round up
Result: 1.36 cubic yards, or 62 bags of 80 lb (82 of 60 lb, 123 of 40 lb).
Round column or post hole
Cubic feet = π × (diameter ÷ 2)² × depth, with diameter and depth in feet
Worked example
A post hole 12 inches across and 4 feet deep.
- Radius = 0.5 ft
- π × 0.5² × 4 = 3.14 ft³
- 3.14 ÷ 0.60 = 5.2
Result: About 3.14 ft³ — six 80 lb bags per hole.
Stairs
Cubic feet = width × tread depth × riser height × n(n + 1) ÷ 2
- n
- Number of steps; step k is k risers tall, so the steps stack like a staircase of blocks
Worked example
4 steps, 4 ft wide, 11 in treads, 7 in risers.
- 4 × (11 ÷ 12) × (7 ÷ 12) × (4 × 5 ÷ 2) = 21.39 ft³
- 21.39 ÷ 27
Result: 0.79 cubic yards before waste.
Premixed Concrete Bag Yields
| Bag size | Yield per bag | Bags per cubic yard | Weight per cubic yard of bags |
|---|---|---|---|
| 80 lb | 0.60 ft³ | 45 | 3,600 lb |
| 60 lb | 0.45 ft³ | 60 | 3,600 lb |
| 40 lb | 0.30 ft³ | 90 | 3,600 lb |
How Far One Cubic Yard Goes
Area covered by one cubic yard (27 ft³) at each slab thickness, before waste.
| Thickness | Area covered |
|---|---|
| 4 inches | 81 sq ft |
| 5 inches | 64.8 sq ft |
| 6 inches | 54 sq ft |
| 8 inches | 40.5 sq ft |
How to Read Your Result
Why add 5–10% extra
Excavations are never perfectly flat, forms bow slightly under the weight of wet concrete, and some material is always left in the mixer or wheelbarrow. Running short in the middle of a pour leaves a cold joint, which is weaker and shows as a line, so it is cheaper to have a little left over.
Bags or ready-mix?
One cubic yard of bagged concrete is 45 bags weighing 3,600 lb before water — a lot to mix by hand. For jobs of about a yard or more, ready-mix delivery is usually faster and gives a more consistent pour. Suppliers often add a short-load fee for small orders, so compare both totals.
Limitations & Accuracy Notes
- Volumes assume a flat, even sub-base and straight forms; slopes, thickened slab edges and footings under a slab need to be added separately.
- Post-hole volumes ignore the space taken by the post itself, so they run slightly high.
- Rebar, wire mesh, gravel base and forms are not included.
- Thickness and reinforcement for driveways, footings and structural work are set by local building codes — confirm with your building department.
Frequently Asked Questions
How do I calculate how much concrete I need?
How many 80 lb bags of concrete make a yard?
How many square feet does a yard of concrete cover?
How thick should a concrete slab be?
Should I buy bags or order ready-mix?
How much does a cubic yard of concrete weigh?
How do I calculate concrete for a post hole?
References & Further Reading
- QUIKRETE Concrete Mix No. 1101 — product data sheet — 80 lb bag yields 0.60 ft³
- QUIKRETE — Concrete Mix product line