This concrete slab calculator turns a rectangle into a volume and then into bags. Enter the length and width in feet, the thickness in inches, the bag size, and a waste allowance. The sample is a 10 foot by 10 foot slab, 4 inches thick, using 80 lb bags and 10% extra for spillage and an uneven subgrade. The headline is the number of whole bags to buy after that waste is included.
The bag yields are the published yields for standard concrete mix, not a particular store’s price. An 80 lb bag is counted as 0.60 cubic feet, a 60 lb bag as 0.45 cubic feet, and a 40 lb bag as 0.30 cubic feet. Those three yields are in proportion to the bag weight, and 27 cubic feet make one cubic yard, so an 80 lb bag is 1/45 of a cubic yard. The page also rounds the waste-adjusted volume up to the next quarter yard for a ready-mix order. A driveway with curves, a footing, or a slab that changes thickness needs a different volume before you use the bag step.
How to use this calculator
- Enter the length and the width in feet. The sample is 10 by 10. Measure the slab you will actually pour, not the property line.
- Enter the thickness in inches. A typical patio slab in this kind of estimate is 4 inches. Type 4, not 0.33 feet. The form converts inches to feet.
- Choose 80 lb, 60 lb, or 40 lb. The yield used is 0.60, 0.45, or 0.30 cubic feet. Then enter a waste allowance from 0 to 100 percent. The sample is 10.
- Press Calculate. Read the bag count, the cubic feet and cubic yards, the bag count before waste, and the ready-mix quantity rounded up to the next 0.25 yard.
How slab volume becomes a bag count
Volume in cubic feet is length times width times thickness, and the thickness has to be in feet. Four inches is 4/12 of a foot, so a 10 by 10 slab at 4 inches is 10 × 10 × 4 / 12 = 100/3 cubic feet, about 33.33 cubic feet on the display. Divide by 27 to get cubic yards. 100/3 / 27 is about 1.235 cubic yards. That is the geometric volume of a perfect rectangular prism. Real ground is not perfect, and a little concrete stays in the mixer and the wheelbarrow.
The waste allowance multiplies that volume. Ten percent turns 100/3 cubic feet into 110/3 cubic feet, about 36.67 cubic feet, or about 1.358 cubic yards. Bags are that volume divided by the yield of one bag. For an 80 lb bag the yield is 0.60 cubic feet, so 110/3 / 0.60 = 61.11 bags. You cannot buy 0.11 of a bag. The headline rounds up to the next whole bag unless the division is exact. The sample therefore reads 62 bags (80 lb). The line “bags before rounding up” keeps the 61.11 so you can see that the extra bag is the remainder, not a second waste factor.
Ready-mix suppliers often take orders in quarter-yard steps. The calculator takes the waste-adjusted yardage and rounds up to the next 0.25 cubic yard. The sample’s 1.358 yards becomes 1.5 cubic yards on that line. If you order ready-mix, you do not also buy the 62 bags. The two counts are alternative ways to obtain the same volume. Adding them would double the pour.
A zero waste allowance is allowed. On the same 10 by 10 by 4 inch slab it shows 56 bags of the 80 lb mix, because 100/3 / 0.60 = 55.56, which rounds up to 56, and the ready-mix line rounds 1.235 yards up to 1.25. Use zero only when you have a reason to believe the form will be filled exactly. Most slab estimates keep some waste because a thin spot in the gravel, or a form that bowed, is discovered after the truck has left.
The same rectangle is not a mulch order. Mulch is sold by the cubic yard or by a 2 or 3 cubic foot bag, and the usual shortcut is length times width times depth in inches, divided by 324. That divisor is 12 times 27: inches become feet, then cubic feet become yards. A 10 foot by 20 foot bed at 3 inches is 600/324 cubic yards, which is 1.852 cubic yards and exactly 50 cubic feet, or 25 bags if each bag is 2 cubic feet. This concrete page would be the wrong tool for that bed. An 80 lb bag is 0.60 cubic feet of concrete, not 2 cubic feet of mulch, and 45 of those bags fill a yard. Thirteen and a half of the 2 cubic foot mulch bags fill a yard. Mixing those yields buys the wrong material.
Formula
Thickness in feet = thickness in inches / 12 Cubic feet = length (ft) × width (ft) × thickness (ft) Cubic yards = cubic feet / 27 Volume with waste = cubic feet × (1 + waste percent / 100) Bag yield: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 40 lb = 0.30 cu ft Exact bags = volume with waste / bag yield Bags to buy = exact bags, rounded up to the next whole bag Ready-mix order = volume with waste, in cubic yards, rounded up to the next 0.25 yard 80 lb bags per cubic yard with no waste = 27 / 0.60 = 45
The 10 × 10 × 4 inch sample is exactly 100/3 cubic feet. The screen shows 33.33 cubic feet and 1.235 cubic yards because those lines are rounded for display. The bag math uses the full fraction. With 10% waste the exact bag count is 61.111…, the screen shows 61.11 bags before rounding, and the headline is 62. With no waste the exact count is 55.555…, shown as 55.56, and the headline is 56 bags.
