In this guide
- What is a concrete calculator?
- How to use this concrete calculator
- How to calculate concrete yards
- Concrete calculation examples
- Concrete slab yardage chart
- How far does a cubic yard go?
- How thick should concrete be?
- How many bags of concrete do I need?
- Ready-mix or bagged concrete?
- Concrete slab cost
- Concrete strength (PSI)
- Why add extra concrete?
- Weather and curing
- Common concrete estimating mistakes
- Metric and unit conversions
- Accuracy and limitations
- Frequently asked questions
What is a concrete calculator?
A concrete calculator, also called a concrete yards calculator, works out how much concrete a project needs. You enter the shape and size, and it returns the volume in cubic yards, cubic feet, cubic meters and liters. Add your prices and it also gives a rough slab cost, plus a bag count for small pours.
Ordering the right amount matters. Too little and you risk a cold joint or a second delivery with a short-load fee. Too much and you pay for concrete you can’t use, since it sets quickly and can’t be stored. A quick, accurate estimate before you call your supplier avoids both.
How to use this concrete calculator
- Pick US or metric units. US is selected by default.
- Choose the shape: slab, footing, round column, post hole or a custom rectangular block.
- Enter the measurements. In US mode, length and depth take feet and inches, while thickness and diameter take inches.
- Set the quantity if you are pouring several identical pieces.
- Choose how much extra to allow for waste, or enter your own percentage.
- Optionally add your supplier’s order step and price, or a bag yield, then press Calculate Concrete.
How to calculate concrete yards
For a slab, footing or any rectangular pour, the steps are:
- Volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12
- Cubic yards = cubic feet ÷ 27
- Adjusted yards = cubic yards × (1 + extra % ÷ 100)
Round shapes, such as columns, piers and post holes, use the circle formula instead:
- Volume = π × radius² × height
- Radius = diameter ÷ 2
Keep every measurement in the same unit before you multiply, and remember that the radius is half the diameter. A quick rule for slabs is square feet times thickness in inches, divided by 324, which gives cubic yards directly.
Concrete calculation examples
Example 1: a patio slab. A slab 20 feet long, 15 feet wide and 4 inches thick holds 100 cubic feet, or 3.70 cubic yards. With 10 percent extra that becomes 4.07 cubic yards, and a supplier that sells in quarter-yard steps would have you order 4.25.
Example 2: a strip footing. A footing 40 feet long, 16 inches wide and 8 inches deep holds 35.56 cubic feet, or 1.32 cubic yards. With 10 percent extra, about 1.45 cubic yards.
Example 3: round columns. A column with a 12-inch diameter and 8 feet of height holds 6.28 cubic feet, or 0.23 cubic yards. Four of them need about 0.93 cubic yards.
Example 4: fence post holes. A hole 10 inches across and 3 feet deep, with a 4-inch round post set in it, takes about 1.37 cubic feet of concrete once the post’s volume is subtracted. Ten holes need 13.7 cubic feet, which is about 23 bags if each bag yields 0.60 cubic feet.
Example 5: a driveway. A driveway 12 feet wide and 40 feet long, poured 5 inches thick, needs 7.41 cubic yards. With 10 percent extra it comes to 8.15 cubic yards, so you would ask your supplier about 8.25.
Example 6: a metric slab. A slab 6 meters by 4 meters and 100 millimeters thick holds 2.4 cubic meters, which is about 3.14 cubic yards.
Concrete slab yardage chart
These are base volumes in cubic yards, with no extra added. Add your waste allowance on top.
| Slab size | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.54 | 1.85 |
| 12 × 12 ft | 1.78 | 2.22 | 2.67 |
| 12 × 24 ft | 3.56 | 4.44 | 5.33 |
| 16 × 16 ft | 3.16 | 3.95 | 4.74 |
| 20 × 20 ft | 4.94 | 6.17 | 7.41 |
| 24 × 24 ft | 7.11 | 8.89 | 10.67 |
| 30 × 30 ft | 11.11 | 13.89 | 16.67 |
| 12 × 40 ft | 5.93 | 7.41 | 8.89 |
How far does a cubic yard go?
