Concrete Calculator
Enter your dimensions to instantly calculate how much concrete you need — cubic yards for ready-mix orders or bags for smaller pours.
Bag size
Enter your dimensions above to see results
How to Use This Calculator
Select your pour shape at the top — Slab for flat pours like patios and driveways, Footing for rectangular footings and walls, or Column for round posts and pillars using tube forms.
Enter your dimensions in whichever unit is most natural — feet, inches, meters, or centimeters. You can mix units between fields (for example, length in feet and thickness in inches). The calculator converts everything automatically.
Use the Quantity field if you're pouring multiple identical shapes (for example, 4 deck footings of the same size). Set the Waste % to add a safety buffer — 10% is standard for most residential pours. Choose your bag size to see the number of bags you'll need, or select "Cu yds" if ordering ready-mix by the cubic yard.
Results update instantly as you type. There's no calculate button. If any required dimension is missing or zero, the result area will show the prompt state until all values are entered.
Formula
This calculator uses standard geometric formulas converted to cubic yards, the unit concrete suppliers and most DIY bags reference.
Slab / Footing Volume (cu ft) = Length × Width × Depth (all in feet) Round column Volume (cu ft) = π × (Diameter ÷ 2)² × Height Convert to cubic yards Cu yds = Volume (cu ft) ÷ 27 Apply waste Final cu yds = Cu yds × (1 + Waste% ÷ 100) Bag count Bags = ⌈ Final cu ft ÷ yield per bag ⌉
One cubic yard equals exactly 27 cubic feet — this is why concrete volumes are expressed in cubic yards for larger pours. A standard 80 lb bag yields approximately 0.60 cubic feet of finished concrete; a 60 lb bag yields 0.45 cu ft; a 40 lb bag yields 0.30 cu ft. These are industry-standard values used by major manufacturers including Quikrete and Sakrete.
Bag counts always round up (ceiling function) because you can't purchase a fraction of a bag. Concrete weight is estimated at 150 lb per cubic foot — the standard for normal-weight concrete. Lightweight or heavyweight mixes will vary.
When ordering ready-mix, your supplier will quote by the cubic yard. Share your calculated volume with them and they can advise on mix design (PSI rating, slump, aggregate size) for your application.
Common Project Sizes
Base quantities without waste factor. Add 10% for a typical residential pour.
| Slab Size (4" thick) | Cubic Yards | Cubic Feet | 80 lb Bags |
|---|---|---|---|
| 4 × 4 ft | 0.20 | 5.3 | 9 |
| 4 × 8 ft | 0.40 | 10.7 | 18 |
| 6 × 6 ft | 0.44 | 12.0 | 20 |
| 8 × 8 ft | 0.79 | 21.3 | 36 |
| 10 × 10 ft | 1.23 | 33.3 | 56 |
| 12 × 12 ft | 1.78 | 48.0 | 80 |
| 16 × 16 ft | 3.16 | 85.3 | 143 |
| 20 × 20 ft | 4.94 | 133.3 | 223 |
Frequently Asked Questions
How much extra concrete should I order?
Order 10% more than your calculated volume for most pours. Increase to 15% for larger slabs or if your forms are irregular. The 10% default in this calculator accounts for typical spillage, over-pour at edges, and uneven subgrades. Running short means a cold joint (a visible seam where old and new concrete meet) — always worse than having a small amount left over.
How many 80 lb bags of concrete are in a cubic yard?
You need approximately 45 bags of 80 lb concrete mix per cubic yard. An 80 lb bag yields about 0.60 cubic feet of finished concrete, and one cubic yard equals 27 cubic feet: 27 ÷ 0.60 = 45 bags exactly. Use the bag calculator above for your specific pour volume with waste factored in.
What's the standard thickness for a concrete slab?
A residential patio or walkway is typically 4 inches thick. A driveway should be at least 4 inches — ideally 6 inches for heavy vehicles. Interior floors are commonly 4 inches. Structural slabs and commercial applications often require 6 inches or more. Always check local building codes for your project type.
Should I use bags or order ready-mix concrete?
For pours under about 1 cubic yard (a 10×10 slab at 4"), bags from a hardware store are practical. For larger pours, a ready-mix truck is far more economical and saves hours of mixing. The breakeven is typically around 0.5–1 cubic yard — at that point, the cost and labor of bagged concrete usually exceeds the truck delivery minimum.
How much does a cubic yard of concrete weigh?
One cubic yard of standard concrete weighs approximately 4,000–4,050 lbs (just over 2 tons). The weight varies slightly by mix design, aggregate type, and water content. This calculator uses 150 lb per cubic foot (4,050 lb/yd³) — the standard figure for normal-weight concrete.
Can I use this calculator for round deck posts or fence posts?
Yes — select the "Column" mode and enter the diameter of your tube form (commonly 8, 10, or 12 inches) and the total pour height including underground depth. For a typical fence post in a 10-inch tube form 2 feet deep, expect roughly 1–2 bags of 60 lb mix per post.
What does the waste percentage account for?
The waste factor covers spillage during pouring, over-pour at form edges, uneven subgrade that adds volume, and any concrete left in the mixer or chute. For a clean slab on level ground, 5–10% is typical. For irregular shapes, rough subgrades, or first-time pourers, use 10–15%.
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