Concrete Calculator — Cement Bags, Sand & Aggregate
Enter the size of your slab, footing, column or beam, pick a mix ratio, and instantly find out how many cement bags, sand (cft) and aggregate to order.
Cement bags required
–
50 kg bags, rounded up
Sand (fine aggregate)
–
cubic feet
Aggregate (coarse)
–
cubic feet
Wet concrete volume
–
cubic metres
How this concrete calculator works
Concrete is ordered by volume but bought as bags of cement, brass/cft of sand and aggregate. This calculator bridges the two. You enter the finished (wet) dimensions of the element you are casting — a slab, footing, column or beam — and it works backwards to the raw materials using the standard dry-volume method taught to Indian quantity surveyors.
The formula
Mixing concrete packs cement and sand into the voids between aggregate stones, so the dry ingredients occupy more space than the finished concrete. The thumb rule is:
- Wet volume = length × width × depth (all converted to metres)
- Dry volume = wet volume × 1.54
- Cement volume = dry volume × cement parts ÷ total parts
- Cement bags = cement volume ÷ 0.0347, rounded up (one 50 kg bag ≈ 0.0347 m³)
- Sand / aggregate (cft) = dry volume × their share × 35.3147
Worked example
Take a 10 ft × 12 ft slab, 5 inches (0.42 ft) thick, with a 1:2:4 mix. In metres that is 3.048 × 3.658 × 0.127 = 1.42 m³ of wet concrete. Dry volume = 1.42 × 1.54 = 2.18 m³. Cement's share is 1/7, so 0.31 m³ of cement = 8.98 bags → 9 bags. Sand is 2/7 of dry volume = 23 cft, and aggregate is 4/7 = 45 cft.
Notes for Indian sites
Sand in India is commonly sold by the brass (100 cft) or by the truckload, so convert cft to brass when talking to your supplier. Always order 5–10% extra for spillage and uneven formwork — the calculator rounds bags up, but a small buffer on sand and aggregate saves a mid-pour shortage. For RCC work (slabs, beams, columns), confirm the grade with your structural engineer; using a weaker mix than specified to save cement is a false economy. Finally, buy cement from a fresh stock (check the manufacturing date) and use it within 90 days, since old cement loses strength.
Frequently asked questions
How many cement bags are needed for 1 cubic metre of concrete?
It depends on the mix. For 1:2:4 (M15), 1 m³ of wet concrete needs about 1.54 m³ of dry materials. Cement's share is 1/7 of that (0.22 m³), which is roughly 6.34 bags — so order 7 bags of 50 kg cement per cubic metre of 1:2:4 concrete.
What does a 1:2:4 concrete mix ratio mean?
It means 1 part cement, 2 parts sand and 4 parts coarse aggregate by volume. This is the nominal M15 grade mix commonly used for flooring, PCC and non-structural work in Indian homes.
Why is the volume multiplied by 1.54 in concrete estimation?
Wet concrete occupies less volume than its dry ingredients because cement and sand fill the voids between aggregate particles. The factor 1.54 converts wet volume to dry volume — a standard thumb rule used by Indian quantity surveyors.
Which concrete mix should I use for a house slab in India?
For RCC roof slabs, beams and columns, use M20 grade (1:1.5:3) or whatever your structural engineer specifies. 1:2:4 (M15) suits PCC, flooring and pathways. Never downgrade the grade your engineer specified to save on cement.
How much concrete do I need for a 10 ft × 12 ft slab, 5 inches thick?
That slab is about 1.42 m³ of wet concrete. With a 1:2:4 mix you need roughly 9 bags of cement (50 kg), 23 cft of sand and 45 cft of aggregate — plus 5–10% extra for wastage.