Concrete Calculator: Estimate Yards, Bags & Cost
Use our Concrete Calculator to quickly estimate how many cubic yards, bags, and total cost you need for your project. Just enter your measurements and get a simple concrete estimate in seconds.
Table of Contents
How much concrete do I need for a 10×10 ft slab?
For a standard 10×10 ft slab at 4 inches thick, you need about 1.23 cubic yards (33 cu ft) of concrete — roughly 56 bags of 80 lb mix. Add the usual 10% waste allowance and you’re ordering closer to 1.35 cu yd or 61 bags. Use the calculator below to plug in your own dimensions and get exact numbers instantly.
You need
1.23 cu yd
33.3 cu ft total volume
- With waste factor included1.35 cu yd
- Bags needed61 bags
- Cost — bagged concrete$427
- Cost — ready-mix delivered$189
In this guide
Why use a concrete calculator
Picture this: you've decided to pour your own patio, fence posts, or a small slab for a shed. You head to the store, grab a stack of bags that feels like plenty, and get to work feeling pretty good about yourself. Halfway through the pour, you notice the pile is shrinking fast — and you're still short of the finish line. So you drive back, grab more bags, mix again, and... it's still not enough.
By the time the job is actually done, the store's closed, your arms are dead, and you've made two extra trips you didn't plan for. Or the opposite happens — you way overbuy "just to be safe," and now there are a dozen unused bags sitting in the garage that have already started curing before you even opened them.
A concrete calculator fixes this in one step. Enter your dimensions once, and you'll know exactly how many cubic yards, how many bags, and how much money the job actually needs — before you ever load the cart.
How much concrete do I need?
Every concrete estimate — whether it's a slab, a footing, or a row of fence-post holes — comes down to the same four steps.
The 4-step method
- Find the volume. Measure the length, width, and thickness (or diameter and depth, for round posts) of what you're pouring, and multiply. That gives you cubic feet.
- Convert to cubic yards. Ready-mix concrete is sold by the cubic yard, not the cubic foot, so divide your total by 27 (there are 27 cubic feet in one cubic yard).
- Match it to a bag size. Bagged concrete lists its yield right on the packaging — usually 0.3 to 0.6 cubic feet per bag depending on weight. Divide your total cubic feet by that yield to get your bag count.
- Add a waste factor. Uneven ground, spillage, and the occasional cracked form board all eat into your total. Add 5–10% on top so you're not stopping mid-pour to make an emergency run.
All of that math is exactly what the calculator above handles automatically — just switch between the rectangular and round tabs depending on your shape, and it does the conversion, the bag rounding, and the cost comparison for you.
A worked example
Let's run actual numbers. Say you're pouring 4 concrete footings, each 3 feet long, 3 feet wide, and 3 feet deep.
Step-by-step
- Volume per footing: 3 × 3 × 3 = 27 cubic feet.
- Total for 4 footings: 4 × 27 = 108 cubic feet.
- Convert to yards: 108 ÷ 27 = 4 cubic yards.
- In 80 lb bags (0.6 cu ft yield each): 108 ÷ 0.6 = 180 bags before waste.
- Add 5% waste: 180 × 1.05 ≈ 189 bags.
That's a big enough pour that ready-mix delivery would almost certainly beat hauling and mixing 189 bags by hand — which is exactly the kind of trade-off the calculator flags for you automatically once your total crosses about 1.5 cubic yards.
How thick should your slab be?
Thickness has a direct, 1-to-1 effect on how much concrete you'll use — a 6-inch slab needs exactly 50% more material than the same 4-inch slab. Getting this number right matters more than almost any other input.
| Project | Typical thickness | Notes |
|---|---|---|
| Walkway / stepping path | 4 in | Light foot traffic only |
| Patio | 4 in | Add wire mesh for crack control |
| Standard driveway | 4–5 in | 6 in for trucks or heavy vehicles |
| Garage floor | 4–6 in | 6 in recommended for storage/lift loads |
| Shed / post footing | varies by frost depth | Check your local frost line |
Bags or ready-mix?
What if your project is small — just a few stepping stones or one post hole? Bags win, no question. What if it's a full driveway? Ready-mix wins just as clearly. The tricky part is the middle ground, and that's where most of the wasted money happens.
As a rule of thumb, under 1 cubic yard, bags are usually more convenient — most ready-mix suppliers charge extra "short load" fees for anything under a full yard anyway. Above roughly 1.5 cubic yards, ready-mix is almost always the cheaper option once you factor in your own time spent mixing.
Not sure which side of that line your project falls on?
Run your numbers in the calculator above →Common mistakes to avoid
- ✕Forgetting to convert inches to feet. Thickness is almost always measured in inches — divide by 12 before multiplying, or the whole estimate comes out 12 times too high.
- ✕Skipping the waste factor. A "perfect" volume calculation ignores uneven subgrade and spillage. Always add 5–10% on top.
- ✕Assuming every bag yields the same amount. A 40 lb, 60 lb, and 80 lb bag all cover different volumes — check the label, don't guess.
- ✕Ordering ready-mix in fractions the supplier won't sell. Most yards round up to the nearest quarter-yard or whole yard, and short loads carry extra fees.
Why trust this guide
Every formula on this page follows standard industry yield rates — 27 cubic feet per cubic yard, and bag yields as printed by major concrete mix manufacturers. Pricing ranges reflect typical 2026 US market rates for bagged and ready-mix concrete. Always confirm exact yield and pricing with your local supplier, since both vary by brand and region.
Frequently Asked Questions
Related Article: Concrete Slab Calculator
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