A 10 × 10 ft slab 4 inches thick is 33.3 cu ft, or 1.23 cubic yards. With 10 percent extra, that's 1.36 cubic yards, or 62 bags of 80 lb premix.
To find how much concrete you need, use the concrete calculator above or multiply length × width × thickness to get cubic feet, divide by 27 for cubic yards, and add about 10 percent for spillage and an uneven base.
Rule of thumb: one cubic yard poured 4 inches thick covers about 81 sq ft, and it takes 45 bags of 80 lb mix to make that yard. Enter your slab above to get cubic yards plus the count of 60 lb and 80 lb bags.
How to measure for a slab, pad, or walkway
- Measure length and width in feet. Measure the inside of the forms, since that's the space the concrete fills. Convert inches to decimal feet by dividing by 12, so 10 ft 6 in is 10.5 ft.
- Choose the thickness in inches. Most patios and walkways are 4 inches. Driveways for heavy vehicles and small slabs that carry weight go thicker. See the thickness table below. For a paver patio instead, see the paver calculator. For an asphalt driveway, see the asphalt calculator.
- Check the base for low spots. Stretch a string across the forms and measure down to the compacted base in several places. If the depth varies, use the average, or raise the extra percentage.
- Split odd shapes into rectangles. An L-shaped patio is two rectangles, so run each one through the calculator and add the results. For a round pad, the area is 3.1416 × radius × radius.
- Set the extra percentage. The default 10 percent matches Sakrete's advice to add 10 percent to every estimate. Quikrete lists its bag yields as approximate, so a little extra covers spills and a base that isn't perfectly flat.
The concrete formula
The calculator uses:
- Volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12 × (1 + extra % ÷ 100)
- Cubic yards = cubic feet ÷ 27
- 80 lb bags = cubic feet ÷ 0.60, rounded up
- 60 lb bags = cubic feet ÷ 0.45, rounded up
Dividing the thickness by 12 converts inches to feet. The bag yields come straight from the data sheets: Quikrete lists its Concrete Mix at about 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag, and Sakrete's High-Strength Concrete Mix lists the same figures.
Worked example (the calculator's default values)
A 10 × 10 ft slab, 4 inches thick, with 10 percent extra:
- Area: 10 × 10 = 100 sq ft
- Volume: 100 × 4 ÷ 12 = 33.3 cu ft
- With 10 percent extra: 33.3 × 1.10 = 36.7 cu ft
- Cubic yards: 36.7 ÷ 27 = 1.36
- 80 lb bags: 36.7 ÷ 0.60 = 61.1, rounded up to 62
- 60 lb bags: 36.7 ÷ 0.45 = 81.5, rounded up to 82
The calculator shows: "You need 1.36 cubic yards (36.7 cu ft). That is 62 bags of 80 lb or 82 bags of 60 lb premix, including 10% extra."
Bag yields and bags per cubic yard
| Bag size | Yield per bag | Bags per cubic yard | Bags per 100 sq ft at 4 in (no extra) |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 112 |
| 50 lb | 0.375 cu ft | 72 | 89 |
| 60 lb | 0.45 cu ft | 60 | 75 |
| 80 lb | 0.60 cu ft | 45 | 56 |
| 90 lb | 0.675 cu ft | 40 | 50 |
Yields are from the Quikrete Concrete Mix data sheet. Sakrete lists its 90 lb bag a little lower, at 0.66 cu ft, so always check the yield printed on the bag you buy. Bags per cubic yard is simply 27 ÷ the yield.
Bags of concrete for common slab sizes
Straight from the calculator with its default 10 percent extra. The 40 lb column uses the same math with Quikrete's 0.30 cu ft yield for that bag.
