Concrete calculator
Enter your slab, footing, wall, or column and get the cubic yards, the number of 40/60/80 lb bags, and ready-mix cost. Plus a gravel, sand, topsoil, and mulch calculator. Free, instant, no signup.
Short answer
To calculate concrete, multiply length × width × thickness in feet, then divide by 27 for cubic yards, and add ~10% waste. A 10×10 slab at 4 inches is about 1.2 cubic yards — roughly 45 80-lb bags per yard. Above one yard, ready-mix (about $120–$180 per yard) is usually cheaper than bags.
Cubic yard
27 ft³
= 1 yd³
80-lb bags/yd
~45
0.6 ft³ each
10×10 @4"
~1.2 yd³
+10% waste
Ready-mix
$120–180
per yard
Concrete bag yields & bags per cubic yard
| Bag size | Yield per bag | Bags per cubic yard | Typical price |
|---|---|---|---|
| 80 lb bag | 0.6 cu ft | 45 | $6 |
| 60 lb bag | 0.45 cu ft | 60 | $5 |
| 50 lb bag | 0.375 cu ft | 72 | $5 |
| 40 lb bag | 0.3 cu ft | 90 | $4 |
Manufacturer-standard yields printed on the bag. Delivered ready-mix runs $125–$165 per cubic yard (2026 national range) — usually cheaper than bags beyond about a half yard, before short-load fees.
Concrete by slab size
Concrete slab cost
Concrete guides
How we calculate this
Every number on this page comes from the same formulas the calculator runs — straight geometry plus the yields printed on the bag:
- Volume. length × width × thickness in feet gives cubic feet (a 10×10 slab at 4″ is 100 × 0.333 = 33.3 cu ft); ÷ 27 for cubic yards. Round shapes use πr²h; tubes subtract the inner void.
- Waste allowance. 10% is added for uneven subgrade, spillage, and over-excavation — pouring short mid-slab is the expensive failure.
- Bags. volume with waste ÷ the bag's yield (0.30 cu ft for 40 lb up to 0.60 cu ft for 80 lb), rounded up to whole bags.
- Ready-mix vs bags. the cost view compares delivered ready-mix per cubic yard against 80-lb bags and recommends whichever is cheaper at your volume.
Assumptions
- Uniform thickness on compacted, level subgrade — low spots eat concrete fast.
- Standard-strength pre-mix; high-early or fiber mixes yield the same volume at higher cost.
- Ready-mix trucks carry ~10 yards; orders under a full truck often pay a short-load fee.
Sources
Last reviewed: July 13, 2026
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Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. A 10×10 ft slab at 4 inches thick is about 1.23 cubic yards. The calculator does this for slabs, footings, walls, and round columns, and adds a 10% waste allowance.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. An 80-lb bag yields about 0.6 cubic feet, so it takes about 45 80-lb bags, 60 60-lb bags, or 90 40-lb bags to make one cubic yard. Above roughly one yard, ready-mix is usually cheaper than bags.
How much does a yard of concrete cost?
Ready-mix concrete typically runs about $120–$180 per cubic yard delivered, before short-load or delivery fees. Enter a price per yard in the calculator to estimate your total, including the waste allowance.
How much gravel or mulch do I need?
Multiply the area in square feet by the depth in feet, then divide by 27 for cubic yards. The Gravel & aggregate tab also converts yards to tons using each material’s density, since bulk gravel, sand, and stone are usually sold by the ton.