How to use the concrete calculator calculator
- 1
Choose the shape you are pouring
Start with the shape that matches your pour: a rectangle or square for most slabs, a circle for a round pad, a triangle for a corner, or a known area if you have already measured the square footage. Complex outlines are handled by adding sections rather than forcing a single shape.
- 2
Enter the measurements and pick your units
Type the dimensions exactly as you measured them and select the matching unit. Feet and inches both work, and the calculator converts internally, so there is no need to turn 8 feet 6 inches into a decimal by hand.
- 3
Set the thickness
Enter the thickness for the pour and choose its unit. Thickness is a project decision that belongs to your plans, your designer, or local code, so the calculator asks for it rather than assuming one.
- 4
Add more sections if the project has several parts
Use the add-section control for an L-shaped patio, a slab plus a walkway, or a pad with a separate landing. Each section is measured and calculated on its own, and the totals are added before the conversion to cubic yards.
- 5
Add an allowance if you want one
An optional allowance percentage increases the ordered volume to cover measurement variation, an uneven subgrade, and spillage. Choose the figure yourself based on how confident you are in the measurements and how level the base is.
- 6
Add a bag yield or your own prices
To work in bags, enter the yield printed on the bag you plan to buy. To estimate cost, enter your quoted price per cubic yard, your price per bag, or both, along with any delivery or fixed fees. Enter both pricing routes to see the arithmetic difference between them.
- 7
Check the result against your supplier
Treat the output as a planning estimate. Confirm the mix, the thickness, the reinforcement, the order increments, and the final quantity with your supplier or a qualified professional before pouring. See the concrete ordering guide for a checklist.
Formula
Cubic yards = (square feet x thickness in inches) / 324
This is the same calculation as area times thickness in feet, divided by 27. Because 27 cubic feet times 12 inches per foot equals 324, dividing by 324 converts square feet and inches straight to cubic yards in one step.
Concrete formulas by shape
Every shape resolves to the same idea: find the area in square feet, multiply by the thickness in feet, then divide by 27. Round elements such as piers and post holes use the cylinder formula instead, because their depth is a height rather than a thin layer.
- Rectangle or square slab
- Cubic feet = length(ft) x width(ft) x thickness(ft)
- Divide the thickness in inches by 12 to get feet.
- Circular pad
- Cubic feet = pi x (diameter(ft) / 2)^2 x thickness(ft)
- Measure the diameter across the widest point through the center.
- Triangle
- Cubic feet = 0.5 x base(ft) x height(ft) x thickness(ft)
- Height is the perpendicular distance from the base to the opposite point.
- Cylinder (pier, post hole, form tube)
- Cubic feet = pi x (diameter(ft) / 2)^2 x height(ft)
- Used by the footing, post hole, and form tube calculators.
- Cubic feet to cubic yards
- Cubic yards = cubic feet / 27
- Exact. One cubic yard is a cube three feet on each side.
- Bags from volume
- Bags = cubic feet needed / bag yield in cubic feet
- Round up, because partial bags cannot be bought.
Worked example
A rectangular slab 20 feet long and 12 feet wide, poured 4 inches thick
- 1Find the area: 20 ft x 12 ft = 240 square feet.
- 2Convert the thickness to feet: 4 in / 12 = 0.3333 ft.
- 3Find the volume in cubic feet: 240 sq ft x 0.3333 ft = 80 cubic feet.
- 4Convert to cubic yards: 80 / 27 = 2.96 cubic yards.
- 5Check with the shortcut: 240 x 4 / 324 = 2.96 cubic yards, the same answer.
The slab needs about 2.96 cubic yards, or 80 cubic feet, of concrete before any allowance.
Worked examples
Circular pad
A round pad 10 feet across, poured 4 inches thick
- 1Halve the diameter to get the radius: 10 ft / 2 = 5 ft.
- 2Find the area: pi x 5^2 = 78.54 square feet.
- 3Convert the thickness: 4 in / 12 = 0.3333 ft.
- 4Multiply: 78.54 sq ft x 0.3333 ft = 26.18 cubic feet.
- 5Convert: 26.18 / 27 = 0.97 cubic yards.
The round pad needs about 0.97 cubic yards, or 26.18 cubic feet.
Two-section project
A patio 12 ft x 10 ft plus a walkway 20 ft x 3 ft, both 4 inches thick
- 1Patio area: 12 x 10 = 120 square feet.
- 2Walkway area: 20 x 3 = 60 square feet.
- 3Add the areas: 120 + 60 = 180 square feet.
- 4Apply the shortcut once: 180 x 4 / 324 = 2.22 cubic yards.