| Slab and thickness | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 4 × 4 ft, 4 in | 0.22 | 10 | 14 | 20 |
| 3 × 10 ft, 4 in | 0.41 | 19 | 25 | 37 |
| 8 × 8 ft, 4 in | 0.87 | 40 | 53 | 79 |
| 10 × 10 ft, 4 in | 1.36 | 62 | 82 | 123 |
| 10 × 12 ft, 4 in | 1.63 | 74 | 98 | 147 |
| 12 × 12 ft, 4 in | 1.96 | 88 | 118 | 176 |
| 10 × 20 ft, 4 in | 2.72 | 123 | 163 | 245 |
| 4 × 4 ft, 6 in | 0.33 | 15 | 20 | 30 |
| 10 × 10 ft, 6 in | 2.04 | 92 | 123 | 184 |
Bags of concrete for Sonotubes, round columns, and post holes
1 bag of 80 lb mix per foot of 10 in Sonotube, according to Sonoco's chart for its Builder's Tube forms, which figures 0.55 cu ft of concrete per foot of height; an 8 in tube takes about 3/5 of a bag per foot and a 12 in tube about 1 1/3. For any round form or hole, the volume in cubic feet is 0.7854 × diameter × diameter (in inches) ÷ 144 × the height in feet:
| Diameter | Cu ft per foot of height | 80 lb bags per foot | 80 lb bags, one 4 ft tube | 60 lb bags, one 4 ft tube | 4 ft tubes per cubic yard |
|---|---|---|---|---|---|
| 6 in | 0.20 | 0.33 | 2 | 2 | 34.4 |
| 8 in | 0.35 | 0.58 | 3 | 4 | 19.3 |
| 10 in | 0.55 | 0.91 | 4 | 5 | 12.4 |
| 12 in | 0.79 | 1.31 | 6 | 7 | 8.6 |
| 14 in | 1.07 | 1.78 | 8 | 10 | 6.3 |
| 16 in | 1.40 | 2.33 | 10 | 13 | 4.8 |
| 18 in | 1.77 | 2.95 | 12 | 16 | 3.8 |
| 24 in | 3.14 | 5.24 | 21 | 28 | 2.1 |
Bag counts are for one tube with no extra, rounded up. For several tubes, multiply the cubic feet first, add the extra, then round up, or divide the number of tubes by the last column for cubic yards of ready-mix. Sonoco rounds its per-foot figures up a little (0.20, 0.35, 0.55, and 0.80 cu ft for 6, 8, 10, and 12 in) and makes Builder's Tube in those 4 diameters, in 4, 8, and 12 ft lengths. The calculator above works with rectangles only, so use this table or the formula for round forms.
For a post hole, subtract the post. Quikrete and Sakrete both say to dig the hole about 3 times as wide as the post and 1/3 of the post's length deep. A 4x4 measures 3.5 × 3.5 in, so it takes up 0.085 cu ft per foot, and a 10 in hole 2 ft deep around it holds (0.545 − 0.085) × 2 = 0.92 cu ft: 2 bags of 80 lb mix or 3 bags of 50 lb (0.375 cu ft each). Sakrete puts 3 to 4 in of gravel in the bottom of the hole for drainage, so take that depth off before you figure the concrete.
How thick should the concrete be?
| Project | Typical thickness | What the source says |
|---|---|---|
| Sidewalks and walkways | 4 in | Quikrete: at least 4 in thick |
| Patio | 3.5 to 4 in | Quikrete's patio plan uses 2 × 4 forms (3.5 in) over 4 in of compacted gravel |
| Small slabs and pads | 4 to 6 in | Quikrete: depends on the load the slab must carry |
| Driveway, cars and SUVs | 4 in minimum | Concrete Network, for vehicles of 3,000 to 6,000 lb |
| Driveway, trucks or RVs | 5 to 6 in | Concrete Network, for vehicles up to 12,000 lb or more |
Thickness drives volume fast. Here's what 100 sq ft takes at common thicknesses, before any extra:
| Thickness | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3.5 in | 29.2 | 1.08 | 49 | 65 |
| 4 in | 33.3 | 1.23 | 56 | 75 |
| 5 in | 41.7 | 1.54 | 70 | 93 |
| 6 in | 50.0 | 1.85 | 84 | 112 |
Anything structural, such as footings, a garage slab, or a pad under a building, follows your local building code. Get the thickness, base, and reinforcement from your plans or your building department, not from a rule of thumb. Once your plans give the bar size and spacing, the rebar calculator counts the bars to buy.
