How Much Concrete Do You Need? A Simple Volume Calculation Guide
Ordering concrete sounds easy—until you realize too late that you ordered too little or paid for far too much. What’s more, ready-mix concrete can’t be returned, and running short mid-pour can ruin a slab.
The good news: calculating exactly how much concrete you need takes one simple formula and about two minutes. This guide shows you how to do it for slabs, footings, and round columns, with worked examples you can copy.
In a hurry? Use our free Concrete Volume Calculator—enter your dimensions and get volume in cubic yards and meters, plus how many 60-lb or 80-lb bags to buy.
Why Getting the Volume Right Matters
- Order too little: You’ll pay a “short load” fee for a second delivery, and the cold joint between pours weakens the slab.
- Order too much: You pay for concrete that gets thrown away.
- The sweet spot: Calculate your exact volume, then add 10% for spillage, uneven ground, and over-excavation.
The Basic Concrete Volume Formula
Every concrete calculation—no matter the shape—is just
Volume = Length × Width × Depth
However, there is one rule: use the same units for all three measurements. The easiest approach is to measure everything in feet, then convert to cubic yards (the way concrete is sold) by dividing by 27, because 1 cubic yard = 27 cubic feet.
Example 1: How Much Concrete for a 10×10 Slab?
A typical patio or shed slab is 10 ft × 10 ft × 4 inches deep.
1—Convert depth to feet: 4 inches ÷ 12 = 0.33 ft
2—Multiply: 10 × 10 × 0.33 = 33.3 cubic feet
3—Convert to cubic yards: 33.3 ÷ 27 = 1.23 cubic yards
4—Add 10% waste: 1.23 × 1.1 = 1.36 cubic yards → order 1.5 cubic yards (suppliers usually deliver in half-yard steps).
In fact, that same slab would need 56 bags of 80-lb concrete… (or 75 bags of 60-lb) if you mix it yourself—which is why bags make sense for small jobs only.
Example 2: Concrete for a Footing
For footings, the process is the same. Take a footing 20 ft long, 1 ft wide, and 6 inches deep:
- 20 × 1 × 0.5 = 10 cubic feet
- 10 ÷ 27 = 0.37 cubic yards
On the other hand, round shapes use a slightly different formula:
Example 3: Concrete for a Round Column or Post Hole
For round shapes, use the cylinder formula:
Volume = π × radius² × height (π ≈ 3.14)
A deck post hole 12 inches in diameter (radius = 0.5 ft) and 4 ft deep:
- 3.14 × 0.5 × 0.5 × 4 = 3.14 cubic feet
That’s only about 0.12 cubic yards—a bag or two of post-mix does the job.
Quick Reference Table: Square Footage to Cubic Yards (4-inch Slab)
Table
| Slab size | Volume | Order (with 10% waste) |
|---|---|---|
| 50 sq ft | 0.62 yd³ | 0.75 yd³ |
| 100 sq ft | 1.23 yd³ | 1.5 yd³ |
| 200 sq ft | 2.47 yd³ | 2.75 yd³ |
| 400 sq ft | 4.94 yd³ | 5.5 yd³ |
| 500 sq ft | 6.17 yd³ | 7 yd³ |
Rule of thumb: at 4 inches thick, 1 cubic yard covers about 81 square feet.
How Many Bags of Concrete Do I Need?
For small projects (under about 1 cubic yard), bagged concrete is often easier than ordering a truck:
- One 80-lb bag ≈ 0.6 cubic feet → 45 bags per cubic yard
- One 60-lb bag ≈ 0.45 cubic feet → 60 bags per cubic yard
Table
| Project size | 80 lb bags | 60 lb bags |
|---|---|---|
| 0.5 yd³ | 23 | 30 |
| 1 yd³ | 45 | 60 |
| 1.5 yd³ | 68 | 90 |
However, for anything over 1–2 cubic yards, ready-mix delivery is usually cheaper and far less work than mixing 90+ bags by hand.
Don’t Want to Do the Math?
Our free Concrete Volume Calculator handles all of this instantly:
- Enter your length, width, and depth
- Get volume in cubic yards and cubic meters
- See exactly how many 60-lb and 80-lb bags you need
- Use the built-in 10–15% waste allowance tip before ordering
It works for slabs, footings, and columns—on any device.
Frequently Asked Questions
How much concrete do I need for a 10×10 slab? A 10×10 slab at 4 inches deep needs 1.23 cubic yards—order 1.5 yards after adding 10% for waste.
How many square feet does 1 yard of concrete cover? At 4 inches thick, In general, 1 cubic yard covers about 81 square feet. At 6 inches thick, it covers 54 square feet.
How many 80-lb bags of concrete make a yard? 45 bags of 80-lb concrete equal roughly 1 cubic yard.
Should I order extra concrete? Yes—add 10% to your calculated volume. This covers spillage, uneven ground, and slight over-digging. That’s because ordering a little extra is always cheaper than paying for a second short-load delivery. Pouring a slab? You’ll also need rebar—check our Rebar Weight Calculator.
What size slab can I pour with bagged concrete? Hand-mixing gets exhausting fast. Most DIYers find bags practical up to about 0.5–1 cubic yard (a 10×10 slab at 4 inches). Beyond that, order ready-mix.