Pouring a patio, driveway, or setting fence posts? Just enter your dimensions and our free concrete calculator tells you exactly how many cubic yards or bags you need โ plus estimated cost. It even adds a waste factor so you never run short. No math, no guesswork, just the right amount of concrete every time.
You're building something. A patio. A driveway. Maybe a set of steps. And now you're staring at a pile of numbers, trying to figure out how much concrete to buy.
It's a pain, right? Order too little and you're stuck with a half-finished slab. Order too much and you've got a pile of hardened concrete you can't return.
That's where our concrete calculator comes in. It does all the math for you. But here's the thing โ you still need to know what numbers to put in. And that's what this guide is for.
Think of it as a smart measuring cup for your project. You tell it the size of the space you're filling, and it tells you exactly how much concrete you need.
It's not magic. It's just a formula that turns your length, width, and depth into a volume. But it saves you from doing the math yourself. And trust me, that's a good thing.
Here's what our calculator does for you:
Gives you the volume in cubic yards (that's how concrete is sold)
Tells you how many bags of concrete mix to buy
Estimates the total cost
Accounts for waste so you don't run short
This is the part that confuses everyone. You measure your project in feet and inches. But concrete is sold in cubic yards. Why?
Because concrete is heavy. A cubic foot of concrete weighs about 150 pounds. A cubic yard? That's 27 cubic feet. So a cubic yard of concrete weighs over 4,000 pounds. That's as much as a small car.
So when you order concrete, you're ordering by the truckload. And trucks measure in cubic yards. It's just how the industry works.
1 cubic yard = 27 cubic feet
That's it. Our calculator does this for you automatically.
Okay, here's the math. But I promise it's not scary.
Volume = Length ร Width ร Depth
That's it. That's the whole formula. You just multiply the three dimensions together.
But here's where people mess up: all three measurements need to be in the same unit. If your length is in feet and your depth is in inches, you can't just multiply them together. You need to convert the inches to feet first.
Quick Conversions:
So if you're pouring a slab that's 10 feet long, 8 feet wide, and 4 inches deep, the math looks like this:
10 ร 8 ร 0.33 = 26.4 cubic feet
Then divide by 27 to get cubic yards: 26.4 รท 27 = 0.98 cubic yards
So you'd need about 1 cubic yard of concrete. See? Not so bad. But our calculator does all this for you, so you don't have to think about it.
Here's a secret that most calculators don't tell you: you always need more concrete than the math says. Why? Because concrete isn't perfect. It spills. It gets stuck in the mixer. The ground isn't perfectly level. And you might make a small mistake in your measurements.
Our calculator lets you set your own waste factor. But if you're not sure, go with 10%. It's the safest bet.
This is the big decision. And it depends on how much concrete you need.
Use it when: You need more than 1 cubic yard, have good truck access, and want the strongest concrete.
Downsides: Must use all at once, minimum orders (3-5 yards), pricier for small jobs.
Use it when: You need less than 1 cubic yard, can't get a truck there, or want to work at your own pace.
Downsides: Lots of physical work, harder to mix consistently, pricier per yard for large projects.
Rule of thumb: if your project needs more than 50 bags of concrete, order ready-mix. Your back will thank you.
This is the most common question we get. And the answer depends on the bag size.
| Bag Size | Makes (cubic feet) | Bags per Cubic Yard |
|---|---|---|
| 40-pound bag | 0.3 cu ft | 90 bags |
| 60-pound bag | 0.45 cu ft | 60 bags |
| 80-pound bag | 0.6 cu ft | 45 bags |
Our calculator does this math for you. Just tell it what bag size you're using, and it'll tell you exactly how many to buy.
12 feet by 10 feet, 4 inches deep (standard). With 10% waste, you need about 1.5 cubic yards or 68 bags of 80-pound mix.
30 feet long, 3 feet wide, 4 inches deep. You'd need about 1.1 cubic yards or 50 bags of 80-pound mix.
10 posts, each hole 12 inches wide and 3 feet deep. You'd need about 2.4 cubic yards or 108 bags of 80-pound mix. Remember: post holes are wider at the bottom โ measure the widest part.
Mistake #1: Using the wrong depth.
A patio needs 4 inches. A driveway needs 6 inches. A foundation needs 12 inches or more. Don't guess. Look up the recommended depth for your project.
Mistake #2: Forgetting about the waste factor.
You will spill some. You will make a mistake. Add 10% to your total. It's cheap insurance.
Mistake #3: Not accounting for the bell shape of post holes.
Post holes are wider at the bottom. If you measure only the top, you'll be short. Measure the widest part of the hole.
