Concrete Calculator
Enter your slab size to find the volume in cubic feet and cubic yards, plus how many bags you need.
Bag counts are rounded up, so you buy whole bags. One 80 lb bag yields about 0.60 cubic feet; one 60 lb bag yields about 0.45 cubic feet.
Use this concrete calculator to work out how much concrete a rectangular slab needs. Enter the length and width in feet, the thickness in inches, and a waste allowance, and it returns the volume in cubic feet and cubic yards plus the number of 80 lb and 60 lb bags to buy.
Ordering the right amount of concrete saves money and avoids a mid-pour shortage. Order too little and you face a cold joint where fresh concrete meets concrete that has already started to set. Order far too much and you pay for material you must dispose of.
How a Concrete Calculator Works
A concrete calculator turns three measurements into a volume. First, it converts the thickness from inches to feet by dividing by 12. Then it multiplies length by width by thickness in feet to get cubic feet. Finally, it divides by 27, because one cubic yard holds 27 cubic feet, to give the figure ready-mix suppliers use. Bag counts come from dividing the cubic feet by the yield of one bag and rounding up to whole bags.
- Cubic feet = length (ft) x width (ft) x (thickness (in) / 12) x (1 + waste % / 100)
- Cubic yards = cubic feet / 27
- Bags = cubic feet / yield per bag, rounded up
One 80 lb bag of premixed concrete yields about 0.60 cubic feet, while one 60 lb bag yields about 0.45 cubic feet. Yields vary slightly by brand and mix, so check the bag label and adjust if your product states a different yield.
How to Use This Concrete Calculator
- Measure the length of the slab in feet, from one end of the form to the other.
- Measure the width in feet at the widest point, and use the average width if the shape tapers slightly.
- Enter the planned thickness in inches. Patios and walkways are commonly 4 inches thick, while driveways and garage floors are often 5 or 6 inches.
- Set the waste allowance. A 10% allowance covers spillage, uneven subgrade, and slight bulging of the forms, and it is the default here.
- Read the results: cubic feet, cubic yards, and the number of 80 lb and 60 lb bags, rounded up.
- Round ready-mix orders up to the nearest quarter or half yard, as most suppliers charge short-load fees.
Choosing the Right Thickness and Waste Allowance
Thickness depends on what the slab must carry. A 4 inch patio slab suits foot traffic and light furniture. A driveway that carries cars needs 5 to 6 inches, and areas used by heavy trucks may need more, plus reinforcement.
The waste allowance is not padding for careless measuring. Ground is rarely level, forms flex under wet concrete, and some material always stays in the wheelbarrow or mixer. A 5% allowance may suit a small indoor slab, while 10% is a safer default for outdoor work on rough ground.
Related jobs around the same project often need their own estimates. If the slab sits beside garden beds, a mulch calculator can help you price the edging material for those beds. A compacted stone layer under the slab matters too, and a gravel calculator works out how much sub-base stone that layer needs. Once the concrete has cured, a paint coverage calculator estimates the gallons needed to seal or coat it. If you are regrading the lawn around the new slab, a topsoil calculator gives you the volume of fill for the surrounding ground.
Concrete Calculator Examples
The table below shows verified results from the calculator. Each row uses the same formulas, with bag counts rounded up to whole bags.
| Slab size | Thickness | Waste | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|---|
| 10 x 10 ft | 4 in | 0% | 33.33 | 1.23 | 56 | 75 |
| 10 x 10 ft | 4 in | 10% | 36.67 | 1.36 | 62 | 82 |
| 20 x 10 ft | 6 in | 0% | 100.00 | 3.70 | 167 | 223 |
Take the first row as a worked example. A 10 foot by 10 foot slab at 4 inches thick gives 10 x 10 x (4 / 12) = 33.33 cubic feet, or 1.23 cubic yards. Dividing 33.33 by 0.60 gives 55.55, which rounds up to 56 bags of 80 lb mix, and dividing by 0.45 gives 74.07, which rounds up to 75 bags of 60 lb mix.
Ready-Mix or Bags: Which Should You Choose?
Bags suit small jobs such as a single step, a small pad, or a repair. You can buy exactly what you need and mix at your own pace. Once the job passes about one cubic yard, however, bag mixing becomes slow and tiring, and the cost per yard climbs above ready-mix prices.
Ready-mix concrete arrives in a truck, pours fast, and gives a consistent mix across the whole slab. Ask the supplier about minimum load charges, short-load fees, and how far the truck can reach from the street.
Concrete Calculator FAQs
How many cubic yards of concrete do I need for a 10x10 slab?
A 10 foot by 10 foot slab at 4 inches thick needs 1.23 cubic yards with no waste allowance, or about 1.36 cubic yards with a 10% allowance. Most suppliers round orders up, so plan on ordering 1.5 cubic yards for this size.
How many 80 lb bags of concrete are in a cubic yard?
One cubic yard holds 27 cubic feet, and one 80 lb bag yields about 0.60 cubic feet, so a cubic yard takes 45 bags. Buy 45 bags per yard as a baseline, then add your waste allowance on top.
How many 60 lb bags of concrete do I need per cubic yard?
At about 0.45 cubic feet per bag, one cubic yard needs 60 bags of 60 lb mix. With a 10% allowance, plan on 66 bags.
Should I add a waste allowance to my concrete order?
Yes. A 10% allowance covers spillage, uneven ground, and form movement, and costs far less than a second delivery. Only skip it for a small, precisely formed indoor slab.
Is it cheaper to buy bags or order ready-mix concrete?
Bags are cheaper for tiny jobs, but ready-mix wins past about one cubic yard. Compare the total bag price against the ready-mix quote with delivery and any short-load fee.
Learn More
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