Estimating materials is the first step of any slab, footing or column. Enter the dimensions and choose the mix to get the concrete volume, cement bags, sand and aggregate in cubic metres and cubic feet.
Enter length, width and depth in metres.
Choose the concrete grade.
Read the cement bags, sand and aggregate.
Dry volume: Wet volume × 1.54
Cement bags: Dry volume × (1 ÷ total parts) × 1440 ÷ 50
Before pouring a slab, footing or column, a builder needs to order cement, sand and aggregate. This calculator works out the wet volume of concrete from the length, width and thickness, converts it to a dry volume of materials, and splits that in the ratio of a nominal mix: M10 (1 : 3 : 6), M15 (1 : 2 : 4), M20 (1 : 1.5 : 3) or M25 (1 : 1 : 2). It returns cement in 50 kg bags and sand and aggregate in both cubic metres and cubic feet.
Cubic feet matter because sand and aggregate are often sold by the cft or by the truckload in India, while engineers design in cubic metres. The calculator follows the rule of thumb used on Indian sites, a dry-volume factor of 1.54 and cement at 1440 kg/m³, which makes it useful for quick estimates, house construction budgets and civil engineering coursework.
1. Measure the element: wet volume = length × width × thickness, all in metres.
2. Multiply by 1.54 to get the dry volume of materials, allowing for voids and bulking of sand.
3. Add up the parts of the mix ratio, for example 1 + 1.5 + 3 = 5.5 for M20.
4. Cement volume = dry volume × cement part ÷ total parts. Multiply by 1440 kg/m³ and divide by 50 to get bags.
5. Sand volume = dry volume × sand part ÷ total parts; aggregate volume = dry volume × aggregate part ÷ total parts.
6. Convert m³ to cft by multiplying by 35.315, and add 5 to 10% for site wastage.
When cement, sand and aggregate are mixed with water and compacted, the finer particles fill the gaps between coarser ones and the mix settles. So the loose dry materials you must buy occupy more volume than the finished concrete. Sand also bulks when damp, taking up more space than dry sand. Site practice in India takes the dry volume as about 1.52 to 1.57 times the wet volume, and 1.54 is the value used here. It is a rule of thumb, not a precise property.
The M stands for mix, and the number is the characteristic compressive strength in N/mm² after 28 days of curing, measured on 150 mm cubes. M20 should reach 20 N/mm². Nominal mixes use fixed volume ratios that experience shows will usually achieve that strength for ordinary work. For M25 and above, Indian practice increasingly uses design mixes, where proportions are worked out from tests on the actual materials. Design mixes can use less cement for the same strength, so this calculator's figures are an estimate, not a specification.
The calculator does not estimate water, because the water-cement ratio should be set for strength and workability, typically around 0.45 to 0.55 for nominal mixes. Too much water makes concrete easier to place but weaker and more porous. Cement is converted from volume to bags using a loose bulk density of 1440 kg/m³; one 50 kg bag is therefore about 0.0347 m³, or roughly 1.23 cft. This is why 1 m³ of M20 by this method needs about 8 bags.
Suresh is building a room on his terrace in Jaipur and needs an M20 roof slab 6 m long, 4.5 m wide and 0.125 m thick.
Wet volume: 6 × 4.5 × 0.125 = 3.375 m³
Dry volume (× 1.54 for voids and bulking): 3.375 × 1.54 = 5.198 m³
Split M20 ratio 1 : 1.5 : 3 (total 5.5): Cement = 5.198 × 1/5.5 = 0.945 m³ → × 1440 kg/m³ ÷ 50 kg = 27.22 bags Sand = 5.198 × 1.5/5.5 = 1.418 m³ Aggregate = 5.198 × 3/5.5 = 2.835 m³
Answer: Cement 28 bags (50 kg); Concrete volume 3.375 m³; Sand 1.418 m³ (50.1 cft)
Entering slab thickness in inches or millimetres instead of metres.
Ordering the wet volume of materials and forgetting the 1.54 dry-volume factor.
Leaving out wastage, spillage and uneven formwork, which can add 5 to 10%.
Treating a nominal-mix estimate as the exact requirement for a design-mix job.
Adding extra water on site to ease pouring, which lowers the strength.
Estimating materials for house roof slabs, footings, columns and beams.
Budgeting cement, sand and aggregate orders for small construction projects.
Civil engineering and diploma coursework on quantity estimation.
Checking contractor quotes for material quantities.
Planning PCC beds and floors using M10 or M15 mixes.
How many bags are in 1 m³ of M20?
About 8 bags with this nominal-mix method.
Should I add wastage?
Yes, 5–10% is typical on site.