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Concrete Calculator: Yards and Bags for Slabs, Sidewalks and Footings
To figure concrete, multiply length × width × thickness in feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab at 4 inches is 33.3 cubic feet, or 1.23 cubic yards, before waste: about 56 bags of 80 lb premix, or 62 with 10% extra. Use the concrete calculator below for slabs, footings and sonotubes, or read on for the formula, a thickness table and worked examples.
Key takeaways
- Cubic yards = L × W × D (all in feet) ÷ 27. For thickness in inches, divide inches by 12 first.
- One cubic yard covers 81 SF at 4 in, 54 SF at 6 in. Shortcut: 324 ÷ thickness (in).
- About 45 bags of 80 lb mix make a yard (0.60 ft³ per bag), or 60 bags of 60 lb, or 90 bags of 40 lb, per Quikrete's published yields.
- Add waste (5–10% is common) and round a ready-mix order up. Running short costs more than a little leftover.
Concrete calculator
Add a line for each slab, footing or set of columns. Enter feet and inches together (10 ft 6 in = 10 and 6). Results update as you type.
Bag counts include waste and are rounded up to whole bags, using the yields in the Quikrete Concrete Mix (No. 1101) data sheet: 0.30, 0.45 and 0.60 ft³ for 40, 60 and 80 lb bags. Other products differ; check your bag. Weight is the net volume at 150 lb/ft³; see how much a yard of concrete weighs. Estimates only.
How to calculate cubic yards of concrete
Concrete is ordered by the cubic yard and bagged mix is sold by weight, but you measure in feet and inches. Every concrete yardage calculator, this one included, does the same three steps:
- Convert everything to feet. Divide inches by 12: 4 in = 0.333 ft, 6 in = 0.5 ft, 8 in = 0.667 ft. A length of 10 ft 6 in is 10.5 ft.
- Multiply to get cubic feet. Length × width × depth.
- Divide by 27. There are 27 cubic feet in a cubic yard (3 × 3 × 3).
Then add your waste allowance and round up to what you can actually buy: whole bags, or the yardage increment your ready-mix plant batches in.
Most flatwork thickness is given in inches, so it's handy to keep inches in the formula:
That second form is exactly how the Jobsite Reel app figures replacement yards for concrete sidewalk items, with an optional waste percentage on top.
Concrete slab calculator, footings, columns and curb
Slabs, sidewalks and pads
Pick Slab / sidewalk / pad in the calculator. It covers patios, driveways, garage floors, sidewalk panels and equipment pads: anything that's a flat rectangle with a uniform thickness. Volume is length × width × thickness. For an L-shaped or notched slab, break it into rectangles and add each one as its own line item. That's also how to use it as a concrete pad calculator for a set of identical pads: one line, quantity set to the number of pads.
If the slab has a thickened edge or turned-down footing, add the thickened part as a separate footing line rather than averaging the thickness by eye.
Footings and walls
Pick Footing / wall. Enter the run length, the width (or wall thickness) in inches, and the depth (or wall height) in feet and inches. A continuous footing 16 in wide and 12 in deep is 16/12 × 12/12 = 1.33 cubic feet per linear foot. A wall 8 in thick and 4 ft tall is 0.667 × 4 = 2.67 cubic feet per linear foot. For a rectangular perimeter, add the four sides but don't double-count the corners: use the outside length on two sides and the inside length on the other two.
Round columns and sonotubes
Pick Round column / sonotube, enter the tube diameter in inches and the height in feet. The volume of a cylinder is π × radius² × height, with the radius in feet:
Dividing the diameter in inches by 24 converts it to a radius in feet in one step. Per foot of height, before waste:
| Tube diameter | Cubic feet per ft of height | 80 lb bags per ft |
|---|---|---|
| 8 in | 0.349 | 0.58 |
| 10 in | 0.545 | 0.91 |
| 12 in | 0.785 | 1.31 |
| 16 in | 1.396 | 2.33 |
| 18 in | 1.767 | 2.95 |
If the tube sits on a belled or spread footing, add the footing as its own line.
Curb and gutter
Curb sections aren't rectangles, and the math works better in square inches of cross-section times linear feet. Rather than squeeze that in here, use our dedicated curb and gutter calculator, which handles curb, gutter and monolithic curb & gutter and also gives removal tons.
Concrete per 100 square feet by thickness
A quick field reference for flatwork, before waste. The SF-per-yard column is 324 ÷ thickness in inches.
| Thickness | CY per 100 SF | SF per CY | 80 lb bags per 100 SF | 60 lb bags per 100 SF |
|---|---|---|---|---|
| 3 in | 0.93 | 108 | 41.7 | 55.6 |
| 4 in | 1.23 | 81 | 55.6 | 74.1 |
| 5 in | 1.54 | 64.8 | 69.4 | 92.6 |
| 6 in | 1.85 | 54 | 83.3 | 111.1 |
| 8 in | 2.47 | 40.5 | 111.1 | 148.1 |
Thickness drives the number more than people expect. Going from 4 to 5 inches adds 25% to the yardage; 4 to 6 adds 50%. Confirm the thickness the job calls for before you order, and measure actual form depth, not the nominal lumber size.
