Concrete Calculator
Pick the shape you're pouring, enter its size in feet and inches, and press Calculate to get the volume in cubic yards plus how many premix bags to buy. A 10% allowance for waste and spillage is included by default.
Concrete Calculator
Concrete volume formulas
Convert every measurement to feet first (inches ÷ 12), then:
Slab: length × width × thickness = cubic feet Round column: 3.1416 × (diameter ÷ 2)² × height = cubic feet Cubic yards = cubic feet ÷ 27Example: a 12 × 16 ft patio poured 4 inches thick is 12 × 16 × 0.333 = 64 cubic feet = 2.37 cubic yards. With 10% extra, order about 2.6 yards.
Bags per cubic yard
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Yields are typical for standard concrete mix. Check the bag, because high-strength and fast-setting mixes can differ slightly.
Slab thickness guide
- 4 in: patios, sidewalks, shed and garage floors for cars.
- 5–6 in: driveways that carry heavier vehicles, and hot tub pads.
- 6 in or more: RV pads, and slabs for heavy equipment. These usually need rebar and a compacted gravel base.
Post holes
For fence posts, use the round column tab with the hole diameter and depth, not the post size. A 10-inch hole 3 feet deep holds about 1.6 cubic feet, or four 60 lb bags. The post itself takes up some of that space: a 4 × 4 post fills about 0.25 cubic feet over 3 feet, which brings it down to about three bags.
Frequently asked questions
How many cubic yards of concrete do I need?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Add 5–10% for waste.
How many 80 lb bags of concrete make a yard?
An 80 lb bag yields about 0.60 cubic feet, so one cubic yard (27 cubic feet) takes 45 bags. With 60 lb bags (0.45 cu ft) you need 60, and with 40 lb bags (0.30 cu ft) you need 90.
How thick should a concrete slab be?
4 inches is standard for patios, walkways and shed floors. Driveways for cars are usually 4–5 inches, and 6 inches is better for heavy trucks, RVs or poor soil. Check your local building code.
Should I use bags or order ready-mix?
Bags make sense for small jobs up to about 1 cubic yard, such as post holes and small pads. Beyond that, ready-mix delivered by truck is usually cheaper and gives a more consistent pour. Many suppliers have a minimum order or charge a short-load fee under a few yards.
How much extra concrete should I order?
Order 5–10% more than the calculated volume. Uneven ground, forms that bow outward and spillage all use extra. Running short in the middle of a pour leaves a weak cold joint.
See also
- Square Footage Calculator: Area of rooms and odd-shaped spaces, plus a cost estimate.
- Ratio Calculator: Simplify ratios, convert to 1:n, and split a total in a ratio.
- Percentage Calculator: Percent of a number, percent change, and increase or decrease by a percent.
Last updated: November 4, 2026