Concrete Mix Ratios

What a mix ratio means

Traditional recipes describe concrete as cement : sand : coarse aggregate by volume. A 1:2:4 mix has one part cement, two parts sand and four parts gravel. A “part” can be any container as long as you use the same one for all three, such as a bucket or a box with fixed dimensions.

Ratios by volume are a convenient shorthand for small hand-mixed jobs and for estimating materials. They are not precise, because sand bulks when damp, aggregates have different densities and the amount of water changes the final volume. Modern ready-mix is specified by strength and durability, and the plant works out the proportions by weight.

Common volume ratios and what they are used for

Why dry materials make less concrete

When water is added, the cement paste fills the voids between sand grains and the sand-paste mortar fills the voids between stones. The finished concrete occupies less volume than the dry materials did. The ratio of dry to wet volume is usually taken as about 1.54, and in practice it falls between about 1.52 and 1.57. That is the factor in the mix ratio calculator. To make one cubic meter of concrete you need about 1.54 m³ of dry material, plus an allowance for waste.

Materials for one cubic meter

The cement figure uses a loose density of 1,440 kg per cubic meter, which gives 0.0347 m³ per 50 kg bag. These are estimates. Add waste, usually 5 to 10%, and round the bags up. Sand and gravel are bought by volume or weight, and damp sand takes more space than dry sand, so leave a margin.

The water–cement ratio

The water–cement ratio (w/c) is the weight of water divided by the weight of cement. It is the single biggest factor in strength and durability. Lower w/c gives stronger, less permeable concrete. Higher w/c gives easier placement but weaker, more porous concrete that scales and cracks sooner. Typical structural mixes have a w/c between about 0.40 and 0.55, and exposure classes in design codes set maximums for concrete exposed to freezing, deicers or sulfates.

Cement needs only part of the water in the mix to react. The rest is there to make the concrete workable, and it eventually leaves as the concrete dries, creating tiny pores. This is why adding a bucket of water to make a stiff mix easier to place is a bad trade. If you need more workability use a water reducer admixture, not extra water.

Mixing by hand or with a mixer

1. Measure the dry ingredients with the same container for each part.
2. Dry-mix the cement and sand first, then add the gravel.
3. Add about three-quarters of the water and mix, then add small amounts until the mix is workable.
4. Mix for at least a few minutes. Undermixed concrete has pockets of dry cement.
5. Use the batch within about an hour in warm weather.

Bags, ready-mix and site mixes compared

Bagged mix has the ratio built in, and the label tells you how much water to add. It is the easiest way to get a consistent result in small batches. Ready-mix is delivered to a specified strength. Site mixing from separate cement, sand and gravel is the cheapest at larger volumes, but it needs space, a mixer, careful measuring and cannot match batch plant accuracy. For anything structural, use a designed mix, not a rule-of-thumb ratio. The PSI guide shows how strength relates to use.

Common mistakes

• Using shovels as parts when the shovels are different sizes.
• Treating the volume ratio as the weight ratio.
• Ignoring bulking of damp sand.
• Adding extra water for ease of placement.
• Using the dry volume as the finished volume.
• Using beach sand or unwashed aggregate, which can contain salts and clay that weaken the mix.

Check the mix with a slump test

1. Wet a cone (a 12 in tall, 8 in bottom diameter, 4 in top diameter) and place it on a flat, damp surface.
2. Fill it in three layers, rodding each layer 25 times.
3. Strike off the top, then lift the cone straight up in a few seconds.
4. Place the cone beside the slumped concrete and measure how far the top has dropped.

Aggregates change the recipe

The ratio assumes well-graded sand and angular crushed stone. Rounded gravel, pea gravel and very fine sand need more cement paste to get the same workability, and mixes with large stone need less. Moist sand takes up more volume than dry sand, so a measure of moist sand has less sand in it. Wash any aggregate that is dirty or silty, because clay and silt weaken the bond between paste and stone.

Mix a trial batch

Before committing to a big pour, make a small batch with the intended ratio, add water slowly until the mix is workable, and write down the water amount. Cast a few test blocks and let them cure; if you can, crush them with a lever or have them tested. Adjust the ratio if the batch is too harsh or too wet, then scale up with the same measures.

When to leave ratios behind

Ratios are a convenient way to describe a mix and estimate materials, but modern practice specifies concrete by performance: strength, slump, air content, maximum aggregate size and maximum water–cement ratio. A designed mix contains the right amounts for these targets, adjusted for the local materials. If your project is anything more than a small slab or post hole, ask a ready-mix plant for a mix with the strength you need rather than mixing from a ratio, and use the ratio only to estimate materials for small batches.

Frequently asked questions