Concrete Mix Calculator

Mix ratios express the proportion of cement, sand, and aggregate by volume. The most common ratio is 1:2:4 — one part cement, two parts sand, four parts aggregate (gravel). To calculate quantities for a project, multiply your wet concrete volume by 1.54 (the dry-to-wet conversion factor — dry materials need to be 54% more than the finished concrete volume because water fills voids during mixing). Then divide that dry volume by the total parts (1+2+4=7) to find the volume per part. The calculator above handles all of this automatically.

1:2:4 yields approximately 3000 PSI strength (general-purpose for sidewalks, light foundations, and most residential slabs). 1:1.5:3 yields approximately 4000 PSI (structural columns, reinforced beams, and heavy load-bearing applications). The richer cement content in 1:1.5:3 produces stronger but more expensive concrete. Most residential projects use 1:2:4. Engineers specify 1:1.5:3 when structural strength matters — high-rise foundations, retaining walls, or seismic zones.

Standard water-cement ratio is 0.45-0.50 by weight. For a 94-lb bag of Portland cement at a 0.50 ratio, you need 47 lbs of water — about 5.6 gallons. Lower ratios (0.40-0.45) make stronger concrete but are harder to work with. Higher ratios (0.55-0.60) are workable for hand mixing but produce weaker concrete. The calculator above defaults to 0.50, which balances strength and workability for most projects.

PSI (pounds per square inch) is the compressive strength of cured concrete. International standards use M-grade nomenclature where M followed by a number indicates strength in megapascals (MPa). Approximate equivalents: M5 = ~700 PSI (1:4:8, fill), M10 = ~1500 PSI (1:3:6, non-structural), M15 = ~2200 PSI (1:2:4 lean, light structural), M20 = ~3000 PSI (1:1.5:3, residential structural), M25 = ~3600 PSI (richer mix, heavy structural). The calculator above uses standard ratios — for engineered applications, your local building code or structural engineer will specify the required grade.

For most DIY projects, pre-mixed bagged concrete is faster and more reliable. Quikrete and Sakrete blend cement, sand, and aggregate in pre-measured ratios — you just add water. Mixing from scratch is cheaper for large projects (especially with a tow-behind mixer or onsite delivery of bulk materials), and gives you control over the ratio. For projects under 1 cubic yard, bagged is almost always the better choice. Above 5 cubic yards, ready-mix delivery beats both options on cost and time.

When you mix dry cement, sand, and aggregate with water, the final wet volume is LESS than the sum of the dry volumes — water fills the voids between particles. The standard factor is 1.54, meaning you need 1.54 cubic feet of dry materials to produce 1 cubic foot of finished concrete. This factor accounts for void filling, water absorption, and slight compaction during placement. Always include this factor when calculating dry materials, or you will come up short.

M20 is an Indian Standard (IS 456) grade designation indicating concrete with a minimum compressive strength of 20 MPa (about 2,900 PSI) at 28 days. The M stands for mix, and the number is the characteristic compressive strength in megapascals. M20 is one of the most common grades for residential and small commercial structural work. The corresponding nominal mix ratio is approximately 1:1.5:3 (cement:sand:aggregate). Other common grades: M10 (1:3:6, non-structural), M15 (1:2:4, light structural), M25 (1:1:2, heavier structural).

Workability is checked using a slump test. Fill a standard slump cone (12 inches tall, 8 inch base, 4 inch top) with fresh concrete in three layers, tamp each layer 25 times, then carefully lift the cone straight up. Measure how much the concrete slumps downward. Typical slump values: 1-2 inches for stiff foundation concrete, 2-4 inches for general slabs and walls, 4-6 inches for pumped or pourable concrete. Excessive slump means too much water (weak concrete); too little slump means the mix is hard to work. Slump tests are not required for small DIY projects but are standard for commercial pours.