Choose a rectangular pour or a round hole or column.
Thickness is the pour depth of the slab or footing.
Enter a post diameter to subtract the space the post takes up.
Yield per bag: 40 lb = 0.30, 50 lb = 0.375, 60 lb = 0.45, 80 lb = 0.60 cu ft.
10 percent is a common allowance for spillage and uneven subgrade.
Enter dimensions to see how many bags you need.
Top 10 Tips for Buying Concrete by the Bag
Tip 1: Bagged Concrete Is Built for Small, Controlled Pours
Bagged mix combines cement, sand, and gravel in one product — add water and you’re pouring. It exists for jobs where a ready-mix truck is overkill: fence posts, small pads, repairs, and slabs under about a cubic yard.
Tip 2: Bag Size Changes How Many Trips to the Truck You’re Making
40, 50, 60, and 80 pound bags all yield different volumes per bag, with the 80 pound bag covering the most per bag but also being the heaviest to lift and mix alone. Pick the size you can actually carry and mix in one batch, not just the one with the fewest bags on paper.
Tip 3: Slab Mode and Round Mode Solve Different Problems
Use slab or footing mode for patios, walkways, and pads measured by length, width, and thickness. Use round or post hole mode for fence posts and columns, measured by diameter and depth instead — and if you’re setting a post in the hole, enter its diameter to deduct so you’re not over-ordering concrete for space the post itself takes up.
Tip 4: Thickness Depends on What’s Going to Sit on Top of It
Four inches is standard for patios, walkways, and most residential pads. Anywhere a vehicle will be parked, like a driveway, needs six inches instead. Getting this wrong doesn’t show up until the slab is already cracking under weight it wasn’t poured for.
Tip 5: Measure in the Units You’ll Actually Use to Order
Enter length, width, and thickness in feet and inches, or diameter and depth for round pours, matching whichever unit toggle you’re working in. A mismatched unit entry throws off every number downstream, so double-check the toggle before you start typing dimensions.
Tip 6: Cubic Feet and Cubic Yards Aren’t Interchangeable When You’re Ordering
One cubic yard equals 27 cubic feet. Bags are sold and yield by the cubic foot, while ready-mix trucks deliver by the cubic yard — so know which unit you’re working in before you call around for pricing.
Tip 7: Never Order the Bare Minimum
The calculator defaults to a 10 percent waste buffer, and that’s not padding — it’s insurance. Running short mid-pour means fresh concrete starts setting before you finish, creating a cold joint: a visible seam and a genuine structural weak point, not just a cosmetic flaw.
Tip 8: Know the Point Where Bags Stop Making Sense
Once your pour exceeds roughly one cubic yard, about 45 to 50 eighty-pound bags, ready-mix delivery is usually cheaper per yard and saves you from mixing dozens of bags by hand. Run the numbers both ways before committing to bags for anything approaching that volume.
Tip 9: Avoid the Mistakes That Turn One Pour Into Two
Common errors include skipping the waste buffer, pouring 4 inches where 6 was needed, forgetting to deduct post diameter on round pours, and mixing too slowly and losing workability before the pour is finished. Any one of these means a second trip or a weaker finished slab.
Tip 10: Curing Time Protects the Work You Just Did
Concrete reaches walkable strength in 24 to 48 hours under normal conditions, but full cure takes 28 days. Loading it too soon, with furniture, vehicles, or heavy foot traffic, can compromise a slab that otherwise would have lasted decades.
Where to Go From Here
Now that you understand bag size tradeoffs, slab thickness by use, the waste buffer, and when to switch to ready-mix, use our Concrete Bag Calculator above to get your exact bag count. You’ll know exactly what to enter because you understand why each choice affects the number.
Frequently Asked Questions About Buying Concrete
How do I calculate how much concrete I need for a slab? Multiply the length by the width by the thickness (all in feet) to get cubic feet. Divide by 27 to convert to cubic yards. Add 10 percent for waste. The calculator above does this automatically.
How many bags of concrete do I need for a 10×10 slab? At 4 inches thick, a 10×10 slab is about 33 cubic feet. Using 80-pound bags (0.6 cubic feet each), you need roughly 56 bags before waste — closer to 62 with a 10 percent overage.
What is the difference between cubic feet and cubic yards? One cubic yard equals 27 cubic feet. Bags are sold by cubic feet of yield. Ready-mix trucks deliver by the cubic yard. Make sure you’re working in the right unit before you order.
How thick should a concrete slab be? Four inches is standard for patios, walkways, and most residential pads. Use 6 inches for driveways or anywhere vehicles will be parked. Structural applications require engineered specifications.
When should I use ready-mix instead of bags? Once your pour exceeds about 1 cubic yard — roughly 45 to 50 eighty-pound bags — ready-mix delivery is usually cheaper per yard and significantly less labor. It also reduces the risk of inconsistent mixing across batches.
Can I use this calculator for round pours like fence posts or columns? Yes. Switch to the round/post hole mode, enter the diameter and depth, and the calculator handles the volume formula for cylindrical pours. You can also enter the post diameter to deduct the space the post itself takes up.
What happens if I run out of concrete mid-pour? Wet concrete that begins to set before the pour is complete creates a cold joint — a visible seam and structural weak point. This is why ordering with a waste factor matters. It is much easier to have a small amount left over than to run short.
How long does concrete take to cure? Concrete reaches working strength — enough to walk on — within 24 to 48 hours under normal conditions. Full cure takes 28 days. Avoid heavy loads on the slab during that window.
Doing more home improvement projects? Be sure to visit our Outdoor and Structures page for more help and ideas!