Switching bag size does not change the cubic yards. It changes only the bag lines. The same 10 by 10 by 4 inch slab with 10% waste needs 82 bags of the 60 lb mix, because 36.67 / 0.45 is about 81.48, which rounds up to 82. A 40 lb bag on a 20 by 10 by 4 inch slab with 10% waste is a different volume: 200/3 cubic feet before waste, about 73.33 cubic feet after, and 245 bags. The ready-mix line for that larger slab is 2.75 cubic yards.
An exact hit is left alone. A 12 foot by 24 foot slab at 6 inches is 144 cubic feet, which is 5.333 cubic yards. Five percent waste makes 151.2 cubic feet and 5.6 cubic yards. Divided by 0.60, that is exactly 252 bags, so the headline is 252 rather than 253. The ready-mix quarter-yard round-up of 5.6 is 5.75 cubic yards. The bag count and the truck order answer different purchasing questions.
Dividing by 324 is only a yardage shortcut, and only when the depth is typed in inches. It does not know a bag yield. 10 times 10 times 4, divided by 324, is about 1.235, the same yardage this page shows for the sample slab before waste. The bag count still has to divide cubic feet by 0.60, 0.45, or 0.30. The mulch calculator divides by 2 or 3 instead, and it rounds a bulk order up to the next half yard rather than the next quarter yard, because that is how bulk mulch is commonly sold. Keep the two round-ups on their own pages.
Worked concrete slab examples
10 by 10 feet, 4 inches thick, 80 lb bags, 10% waste
The calculator shows 62 bags (80 lb). Slab volume is 33.33 cubic feet, or 1.235 cubic yards. With 10% waste the volume is 36.67 cubic feet, or 1.358 cubic yards. Bags before rounding up are 61.11. Bags with no waste allowance, rounded up, are 56. The ready-mix order is 1.5 cubic yards. The yield line says each 80 lb bag counts as 0.6 cubic feet, which is the 0.60 cubic foot yield displayed without a trailing zero.
Buying 62 bags and also ordering 1.5 yards would pour the slab twice. Pick one. The bag figure is for mixing on site. The yard figure is for a truck. The 6-bag gap between 56 and 62 is the 10% waste plus the effect of rounding two different remainders. It is not a second thickness.
The same slab with no waste, and the same slab in 60 lb bags
Set the waste allowance to 0 and keep 80 lb bags. The headline is 56 bags (80 lb). Volume stays 33.33 cubic feet and 1.235 cubic yards, and the waste line repeats that volume because 0% changes nothing. Bags before rounding up are 55.56. The ready-mix line is 1.25 cubic yards, which is the next quarter yard above 1.235. If your supplier will sell 1.25 and you have no waste, that is the order. If the supplier’s minimum is a full yard above the calculated volume, the minimum wins. This page does not know the supplier’s minimum.
Return the waste to 10 and choose the 60 lb bag. The headline is 82 bags (60 lb). Cubic yards do not change: still 1.235 before waste and 1.358 after, and the ready-mix line is still 1.5 cubic yards. The bag yield line now reads 0.45 cubic feet. A 60 lb bag holds three-quarters of the concrete an 80 lb bag holds, and 62 / 0.75 is about 82.7 before you account for the fact that each count was rounded up separately. Trust the 82 the form shows for the 60 lb choice, not a rescaled 62.
A small pad that comes out even, and a larger 6 inch slab
A 3 foot by 3 foot pad, 4 inches thick, with 80 lb bags and no waste, is 3 cubic feet. Divided by 0.60 that is exactly 5 bags. The headline is 5 bags (80 lb), bags before rounding up are 5, and the no-waste bag line is also 5. Volume is 3 cubic feet, or 0.111 cubic yards. Ready-mix rounded up to the next quarter yard is 0.25 cubic yards. Ordering a truck for a quarter yard is often unrealistic. The useful answer on a pad this small is the 5 bags. The yard line is there so the arithmetic stays visible, not as a suggestion to schedule a ready-mix delivery.
A 12 foot by 24 foot slab at 6 inches, 80 lb bags, and 5% waste, is 144 cubic feet before waste and 151.2 cubic feet after. The headline is 252 bags (80 lb), an exact division, so nothing extra was added for rounding. Ready-mix is 5.75 cubic yards. At a 45-bag-per-yard yield, 5.6 yards times 45 is 252, which is the check. A garage slab of this size is where bags stop being practical for many people. The calculator still counts them, because the yield math does not change at the point where your back does.