- 5In cubic feet that is 180 x 0.3333 = 60 cubic feet.
The two sections together need about 2.22 cubic yards, or 60 cubic feet.
Bag count from a yield you enter
A 4 ft x 4 ft equipment pad at 4 inches, using a bag whose label states a 0.6 cubic foot yield
- 1Find the area: 4 x 4 = 16 square feet.
- 2Find the volume: 16 x 0.3333 = 5.33 cubic feet.
- 3Divide by the yield printed on the bag: 5.33 / 0.6 = 8.89 bags.
- 4Round up to whole bags: 9 bags.
This pad needs 9 bags at that yield. The 0.6 figure is the example yield entered here, not a product specification: read the yield off the bag you actually buy.
Cost estimate from your own price
Ordering 3 cubic yards of ready-mix at a quoted price of $160 per cubic yard with a $90 delivery fee
- 1Multiply the volume by the quoted price: 3 x $160 = $480.
- 2Add the delivery fee: $480 + $90 = $570.
- 3Divide by the volume for a delivered rate: $570 / 3 = $190 per cubic yard.
The estimated total is $570. The $160 price and $90 fee are figures entered for this example. Use the numbers your own supplier quotes, because concrete pricing varies by mix, quantity, and location.
How concrete volume is calculated
Concrete is sold and ordered by volume, so every concrete estimate is a volume problem before it is anything else. The volume of a slab is its surface area multiplied by its thickness. That single sentence covers most residential pours, and the only real work is keeping the units consistent while you do it.
The complication is that areas are usually measured in feet while thickness is usually specified in inches. A 4 inch slab is not 4 feet thick, so the thickness has to be converted before it is multiplied. Dividing inches by 12 turns them into feet, after which area in square feet times thickness in feet gives a volume in cubic feet.
The last step converts cubic feet into cubic yards, because that is the unit ready-mix suppliers quote and deliver in. One cubic yard is exactly 27 cubic feet, since a cubic yard is a cube three feet on each side and 3 x 3 x 3 equals 27. That factor is exact and never rounds. If you want to work through the underlying conversions in more detail, see how to calculate cubic yards.
The concrete formula using square feet and thickness
Because the two conversions are fixed, they can be folded into one number. Dividing by 12 converts inches to feet, and dividing by 27 converts cubic feet to cubic yards. Doing both at once means dividing by 12 times 27, which is 324.
That gives the shortcut most estimators use: square feet multiplied by thickness in inches, divided by 324, equals cubic yards. A 500 square foot slab at 6 inches works out to 500 times 6 divided by 324, which is 9.26 cubic yards. The long route through cubic feet gives the identical answer, so use whichever you find easier to check.
The shortcut is worth memorizing because it makes mental checks easy. If a supplier quotes you a volume that is wildly different from your own square feet times inches divided by 324, one of the two numbers deserves a second look before the truck is booked. For a dedicated tool that does only this step, see the square feet to cubic yards calculator.
Ready-mix versus bagged concrete
Both routes deliver the same calculated volume. Which one suits a project depends on the size of the pour, site access, how fast the concrete has to be placed, and what you can source locally. This calculator does not recommend either one, and it does not know your site.
| Consideration | Ready-mix delivered | Bags mixed on site |
|---|---|---|
| How it is quoted | By the cubic yard, usually with delivery and other fees listed separately | By the bag, with the yield printed on the bag |
| What the calculator needs | Your quoted price per cubic yard and any fees | The bag yield in cubic feet and your price per bag |
| Placement | Arrives mixed and is placed continuously | Mixed in batches, so placement is paced by your mixing capacity |
| Common fit | Larger continuous pours where a single monolithic placement matters | Small pours, repairs, posts, and piers, or sites a truck cannot reach |
| What to confirm | Order increments, scheduling, access, and any minimum or short-load terms | Yield per bag, how many bags you can mix before the first batch sets |
How concrete bags are calculated
A bag of concrete mix is sold by weight, but it is used by volume, so every bag prints a yield: the volume of finished concrete one bag produces. That yield is the only number that turns a volume into a bag count, and it differs between products and between bag weights of the same product.
The arithmetic is a single division. Take the volume you need in cubic feet, divide it by the yield in cubic feet, then round up, because you cannot buy part of a bag. A pour needing 12 cubic feet at a 0.6 cubic foot yield takes 20 bags exactly. The same pour at a 0.45 cubic foot yield takes 26.67 bags, which rounds up to 27.
YardCalc does not publish bag yields for specific products, and this calculator does not assume one. The yields used in the table below are round numbers chosen to demonstrate the division, not product data. Read the yield off the bag you intend to buy, or take it from the manufacturer documentation, and enter that. Working the other way is how projects end up several bags short. The concrete bag calculator covers this in more detail.
