Concrete Calculator
Work out how much concrete you need for a slab, footing, or post holes. Enter your dimensions and get cubic yards, cubic feet, and the number of 40, 60, and 80 lb bags — with a waste allowance and an optional ready-mix cost estimate.
- Volume (cubic feet)
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- Volume (cubic yards)
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- Volume (cubic meters)
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- 40 lb bags
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- 60 lb bags
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- 80 lb bags
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- Estimated ready-mix cost
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Bag yields: 40 lb ≈ 0.30 ft³ · 60 lb ≈ 0.45 ft³ · 80 lb ≈ 0.60 ft³
Common questions
How much concrete do I need for a slab?
Multiply length × width × thickness with every dimension in feet, then divide by 27 for cubic yards. A 10 ft × 10 ft pad at 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards — about 56 eighty-pound bags, or 1.4 cubic yards of ready-mix once you add 10% waste.
How thick should a concrete pad be?
Four inches is the standard for patios, sheds, and walkways. Go to 5–6 inches where vehicles park or heavy equipment sits, and follow your local code for anything structural. Thickness drives cost almost linearly: going from 4 to 6 inches adds 50% to the volume.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet, so it takes 45 eighty-pound bags (0.60 ft³ each), 60 sixty-pound bags (0.45 ft³), or 90 forty-pound bags (0.30 ft³). Those are the manufacturer-published yields used by the calculator.
Should I use bags or order ready-mix?
Past about 2 cubic yards — roughly 90 eighty-pound bags — ready-mix delivery is usually cheaper and far less work than mixing by hand, and it gives you one continuous pour instead of a cold joint every batch. The calculator flags the crossover point for your volume.
How do I measure for a concrete footing or post holes?
Footings use the same length × width × depth math as a slab. Post holes are cylinders: π × (diameter ÷ 2)² × depth, times the number of holes. A 10-inch hole 3 feet deep is about 1.6 cubic feet, so a 12-post fence needs roughly 20 cubic feet — about 33 sixty-pound bags.
How much waste should I add to a concrete order?
Ten percent is a sensible default. Concrete spills, forms bow, and subgrades are never perfectly level — and running short mid-pour means a cold joint you cannot undo. Overordering by a wheelbarrow costs far less than a second delivery.
How this is calculated
- Bag yields are the standard values published by major pre-mix brands (Quikrete, Sakrete); actual yield varies slightly by product.
- Shapes assume uniform thickness/depth. Sloped or stepped pours need to be broken into sections.
- The default 10% waste allowance covers spillage, over-excavation, and uneven subgrade.
- Above roughly 2 cubic yards, ready-mix delivery is usually cheaper and far less work than bags — the tool flags this.
- Manufacturer bag-yield tables (Quikrete, Sakrete) — Published yields: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³ per bag.
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Related reading
Short answers with the math shown- Best Concrete Estimator Apps for Construction Projects (2026)Which concrete estimator to use depends on the job: a free calculator for a pad, a phone app for site quotes, takeoff software for plan-based bids. What each does and when to pick it.
- How Much Concrete Do I Need for a Slab or Pad?Length × width × thickness ÷ 27 gives cubic yards. Slab sizes and bag counts, how thick a pad should be, and the point where ready-mix beats mixing bags.
- How Much Gravel Do I Need?Measure the area, pick a depth, divide by 27. A worked example, a coverage table by depth, and how to convert cubic yards of gravel to tons before you call the supplier.