How much concrete do I need?
Measure the area you will fill and how thick the concrete will be. QUIKRETE's own calculator works with slabs 4 or 6 inches thick, and its concrete mix is made for pours 2 inches thick or more. Use "How many" for several pads or post holes the same size.
Bag yields leave out waste and an uneven base, so the extra-for-waste box starts at 0 and it's up to you how much to add.
Source QUIKRETE · Concrete calculator (bags rounded up; yields leave out waste) ↗QUIKRETE · Concrete Mix (No. 1101) data sheet: yield, unit weight, curing ↗
How to calculate concrete
Here it is with your numbers:
- Change 4 in to feet: 0.3333 ft.
- Multiply length × width × thickness: 10 × 10 × 0.3333 = 33.3333 cu ft.
- Divide by 27 to get cubic yards: 33.3333 ÷ 27 = 1.2346 cu yd, rounded to 1.23.
- Divide by what one 80 lb bag makes (0.6 cu ft) and round up: 33.3333 ÷ 0.6 = 55.5556, so 56 bags.
It means buying 56 bags of 80 lb concrete mix (or 75 of 60 lb, or 112 of 40 lb), or ordering 1.23 cubic yards of ready-mix, with nothing added for waste.
Change every size to feet first (divide inches by 12), multiply, and divide cubic feet by 27 to get cubic yards. For a round hole, halve the distance across to get the radius.
Source Common Core State Standards · Grade 5, volume = length × width × height (5.MD.C.5b) ↗Common Core State Standards · Grade 8, volume of a cylinder (8.G.C.9) ↗NIST Handbook 44 (2026) · Appendix C, General Tables of Units of Measurement ↗
How many bags of concrete in a yard?
What one bag of QUIKRETE Concrete Mix makes once it's mixed with water. You can't buy part of a bag, so always round up.
| Bag | Makes | 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 |
Concrete for common slab sizes
All 4 inches thick, with nothing added for waste.
| Slab | Cubic yards | 80 lb bags |
|---|---|---|
| 4 × 4 ft | 0.20 | 9 |
| 10 × 10 ft | 1.23 | 56 |
| 10 × 12 ft | 1.48 | 67 |
| 12 × 12 ft | 1.78 | 80 |
| 12 × 20 ft | 2.96 | 134 |
| 20 × 20 ft | 4.94 | 223 |
| 24 × 24 ft | 7.11 | 320 |
Concrete for fence posts
QUIKRETE suggests a hole about 3 times as wide as the post and a third of the post's length deep. Choose "Round hole", enter the hole's size and how many posts. The answer fills the whole hole, so it is a little more than you need once the post is in.
FAQs
How many bags of concrete do I need?
Work out the volume in cubic feet, then divide by what one bag makes and round up. QUIKRETE says an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag 0.45 and a 40 lb bag 0.30. A 10 × 10 ft slab 4 in thick is 33.33 cubic feet, so 56 bags of 80 lb.
Source QUIKRETE · Concrete Mix (No. 1101) data sheet: yield, unit weight, curing ↗
How many bags of concrete are in a yard?
A cubic yard is 27 cubic feet, so it takes 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb, using QUIKRETE's yields.
How do you estimate concrete in yards?
Measure the length, width and thickness in feet (4 inches is 0.33 ft), multiply them and divide by 27. A 12 × 12 ft slab 4 in thick is 48 cubic feet, or 1.78 cubic yards.
Source NIST Handbook 44 (2026) · Appendix C, General Tables of Units of Measurement ↗
How long does concrete take to cure?
For QUIKRETE Concrete Mix, keep it curing (under plastic sheeting, wet burlap or a curing compound) for 5 days in warm weather (70 °F or more) or 7 days at 50 – 70 °F. It reaches about 2,500 psi in 7 days and its full 4,000 psi in 28 days. Wait 24 hours before putting any strain on a post.
Source QUIKRETE · Concrete Mix (No. 1101) data sheet: yield, unit weight, curing ↗
How much does a yard of concrete weigh?
Mixed QUIKRETE concrete weighs about 140 lb per cubic foot, so a cubic yard is about 3,780 lb (140 × 27).
Sources
- QUIKRETE: Concrete Mix (No. 1101) data sheet: yield, unit weight, curing
- QUIKRETE: Concrete calculator (bags rounded up; yields leave out waste)
- NIST Handbook 44 (2026): Appendix C, General Tables of Units of Measurement
- Common Core State Standards: Grade 5, volume = length × width × height (5.MD.C.5b)
- Common Core State Standards: Grade 8, volume of a cylinder (8.G.C.9)