Concrete Calculator — Cement, Sand & Aggregate

Concrete mix ratios at a glance

Indian site practice mostly uses nominal mixes — fixed volumetric ratios of cement, sand and coarse aggregate (written as C : S : A) that are proven to hit a target strength without a lab mix design. The grade number (M10, M20, M25…) is the characteristic 28-day compressive strength in N/mm², so M20 concrete is designed to reach 20 N/mm². Higher grades use proportionally more cement relative to sand and aggregate, which is why the ratio tightens as the grade number climbs:

Grade	Mix (C : S : A)	Strength (28-day)	Typical Use
M10	1 : 3 : 6	10 N/mm²	Levelling course below footings, non-structural beds
M15	1 : 2 : 4	15 N/mm²	PCC, flooring base, pathways
M20	1 : 1.5 : 3	20 N/mm²	Residential RCC — slabs, beams, columns
M25	1 : 1 : 2	25 N/mm²	Footings, structural members, G+2 and above

How to calculate concrete volume for a slab, beam or column (step-by-step)

Whether you're pricing a single slab or checking a contractor's material order, working out concrete quantity takes four steps — the same ones this calculator runs automatically:

Find the wet volume. For a slab, beam, footing or rectangular column: length × breadth × depth. For a circular column: π/4 × diameter² × height. Multiply by the number of identical elements.
Convert to dry volume. Multiply the wet volume by 1.54. Loose cement, sand and aggregate carry air voids; once mixed, watered and compacted, the same materials settle into about 35% less space — so you must batch more dry material than the finished concrete volume.
Split by the mix ratio. For M20 (1 : 1.5 : 3) the parts add up to 5.5. Cement gets 1/5.5 of the dry volume, sand 1.5/5.5, aggregate 3/5.5.
Convert to site units. One 50 kg cement bag occupies 0.0347 m³; sand and aggregate are ordered in cft (1 m³ = 35.31 cft) or by the tonne.

Worked example: a 10 ft × 10 ft slab, 6 inches thick, in M20 — wet volume = 50 cft = 1.42 m³, dry volume = 2.18 m³. Cement: 2.18 ÷ 5.5 = 0.40 m³ ≈ 11.4 bags. Sand: 0.59 m³ ≈ 21 cft. Aggregate: 1.19 m³ ≈ 42 cft. Water at a 0.5 ratio: about 285 litres.

Why the water–cement ratio decides concrete strength

Concrete doesn't gain strength by drying out — it gains strength through hydration, a chemical reaction between cement and water that continues for weeks after the pour. Every mix needs enough water to hydrate the cement fully, plus a little more to make it workable enough to place and compact. The water–cement (w/c) ratio — the weight of water divided by the weight of cement — is the single biggest lever on both strength and durability.

Too little water and the mix can't be compacted properly, trapping air voids that weaken the concrete just as much as excess water would.
Too much water is the far more common site mistake — masons add water to make pouring easier, but every surplus litre eventually evaporates and leaves a void behind. Raising the ratio from 0.5 to 0.6 can cut 28-day compressive strength by 20–30% and make the concrete more porous, letting moisture reach the reinforcement and accelerate rebar corrosion.
Typical site ratios: 0.45–0.50 for M20 residential work, tightening to 0.40–0.45 for M25 and above, or wherever the structure faces moisture, chloride or sulphate exposure — foundations, water tanks, basements, coastal sites.

The practical rule: use the lowest w/c ratio that still lets you compact the concrete fully with a mechanical vibrator, not extra water. If a pour is genuinely hard to work with, add a plasticizer admixture instead of water — it improves flow without diluting strength.

Site tips that save real money

Order 2–5% extra for spillage and uneven shuttering — running short mid-pour costs far more than the buffer.
Never add water beyond the w/c ratio to “make pouring easier” — every extra litre permanently weakens the slab.
For pours above ~5 m³, compare the cost of ready-mix concrete (RMC) — site-mixed batches often lose consistency at that scale.
Log every cement bag and sand trip against the estimate — the gap between calculated and delivered quantity is where site leakage hides.
Cure every pour for at least 7 days (10–14 for M25+) by keeping it continuously moist — skipping curing throws away a large share of the strength the mix design promised.

Frequently asked questions

How many cement bags are needed for 1 m³ of M20 concrete?

About 8 bags. For M20 (1 : 1.5 : 3), dry volume = 1 × 1.54 = 1.54 m³; the cement share is 1.54 ÷ 5.5 = 0.28 m³, and 0.28 ÷ 0.0347 m³ per bag ≈ 8 bags (roughly 400 kg of cement).

Why is wet volume multiplied by 1.54?

Cement, sand and aggregate contain air voids when measured loose. When mixed with water and compacted, the same materials occupy about 35% less space — so you must batch roughly 1.54× the finished (wet) concrete volume in dry materials.

Which concrete grade should I use for a house slab?

M20 is the minimum grade for reinforced concrete work as per IS 456, and it is the standard choice for residential slabs, beams and columns. M25 is common for footings and for buildings of G+2 and above, or wherever the structural engineer specifies it.

How much cement, sand and aggregate for a 100 sq ft slab?

A 100 sq ft slab at 5 inch thickness is about 1.18 m³ of wet concrete. In M20 that needs roughly 9.5 bags of cement, 17.5 cft of sand and 35 cft of aggregate — you can verify these exact numbers in the calculator above.

How much water does the mix need?

Site practice uses a water–cement ratio of 0.45–0.5, i.e. 22.5–25 litres per 50 kg bag. More water makes concrete easier to pour but permanently weaker — keep the ratio tight and compact well.

Does this work for RCC (reinforced) elements?

Yes — the concrete volume is the same; reinforcement displaces a negligible volume for estimation purposes. Steel quantity is calculated separately, typically 80–120 kg per m³ of concrete for residential RCC members.

What water–cement ratio should I use, and does more water make concrete stronger?

Use 0.45–0.50 for M20 residential work, and 0.40–0.45 for M25 and above or in wet/exposed locations. More water does the opposite of making concrete stronger — every 0.05 increase above 0.5 can cut 28-day strength by roughly 10%, because surplus water leaves voids behind as it evaporates instead of reacting with cement.

How do I calculate concrete volume for a slab, beam or column?

Multiply length × breadth × thickness for a slab, beam or rectangular column (in consistent units), or π/4 × diameter² × height for a circular column, to get the wet concrete volume. Multiply by 1.54 to get the dry volume, then split it by the mix ratio to find cement, sand and aggregate — exactly what this calculator does for you.

Also try the free construction cost calculator for your full project budget, and the GST contractor bill generator for billing the work.