Brazil Concrete Mix Calculator: Cement, Sand and Gravel
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Enter a volume — or an area and a thickness — pick what the concrete is for, and get the number of 50 kg cement bags, the cubic meters of sand and gravel, and the liters of water the batch requires. Mix designs come from the official SINAPI compositions for site-mixed concrete, and the result already includes a 5% waste allowance.
Concrete mix table by strength
The mix ratio sets how much sand and coarse aggregate go in for each part of cement. The higher the characteristic strength (fck, measured at 28 days), the more cement and the less water the batch takes. The table reproduces the per-cubic-meter consumption of the SINAPI compositions for concrete mixed in a 400-liter drum mixer using 9.5–19 mm crushed stone.
All ratios are by dry mass. That matters: damp sand bulks by up to 30%, so anyone measuring by the bucket on site ends up with less real sand than they think.
| Typical use | fck | Ratio (dry mass) | Cement per m³ | 50 kg bags | Sand | Gravel | Water (w/c) | Source |
|---|---|---|---|---|---|---|---|---|
| Floor slab on grade, sidewalk, screed | 15 MPa | 1 : 3.4 : 3.5 | 273.1 kg | 5.5 | 0.80 m³ | 0.58 m³ | 205 L (0.75) | SINAPI 94963 |
| Garden wall, footing beam, light elements | 20 MPa | 1 : 2.7 : 3.0 | 323.0 kg | 6.5 | 0.76 m³ | 0.59 m³ | 203 L (0.63) | SINAPI 94964 |
| Slab, beam, column (structural) | 25 MPa | 1 : 2.3 : 2.7 | 362.7 kg | 7.3 | 0.72 m³ | 0.59 m³ | 203 L (0.56) | SINAPI 94965 |
| Foundation and aggressive exposure | 30 MPa | 1 : 2.1 : 2.5 | 388.9 kg | 7.8 | 0.71 m³ | 0.59 m³ | 202 L (0.52) | SINAPI 94966 |
Working out the volume of each element
Concrete is always sold by the cubic meter, and volume is always length × width × height. What changes is which dimension plays the role of height. A 20 m² floor at 8 cm is 20 × 0.08 = 1.6 m³. A 4.00 × 5.00 m solid slab at 12 cm is 2.4 m³. A 5.00 m beam with a 14 × 40 cm section is 0.28 m³. Ten 14 × 30 cm columns at 3.00 m tall add up to 1.26 m³. A 1.20 × 1.20 m spread footing 40 cm deep is 0.576 m³.
Precast joist-and-block slabs are the exception: only the topping and the joist infill are concrete, typically 0.05 to 0.07 m³ per square meter of slab. Confirm the figure with the system supplier.
Why a 5% waste allowance is built in
The theoretical volume is never the volume that reaches the element. Concrete sticks to the mixer, the wheelbarrow and the tools; some escapes through gaps in the formwork; and real thickness almost always exceeds the design value because the ground or the forms are not perfectly level. The calculator therefore adds 5% to whatever volume you enter or derive, and sizes every material against that corrected figure.
Five percent is conservative for site-mixed concrete in well-built forms. On blinding over uneven soil, in reused timber formwork, or on small pours, real waste easily reaches 10%. Running short mid-pour is far worse than running over: it creates an unplanned cold joint, a permanent weak line and a future leak path.
Common mistakes and curing
Adding water to improve workability is the single most damaging habit on a small site. The water-to-cement ratio governs strength, and every extra liter per bag of cement lowers the final fck irreversibly. Too-wet concrete also segregates, letting the gravel sink. Other frequent errors: short mixing time (the drum needs three to five minutes after the last material goes in), dropping concrete from more than two meters, and skipping compaction, which leaves honeycombing around the reinforcement.
Curing — simply keeping the concrete moist while the cement hydrates — is free and decisive. Brazil’s ABNT NBR 14931:2023 requires curing to continue until the concrete has gained sufficient strength; the established site minimum is seven days of wet curing, stretching to 14 or 21 days in cold weather or when relative humidity is below 50%. Wetting the surface three times a day, ponding water on a slab, or covering with plastic sheeting all work. Start as soon as the surface can be touched without marking, usually 6 to 12 hours after the pour.
One more decision point: below roughly 3 m³ per pour, mixing on site is usually cheaper. Above that, ready-mixed concrete delivered by truck wins on total cost, speed, strength control and waste — and it removes the sand and gravel stockpiles from the site entirely. Pumping is priced separately and only pays off when the truck cannot reach the element.
Frequently asked questions
How many cement bags does 1 m³ of concrete need?
About 5.5 fifty-kilo bags for 15 MPa, 6.5 for 20 MPa, 7.3 for 25 MPa and 7.8 for 30 MPa. Adding the 5% waste allowance, a 1 m³ pour at 25 MPa works out to 8 bags. Figures come from SINAPI compositions 94963 to 94966.
What is the mix ratio for 25 MPa concrete?
1 : 2.3 : 2.7 by dry mass of cement, medium sand and 9.5–19 mm crushed stone, with a water-to-cement ratio of 0.56 — that is 362.7 kg of cement, 0.72 m³ of sand, 0.59 m³ of gravel and roughly 203 liters of water per cubic meter.
Can I use 15 MPa concrete for a slab or a column?
No. ABNT NBR 6118 sets a minimum of 20 MPa (class C20) for structural concrete with passive reinforcement, and higher classes as environmental aggressiveness increases — typically C25 in urban settings and C30 for foundations and coastal areas. The 15 MPa mix is for non-structural work only: blinding, screed and sidewalks.
How much water should I actually add?
The table figure of about 200 liters per cubic meter assumes dry materials. Since site sand always arrives damp, start with 70–80% of that water, mix, then add the rest gradually until the batch reaches the right consistency. Never add water to a batch that has already begun to stiffen — it destroys the strength for good.
Sources
Mix designs are indicative and reproduce the official SINAPI compositions for site-mixed concrete. Real strength depends on a dosage study with locally available aggregates, actual sand moisture, cement quality, compaction and curing. For structural elements — slabs, beams, columns and foundations — the mix and volume must be specified by the licensed civil engineer responsible for the project.
Related calculators
Also available in Portuguese: Calculadora de Concreto: Cimento, Areia e Brita