Concrete Calculator
Choose a shape, enter your measurements and get cubic yards, bag counts and weight. For a specific project, use one of the focused calculators below.
One cubic yard of concrete takes 45 bags of 80 lb mix, 60 bags of 60 lb or 90 bags of 40 lb (27 ft³ ÷ bag yield).
ft³ · m³ · incl. % extra
Weight ≈ lb (exact volume, 145 lb/ft³)
Bags are rounded up. Yields: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.
Concrete for common jobs
| Job | Size | Volume | 80 lb bags (+10%) | 60 lb bags (+10%) |
|---|---|---|---|---|
| Fence post hole | 10 in × 3 ft deep | 1.64 ft³ | 3 | 4 |
| Deck footing | 12 in × 4 ft deep | 3.14 ft³ | 6 | 8 |
| Patio or shed pad | 10 × 10 ft × 4 in | 1.23 yd³ | 62 | 82 |
| Walkway | 30 × 3 ft × 4 in | 1.11 yd³ | 55 | 74 |
| Front steps (3 steps) | 4 ft wide, 7 in rise, 11 in run | 12.83 ft³ | 24 | 32 |
| Wall footing | 20 ft × 16 in wide × 8 in deep | 0.66 yd³ | 33 | 44 |
Post holes are calculated before the post goes in, so they are a safe upper estimate.
Concrete calculators
Concrete slab & pad calculator
Patios, pads, walkways and driveways. Bags, rebar, weight, cost and a scaled diagram.
Concrete bag calculator
How many 40, 60 or 80 lb bags for any volume, by brand.
Post hole & column calculator
Round forms and Sonotubes by diameter and depth.
Footing calculator
Continuous footings and footers for walls and decks.
Concrete steps calculator
Volume for stairs with any rise, run and width.
Concrete block calculator
CMU blocks, mortar and fill for a wall.
Quikrete calculator
Every QUIKRETE® mix, by its data-sheet yield.
Sakrete calculator
Sakrete mixes and bag sizes, by data-sheet yield.
How to calculate concrete by shape
Slabs, pads and walkways
Length × width × thickness, with thickness in feet. A 10×10 ft patio at 4 in is 10 × 10 × 0.333 = 33.33 ft³, or 1.23 yd³. For bag counts, rebar, cost and a drawing, use the slab & pad calculator.
Post holes, piers and Sonotubes
A round hole is a cylinder: π × radius² × depth. Keep the units straight: a 12 in diameter is a 0.5 ft radius, so a 4 ft deep deck footing holds 3.1416 × 0.5² × 4 = 3.14 ft³, which is 6 bags of 80 lb mix.
Steps
Solid steps are a stack of blocks. The bottom step is one rise tall, the next two rises, and so on, so volume = width × run × rise × (1 + 2 + … + n). Three 7 in steps with 11 in treads, 4 ft wide, need 12.83 ft³.
Footings
A continuous footing is a long rectangle: length × width × depth, with width and depth converted from inches. A 20 ft run, 16 in wide and 8 in deep, is 17.78 ft³ (33 × 80 lb bags with 10% extra). Footing size is set by your plans and local code.
Bags of concrete per cubic yard
| Concrete | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags | Weight |
|---|---|---|---|---|---|
| 0.5 yd³ | 13.5 | 23 | 30 | 45 | 1,958 lb |
| 1 yd³ | 27 | 45 | 60 | 90 | 3,915 lb |
| 2 yd³ | 54 | 90 | 120 | 180 | 7,830 lb |
| 3 yd³ | 81 | 135 | 180 | 270 | 11,745 lb |
| 5 yd³ | 135 | 225 | 300 | 450 | 19,575 lb |
No waste allowance included. Add 5–10% for uneven ground and spillage.
Bags or ready-mix?
Bagged mix makes sense for post holes, steps, small pads and repairs. Once a job passes about 1 yd³ (45 × 80 lb bags), compare a ready-mix quote: delivery and short-load fees add up on small orders, but on larger pours ready-mix saves a day of mixing and gives a more consistent slab. Ready-mix is ordered by the cubic yard, usually in ¼ yd³ steps. The slab calculator compares both with your local prices.
How to use this calculator
- Pick the shape. Slabs, footings and pads are rectangles; post holes and Sonotubes are cylinders; steps are a stack of rectangles.
- Measure in feet and inches. Length and width in feet, thickness or depth in inches for slabs, diameter in inches for holes.
- Work out cubic feet. Rectangle: L × W × (thickness ÷ 12). Cylinder: π × (diameter ÷ 24)² × depth.
- Convert to cubic yards or bags. Divide by 27 for cubic yards. Divide by the bag yield (0.60 ft³ for an 80 lb bag) and round up for bags.
- Add 5 to 10% extra. Uneven ground, form flex and spillage always use more than the exact volume.
Rectangle: L (ft) × W (ft) × T (in) ÷ 12 = ft³
Cylinder: π × (D (in) ÷ 24)² × H (ft) = ft³
Steps: W × run × rise × n(n+1)/2 (all in ft) = ft³
yd³ = ft³ ÷ 27 · bags = ft³ ÷ bag yield, rounded up · weight ≈ ft³ × 145 lb
Bag yields are taken from the manufacturer data sheet for standard concrete mix (40, 60 and 80 lb bags). Rapid-set and high-strength mixes can yield slightly differently, so check the back of your bag.
Questions
How many bags of concrete are in a yard?
45 bags of 80 lb mix, 60 of 60 lb or 90 of 40 lb. A cubic yard is 27 ft³; divide by the bag yield.
How many bags of concrete for a fence post?
A 10 in hole 3 ft deep holds 1.64 ft³ before the post goes in: 3 bags of 80 lb mix or 4 of 60 lb. The post itself takes up a little of that space, so this is a safe upper estimate.
Is it cheaper to buy bags or ready-mix?
For small jobs under about 1 yd³, bags are usually cheaper because ready-mix carries delivery and short-load fees. Above 2 to 3 yd³, ready-mix usually wins on price and labor. Compare with your local prices in the slab calculator.
How much does a yard of concrete weigh?
About 3,800 to 4,050 lb. Normal-weight concrete is 140 to 150 lb per cubic foot, and a cubic yard is 27 ft³.
What thickness should I use?
4 in for patios and walkways, 5 to 6 in for driveways and garage floors. Footings follow local code and frost depth.
Sources: bag yields from the QUIKRETE® Concrete Mix No. 1101 data sheet; concrete density 140–150 lb/ft³ from Iowa DOT Materials IM 340. Calculations last checked 2026-11.