How it’s calculated
A volumetric mix such as 1 : 2 : 3 measures cement, sand and gravel in equal-sized buckets. Because dry ingredients contain voids that the paste fills, the loose dry volume is about 1.54 times the finished (wet) volume.
Example: 1 yd³ (27 ft³) at 1 : 2 : 3. Dry volume = 27 × 1.54 = 41.58 ft³; 6 parts → cement 6.93 ft³ = 651 lb = 7 bags (6.93 rounded up), sand 13.86 ft³ (0.51 yd³), gravel 20.79 ft³ (0.77 yd³). Water at w/c 0.50: 651 × 0.5 = 326 lb ≈ 39 gal.
Volume proportioning is the old “arbitrary” method; engineered concrete is proportioned by weight to a specified strength. Use a tested mix or ready-mix for structural work.
Frequently asked questions
How many bags of cement are in a yard of 1:2:3 concrete?
About 7 bags of 94 lb portland cement, with roughly 0.51 yd³ of sand and 0.77 yd³ of gravel.
Why multiply by 1.54?
Sand and gravel have voids, and the cement paste fills them when water is added. The loose dry materials therefore occupy roughly 52–54% more volume than the concrete they make.
How much water per bag of cement?
At a water–cement ratio of 0.5 a 94 lb bag needs 47 lb of water, about 5.6 US gallons, less any moisture already in the sand.
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Sources
- Slump Test: theory (dry volume = 1.52–1.54 × wet volume) — Virtual Labs, National Institute of Technology Karnataka (Govt. of India MoE)
- Concrete Mix Design Manual (1 bag cement = 94 lb = 1.00 ft³ loose) — North Carolina Department of Transportation
- Design and Control of Concrete Mixtures, Ch. 9: Designing and Proportioning Normal Concrete Mixtures — Portland Cement Association (via University of Memphis)
- Concrete: The Basic Mix (1 cement : 2 sand : 3 gravel by volume) — University of Illinois, Materials Science and Engineering
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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