The area one cubic yard covers depends only on the thickness. Divide 324 by the thickness in inches:
| Thickness | Area per cubic yard |
|---|---|
| 3 in | 108 sq ft |
| 4 in | 81 sq ft |
| 5 in | 64.8 sq ft |
| 6 in | 54 sq ft |
| 8 in | 40.5 sq ft |
How thick should concrete be?
These are common thicknesses for residential work. Your local building code, soil and loads can call for more, so treat them as a starting point and confirm with your contractor or building department.
- Sidewalks, patios and shed pads: usually 4 inches.
- Driveways: often 4 to 6 inches, with 5 or 6 for heavier vehicles.
- Garage floors: commonly 4 to 6 inches.
- Footings: sized by code and project plans, not by a general rule.
Thickness has a big effect on volume, because every extra inch adds about a quarter of a yard for each 80 square feet. A thicker slab also needs a properly prepared base, which is not part of this estimate.
How many bags of concrete do I need?
The number of bags depends on the yield printed on the label, which differs by product and manufacturer. These are common yields for premixed bags, and the bag counts follow from them:
| Bag size | Typical yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
Always use the yield from the bags you actually buy. To estimate, divide the adjusted volume in cubic feet by the yield per bag and round up. A 10 by 10 foot slab at 4 inches holds about 33 cubic feet, which is roughly 56 bags at 0.60 cubic feet, and about 62 with 10 percent extra.
Ready-mix or bagged concrete?
Bags suit small jobs such as a few post holes, a stepping pad or a repair. Mixing them yourself takes time and effort, and consistency can vary from batch to batch. A cubic yard of concrete weighs roughly 4,000 pounds, which is why larger pours are usually ordered as ready-mix and delivered by truck. A standard mixer truck carries roughly 8 to 10 cubic yards.
Suppliers sell in different steps and often charge extra for loads below a minimum size, so ask before you order. They can also advise on the mix, additives and the best time for delivery. If you are paving with asphalt instead, the Asphalt Calculator estimates tons for your area.
Concrete slab cost
To estimate slab cost, multiply the cubic yards you will order by your supplier’s price per yard. As an illustration only, 4.25 yards at $150 per yard comes to $637.50 for the concrete. For a quick price per square foot, divide your price per yard by the area one yard covers: at 4 inches, that is your price divided by 81.
Real prices change with your location, the strength of the mix and the delivery distance, so use a current quote. A full slab project also includes costs this concrete slab cost calculator does not cover:
- Delivery, fuel and short-load fees
- Excavation and a gravel base
- Forms, rebar or wire mesh
- Finishing, sealing and control joints
- Pumping, if the truck can’t reach the site
- Labor, permits and removal of old concrete
Concrete strength (PSI)
Concrete strength is measured in pounds per square inch (psi) after curing. Residential slabs and sidewalks commonly use mixes in the range of about 3,000 to 4,000 psi, and driveways and garage floors often sit toward the higher end. In regions with freezing weather, suppliers usually recommend air-entrained concrete to resist freeze and thaw damage. Strength does not change the volume you need, but it does affect the price, so mention your project when you call for a quote.
Why add extra concrete?
A pour rarely matches the exact geometry. Uneven ground, forms that shift and spillage all use more concrete than the math predicts, and running short mid-pour can cause a cold joint. The extra allowance is shown separately from the base volume, so you can always see what the shape alone requires.
A common range is 5 to 10 percent. Use the higher end for rough or uneven subgrades, tight budgets for delivery and complex forms. The right figure is a judgment call, so ask your supplier or contractor.
Weather and curing
Hot, dry or windy weather makes concrete set faster and can cause cracking, and cold weather slows setting and can damage fresh concrete if it freezes. As a rough guide, take extra care above about 90°F and below about 40°F. Keep the surface from drying too quickly, for example by curing with water or a curing compound.
Concrete generally reaches most of its design strength in about 28 days. Many people allow light foot traffic after a day or two and vehicles after about a week, but your mix and the conditions change the timing, so follow your supplier’s guidance.
Common concrete estimating mistakes
- Mixing inches and feet. Convert everything to the same unit before multiplying.
- Using diameter as radius. This makes a round column four times too big.
- Ordering the exact calculated amount. Leave room for waste and uneven ground.
- Forgetting thickened edges and footings. They add volume beyond the flat slab.
- Confusing cubic feet and cubic yards. One cubic yard is 27 cubic feet.
