Concrete Calculator (2026)
Find out how much concrete your slab needs, how many bags that means, and whether bags or ready-mix is cheaper.
Calculate Your Slab
Measure the longest side of the slab in feet.
Measure the shorter side of the slab in feet.
Most patios and walkways are 4 inches thick; driveways are usually 5 to 6 inches.
Yield is the mixed volume each bag produces, per the manufacturer.
Get a quote from a local ready-mix supplier; prices vary by region and mix.
The store price for one bag of the selected size.
Ready-mix cost is calculated from the exact, unrounded cubic yards, then rounded to cents. The cubic yards shown above are rounded to 2 decimals, so multiplying the displayed yards by the price may differ by a few cents. Bags are always rounded up: you cannot buy part of a bag.
How the Math Works
A slab is a rectangular box, so its volume is length times width times thickness. The catch is the units: length and width are in feet, but thickness is in inches, so thickness must be divided by 12 first. Once you have cubic feet, divide by 27 to get cubic yards, because one cubic yard equals 27 cubic feet. Ready-mix is sold by the cubic yard, while bagged concrete is sold by the bag, so the calculator converts the same volume both ways and prices each option.
Volume (cu yd) = volume (cu ft) / 27
Bags needed = volume (cu ft) / yield per bag, rounded UP
Bags cost = bags needed x price per bag
Ready-mix cost = exact volume (cu yd) x price per cubic yard
Slab Thickness Guide
Thickness is the single biggest driver of how much concrete you need. Going from 4 to 6 inches raises the volume by 50 percent for the same slab size, so it pays to match the thickness to the job rather than guessing. This table shows common uses and the thickness each one normally calls for.
| Project | Typical thickness | Why |
|---|---|---|
| Walkway or garden path | 4 inches | Foot traffic only, light loads |
| Patio or shed base | 4 inches | Standard for outdoor living slabs |
| Driveway (cars) | 5 to 6 inches | Vehicle weight needs extra depth |
| Driveway (heavy trucks or RV) | 6 to 8 inches | Heavy axle loads crack thin slabs |
| Garage floor | 5 to 6 inches | Cars plus stored weight, often with reinforcement |
| Workshop or barn floor | 6 inches | Equipment and point loads |
Worked Example: 10 x 10 Foot Slab, 4 Inches Thick
| Calculation | Result | |
|---|---|---|
| Volume in cubic feet | 10 x 10 x (4 / 12) | 33.33 cu ft |
| Volume in cubic yards | 33.33 / 27 | 1.23 cu yd |
| 80 lb bags needed | 33.33 / 0.60, rounded up | 56 bags |
| Bags cost | 56 x $5.50 | $308.00 |
| Ready-mix cost | 1.2346 x $150 (exact yards) | $185.19 |
| Cheaper option | $185.19 is less than $308.00 | Ready-mix |
Notice how much cheaper ready-mix is here, even for a small slab. Bagged concrete makes sense for tiny jobs, repairs, and places a truck cannot reach, but once you pass about one cubic yard, ready-mix usually wins on price alone, before you even count the labor of mixing dozens of bags by hand.
Order About 10% Extra for Waste and Spillage
Real pours never match the textbook volume exactly. Forms bow slightly, the ground is never perfectly level, and some concrete is always lost to spillage, over-excavation, and the wheelbarrow. The standard advice is to order about 10% more than the calculated volume. This is a guide note only: the calculator above shows the exact calculated volume, and the 10% allowance is NOT included in its results, so add it yourself when you order. For the example slab, 10% extra means ordering about 1.36 cubic yards instead of 1.23, or buying 62 bags instead of 56. Running short mid-pour is far worse than having a little left over, because a cold joint where fresh concrete meets partially set concrete weakens the slab.
Footings, Steps, and Other Shapes Need a Different Calculation
This calculator is built for flat rectangular slabs only. Footings are long, narrow trenches with their own depth and width, steps are a stack of boxes of different sizes, and circular pads, columns, and sonotubes need circle math, not slab math. If your project includes footings or steps, calculate those volumes separately and add them to the slab volume. For a footing, multiply its length by its width by its depth, all in feet. For steps, work out each step as its own box and add the boxes together. Mixing shapes into a slab calculation is the most common reason a pour comes up short.
Bags vs Ready-Mix: Which Should You Choose?
- Choose bags for small jobs. Under about half a cubic yard, a few bags mixed in a wheelbarrow are simpler and cheaper than a truck delivery, which often carries a minimum-load or short-load fee.
- Choose ready-mix for anything over about one cubic yard. The per-yard price is usually far lower, the mix is consistent, and the pour finishes in minutes instead of hours.
- Check truck access. If a mixer truck cannot reach the slab, bags may be your only option, or you may need a pump, which adds cost.
- Count your labor. Mixing 56 bags by hand is a full day of hard work for two people. Ready-mix turns that into a morning.
- Compare delivered prices, not just material prices. Ready-mix quotes may add delivery, fuel, or short-load fees, and bag prices may not include tax. Put the all-in numbers into the calculator for a fair comparison.
