Concrete Mix Ratio Calculator

How the concrete mix ratio calculator works

The mix ratio calculator takes a target concrete volume in cubic yards and a mix ratio (cement:sand:aggregate by volume), and produces the quantities of each ingredient needed for site-mixed concrete. The standard 1:2:3 mix (1 part cement, 2 parts sand, 3 parts aggregate) produces concrete at about 3000 psi — strong enough for most residential applications.

The math accounts for shrinkage: dry ingredients occupy more volume than the wet concrete they produce. The standard multiplier is 1.54 — meaning you need 1.54 cubic feet of dry ingredients to produce 1 cubic foot of finished concrete. The calculator multiplies the target wet volume by 1.54 to get dry volume, then divides by the total ratio parts to get each ingredient's volume.

A worked example: 1 cubic yard of 1:2:3 mix. Wet volume = 27 ft³. Dry volume = 27 × 1.54 = 41.58 ft³. Total parts = 1 + 2 + 3 = 6. Cement volume = 41.58 × (1/6) = 6.93 ft³. At 1.25 ft³ per 94-lb bag of Portland cement, that's 5.5 bags — round up to 6 bags. Sand volume = 41.58 × (2/6) = 13.86 ft³. At 80 lb/ft³, that's 1,109 lb or 0.55 tons. Aggregate volume = 41.58 × (3/6) = 20.79 ft³. At 100 lb/ft³, that's 2,079 lb or 1.04 tons.

Water-cement ratio — the most important number in concrete

The water-cement ratio (W/C) is the weight of water divided by the weight of cement, and it's the single most important factor in concrete strength. A W/C of 0.45 produces 5000 psi concrete; 0.55 produces 4000 psi; 0.65 produces 3000 psi. Every 0.1 increase in W/C reduces compressive strength by about 1000 psi.

For a typical 1:2:3 mix with 6 bags of cement per cubic yard (564 lb of cement), a W/C of 0.50 requires 282 lb of water — about 34 gallons. The water in the sand and aggregate (called "absorbed moisture") adds another 5–10 gallons. The mistake most DIYers make is adding water until the mix "looks right" — which usually means too wet, which produces weak concrete.

Workability (the ease with which concrete can be placed and finished) is determined by water content, not by W/C ratio. Adding water increases workability but reduces strength. The right way to increase workability without weakening the mix is to use a plasticizer (water reducer) — a chemical admixture that lets you cut water by 10–20% while keeping the same workability. A 1-gallon jug of plasticizer ($20) treats about 5 cubic yards of concrete and pays for itself in increased strength.

Test the mix with a slump cone: a 12-inch tall truncated cone filled with concrete in 3 layers and rodded 25 times per layer. Lift the cone straight up — the concrete will slump. A 4-inch slump is standard for slabs; 2-inch for footings; 6-inch for walls (with a plasticizer). Slump over 6 inches means the mix is too wet and will be weak. Slump under 2 inches is too stiff to place without vibration.

Site mixing vs ready-mix — when to choose each

Site mixing is economical for small jobs (under 1 cubic yard total) and for jobs where ready-mix delivery is impractical (remote locations, after-hours work, very small quantities). The cost crossover is around 2–3 cubic yards — above that, ready-mix is cheaper per cubic yard of finished concrete, even accounting for delivery and short-load fees.

Ready-mix has quality advantages that site mixing can't match. A batch plant weighs ingredients to the pound, controls water to the gallon, and adds admixtures precisely. Site mixing relies on shovelfuls and bucket counts — typically 10–20% less accurate. The result is that site-mixed concrete tests 10–15% weaker than the equivalent ready-mix design, batch for batch. For structural concrete (footings, walls, slabs carrying loads), specify ready-mix.

For site mixing, use a towable drum mixer ($40/day rental) — a paddle mixer in a drill is acceptable for jobs under 0.5 yard. Add water first (about 1 gallon for a 2-bag batch), then half the aggregate, then the cement, then the rest of the aggregate and sand. Mix for 3 minutes after everything is in. Pour immediately; the mix has a working time of 30–60 minutes depending on temperature. In hot weather, add ice to the mix water to slow set time.

What the mix ratio calculator excludes: water (figure 1.5 gallons per 94-lb bag of cement, so 9 gallons for a 6-bag yard), plasticizer ($5 per cubic yard), mixing equipment ($40/day rental), forms, finishing tools, and labour (figure 2 hours of mixing labour per cubic yard of finished concrete). Site mixing is also messier — plan for 10–15% spillage and waste, vs 5% for ready-mix.

Portland cement comes in 94-lb bags (1 cubic foot per bag) and costs $10–14 per bag as of 2026. A 6-bag yard uses $60–84 of cement; sand at $35/ton is $19; aggregate at $35/ton is $36. Total: $115–140 per cubic yard of materials, vs $130–180 for ready-mix delivered. The cost savings is real but the labour cost is 4–6× higher — for any job over 1 yard, ready-mix wins on total cost including labour.

Common mistakes and how to avoid them

The single most common mistake in mix-ratio calculation is ignoring the 1.54 dry-volume factor. Wet concrete occupies less volume than the dry ingredients that go into it — cement and water chemically combine, and the smaller particles fill the voids between larger ones. The industry-standard multiplier is 1.54: you need 1.54 cubic feet of dry ingredients to produce 1 cubic foot of wet concrete. A calculator that does not apply this factor will understate cement, sand, and aggregate by about 35%. This calculator applies it; many do not.

A second common mistake is using the wrong water-cement ratio. The W/C ratio — the weight of water divided by the weight of cement — is the single most important factor in concrete strength. A W/C of 0.45 produces 5000 psi; 0.55 produces 4000 psi; 0.65 produces 3000 psi. Every 0.1 increase reduces strength by about 1000 psi. The mistake most DIYers make is adding water until the mix looks right — which usually means too wet, which produces weak concrete. Measure the water: for a 94-lb bag of cement at W/C 0.5, use 47 lb of water (about 5.6 gallons).

Aggregate gradation is the third common error. The aggregate should be 3/4-inch minus crushed stone — large stones for strength, smaller stones and fines to fill the voids. Pea gravel (round, smooth) does not bond well with cement paste and produces weak concrete. River rock (smooth) is the same. Clean crushed stone (no fines) has voids that weaken the mix. The correct material is crushed stone with a range of sizes from 3/4 inch down to dust — the fines are not waste, they are what fills the voids.

Mixing order matters more than people expect. The correct order is: water first (about 1 gallon for a 2-bag batch), then half the aggregate, then the cement, then the rest of the aggregate and sand. This prevents the cement from balling up on the drum walls and ensures even distribution. Mix for 3 minutes after everything is in — under-mixing leaves dry pockets that show up as weak spots in the finished concrete. Over-mixing adds air and reduces strength slightly, but under-mixing is the bigger risk.

Finally, do not add water to extend working time. Concrete has a working time of 30-60 minutes once mixed; in hot weather, less. Adding water to re-tempered concrete extends workability but reduces strength by about 10% per gallon of added water per cubic yard. If you need more working time, use a plasticizer (water reducer) or a set retarder — chemical admixtures that extend workability without weakening the mix. A $20 jug of plasticizer treats 5 cubic yards and pays for itself in increased strength.