Introduction
This Concrete Mix Calculator tells you how much concrete you need for a slab, patio, driveway, walkway, or footing. Type in the length, width, and depth of your pour, or just enter the total volume if you already know it. The calculator does the math and shows your answer in cubic yards, cubic feet, and cubic meters.
You also get the number of bags to buy. The tool works out counts for 40 lb, 50 lb, 60 lb, 80 lb, 25 kg, and 30 kg bags, and shows results for 3-inch, 4-inch, and 6-inch slab thickness side by side. Bag counts are always rounded up, because you cannot buy half a bag.
Mixing your own? Pick a mix ratio or concrete grade, like 1:2:4 (M15) or 1:1.5:3 (M20). The calculator splits the job into cement, sand, and gravel so you know how much of each to order. You can also add a waste factor for spills and uneven ground, and open the cost tool to see a price estimate.
Every answer comes with a step-by-step solution, so you can check the math yourself and see exactly where each number came from.
How to use our Concrete Mix Calculator
Enter the size of your pour, pick your mix, and the calculator shows how much concrete you need, how many bags to buy, the cement, sand and gravel amounts, and an estimated cost.
Dimensions or Volume tab: Pick Dimensions if you know the size of the slab. Pick Volume if you already know how much concrete you need.
Area: Type the area of your slab and pick sq ft, sq yd, or sq m. Use this only if you already know the area.
Length: Type how long the slab is and pick the unit (in, ft, yd, cm, or m). Length and width beat the area box if you fill in both.
Width: Type how wide the slab is and pick the unit.
Depth (Slab Thickness): Type how thick the concrete will be, like 4 inches for a patio. Pick the unit.
Volume Needed: On the Volume tab, type the amount of wet concrete you need and pick cu ft, cu yd, or cu m.
Result Unit System: Choose Imperial for cubic yards and pound bags, or Metric for cubic meters and kilogram bags.
Mix Type: Choose Standard Mix for walkways, patios, and general slabs. Choose High-Strength / Crack-Resistant for driveways, structural slabs, and cold, freeze-thaw areas.
Mix Ratio / Concrete Grade: Pick the cement : sand : gravel ratio you want, such as 1:2:4 (M15). A stronger grade uses more cement.
Waste Factor (%): Set how much extra to add for spills and uneven ground. Type a number or drag the slider. 10% is a safe start.
Calculate: Click this to see your results. Results also update as you type.
Reset: Click this to clear your entries and go back to the default values.
Cement unit buttons: In the material breakdown, choose to see cement as 94 lb bags, cubic yards, or cubic meters.
Estimate Project Cost: Click this button to open the cost tools.
Price per Bag: Type what one bag costs at your store.
Bag Size: Pick the bag size you plan to buy, from 40 lb up to 80 lb, or 25 kg and 30 kg.
Concrete Mix: What It Is and How It Works
Concrete is made from four things: cement, sand, gravel (stone), and water.1 Cement is the glue. Sand fills the small gaps. Gravel gives the concrete its bulk and strength. Water starts a chemical change called hydration that makes the mix harden into rock.1 Getting the amounts right is what makes a slab strong instead of weak and crumbly.
Mix Ratios and Concrete Grades
A mix ratio like 1:2:4 means 1 part cement, 2 parts sand, and 4 parts gravel by volume. More cement means more strength. Each ratio matches a grade, such as M15, where the number is the strength in megapascals (MPa) after 28 days of curing.
- 1:5:10 (M5) and 1:4:8 (M7.5): weak fill, blinding layers, under footings.
- 1:3:6 (M10): light paths, small pads, garden work.
- 1:2:4 (M15): the common all-around mix for patios, walkways, and sheds.
- 1:1.5:3 (M20): driveways, garage floors, and slabs that hold weight.
- 1:1:2 (M25): columns, beams, and other structural jobs.
Wet Volume vs. Dry Volume
The space you fill is the wet volume. But dry cement, sand, and gravel are full of tiny air pockets. When you add water, those pockets collapse and the pile shrinks. So you must buy more dry material than the hole holds. Builders multiply the wet volume by about 1.54 (or 1.57 for richer mixes) to get the dry volume they need to order.
Slab Thickness
Thickness controls how much concrete you need and how much load the slab can carry. Common choices:
- 3 inches (7.6 cm): walkways and foot traffic only.
- 4 inches (10.2 cm): patios, sheds, and most home slabs.
- 6 inches (15.2 cm): driveways, RV pads, and heavy vehicles.
For a slab-only estimate, the Concrete Slab Calculator works from the same numbers.
Bagged Concrete Yields
Pre-mixed bags already have cement, sand, and gravel inside. You just add water. Each bag makes a set amount of wet concrete:
- 40 lb bag ≈ 0.30 cu ft
- 60 lb bag ≈ 0.45 cu ft
- 80 lb bag ≈ 0.60 cu ft2
- 25 kg bag ≈ 0.41 cu ft3, about 0.012 m³
That means one cubic yard takes about 45 bags of 80 lb mix. High-strength and crack-resistant bags yield no less. Quikrete's 5000 high-strength mix also makes about 0.60 cu ft per 80 lb bag.4
Why You Add a Waste Factor
Real job sites are never perfect. Some concrete spills, the ground under the slab dips, and forms bulge a little. The National Ready Mixed Concrete Association advises ordering 4% to 10% more than the volume worked out from plan dimensions.6 That keeps you from running short in the middle of a pour, which can leave a cold joint and a weak spot. For rough or uneven subgrades, go higher.
Bags or Ready-Mix?
Bags work well for small jobs, usually under 1 cubic yard. Past that, mixing by hand takes too long and the pour can set unevenly. For larger slabs, a ready-mix truck is cheaper and gives a more uniform result. Most suppliers have a minimum order of about 1 cubic yard. Check the load and hauling limits with the Concrete Weight Calculator before you book delivery.
Quick Tips for a Strong Pour
- Do not add extra water to make the mix easier to move. More water means weaker concrete.7
- Compact the base gravel before you pour so the slab does not settle and crack.2
- Keep the fresh slab damp while it cures, typically for 3 to 7 days.8 Concrete left to dry out can lose as much as 50 percent of its potential strength.8
- Space control joints no more than 24 to 36 times the slab thickness apart, about 10 feet for a 4-inch slab, so cracks form where you want them.9
- Add steel where the slab carries load. The Rebar Calculator lays out spacing and total bar length.
- Normal-weight concrete weighs about 140 to 150 lb per cubic foot (2,240 to 2,400 kg per m³).10 That is roughly 4,000 lb per cubic yard, so plan your hauling.