- Assuming every bag has the same yield. Check the label.
- Ignoring the base. Gravel and forms are separate materials.
Metric and unit conversions
Switch the calculator to metric to enter millimeters, centimeters or meters and see cubic meters and liters. These conversions are the ones used behind the scenes:
- 1 cubic yard = 27 cubic feet
- 1 cubic yard ≈ 0.7646 cubic meters
- 1 cubic foot ≈ 0.0283 cubic meters
- 1 cubic meter = 1,000 liters
Accuracy and limitations
This concrete calculator gives a planning estimate, not an engineering design. Footing size, slab thickness and reinforcement are set by your local building code and project plans. Confirm the quantity, mix and delivery details with your concrete supplier before ordering.
Frequently asked questions
How many cubic yards of concrete do I need?
Multiply length by width by thickness, all in feet, then divide the cubic feet by 27. For slabs there is a shortcut: square feet times thickness in inches, divided by 324, gives cubic yards. A 10 by 10 foot patio poured 4 inches thick needs about 1.23 cubic yards before any extra for waste.
How much does a concrete slab cost?
The concrete itself costs your local price per cubic yard times the yards you order. Delivery, short-load fees, base preparation, forms, rebar, finishing and labor are extra and vary a lot by region and contractor. Enter your supplier’s price in the calculator to see a material estimate.
How many bags of concrete are in a cubic yard?
It depends on the yield printed on the bag. If an 80 lb bag yields 0.60 cubic feet, a cubic yard (27 cubic feet) takes about 45 bags. A 60 lb bag at 0.45 cubic feet takes about 60 bags, and a 40 lb bag at 0.30 cubic feet takes about 90. Check the label on the bags you plan to buy.
How many 80 lb bags do I need for a 10 by 10 slab?
A 10 by 10 foot slab that is 4 inches thick holds 33.3 cubic feet. At a yield of 0.60 cubic feet per bag, that is about 56 bags. With 10 percent extra, plan on about 62 bags. At that size, many people choose ready-mix delivery instead.
How thick should a concrete slab be?
Patios, sidewalks and shed pads are commonly poured 4 inches thick. Driveways and garage floors are often 4 to 6 inches, with the thicker end for heavier vehicles. Footings are sized by your local building code and project plans, so confirm the requirements with your building department.
How many square feet does a cubic yard of concrete cover?
Divide 324 by the thickness in inches. A cubic yard covers about 81 square feet at 4 inches, about 65 square feet at 5 inches and about 54 square feet at 6 inches.
How much extra concrete should I order?
Many people add roughly 5 to 10 percent for uneven ground, spillage and forms that bow slightly, but there is no universal rule. The calculator starts at 10 percent as an example, and you can change it or enter your own percentage.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27, because one cubic yard is 3 feet by 3 feet by 3 feet. For example, 100 cubic feet is about 3.7 cubic yards. To go the other way, multiply cubic yards by 27.
Can I use this for footings, columns and post holes?
Yes. Choose the shape at the top of the calculator and enter only the measurements it asks for. The post hole option can subtract the space taken up by the post if you enter its diameter.
When should I order ready-mix instead of buying bags?
Bags are practical for small jobs, such as a few post holes or a small pad. Once a project needs around a cubic yard or more, ready-mix is usually faster, more consistent and often cheaper than dozens of bags. Ask your supplier about minimum loads and short-load fees.
What PSI concrete should I use?
Strength depends on the project and your climate. Residential slabs and sidewalks commonly use mixes in the range of about 3,000 to 4,000 psi, and colder regions often call for air-entrained mixes. Ask your concrete supplier or contractor what suits your project and local code.
How long does concrete take to cure?
Concrete generally reaches most of its design strength in about 28 days. Many people allow light foot traffic after a day or two and vehicles after about a week, but weather, mix and thickness change the timing, so follow your supplier’s advice.
How do I calculate concrete in cubic meters?
Switch the calculator to metric and enter your measurements in meters, centimeters or millimeters. The result is shown in cubic meters and liters as well as cubic yards. One cubic yard is about 0.7646 cubic meters.
Does the calculator save my numbers?
No. The calculation runs in your browser and your entries are not uploaded. Use Copy results or Print results if you want to keep a record.