How many bags of concrete do I need?
Bagged premix is sold by weight, but what you need is volume, so the number that matters is the bag's yield. Quikrete's data sheet for its standard Concrete Mix (No. 1101), a 4,000 psi blend, lists these yields:
| Bag size | Yield per bag | Bags per cubic yard | Bags per 10 ft³ |
|---|---|---|---|
| 40 lb | ≈ 0.30 ft³ | 90 | 33.3 |
| 60 lb | ≈ 0.45 ft³ | 60 | 22.2 |
| 80 lb | ≈ 0.60 ft³ | 45 | 16.7 |
Source: Quikrete Concrete Mix 1101 data sheet (checked November 2026). The same sheet notes the product is meant for pours 2 inches thick or more. Other brands and specialty mixes (fast-setting, high-strength, fiber) can yield differently, so read the yield printed on the bag you're actually buying and adjust.
Notice that every row works out to the same 3,600 lb of dry mix per cubic yard (45 × 80 = 60 × 60 = 90 × 40). Bag size changes how many trips you make to the truck, not how much material you need.
Worked examples
Each example uses the calculator's default 10% waste and the Quikrete yields above. You can enter any of them in the calculator to check.
Example 1: 10 × 10 ft patio at 4 inches
- Volume: 10 × 10 × (4 ÷ 12) = 33.3 ft³ = 33.3 ÷ 27 = 1.23 CY
- With 10% waste: 33.3 × 1.10 = 36.7 ft³ = 1.36 CY
- 80 lb bags: 36.7 ÷ 0.60 = 61.1, round up to 62 bags (56 with no waste)
- 60 lb bags: 36.7 ÷ 0.45 = 81.5, round up to 82 bags
- Ready-mix rounded to the next quarter yard: 1.5 CY
Sixty-two 80 lb bags is nearly 5,000 lb of material to carry and mix. That's the size of job where many crews start pricing a truck instead.
Example 2: 4 ft × 60 ft sidewalk at 4 inches
- Area: 4 × 60 = 240 SF
- Volume: 240 × 4 ÷ 12 = 80 ft³ = 2.96 CY
- With 10% waste: 88 ft³ = 3.26 CY
- Ready-mix rounded to the next quarter yard: 3.5 CY (3.26 is just over 3.25)
- 80 lb bags, for comparison: 88 ÷ 0.60 = 146.7, so 147 bags
This is a ready-mix job for most contractors. Note how the waste allowance pushed the order past a quarter-yard break; if you trust your subgrade, 3.25 CY at about 9.7% extra may be enough. If the walk is divided into panels, the panel count doesn't change the volume. Only the total length, width and thickness matter. For pricing the whole job, including removal tons, see our sidewalk replacement cost guide.
Example 3: 20 linear feet of footing, 16 in wide × 12 in deep
- Volume: 20 × (16 ÷ 12) × (12 ÷ 12) = 26.7 ft³ = 0.99 CY
- With 10% waste: 29.3 ft³ = 1.09 CY
- 80 lb bags: 29.3 ÷ 0.60 = 48.9, so 49 bags
- Ready-mix rounded to the next quarter yard: 1.25 CY
Trenched footings are where waste goes up. A trench dug with a bucket rarely has clean, straight walls, so measure the actual trench width in a few places, not the bucket width.
Example 4: 6 sonotubes, 12 in diameter × 4 ft deep
- One tube: 3.1416 × (12 ÷ 24)² × 4 = 3.1416 × 0.25 × 4 = 3.14 ft³
- Six tubes: 6 × 3.1416 = 18.8 ft³ = 0.70 CY
- With 10% waste: 20.7 ft³ = 0.77 CY
- 80 lb bags: 20.7 ÷ 0.60 = 34.6, so 35 bags; 60 lb bags: 47; 40 lb bags: 70
In the calculator, that's one line set to Round column / sonotube, 12 in, 4 ft, quantity 6. If you mix bags tube by tube, you'll open 6 bags per tube (3.46 ft³ with waste ÷ 0.60 = 5.8), which is 36 bags for the set, one more than the pooled total. Rounding per pour is the realistic number when each tube gets its own batch.
When to order ready-mix vs bags
There's no universal break-even, and we won't quote prices because they swing by region, season and supplier. What you can do is put both options side by side using the quantities above:
- Labor and handling. A cubic yard is about 45 bags of 80 lb mix, 3,600 lb of dry material, mixed one batch at a time. Many contractors move to ready-mix somewhere around a yard or more for that reason alone.
