How the result is calculated
- 01
Project cubic feet = length x width x (thickness / 12) - 02
Bag count = project cubic feet / selected bag yield, rounded up - 03
Yields used: 40 lb 0.30, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 cu ft
CONSTRUCTION
Concrete Bag Calculator
Calculate how many 40, 50, 60 or 80 lb bags of concrete a rectangular project needs. Enter your measurements below and the estimate updates instantly.
Formula, validation and worked example reviewed November 12, 2026. See our calculation methodology.
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CONSTRUCTION
US UNITSConcrete Bags
YOUR ESTIMATE
56 bags80 lb concrete mix1.23 cu yd · 33.3 cu ft · Before extra waste
Approximate yield 0.6 cu ft/bag for QUIKRETE Concrete Mix No. 1101; verify your product. Whole bags rounded up from unrounded volume.
✓ Instant calculation on your device · Verify code-sensitive work locally
Bag count by product size
Compare bag sizes using published approximate yields.
This concrete bag calculator finds rectangular project volume, divides it by the selected 40, 50, 60 or 80 lb bag yield, and rounds up to a whole bag count.
How do you calculate bags of concrete by bag weight?
Pre-blended concrete bags are sold by dry weight (40 lb, 50 lb, 60 lb, and 80 lb), and each size yields a specific volume of mixed concrete in cubic feet. Divide your slab's cubic footage by the bag yield and round up to the next whole bag:
Nbags=⌈ L · W · (Thickness / 12)Yieldbag ⌉
(Whole bag count)where:
- Yield_bag – Published approximate yield per bag (0.60 ft³ for 80 lb, 0.45 ft³ for 60 lb, 0.375 ft³ for 50 lb, and 0.30 ft³ for 40 lb); and
- N_bags – Whole bags required before jobsite spillage or uneven subgrade allowance. For ready-mix truck yardage, use the concrete calculator, or for fence posts use the post hole concrete calculator.
10 ft by 10 ft slab, 4 in thick
- Four inches becomes exactly 4/12 ft (approximately 0.3333 ft).
- 10 x 10 x (4/12) = 33.333… cu ft.
- With 80 lb bags, 33.3 / 0.60 = 55.6.
- Round up to 56 whole bags.
Quick reference chart
Concrete Bag Yield & Count per Cubic Yard by Bag Weight
Based on published QUIKRETE Concrete Mix approximate yields.
| Bag Weight | Yield per Bag (cu ft) | Bags per 10 sq ft (4" Slab) | Bags per 1 Cubic Yard (27 cf) |
|---|---|---|---|
| 40 lb bag | 0.30 cu ft | 12 bags | 90 bags |
| 50 lb bag | 0.375 cu ft | 9 bags | 72 bags |
| 60 lb bag | 0.45 cu ft | 8 bags | 60 bags |
| 80 lb bag | 0.60 cu ft | 6 bags | 45 bags |
Common mistakes to avoid
- Selecting a bag weight that does not match the actual product being purchased.
- Assuming published yield includes uneven subgrade, over-excavation, spillage, or waste.
- Rounding the volume down or buying the fractional bag result.
- Adding extra water to stretch the mix, which can reduce concrete strength.
- Using a rectangular slab calculation for posts, tubes, steps, or irregular shapes.
Questions answered
Calculator FAQ
How many bags of concrete do I need?+
Enter the project length, width and thickness, then select the actual bag weight. The calculator divides volume by the approximate product yield and rounds up to a whole bag.
How many 80 lb bags of concrete make a cubic yard?+
It takes exactly 45 eighty-pound (80 lb) bags of concrete mix to make one cubic yard (27 cubic feet), based on the standard yield of 0.60 cu ft per bag.
How many 60 lb bags of concrete make a cubic yard?+
It takes 60 sixty-pound (60 lb) bags of concrete mix to make one cubic yard (27 cubic feet), based on the standard yield of 0.45 cu ft per bag.
How many bags of concrete do I need for a 10x10 slab at 4 inches?+
A 10x10 ft slab at 4 inches depth has a volume of 33.3 cu ft. You will need 56 bags of 80 lb mix (33.3 ÷ 0.60) or 74 bags of 60 lb mix before extra spillage allowance.
Why does the calculator always round bag count up?+
The calculated fraction still represents concrete the project needs, so the estimate moves to the next whole bag.
Which approximate yields does the tool use?+
It uses 0.30 cu ft for 40 lb, 0.375 for 50 lb, 0.45 for 60 lb, and 0.60 for 80 lb bags.
Are those yields exact for every concrete mix?+
No. They match the referenced QUIKRETE Concrete Mix data but yields are approximate and can differ by product. Check the bag's data.
Does the estimate include waste?+
No. It is exact rectangular volume divided by approximate bag yield. Add a suitable allowance for actual site conditions.
Can I compare different bag weights?+
Yes. Keep the same project dimensions and switch the bag-size field to compare the estimated whole-bag counts.
Practical selection guide
Concrete bag yield, coverage and ready-mix yardage comparison.
Determine whether bagged concrete mix or ready-mix delivery is best for your slab, footing, or post hole based on standard package yields.
80 lb bags (0.60 cu ft yield)
An 80 lb bag yields approximately 0.60 cubic feet. It takes 45 bags to equal 1 cubic yard (27 cu ft). Recommended for small-to-medium slabs, walkways, and posts.
60 lb bags (0.45 cu ft yield)
A 60 lb bag yields 0.45 cubic feet, requiring 60 bags per cubic yard. Easier to lift and mix by hand in a wheelbarrow or portable mixer.
50 lb & 40 lb bags
50 lb bags yield 0.375 cu ft (72 bags/yd³), and 40 lb bags yield 0.30 cu ft (90 bags/yd³). Popular for light fence posts and mailbox anchors.
When to switch to ready-mix delivery
For jobs requiring more than 1.5 to 2 cubic yards (approx. 70 to 90 eighty-pound bags), ready-mix concrete truck delivery is faster, structurally more uniform, and usually cheaper.
Method references: QUIKRETE Concrete Mix yield data · QUIKRETE concrete calculator notes