Gravel Calculator
Base stone under the slab — tons and yards.
Slabs, footings & piers
How much concrete do I need?
Choose slab or column, enter the dimensions, and the concrete calculator returns cubic yards plus the exact number of bags. A standard 10 × 10 ft patio slab at 4 inches thick is 1.23 cubic yards — about 61 bags of the 80 lb mix — so for anything close to a yard you should price ready-mix delivery.
For anything under about one cubic yard, bagged mix is usually the practical answer — but the bag count climbs fast, and a 10 × 10 ft slab at 4 inches takes roughly sixty 80 lb bags. The estimator covers slabs, footings, walls and round columns, accepts feet or meters, and shows cubic yards, cubic meters and bag counts side by side so you can weigh hand-mixing against a ready-mix delivery before you order.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
An 80 lb bag yields 0.60 cu ft, so one cubic yard takes 45 bags. A 60 lb bag (0.45 cu ft) takes 60 per yard, and a 40 lb bag (0.30 cu ft) takes 90.
How many bags of concrete for a 10×10 slab?
A 10 × 10 ft slab poured 4 inches thick holds 1.23 cu yd. Adding 5% overpour you'll need about 60 of the 80 lb bags — or consider a ready-mix delivery, which gets cheaper around one yard.
Is it cheaper to use bagged concrete or order ready-mix?
Bagged premix wins for jobs under ~1 cubic yard. Above that, ready-mix costs less per yard and saves hours of mixing — many plants enforce a minimum order around 1 yard anyway.
Should I order extra concrete?
Add 5–10% for spillage, an uneven subgrade and finishing waste. For ready-mix, many contractors round up to the next half-yard because a short pour is far worse than a little leftover.
How many bags of concrete for a fence post or deck footing?
A standard post hole about 10 inches wide and 30 inches deep takes roughly 2–3 bags of 80 lb mix (or the equivalent in fast-setting 50 lb concrete). Use the column mode in this calculator with the hole diameter and depth for an exact count across all your posts.
How do I calculate concrete for a round column or tube form?
Volume of a cylinder is π × radius² × height. Enter the form's inside diameter and the fill height in the calculator's column mode — it converts to cubic yards and cubic feet and tells you how many bags each tube needs.
Concrete Calculator
Imperial
Project type
ft
ft
inches
Waste / overpour
Enter your project size to see the result.
–
cubic yards
Bag yields (0.30 / 0.45 / 0.60 cu ft) are standard for Quikrete-style mixes.
How it works
How to use this concrete calculator
1Measure your form
For a slab: length × width in feet and thickness in inches. For posts: count the piers and measure the hole diameter and depth in inches.
- Slab thickness: 4 in for patios & sidewalks, 6 in for driveways.
- Sonotube piers are sized by inside diameter — 8, 10 or 12 in common.
- Footings under a wall: treat as a long, thin slab.
2Pick the bag size
Bags of premix yield different volumes — always the sizes above: 40 lb = 0.30 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft per bag.
- 80 lb bags mean fewer trips and less handling.
- 40 lb bags are easier to lift for small repairs.
- Read the bag: mix yields can vary slightly by brand.
3Bagged or ready-mix?
Bagged mix is convenient up to about 0.5–1 cubic yard (roughly 40–50 of the 80 lb bags). Above that, a ready-mix truck (1–10+ cubic yards) becomes cheaper and much less labor — concrete doesn't pause while you mix a hundred bags.
The math
Concrete quantity formula
Slab: cubic feet = length × width × (thickness ÷ 12) • Column: cubic feet = number × π × (diameter ÷ 2 ÷ 12)² × depth
Cubic yards = cubic feet ÷ 27 • Bags = (cubic feet × (1 + waste)) ÷ bag yield
Worked example — 10 × 10 ft slab at 4 in thick, 5% overpour, 80 lb bags:
Volume: 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cu yd; × 1.05 = 35 cu ft ÷ 0.6 = 59 → buy 60 bags. At 1.23 yards, price ready-mix too.
Volume: 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cu yd; × 1.05 = 35 cu ft ÷ 0.6 = 59 → buy 60 bags. At 1.23 yards, price ready-mix too.
Bag guide
How many concrete bags in a yard?
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 bags |
| 60 lb | 0.45 cu ft | 60 bags |
| 80 lb | 0.60 cu ft | 45 bags |
Handy memory hook: a 10 × 10 slab at 4 inches uses about 1.23 yards — that's 56–60 of the 80 lb bags or one short truckload of ready-mix.
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