Concrete Mix Ratio Calculator

Concrete mix ratios define the proportions of cement, sand (fine aggregate), gravel (coarse aggregate), and water needed to achieve a specific strength. Getting the ratio right is essential for structural integrity, workability, and durability.

Standard Mix Ratios by Grade:

Water-Cement Ratio: The water-cement ratio (w/c) is critical. Too much water weakens concrete; too little makes it unworkable.

Here is the part most mix-ratio tables get backwards, and this page got it backwards too until recently. Water is not a multiple of the cement. How much water a batch needs is set by the aggregate: its surface area, its maximum size, and how stiff you are willing to let the mix be. For 20 mm gravel at a 25 to 50 mm slump, which is about as stiff as you can still turn over with a shovel, that is 185 litres per cubic metre of finished concrete, whatever grade you are mixing. It is the same row of the same water-demand table the concrete mix design calculator works from. The w/c ratio is what falls out of that once you know the cement content.

Work it the other way and the lean grades starve. Multiplying 222 kg of cement by a tidy-looking 0.55 gives an M10 batch 122 litres per cubic metre, which is not a stiff mix, it is a pile of damp powder that will never come together in a wheelbarrow. At the rich end the same mistake runs the other way: 682 kg of cement times 0.35 hands an M30 batch 239 litres, a third more water than structural concrete should ever see.

Formula: w/c = water (liters) ÷ cement weight (kg), with water taken from the aggregate, not from the cement.

Volume Estimation: Working the M20 numbers below backwards, one 50 kg bag of cement makes about 0.124 m³ (4.4 cubic feet) of finished concrete, and a 94 lb (42.6 kg) bag makes about 0.106 m³ (3.7 cubic feet). Those figures move with the mix: a leaner M10 stretches a bag further, a rich M30 uses it up faster.

Calculating Materials per Cubic Meter (M20 mix, 1:1.5:3): Total parts = 1 + 1.5 + 3 = 5.5 parts

• Cement: 1/5.5 × 1.54 (dry volume factor) = 0.28 m³ = 403 kg, so 9 bags of 50 kg
• Sand: 1.5/5.5 × 1.54 = 0.42 m³ (~630 kg)
• Gravel: 3/5.5 × 1.54 = 0.84 m³ (~1,260 kg)
• Water: 185 liters, giving w/c = 185 ÷ 403 = 0.46

Bag counts always round up. 403 kg is 8.06 bags, so you buy 9 and most of the ninth goes back on the shelf for the next job.

Why M25 and M30 sit awkwardly here

A nominal mix is a fixed volumetric recipe you can batch on site with a shovel and a bucket, and every code that defines one stops at M20. Push the recipe richer and the arithmetic keeps working while the concrete stops making sense: 1:1:2 comes out at 554 kg of cement in a cubic metre and 1:0.75:1.5 at 682 kg, against a normal ceiling of 450. You would reach the strength, but you would pay for cement you did not need and get more shrinkage cracking for the trouble. Those two grades are here because people ask for them, and the honest answer for either one is a designed mix.

The 1.54 dry volume factor accounts for the volume increase when converting wet (compacted) concrete volume to dry material volume, because air voids in dry materials occupy more space.

Mixing Tips: Always add water gradually. You can always add more, but you cannot take it back out. Mix dry ingredients first, then add water. For small projects, a wheelbarrow and shovel work fine. For anything over 0.5 cubic meters (about 18 cubic feet), consider renting a mixer or ordering ready-mix.

Temperature Considerations: Do not pour concrete when temperatures are below 40 degrees Fahrenheit (4 degrees Celsius) or above 90 degrees Fahrenheit (32 degrees Celsius). Cold weather slows curing; hot weather causes rapid moisture loss and cracking. In hot conditions, keep materials cool and cure with water or wet burlap. In cold conditions, use heated water and insulating blankets.

Curing Time: Concrete reaches about 70% of its final strength in 7 days and 99% in 28 days. Keep it moist for at least 7 days for best results. Never let fresh concrete dry out too quickly.

This page or the mix design one?

This page hands you a recipe. Pick a grade, give it a volume, and it tells you how many bags, how much sand and gravel, and how much water to add, in quantities you can carry to a wheelbarrow. The proportions are fixed in advance, which is the whole point of a nominal mix: no testing, no aggregate grading, just a ratio that has worked for a century on small pours.

The concrete mix design calculator goes the other way. You give it a target strength and it works out the w/c ratio the concrete actually needs, using Abrams’ law and the ACI 211.1 absolute volume method. That is the route for anything structural, anything at M25 or above, and anything where a specification names a strength rather than a ratio. It will usually reach the same strength on less cement than the recipe above, because it is designing rather than following a rule of thumb.