Concrete mix ratio calculator

Cement, sand, gravel, and water for a batch mixed from scratch. Set the volume ratio and the water-cement ratio; get the bags of Portland cement and the strength the water — not the ratio — actually allows.

01 How the math works

A mix ratio is three numbers — cement : sand : gravel — measured by loose volume. A 1:2:3 batch is one part cement, two parts sand, three parts gravel. The water is set separately, as the water-cement ratio: the weight of water divided by the weight of cement.

To find how much a batch makes, add the absolute (solid) volumes of each ingredient plus a small amount of trapped air. A 94 lb bag of Portland cement is one cubic foot loose, so one bag is one "part." The finished concrete comes out smaller than the loose pile because the cement paste and sand fill the gaps between the gravel.

The water-cement ratio is the part that sets strength. From ACI 211.1, a non-air-entrained mix hits about 4,000 psi (27.6 MPa) at w/c 0.57, 3,000 psi (20.7 MPa) at 0.68, and 2,000 psi (13.8 MPa) at 0.82. Lower water, higher strength. The volume ratio sets the proportions; the water sets how strong they get.

02 Worked example

Take a small equipment pad: 3 ft × 3 ft × 4 in (0.91 m × 0.91 m × 100 mm). That's 3 ft³ (0.085 m³) of concrete. With a 10% waste factor, size for 3.3 ft³ (0.093 m³).

At a 1:2:3 ratio and a water-cement ratio of 0.50, the totals for 3.3 ft³ are about 71 lb of cement (32 kg), 1.5 ft³ of sand (0.043 m³, 152 lb / 69 kg), 2.3 ft³ of gravel (0.065 m³, 228 lb / 103 kg), and 4.3 gal (16.2 L) of water. Cement comes in whole bags, so buy one 94 lb bag.

In practice you mix it as one full-bag batch: a 94 lb bag of cement makes about 4.34 ft³ (0.123 m³) of concrete at this ratio, so the whole bag with 2 ft³ of sand, 3 ft³ of gravel, and 5.63 gal (21.3 L) of water covers the pad on its own, with a little left over.

At w/c 0.50 the strength ceiling is near 4,800 psi (33 MPa). That's a ceiling, not a promise — reach it only if you keep the water there.

03 When this calculator is wrong

The biggest error in volume batching isn't the ratio — it's the water. Most mix-ratio calculators print a fixed strength per ratio, as if "1:2:3" were a strength grade. It isn't. The same 1:2:3 batch mixed at w/c 0.50 can reach about 4,800 psi (33 MPa); loosened to a soupy w/c 0.70 to make it easier to place, it drops to roughly 2,900 psi (20 MPa) — a ~40% loss from added water alone, at an unchanged ratio. The strength readout above tracks the water-cement ratio for exactly this reason.

• Damp sand throws off the sand measure. Moist sand bulks 20–30% in loose volume at 5–6% surface moisture, and fine sand up to ~40% (PCA). Measure damp sand by the shovel or bucket and you under-deliver sand by weight, so the batch runs rich on cement. Dry sand, or increase the measured sand volume by the bulking percentage.
• The 1.54 "dry-volume factor" overstates cement. Many calculators multiply the finished volume by 1.54 before splitting it by ratio. This one uses the ACI 211.1 absolute-volume method, which works out closer to 1.38 for a 1:2:3 mix — the 1.54 figure ignores how the paste fills voids and over-orders cement by about 10%.
• A lean ratio can't reach the w/c strength ceiling. The strength figure assumes enough cement paste to coat and bind the aggregate. A thin mix like 1:3:6 won't make the tabulated strength even at a low water-cement ratio, because there isn't enough paste. The ceiling is necessary, not sufficient.
• Aggregate density varies ±10%. The sand and gravel weights use 100 lb/ft³ (ASTM C29); your pit-run or pea gravel can run lighter or heavier, which shifts the weights but not the volumes you batch by.

04 What to do with the result

Mixing from scratch is the most labor-intensive way to make concrete — you're batching and lifting three materials and water instead of one premix bag. Bagged premix already stops making sense above about 1 yd³ (0.76 m³), where a cubic yard is roughly 60 bags of 80 lb premix, about 4,800 lb to lift and mix inside the roughly 90-minute working time before the concrete sets. From-scratch batching crosses that line sooner, not later. Keep hand-mixing for small pours — pads, posts, a few footings — and call for ready-mix above a yard.

The exception is access: a remote backyard, a basement, or a spot no truck can reach makes hand-mixing the only option regardless of volume. For those, batch by full bags of cement — size up to the next whole bag and keep the ratio and the water-cement ratio fixed across every batch so the pour is uniform.

05 Common questions

What is a 1:2:3 concrete mix?

One part Portland cement, two parts sand, three parts gravel, measured by loose volume. It's the general-purpose residential ratio. At a water-cement ratio of 0.50 one 94 lb bag of cement makes about 4.34 ft³ (0.123 m³) of concrete — roughly 6.2 bags of cement per cubic yard.

How much sand and gravel do I need per bag of cement?

For a 1:2:3 mix, each 94 lb bag of cement (1 ft³ loose) takes 2 ft³ of sand and 3 ft³ of gravel. For 1:2:4 it's 2 ft³ sand and 4 ft³ gravel per bag; for 1:1.5:3 it's 1.5 ft³ sand and 3 ft³ gravel.

What is the right water-cement ratio for concrete?

By weight, 0.45–0.60 covers most work. Lower is stronger: ACI 211.1 puts 0.57 at about 4,000 psi (27.6 MPa) and 0.68 at about 3,000 psi (20.7 MPa), non-air-entrained. For freeze-thaw exposure, air-entrained concrete reaches the same strengths at a lower ratio — about 0.48 for 4,000 psi.

Is 1:2:3 or 1:2:4 stronger?

1:2:3 has the higher strength ceiling because it carries more cement paste per unit of aggregate — about 6.2 bags of cement per cubic yard versus 5.4 for 1:2:4. A 1:2:4 mix is a leaner, more economical fill mix. Either way, the water-cement ratio you mix at still governs the strength you actually get.

How many bags of cement are in a cubic metre of concrete?

For a 1:2:3 mix at w/c 0.50, one cubic metre takes about 8 bags of 50 kg Portland cement (roughly 380 kg), plus about 0.57 m³ of sand and 0.85 m³ of gravel before waste. In cubic yards that's the 6.2 ninety-four-pound bags noted above.