Multiple sections and irregular areas
Real pours are rarely one clean rectangle. A patio wraps around a corner, a driveway widens at the street, a slab has a separate landing at the door. Trying to average an irregular outline into a single rectangle is where most estimates go wrong, usually by underestimating.
The reliable method is to break the outline into shapes you can measure exactly, calculate each one, then add the volumes. A rectangle plus a triangle handles most angled corners. A rectangle plus a half circle handles a rounded end. Nothing is lost by using more sections than you think you need, and every section can carry its own quantity if the same shape repeats.
Add all the sections before converting to cubic yards rather than converting each one and adding the rounded results. Rounding several times and then summing introduces error that rounding once at the end avoids. The calculator above does this for you: it totals the area first, applies the thickness, and converts once.
Allowance and measurement variation
Calculated volume assumes a perfectly flat subgrade and exact dimensions. Neither is ever quite true. A subgrade that dips by half an inch across a large slab holds noticeably more concrete than the geometry predicts, forms bow slightly under pressure, and some material is always lost to spillage and to what stays in the chute or the mixer.
For that reason many estimators order slightly more than the calculated figure. YardCalc does not set that percentage for you, because the right allowance depends on how carefully the subgrade was prepared, how confident you are in the measurements, and how costly a shortfall would be on your particular pour. A shortfall in the middle of a monolithic slab is a much bigger problem than a shortfall on a set of fence posts.
The allowance field is there so you can apply your own judgment, and the result shows both the calculated volume and the adjusted figure so you can see exactly what the allowance added. Discuss the final number with your supplier, who will know the order increments they work in.
Estimating concrete cost
Cost follows volume. Once the cubic yards are known, a ready-mix estimate is the volume multiplied by the price per cubic yard, plus any fixed charges the supplier lists. A bagged estimate is the whole bag count multiplied by the price per bag, plus any fixed charges.
YardCalc does not publish concrete prices. Prices vary by mix design, by quantity, by season, by fuel cost, and by how far the plant is from the site, and a national average would be misleading for any specific project. Every figure in the cost fields is one you enter, taken from a real quote or a current shelf price.
Fixed fees matter more on small pours than most people expect. A delivery charge spread across ten cubic yards is a small addition per yard; the same charge on a single yard can rival the material cost. That is why the calculator shows the material subtotal and the fees separately before the total, and why entering both pricing routes shows the arithmetic difference between them. For a dedicated tool, see the concrete cost calculator.
Common mistakes
Most concrete shortfalls trace back to one of these:
- Multiplying by the thickness in inches instead of converting it to feet first, which overstates the volume by a factor of twelve.
- Measuring the inside of the forms in some places and the outside in others, so the area does not match what will actually be filled.
- Averaging an irregular outline into one rectangle rather than splitting it into measurable sections.
- Assuming a bag yield instead of reading it off the bag, then buying by a count that was never right for that product.
- Rounding each section to the nearest quarter yard and then adding, rather than adding first and rounding once at the end.
- Forgetting that a dished or rutted subgrade holds more concrete than a flat one, and ordering the bare geometric figure with no allowance.
- Treating a calculated volume as an order quantity without checking the increments and terms the supplier actually works in.
- Using a slab thickness copied from an unrelated project rather than the one specified for yours.
Square feet to concrete cubic yards by thickness
Cubic yards required for a given area at a given thickness, calculated as square feet times thickness in inches divided by 324. Thickness is a project decision and is not recommended here.
| Area | 2 in | 3 in | 4 in | 5 in | 6 in | 8 in | 12 in |
|---|---|---|---|---|---|---|---|
| 25 sq ft | 0.154 | 0.231 | 0.309 | 0.386 | 0.463 | 0.617 | 0.926 |
| 50 sq ft | 0.309 | 0.463 | 0.617 | 0.772 | 0.926 | 1.23 | 1.85 |
| 100 sq ft | 0.617 | 0.926 | 1.23 | 1.54 | 1.85 | 2.47 | 3.7 |
| 150 sq ft | 0.926 | 1.39 | 1.85 | 2.31 | 2.78 | 3.7 | 5.56 |
| 200 sq ft | 1.23 | 1.85 | 2.47 | 3.09 | 3.7 | 4.94 | 7.41 |
| 300 sq ft | 1.85 | 2.78 | 3.7 | 4.63 | 5.56 | 7.41 | 11.11 |
| 500 sq ft | 3.09 | 4.63 | 6.17 | 7.72 | 9.26 | 12.35 | 18.52 |
| 750 sq ft | 4.63 | 6.94 | 9.26 | 11.57 | 13.89 | 18.52 | 27.78 |
| 1,000 sq ft | 6.17 | 9.26 | 12.35 | 15.43 | 18.52 | 24.69 | 37.04 |
Concrete coverage by thickness
How much area a given volume covers at each thickness. The first row answers the common question of how far one cubic yard of concrete goes: 81 square feet at 4 inches, and 27 square feet at 12 inches.