Mistake #4: Using the wrong bag size.
An 80-pound bag makes 0.6 cubic feet. A 60-pound bag makes 0.45 cubic feet. Don't mix them up. Our calculator lets you choose the right one.
Mistake #5: Ordering concrete in cold weather.
Concrete doesn't set properly below 40ยฐF. If you're pouring in cold weather, you need special additives. Talk to your concrete supplier.
Tip 1: Use the right mix strength.
Concrete comes in different PSI ratings. 2,500 PSI for sidewalks/patios. 3,000 PSI for driveways/garage floors. 4,000 PSI for foundations. 5,000 PSI for industrial. For most DIY, 3,000 PSI is good.
Tip 2: Add reinforcement.
Concrete is strong under compression but weak under tension. That's why it cracks. Adding rebar or wire mesh makes it much stronger. For slabs, use 1/2-inch rebar on 2-foot centers.
Tip 3: Control joints are your friend.
Concrete will crack. But you can control where by cutting control joints โ grooves that create weak points. The concrete cracks there instead of randomly.
Tip 4: Cure it properly.
Concrete needs to stay moist while curing. If it dries too fast, it'll be weak. Cover with plastic sheeting or wet burlap for at least 7 days. Keep it damp.
Not every project is a perfect rectangle. Here's how to handle other shapes.
Circular slabs:
Measure the diameter (width across the circle). The calculator will use the formula for a cylinder.
L-shaped slabs:
Break it into two rectangles. Calculate each one separately, then add them together.
Triangular slabs:
Measure the base and the height. The formula is (base ร height ร depth) รท 2.
Curved walkways:
Measure the length along the curve. Then measure the width. Treat it like a rectangle. It won't be perfect, but it'll be close enough.
Did you know that concrete is the most widely used man-made material in the world? We use more concrete than all other building materials combined. The only thing we use more of is water.
The ancient Romans were concrete masters. The Pantheon in Rome โ built around 126 AD โ has the world's largest unreinforced concrete dome. It's still standing after almost 2,000 years. Their secret? They mixed volcanic ash into their concrete, which made it waterproof and self-healing.
Multiply length ร width ร depth (all in feet) to get cubic feet. Then divide by 27 to get cubic yards. For example, a 10'ร8'ร4" slab = 10 ร 8 ร 0.33 = 26.4 cu ft รท 27 = 0.98 cubic yards. Or just use our calculator โ it does all this automatically.
About 45 bags of 80-pound mix, 60 bags of 60-pound mix, or 90 bags of 40-pound mix. Each 80-pound bag makes about 0.6 cubic feet, and a cubic yard is 27 cubic feet.
If you need more than 1 cubic yard (or more than 50 bags), order ready-mix from a truck. For smaller projects, bagged concrete is fine. Ready-mix is stronger and more consistent, but you need to use it all at once.
Patios and walkways: 4 inches. Driveways: 6 inches. Garage floors: 6 inches. Foundations: 8-12 inches or more depending on local codes. Always check your local building requirements.
PSI stands for pounds per square inch โ it measures concrete strength. 2,500 PSI is good for sidewalks. 3,000 PSI for driveways. 4,000+ PSI for foundations and heavy loads. Higher PSI = stronger concrete.
Add 10% for most DIY projects. Pros might use 5%. Beginners or complex projects should use 15%. It's better to have a little extra than to run short mid-pour.
Measure the hole's diameter and depth in inches. The formula for one hole: (ฯ ร radiusยฒ ร depth) รท 1,728 gives you cubic feet. Multiply by the number of posts. Remember: holes are often wider at the bottom โ measure the widest part.
It's not recommended below 40ยฐF. Concrete needs to stay above 40ยฐF while curing. In cold weather, use hot water in the mix, special cold-weather additives, and cover the concrete with insulating blankets.
Concrete reaches about 50% strength in 3 days, 75% in 7 days, and 90%+ in 28 days. Keep it moist for at least 7 days for best results. Light foot traffic after 24 hours, vehicles after 7 days.
Concrete shrinks as it cures, which causes cracking. Control joints (grooves cut into the slab) direct cracks to happen in those lines. Also, rapid drying, improper mixing, or missing reinforcement can cause cracking.
Divide cubic feet by 27. For example, 54 cubic feet รท 27 = 2 cubic yards. Our calculator handles this conversion automatically.
Ready-mix concrete typically costs $125-$165 per cubic yard, depending on your location and the mix strength. Bagged concrete costs about $4-$6 per 80-pound bag, which works out to $180-$270 per cubic yard equivalent.
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