Concrete for footings by size, per linear foot
0.5 cu ft per linear foot for a footing 12 in wide and 6 in thick, the smallest the International Residential Code allows, or 1.85 cubic yards per 100 ft. For any footing, multiply the width by the thickness in inches and divide by 144 for cubic feet per foot:
| Footing (width × thickness) | Cu ft per linear foot | Cu yd per 10 ft | Cu yd per 100 ft | 80 lb bags per 10 ft |
|---|---|---|---|---|
| 12 × 6 in | 0.50 | 0.19 | 1.85 | 9 |
| 16 × 6 in | 0.67 | 0.25 | 2.47 | 12 |
| 16 × 8 in | 0.89 | 0.33 | 3.29 | 15 |
| 18 × 8 in | 1.00 | 0.37 | 3.70 | 17 |
| 20 × 10 in | 1.39 | 0.51 | 5.14 | 24 |
| 24 × 12 in | 2.00 | 0.74 | 7.41 | 34 |
The figures have no extra, so add yours before you order. The code also puts exterior footings at least 12 in below undisturbed ground, and below the frost line where the ground freezes, and its footing tables call for wider or thicker footings as the building and the soil change, so take the size from your plans or building department. The calculator handles a footing as a long, narrow slab: enter the length, the width in feet (inches ÷ 12), and the depth in inches as the thickness. For a 60 ft run of 18 × 8 in footing (width 1.5 ft, thickness 8 in) it shows: "You need 2.44 cubic yards (66.0 cu ft). That is 110 bags of 80 lb or 147 bags of 60 lb premix, including 10% extra."
Bags or ready-mix?
Sakrete's rule of thumb is that a pallet of bags is about a cubic yard, and it takes 45 bags of 80 lb mix to make one. Concrete Network recommends bags for small jobs such as walkways, small patios, and footings, and ordering by the yard for larger jobs such as driveways. For fence posts, the fence calculator counts the bags to set them.
Put the default example in physical terms: 62 bags × 80 lb is 4,960 lb of dry mix to haul, mix, and place in one continuous pour. If the calculator shows more than about a cubic yard, get a ready-mix quote and compare. Ready-mix is sold by the cubic yard, so give the supplier the cubic yards from the calculator along with the slab size and thickness.
Cubic yards of concrete for common slab sizes
From the calculator with its default 10 percent extra, the figure to give a ready-mix supplier. By hand, length × width in feet × thickness in inches ÷ 324 gives cubic yards; add the extra after. For the exact volume, divide a cell by 1.1.
| Slab | Area | 4 in thick (cu yd) | 5 in thick (cu yd) | 6 in thick (cu yd) |
|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 1.36 | 1.70 | 2.04 |
| 10 × 12 ft | 120 sq ft | 1.63 | 2.04 | 2.44 |
| 12 × 12 ft | 144 sq ft | 1.96 | 2.44 | 2.93 |
| 12 × 16 ft | 192 sq ft | 2.61 | 3.26 | 3.91 |
| 10 × 20 ft | 200 sq ft | 2.72 | 3.40 | 4.07 |
| 16 × 16 ft | 256 sq ft | 3.48 | 4.35 | 5.21 |
| 20 × 20 ft | 400 sq ft | 5.43 | 6.79 | 8.15 |
| 20 × 24 ft | 480 sq ft | 6.52 | 8.15 | 9.78 |
| 24 × 24 ft | 576 sq ft | 7.82 | 9.78 | 11.73 |
How to order ready-mix concrete
4 to 10 percent more than the volume from your plan dimensions is what the National Ready Mixed Concrete Association (NRMCA) says to order, so the calculator's default 10 percent extra sits at the top of that range. Along with the cubic yards, give the supplier these details:
| What to specify | NRMCA guidance for most work |
|---|---|
| Quantity | Cubic yards from your plan dimensions, plus 4 to 10 percent |
| Compressive strength | 3,500 to 5,000 psi |
| Slump (how wet the mix is) | 3 to 5 in |
| Largest stone size | 3/4 or 1 in |
| Air content | Air-entrained wherever it will be exposed to freezing |
| Delivery | Street address, where and how it will be placed, delivery rate, and start time; truck mixers hold 8 to 12 cubic yards |
The extra covers what plans miss: in NRMCA's example, a 4 inch slab that ends up just 1/8 inch thicker uses 3 percent more concrete, and hardened concrete takes up about 2 percent less volume than it did freshly mixed. On a pour of several trucks, re-measure near the end and adjust the last 2 or 3 loads. Your plans or local code may call for a different strength, slump, or air content, so check them first.
Tips and common mistakes
- Measuring the outside of the forms. The form boards take up space. Measure inside them.
