Concrete Mix Ratio Calculator

How to work out a 1:2:4 concrete mix

This concrete mix ratio calculator takes the volume of concrete you need and splits it into 1 part cement, 2 parts sand, and 4 parts coarse aggregate. The formula is one part = total volume ÷ 7. Cement is one part, sand two, and stone four. The cement volume is then divided by 1.25 ft³ to estimate 94-lb bags. For 27 ft³ (one cubic yard) you get 3.86 ft³ of cement, 7.71 ft³ of sand, and 15.43 ft³ of stone.

Enter the total concrete volume in cubic feet. If you only know the slab size, get the volume first with the concrete slab calculator and multiply cubic yards by 27.

The ratio in plain words

Take a mixing tub and a 5-gallon bucket. For every bucket of cement, you add two buckets of sand and four buckets of stone. The stone forms the skeleton, the sand fills the gaps between stones, and the cement paste glues everything together.

The calculator works like this:

1. One part = total ft³ ÷ 7
2. Cement = 1 part; sand = 2 parts; aggregate = 4 parts
3. Cement bags = cement ft³ ÷ 1.25, shown to one decimal place

Worked examples

1. One cubic yard: 27 ft³

• One part: 27 ÷ 7 = 3.857 ft³
• Cement: 3.86 ft³, which is 3.857 ÷ 1.25 = 3.1 bags
• Sand: 7.71 ft³
• Aggregate: 15.43 ft³

2. A small pad: 10 ft³

• One part: 10 ÷ 7 = 1.429 ft³
• Cement: 1.43 ft³ (1.1 bags)
• Sand: 2.86 ft³
• Aggregate: 5.71 ft³

Before you buy: adjust for bulking

The calculator divides the finished volume exactly. Real batching needs more dry material than that, for two reasons:

• Dry ingredients shrink when mixed. Sand slips into the spaces between the stones, and cement fills the spaces in the sand. Builders commonly multiply the finished volume by about 1.5 to 1.57 to get the dry volume.
• Bag volume. A 94-lb bag of portland cement is conventionally taken as about 1 cubic foot in US practice, which is less than the 1.25 ft³ used here.

For example 1: 27 × 1.54 = 41.6 ft³ of dry material. One part is 41.6 ÷ 7 = 5.94 ft³ of cement, or about 6 bags. Sand comes to about 11.9 ft³ and stone to about 23.8 ft³. Use these adjusted figures when you order.

Common nominal mixes

Ratio (cement : sand : stone)	Parts	Typical use
1 : 1.5 : 3	5.5	Richer mix for slabs and posts that carry more load
1 : 2 : 4	7	General footings, paths, light slabs (this calculator)
1 : 3 : 6	10	Lean mix for blinding layers and mass fill
1 : 4 : 8	13	Very lean fill under foundations

A lower total of parts means more cement in each cubic foot, so the mix is stronger and costs more.

How to batch it on site

• Measure by volume with the same container. Use one bucket for all three ingredients. Level it off each time. Shovel counts drift.
• Mix dry first. Turn the cement, sand, and stone together until the color is even, then add water a little at a time.
• Use clean, washed materials. Clay or silt in the sand weakens the paste. “Concrete sand” and washed 3/4-inch stone are the usual choices.
• Batch only what you can place in about an hour. Concrete starts to stiffen, and adding water to loosen it again costs strength.

Common mistakes

• Ordering the finished volume of dry ingredients. Without the bulking allowance you can run out partway through the pour.
• Too much water. Soupy concrete is easy to pour, but it is weaker, shrinks more, and cracks sooner.
• Ignoring curing. Keep the concrete damp for several days. Slabs that dry out fast lose strength at the surface.
• Site-mixing big pours. Above roughly a cubic yard, ready-mix or bagged mix from the concrete bag calculator is usually faster and more consistent.

For reinforcement, size bars with the rebar calculator.

Nominal mixes are a general-purpose guide. Structural footings, walls, and slabs that carry vehicles should use a specified design mix, checked by a licensed engineer or your local building department.