Concrete bag calculator for slabs, pads and footings
Enter your slab or footing size and the bag size to see how many bags of concrete mix to buy.
Enter your measurements
More options: Extra · Price
- Bags before the extra
- 56 bags · 80 lb (≈ 0.60 cu ft)
- Concrete incl. extra
- 1.36 cu yd (36.7 cu ft)
- Concrete before the extra
- 1.23 cu yd (33.3 cu ft)
- Bags of this size per cubic yard
- 45 bags
- Weight of the bags
- 4,960 lb
10 ft × 10 ft × 4 in = 33.3 cu ft = 1.23 cu yd
33.3 cu ft ÷ 0.6 cu ft per bag = 56 bags before extra
+ 10% = 1.36 cu yd ÷ 0.6 cu ft = 62 bags
- Cut control joints about 1 in deep every 6–8 ft (Quikrete’s guidance) so the slab cracks along straight lines instead of at random.
- Past about a cubic yard, price ready-mix delivery too (ask about small-load minimums), or at least rent a drum mixer: mixing that many bags by hand is a long, heavy day.
Quick answers for common sizes
| Example | Answer |
|---|---|
| How many bags of concrete for a 10×10 slab | 62 bags |
| How many bags of concrete in a cubic yard | 45 bags |
Bags for your slab by thickness (length, width and bag size as above)
| Thickness | Bags of concrete (incl. extra) | Concrete incl. extra | Weight of the bags |
|---|---|---|---|
| 3.5 in | 54 bags | 1.19 cu yd | 4,320 lb |
| 4 in | 62 bags | 1.36 cu yd | 4,960 lb |
| 5 in | 77 bags | 1.7 cu yd | 6,160 lb |
| 6 in | 92 bags | 2.04 cu yd | 7,360 lb |
Bagged concrete mix is sold by weight but used by volume. An 80 lb bag makes about 0.6 cubic feet of concrete, so a cubic yard (27 cubic feet, a 3 ft cube) takes about 45 bags. Check the yield printed on your bag.
How the concrete bag calculation works
Work out the volume of the pour, add a little extra, then divide by the yield of one bag. For a slab that is length × width × thickness; for a round footing or tube form it is 3.1416 × radius² × depth. US bags are labelled by weight, but what matters is the volume of concrete each one makes, which is printed on the bag as its yield.
slab cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12 round footing cubic feet = 3.1416 × (diameter (in) ÷ 24)² × depth (in) ÷ 12 bags = cubic feet × (1 + extra) ÷ bag yield (cu ft) 1 cubic yard = 27 cubic feet
| Bag | Yield (Quikrete Concrete Mix) | Bags per cubic yard |
|---|---|---|
| 40 lb | ≈ 0.30 cu ft (8.5 L) | 90 |
| 50 lb | ≈ 0.375 cu ft (10.6 L) | 72 |
| 60 lb | ≈ 0.45 cu ft (12.7 L) | 60 |
| 80 lb | ≈ 0.60 cu ft (17 L) | 45 |
Worked examples
- How many bags of concrete for a 10×10 slab — a 4 in patio or shed floor, and why 2×4 forms change the answer.
- How many bags of concrete in a cubic yard — 40, 50, 60 and 80 lb bags compared, and where ready-mix starts to make sense.
- Setting posts rather than pouring a slab? The fence post concrete calculator subtracts the post from the hole — see how many bags of concrete per 4×4 fence post and how many bags of concrete for a 6×6 post.
Slabs and pads
Quikrete’s data sheet says its standard mix is for pours 2 in thick or more; 4 in is the usual thickness for a patio, walkway or shed floor, and more for anything a vehicle drives on. Most slabs go on a few inches of compacted gravel so water drains away underneath — the gravel calculator sizes that layer, and how much gravel for a 10×12 shed base shows the pad-overhang rule. Cut control joints about 1 in deep every 6–8 ft so shrinkage cracks follow your lines.
Round footings and tube forms
Choose the round option for cardboard tube forms, deck and porch footings, and post holes without a post in them. Footings that hold up a structure are not a guess: the size and depth come from your plans or the building code, and in cold climates the bottom must be below the frost line. The calculator only counts the bags.
Water and mixing
Quikrete suggests starting with about 6 pints (2.8 L) of water per 80 lb bag and working up to no more than about 9 pints — too much water weakens the concrete. Allow roughly 1 cu ft of mixer drum per 80 lb bag, so a small rental mixer does two or three bags per batch.
What you’ll need
- Mortar / concrete mixing tubBudget price
A wide, shallow tub for mixing one or two bags of concrete at a time with a hoe — much easier than mixing in a wheelbarrow.
See options on Amazon (opens in a new tab) - Mortar / mixing hoeBudget price
A hoe with holes in the blade pulls water through dry mix quickly, so bagged concrete mixes evenly without clumps.
See options on Amazon (opens in a new tab) - Magnesium floatBudget price
Smooths and closes the surface of a small slab or pad after screeding. A hand float is enough for pads up to a few feet across.
See options on Amazon (opens in a new tab) - Wheelbarrow (6 cu ft class)Mid-range price
Most home wheelbarrows hold 4–6 cubic feet — so a cubic yard of mulch is five or six trips. Two wheels are steadier on soft ground.
See options on Amazon (opens in a new tab) - Hand tamper (8 × 8 in or 10 × 10 in)Budget price
Compacts gravel in small areas and along edges where a rented plate compactor won’t reach. For a whole patio base, rent the plate compactor.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much area does an 80 lb bag of concrete cover?
About 1.8 sq ft at 4 in thick, since an 80 lb bag makes roughly 0.6 cubic feet. At 3½ in (a slab formed with 2×4s) it is about 2 sq ft; at 2 in, about 3.6 sq ft.
How many bags of concrete for a 12 inch tube form 4 feet deep?
A 12 in tube 48 in deep holds about 3.14 cubic feet: roughly 5.2 bags of 80 lb, or 6 once you add 10% for spillage. Choose the round option to run your own sizes.
Is bagged concrete or ready-mix better for a small slab?
Bags suit jobs up to roughly a cubic yard (about 45 bags of 80 lb), especially where a truck can’t reach. Beyond that, ready-mix delivery is usually less work and more consistent; ask local suppliers about minimum loads and short-load fees before deciding.
How much water does an 80 lb bag of concrete need?
Quikrete’s data sheet starts at about 6 pints (2.8 L) per 80 lb bag and says the final water should be about 6–9 pints. Add it gradually; soupy concrete is weaker.
Do I need gravel under a concrete slab?
Most patio and shed slabs go on a few inches of compacted gravel so water drains away and the ground underneath stays firm. Follow your local practice or plans; on well-drained, undisturbed soil some small pads skip it.
Sources
- Quikrete Concrete Mix (No. 1101) data sheet (opens in a new tab) — Yields: 40 lb ≈ 0.30, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft; 2 in minimum; control joints every 6–8 ft; water per bag
- Quikrete Fast-Setting Concrete Mix (No. 1004) data sheet (opens in a new tab) — Metric yields: 25 kg ≈ 11.6 L, 30 kg ≈ 14 L