Concrete Calculator by Grade
Cement bags, sand and aggregate for a slab, footing, column or beam in M10, M15, M20 or M25 nominal mix.
How the concrete calculation is done
The wet volume of concrete is the member’s length x width x thickness, converted to cubic metres. Because cement, sand and aggregate lose volume when mixed with water and compacted, the dry volume of materials is taken as 1.54 times the wet volume. That dry volume is split in the nominal mix ratio of the grade: M20 is 1:1.5:3, so cement is 1 part in 5.5, sand 1.5 parts and aggregate 3 parts. Cement is converted to 50 kg bags at 0.0347 cu m per bag, and wastage is added to all three materials.
Worked example (default inputs)
A slab 20 ft x 15 ft x 5 inch: 6.096 x 4.572 x 0.127 = 3.54 cu m wet. Dry volume 5.45 cu m. For M20: cement 5.45 / 5.5 = 0.99 cu m, which is 28.6 bags; with 5% wastage, 31 bags. Sand 1.49 cu m (1.56 cu m with wastage, about 55 cft or 2.5 t). Aggregate 2.97 cu m (3.12 cu m with wastage, about 110 cft or 4.7 t). Water about 775 litres.
How many cement bags per cu m of M20 concrete?
About 8 bags per cu m of finished M20 concrete by nominal mix (0.28 cu m of cement), and about 6.4 bags for M15. A design mix from a lab can be lower or higher.
What is the difference between nominal mix and design mix?
A nominal mix uses fixed volume ratios such as 1:1.5:3 and is allowed for M20 and below in ordinary work. A design mix is proportioned by weight in a lab for the actual materials and is required for M25 and above in most specifications, and for ready-mix concrete.
Should sand be ordered in cft, cu m or tonnes?
Suppliers in north India quote mostly in cft (one truck of 400 or 700 cft); Mumbai and the south often quote in tonnes or brass (100 cft). The calculator shows all three so the indent matches what the supplier bills.
The bags, sand and aggregate for each pour can be indented from these numbers and the receipts matched to the pour in the Trackovo app, so the site knows what was ordered, what came and what was used: track material and labour in Trackovo. Trackovo is a construction site management app with material, labour attendance, DPR and payments, with a 14-day free trial.