- Consistency. One truck delivers one batch. Hand-mixed bags vary with how much water each batch gets, and a slow pour gives earlier batches time to stiffen before the next ones go in.
- Delivery charges. Ready-mix suppliers commonly have a minimum order and charge extra for small, or "short," loads. Ask for a delivered price on your exact yardage with every fee included, then compare.
- Access. Can the truck reach the pour, or will you need a pump, buggies or wheelbarrows? A backyard pad with no truck access can tip the decision toward bags.
- Small, scattered pours. Post holes, a single stair landing, a few sidewalk panel repairs across a property: bags let you pour each one when it's ready.
Run the calculator once, read off both the yardage and the bag count, and price both. Then the decision is a number, not a habit.
How many yards of concrete do I need? Rounding up for short loads
The calculator gives you a geometric volume. What you order should be a little more, for reasons the formula can't see:
- Subgrade. It's never perfectly flat. On a 4-inch slab, a base that runs half an inch low adds 12.5% to the volume by itself (0.5 ÷ 4).
- Forms. Forms bow out under load, and trenches are wider than planned.
- Spillage and leftovers. Some stays in the chute, the mixer or the wheelbarrow.
Many crews add 5–10%, and this calculator defaults to 10%. Then round the ready-mix order up to whatever increment your plant batches in; the calculator lets you choose a quarter, half or whole yard. Ask your supplier what they accept.
Running short is the expensive mistake. A second truck for a fraction of a yard may bring its own minimum and short-load charges, and the crew stands around waiting while the first pour starts to set. A bit of leftover concrete is cheap by comparison; plan a spot for it (a splash block, a pad, a few post bases) before the truck arrives.
The calculator also shows the approximate weight in place. Normal-weight concrete runs about 140–150 lb per cubic foot, so a yard is roughly 2 tons. That matters for truck payloads, for whether a trailer can carry your bags, and for haul-off when you tear out old concrete. Our guide on how much a yard of concrete weighs has the tables.
Getting the measurements right in the field
The math is the easy part. Most bad concrete orders come from measurements: a dimension written on a scrap of cardboard, a thickness assumed instead of checked, or one panel forgotten when the numbers got to the office.
That's the problem Jobsite Reel is built for. On a sidewalk or curb walk, you add an area, take photos with the iPhone camera, type the length, width and depth, and the app shows square feet, replacement cubic yards (with your waste percentage) and removal tons right there. Every area gets a GPS pin, and the one-tap PDF report totals quantities by folder on a satellite overview map. It handles concrete sidewalk, curb / curb & gutter and asphalt items; for footings, columns and bag counts, use the calculator on this page.
Field note: iPhone GPS is typically good to about 10–16 ft in open sky. That's enough to find a panel again or show an owner where the work is, but measure dimensions with a tape or wheel, not from a map.
See pricing, or if you price asphalt too, our asphalt tonnage calculator uses the same measure-then-multiply approach.
Frequently asked questions
How do I calculate how many yards of concrete I need?
Multiply length by width by thickness, all in feet, to get cubic feet, then divide by 27 to get cubic yards. Convert inches to feet by dividing by 12, so a 4-inch slab is 0.333 ft thick. A 10 x 10 ft slab at 4 inches is 33.3 cubic feet, or 1.23 cubic yards, before waste.
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick, a 10 x 10 ft slab is 33.3 cubic feet. Using Quikrete's published yield of about 0.60 cubic feet per 80 lb bag, that is 56 bags with no waste, or 62 bags with 10 percent extra. In 60 lb bags (0.45 cubic feet each) it is 82 bags with 10 percent extra.
How many 80 lb bags of concrete make a yard?
About 45. Quikrete's Concrete Mix data sheet lists an 80 lb bag as yielding approximately 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 / 0.60 = 45 bags. The same math gives 60 bags of 60 lb (0.45 cubic feet each) or 90 bags of 40 lb (0.30 cubic feet each). Other products can yield differently, so check the bag.
How many square feet does a yard of concrete cover?
A cubic yard covers 81 square feet at 4 inches thick, 64.8 square feet at 5 inches, 54 square feet at 6 inches and 40.5 square feet at 8 inches. The formula is 324 divided by the thickness in inches.
How much extra concrete should I order?
Many crews add 5 to 10 percent for uneven subgrade, form bulge and spillage; this calculator defaults to 10 percent. Raise it when the subgrade is rough. On a 4-inch slab, a subgrade that sits half an inch low adds 12.5 percent to the volume by itself.
Should I use bagged concrete or order ready-mix?
There is no single break-even. A cubic yard is about 45 bags of 80 lb mix, which is a lot of mixing and lifting, so many contractors switch to ready-mix somewhere around a yard or more. Compare the bag count and labor against a delivered ready-mix quote for your exact yardage, including any minimum-load or short-load charges your supplier applies.