- Assuming a perfectly flat base. Concrete Network suggests budgeting as if the slab were 1/4 inch thicker than planned. For the 10 × 10 example, 4.25 inches with 10 percent extra comes to 1.44 cubic yards, or 65 bags of 80 lb mix.
- Forgetting thickened edges. Concrete Network notes that driveway edges are sometimes thickened by 1 to 2 inches for 4 to 8 inches in from the edge. Figure that strip as its own rectangle and add it.
- Skipping control joints. Quikrete recommends control joints about 1/4 of the slab's depth, spaced every 6 to 8 ft in a 4 inch slab, and says patios larger than 10 × 10 ft should have them.
- Pouring dead level. Quikrete generally recommends a slope of 1/4 inch per foot for walks. Its patio plan uses 1/8 inch per foot. Either way, slope away from the house.
- Adding too much water. Quikrete calls for about 6 pints of water per 80 lb bag to start, up to about 9 pints. Stay within the range on the bag.
- Curing too briefly. Quikrete's data sheet calls for 5 days of curing in warm weather (70°F and up) and 7 days between 50 and 70°F. Sakrete says to keep the concrete moist for 3 to 5 days, protect it from freezing for 48 hours, and wait 24 hours for foot traffic and 72 hours for vehicles.
- Running short mid-pour. A few spare bags cost far less than stopping partway through a slab. Round up, not down.
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab is 33.3 cu ft. That's 56 bags of 80 lb mix with no extra, or 62 bags with 10 percent extra. In 60 lb bags it's 75 with no extra, or 82 with 10 percent.
How many bags of concrete do I need?
56 bags of 80 lb premix or 75 bags of 60 lb for every 100 sq ft of slab 4 inches thick, or 62 and 82 with the calculator's 10 percent extra. Multiply length × width in feet by the thickness in inches and divide by 12 for cubic feet, then divide by the bag's yield: about 0.60 cu ft for an 80 lb bag and 0.45 cu ft for a 60 lb bag, per Quikrete's data sheet. Round up. A sack is the same as a bag.
How many bags of Quikrete do I need?
45 bags of 80 lb Quikrete Concrete Mix per cubic yard, since Quikrete rates the 80 lb bag at about 0.60 cu ft. Per yard, that's 60 bags of 60 lb (0.45 cu ft each), 72 of 50 lb (0.375 cu ft), or 90 of 40 lb (0.30 cu ft). A 10 × 10 slab 4 inches thick takes 56 bags of 80 lb, or 62 with the calculator's 10 percent extra. Sakrete's High-Strength Concrete Mix lists the same yields for its 40, 60, and 80 lb bags, so the count is the same.
How much does one bag of concrete cover?
About 1.8 sq ft at 4 inches thick for an 80 lb bag, 1.35 sq ft for a 60 lb bag, and 0.9 sq ft for a 40 lb bag. Coverage is the bag's yield in cubic feet × 12 ÷ the thickness in inches, so 0.60 × 12 ÷ 4 = 1.8. At 2 inches, the thinnest pour Quikrete's Concrete Mix is designed for, an 80 lb bag covers about 3.6 sq ft; at 6 inches, about 1.2 sq ft.
How many cement bags do I need?
About 5 to 6 bags of 94 lb portland cement per cubic yard if you mix your own concrete, according to Quikrete's portland cement data sheet, along with sand and gravel (it gives 1 part cement, 2 parts sand, and 3 parts gravel). The default 10 × 10 slab, 1.36 cubic yards with extra, would take about 7 to 9 bags, rounded up. If you are buying premixed concrete, which already contains the sand and gravel, use the 80 lb or 60 lb bag count from the calculator instead.
How many 80 lb bags of concrete make a yard?
45. An 80 lb bag yields about 0.60 cu ft and a cubic yard is 27 cu ft, so 27 ÷ 0.60 = 45. With 60 lb bags (0.45 cu ft each) it takes 60.
How many square feet does a yard of concrete cover?
About 81 sq ft at 4 inches thick, 65 sq ft at 5 inches, and 54 sq ft at 6 inches. The quick math is 324 divided by the thickness in inches.
How much does a yard of concrete weigh?
