Understanding Concrete Mix Ratios
Concrete mix ratios express the proportion of cement, sand, and coarse aggregate by volume. The first number is always cement (1 part), followed by sand and aggregate. Stronger mixes have lower aggregate ratios - M25 at 1:1:2 is stronger than M15 at 1:2:4.
Dry Volume Method
Loose dry materials pack down to about 65 percent of their starting volume once mixed with water, so order 1.54 times the wet volume you need:
Dry Volume=Wet Volume×1.54
Material per Part
Material Volume=Material PartsTotal Parts×Dry Volume
Standard Grades
| Grade | Ratio | PSI | MPa | Use |
|---|---|---|---|---|
| M10 | 1:3:6 | 1450 | 10 | Sub-base, PCC |
| M15 | 1:2:4 | 2175 | 15 | Patios, paths |
| M20 | 1:1.5:3 | 2900 | 20 | Slabs, footings |
| M25 | 1:1:2 | 3625 | 25 | RCC columns |
| M30 | 1:0.75:1.5 | 4350 | 30 | Commercial |
PSI to MPa Conversion
1 MPa equals approximately 145 PSI. When working with US suppliers, order concrete by PSI rating. Elsewhere, specify M grade or MPa. Structural drawings always specify the 28-day characteristic strength.Water-Cement Ratio Impact
The water-to-cement ratio (w/c) is the single most important factor in concrete strength. Lower w/c = stronger concrete but harder to work with:
- 0.40 w/c - high-strength concrete (M30+), requires admixtures
- 0.50 w/c - standard structural mix, typical residential use
- 0.60 w/c - workable but 25 to 40 percent weaker
Do Not Over-Water
Adding extra water on site is the most common way to ruin a good mix. Each additional liter per bag of cement can reduce compressive strength by 10 to 15 percent. If the mix is too stiff, add a plasticizer admixture, never more water.