Bag counts for common rectangular slabs
| Slab | 80 lb bags, 0% waste | 80 lb bags, 10% waste | Ready-mix at 10% waste |
|---|---|---|---|
| 3 ft × 3 ft × 4 in | 5 | 6 | 0.25 cu yd |
| 10 ft × 10 ft × 4 in | 56 | 62 | 1.5 cu yd |
| 12 ft × 12 ft × 4 in | 80 | 88 | 2 cu yd |
| 20 ft × 10 ft × 4 in | 112 | 123 | 2.75 cu yd |
| 12 ft × 24 ft × 4 in | 160 | 176 | 4 cu yd |
| 12 ft × 24 ft × 6 in | 240 | 264 | 6 cu yd |
What a rectangular bag count leaves out
The yields 0.60, 0.45, and 0.30 cubic feet are the usual published yields for US concrete mix in those bag weights. A bag of sand mix, mortar, fast-setting post mix, or a high-strength blend can yield less. Read the yield printed on the bag you are buying and do not assume this table overrides it. If the bag says 0.60 cubic feet, the 80 lb choice matches. If it says something else, scale the volume yourself. This form will not accept a custom yield.
The shape is a rectangle of one thickness. A footing under a thickened edge, a slope for drainage, a circular patio, steps, and sonotube piers are different volumes. Air entrainment, fiber, and color do not change the geometry, but the water you add does change the finished strength. The calculator does not give a water quantity or a strength. It also does not subtract the volume of rebar, which is small next to a 10% waste allowance and is one reason the waste factor exists.
UK and Australian bags are sold by the kilogram, often 20 kg, and the yield is printed in litres or cubic metres. Do not treat an 80 lb bag as a 20 kg bag. Forty kilograms is about 88 lb, so the weights are not interchangeable, and the cubic-foot yield is a US customary figure. You can still use the cubic-yard line as a volume check: 1 cubic yard is 0.764555 cubic metres. Convert the displayed yards if a local supplier sells by the metre, then apply that supplier’s bag yield. The bag headline itself is for the 40, 60, and 80 lb yields above.
Waste of 10% is a planning allowance, not a code requirement. A careful pour on a flat, well-formed base may come in under it. A pour on soft soil may exceed it. The ready-mix quarter-yard step is a common ordering practice, not a rule that every plant uses. Confirm the plant’s minimum load and its rounding before you treat 1.5 cubic yards as an order. Neither count includes delivery, gravel, forms, or the labor.
Frequently asked questions
How many 80 lb bags of concrete do I need for a 10 by 10 slab at 4 inches?
With no waste, this calculator shows 56 bags, because the slab is 33.33 cubic feet and an 80 lb bag yields 0.60 cubic feet. With the sample 10% waste allowance it shows 62 bags. The volume with waste is 36.67 cubic feet, or 61.11 bags before rounding up to a whole bag.
Why does the concrete calculator round the bag count up?
A supplier sells whole bags. On the 10 by 10 by 4 inch slab with 10% waste, the exact count is 61.11 bags of the 80 lb mix, and 61 bags would leave the pour short of the waste-adjusted volume. The headline is 62. When the division is exact, as with 252 bags for the 12 by 24 foot, 6 inch slab at 5% waste, no extra bag is added.
What yield does the calculator use for an 80 lb, 60 lb, or 40 lb bag?
0.60 cubic feet for 80 lb, 0.45 cubic feet for 60 lb, and 0.30 cubic feet for 40 lb. Those are standard concrete-mix yields, and 27 / 0.60 is 45 bags of the 80 lb size per cubic yard before waste. A bag with a different yield printed on it will not match. Sand mix and mortar are not these yields.
Should I order ready-mix yards and the bags?
No. They are two ways to buy the same waste-adjusted volume. The 10 by 10 by 4 inch slab at 10% waste is 62 bags or 1.5 cubic yards of ready-mix, not both. The yard line rounds up to the next quarter yard. A plant with a larger minimum load will not honor a quarter-yard order just because this page printed one.
Does the 10% waste allowance change the cubic yards?
Yes. Waste multiplies the geometric volume before either the bag count or the ready-mix rounding. The 10 by 10 by 4 inch slab is 1.235 cubic yards with no waste and 1.358 cubic yards with 10% waste. The ready-mix line then rounds 1.358 up to 1.5 cubic yards. Set waste to 0 if you want the bare geometry, and expect a real pour to run past it if the base is low.
Can I use these 80 lb bag counts for a metric pour in the UK or Australia?
Use them only as a volume check. One cubic yard is about 0.765 cubic metres. Local bags are commonly 20 kg and publish their own litre yield, which is not 0.60 cubic feet. The headline bag count is for 40, 60, and 80 lb US bags. Convert the cubic-yard line, then divide by the yield on the bag you can actually buy.
Related ClearCalc pages
- Mulch calculator for a rectangular bed Cubic yards and 2 or 3 cubic foot bags. A 10 by 20 foot bed at 3 inches is 25 bags, not these 80 lb concrete bags.
- Fixed-rate mortgage payment calculator The loan payment on a house, not the concrete under a patio.
- Rent affordability calculator Compares rent with 30% of gross income. It does not estimate a pour.
- Length converter from meters If the slab was measured in meters, convert to feet before you type the length and width here. Thickness stays in inches on this form.
- Room paint calculator Gallons for a rectangular room. It uses square feet per gallon, not these bag yields.