About 4,000 lb, or 2 tons. In a yield example from the National Ready Mixed Concrete Association, fresh concrete weighs 151 lb per cu ft, which works out to 4,077 lb per cubic yard (27 cu ft). Bagged mix comes close: the 45 bags of 80 lb that make a yard weigh 3,600 lb dry, before the 6 to 9 pints of water Quikrete calls for per bag. At 151 lb per cu ft, the default 10 × 10 slab with extra (36.7 cu ft) weighs about 5,500 lb.
How much concrete do I need for a 12 × 12 slab?
1.96 cubic yards (52.8 cu ft) at 4 inches thick with 10 percent extra: 88 bags of 80 lb mix or 118 bags of 60 lb mix, the same as the calculator gives. With no extra it's 1.78 cubic yards (48 cu ft), or 80 and 107 bags. Nearly 2 yards is a good size to compare with a ready-mix delivery.
How many bags of concrete do I need for a Sonotube?
6 bags of 80 lb mix for a 12 in Sonotube 4 ft long, 4 bags for a 10 in tube, and 3 bags for an 8 in tube, with no extra. A round form holds 0.7854 × diameter × diameter ÷ 144 cu ft per foot of height (0.35, 0.55, and 0.79 for 8, 10, and 12 in). Multiply by the length in feet, divide by 0.60 for 80 lb bags, and round up. Sonoco's own chart comes to about 1 bag of 80 lb mix per foot of 10 in tube.
How much concrete do I need for a footing?
0.5 cu ft per linear foot for a footing 12 in wide and 6 in thick, the International Residential Code minimum, or 1.85 cubic yards per 100 ft before any extra. For other sizes, multiply the width by the thickness in inches, divide by 144, and multiply by the length in feet: a 16 × 8 in footing is 0.89 cu ft per foot, or 3.29 cubic yards per 100 ft. In the calculator, enter the footing's length, its width in feet, and its depth in inches as the thickness.
Can I pour a slab thinner than 4 inches?
Quikrete's standard Concrete Mix is made for pours 2 inches thick or more, but Quikrete recommends sidewalks be at least 4 inches thick, and Concrete Network puts the minimum for a driveway at 4 inches. For thin layers, look for a product rated for that thickness on the bag.
How much extra concrete should I order?
Sakrete suggests adding 10 percent to every estimate. Go higher if the base is uneven or you're pouring into holes or trenches, where the true volume is harder to measure.
Should I use bags or ready-mix concrete?
Concrete Network recommends bags for small jobs such as walkways, small patios, and footings, and ordering by the yard for larger jobs such as driveways. Sakrete's rule of thumb is that a pallet of bags is about a cubic yard, so if the calculator shows more than about a cubic yard, get a ready-mix quote and compare. Give the supplier the cubic yards along with the slab size and thickness.
How thick should a concrete driveway be?
Concrete Network puts the minimum at 4 inches for cars and SUVs (vehicles of 3,000 to 6,000 lb) and 5 to 6 inches for trucks or RVs up to 12,000 lb or more. Sidewalks and walkways should be at least 4 inches thick. Anything structural, such as a garage slab, follows your local building code.
How long does concrete take to cure?
Quikrete's data sheet calls for 5 days of curing in warm weather (70°F and up) and 7 days between 50 and 70°F. Sakrete says to keep the concrete moist for 3 to 5 days, protect it from freezing for 48 hours, and wait 24 hours for foot traffic and 72 hours for vehicles.
Sources
- QUIKRETE Concrete Mix No. 1101: Product Data Sheet (PDF)
- Sakrete High-Strength Concrete Mix: Technical Data Sheet (PDF)
- Sakrete: Estimating Bagged Concrete Jobs
- QUIKRETE: Concrete Sidewalks and Small Slabs (PDF)
- QUIKRETE: Building a Concrete Patio
- Concrete Network: How Thick Should a Concrete Driveway Be?
- Concrete Network: Concrete Calculator and Ordering Tips
- QUIKRETE Portland Cement: Product Information Sheet (PDF)
- NRMCA CIP 31: Ordering Ready Mixed Concrete (PDF)
- NRMCA CIP 8: Discrepancies in Yield (PDF)
- Sonoco: Sonotube Builder's Tube Concrete Forms (PDF)
- 2021 Seattle Residential Code, Chapter 4 Foundations, IRC R403 Footings (PDF)
- Voluntary Product Standard PS 20-20, American Softwood Lumber Standard (PDF)
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