Concrete Mix Calculator

What the concrete mix calculator does

If you are batching your own concrete instead of buying it pre-mixed, you need to know how much cement, sand, and gravel to buy and in what proportion. This works that out. You tell it how much concrete you need and which mix ratio you want, and it returns the volume of cement, sand, and gravel, plus the number of cement bags.

You can give it the amount either as a volume you already know, or as a length, width, and depth for it to measure. Below is how it turns that into a shopping list, and what the ratios actually mean.

How to use it

1. Choose how you are entering the amount. Pick Volume if you already know the cubic yards, or Length, Width, and Depth to have the calculator work the volume out.
2. Enter the numbers. Either the wet volume in your chosen unit, or the three dimensions with their units.
3. Pick the mix ratio. Each option shows its strength, so you can match it to the job.
4. Press Calculate for the breakdown, or Reset to clear it.

How the materials are worked out

There is one step here that catches people out, so it is worth understanding. The volume of finished concrete you want is the wet volume. But dry cement, sand, and gravel take up more room than the concrete they make, because each one is full of tiny air gaps before water packs them together. Sand is about 20 percent air, gravel about a third, and together that comes to roughly 54 percent extra.

So the first thing the calculator does is bump the wet volume up to a dry volume:

Dry volume = wet volume × 1.54

Then it splits that dry volume by your ratio. In a 1:2:4 mix there are seven parts in total, so cement is one seventh of the dry volume, sand is two sevenths, and gravel is four sevenths. Finally it turns the cement volume into 94 lb bags so you can order them.

The mix ratios and what they mean

A ratio like 1:2:4 means one part cement to two parts sand to four parts gravel, measured by volume. The more cement in the mix, the stronger the concrete, which is what the strength figure next to each ratio tells you.

For most home jobs, a 1:2:4 mix is the common choice. Step up to 1:1.5:3 or richer when the concrete is carrying real load.

Wet volume, dry volume, and how precise this is

You can enter the amount in cubic yards, cubic feet, or cubic metres, or give dimensions in inches, feet, yards, millimetres, centimetres, or metres. The result shows both the wet volume you asked for and the dry volume it is based on, so you can see where the bigger material figure comes from.

Cement bags are rounded up to a whole bag. The split itself is a nominal, by-volume estimate, the same method used on building sites worldwide, and it is meant for planning and ordering rather than as a lab-tested mix design. The exact amounts shift a little with how damp your sand is and where your aggregate comes from, so order to these numbers and adjust the water on the day.

A worked example: one cubic yard of 1:2:4

Say you need one cubic yard of concrete in a 1:2:4 mix.

First the dry volume: 1 × 1.54 = 1.54 cubic yards of dry materials. There are seven parts in a 1:2:4 mix, so:

• Cement = 1.54 × 1/7 = 0.22 cubic yards, which is about 6 bags of 94 lb cement
• Sand = 1.54 × 2/7 = 0.44 cubic yards
• Gravel = 1.54 × 4/7 = 0.88 cubic yards

So one cubic yard of this mix needs roughly 6 bags of cement, just under half a yard of sand, and a bit under a yard of gravel.

Choosing a mix and ordering

Pick the ratio by what the concrete has to do. A garden path or a fence post is happy with a leaner mix, while a slab or a beam wants more cement for strength. When in doubt, a little stronger is the safer way to miss.

Add about 5 to 10 percent to every material when you order, to cover spillage and the bits that stick to the mixer. And for anything structural, where the concrete is holding up part of the building, follow the mix design on the drawings or have an engineer confirm it, since a nominal ratio is a good estimate but not a substitute for a designed mix.

Questions people ask

Which concrete mix is the strongest?

Of the ratios here, 1:1:2 is the strongest at about 25 MPa, because it has the most cement relative to sand and gravel. The leaner the mix, the weaker but cheaper it is.

What does a 1:2:4 mix mean?

One part cement, two parts sand, four parts gravel, measured by volume. It is a common general-purpose mix of about 15 MPa.

Why is the dry volume bigger than the concrete I want?

Because dry sand and gravel are full of air gaps that close up when water is added. The calculator multiplies by 1.54 to allow for that, so you buy enough dry material to end up with the concrete volume you need.

How many bags of cement are in a cubic yard of concrete?

It depends on the mix. A 1:2:4 yard works out to roughly 6 bags of 94 lb cement, with the rest of the volume made up by sand and gravel.

References

A quick note on where these figures come from. The nominal mix ratios and their strength grades, like 1:2:4 giving roughly 15 MPa, are set out in the standards for plain and reinforced concrete, most directly in IS 456. The formal way to proportion a mix by weight is covered by the American Concrete Institute's practice for selecting concrete proportions. The unit conversions follow the US National Institute of Standards and Technology guide. The 1.54 dry-volume factor is a long-standing site-estimating convention, and it comes straight from the air-void content of dry sand and aggregate described above.

1. Bureau of Indian Standards, IS 456, Plain and Reinforced Concrete (nominal mix proportions and grades). https://www.bis.gov.in
2. American Concrete Institute, ACI 211.1, Selecting Proportions for Normal, Heavyweight, and Mass Concrete. https://www.concrete.org
3. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811