| Cubic yards | 2 in | 3 in | 4 in | 5 in | 6 in | 8 in | 12 in |
|---|---|---|---|---|---|---|---|
| 1 | 162 sq ft | 108 sq ft | 81 sq ft | 64.8 sq ft | 54 sq ft | 40.5 sq ft | 27 sq ft |
| 2 | 324 sq ft | 216 sq ft | 162 sq ft | 129.6 sq ft | 108 sq ft | 81 sq ft | 54 sq ft |
| 5 | 810 sq ft | 540 sq ft | 405 sq ft | 324 sq ft | 270 sq ft | 202.5 sq ft | 135 sq ft |
| 10 | 1,620 sq ft | 1,080 sq ft | 810 sq ft | 648 sq ft | 540 sq ft | 405 sq ft | 270 sq ft |
Bag-yield math: bags per cubic yard
Mathematical bag-yield examples showing 27 cubic feet divided by each yield. These are round numbers chosen to demonstrate the arithmetic, not product data. Use the yield printed on the actual concrete bag or supplier documentation.
| Bag volume | Bags per cubic yard | Whole bags for 1 cubic yard |
|---|---|---|
| 0.33 cu ft | 81.82 | 82 |
| 0.45 cu ft | 60 | 60 |
| 0.6 cu ft | 45 | 45 |
| 0.75 cu ft | 36 | 36 |
| 1 cu ft | 27 | 27 |
Frequently asked questions
- How much concrete do I need for a 10 x 10 slab?
- A 10 by 10 foot slab is 100 square feet. At 4 inches thick that is 100 x 4 / 324, which is 1.23 cubic yards, or 33.33 cubic feet. At 6 inches it is 1.85 cubic yards. The thickness you use should come from your project specification rather than from a rule of thumb, which is why the calculator asks for it.
- How many cubic feet are in a cubic yard of concrete?
- Exactly 27. A cubic yard is a cube three feet on each side, and 3 x 3 x 3 equals 27. This holds for any material, not just concrete, and it never rounds.
- How many square feet does a cubic yard of concrete cover?
- It depends entirely on the thickness. One cubic yard covers 162 square feet at 2 inches, 108 square feet at 3 inches, 81 square feet at 4 inches, 54 square feet at 6 inches, and 27 square feet at 12 inches. The coverage table above lists more thicknesses.
- How many bags of concrete make a cubic yard?
- Divide 27 cubic feet by the yield printed on the bag. A bag yielding 0.6 cubic feet takes 45 bags per cubic yard; one yielding 0.45 cubic feet takes 60. Because yields differ by product and bag weight, read the figure off the bag you plan to buy rather than using a remembered number.
- Should I order ready-mix or use bags?
- That depends on the size of the pour, whether a truck can reach the site, how quickly the concrete has to be placed, and what your supplier offers. This calculator prices both routes from figures you enter so you can compare them, but it does not recommend one, because the right answer is specific to your site and schedule.
- How much extra concrete should I order?
- There is no single correct allowance. The right figure depends on how level the subgrade is, how confident you are in the measurements, and how serious a shortfall would be on your pour. The calculator has an optional allowance field so you can apply your own judgment, and your supplier can advise on the increments they deliver in.
- How thick should my concrete be?
- YardCalc does not prescribe a thickness. Required thickness depends on the loads the concrete will carry, the subgrade, the reinforcement, the climate, and local code, so it belongs to your project specification, your designer, or your building department. Enter the thickness that applies to your project and the calculator handles the arithmetic.
- Does this calculator work in metric?
- Yes. Dimensions can be entered in centimeters or meters and thickness in millimeters, centimeters, or meters, and results are shown in cubic meters alongside cubic yards and cubic feet. The underlying conversions are exact.
Volume is calculated from your measurements using exact geometry, with one cubic yard treated as exactly 27 cubic feet. Bag counts come only from the yield you enter, and cost estimates only from prices and fees you enter. No concrete density, bag yield, price, or slab thickness is assumed anywhere on this page. Concrete calculations estimate material quantity from geometry and user-entered values. Confirm mix design, thickness, reinforcement, code requirements, bag yield, delivery, and final order quantity with the appropriate supplier or professional.
Results are planning estimates. Last reviewed 2026-07-28.