What concrete mix ratio should I use for a slab?

M20 grade concrete (1:1.5:3 — cement:sand:aggregate) is commonly used for residential slabs, beams and columns. Higher grades like M25 are used for heavily loaded members.

How do I calculate the volume of concrete needed?

Multiply the length x width x thickness of the slab/member (in the same units) to get the wet volume, then this calculator converts that to cement bags, sand and aggregate quantities.

Why is the dry volume more than the wet volume?

Concrete ingredients have voids between particles before mixing, so roughly 1.54x the wet volume of dry materials (cement + sand + aggregate) is required.

Concrete Calculator — Cement, Sand & Aggregate for Concrete Mix

Understanding Concrete Mix Calculations

Concrete is a mixture of cement, sand (fine aggregate), coarse aggregate (crushed stone) and water, combined in specific proportions to achieve a target strength grade. Whether you're casting a slab, column, beam or footing, knowing the quantities of each material needed helps you order the right amounts, plan labour, and avoid both costly shortages mid-pour and wasted excess material. This calculator estimates cement bags, sand and coarse aggregate quantities for your concrete work based on the member dimensions and mix grade you select.

How the Concrete Calculator Works

The calculator first computes the wet volume of concrete needed (length × width × depth, or the equivalent for your chosen member type). Because dry materials have voids between particles that get filled during mixing and compaction, a dry volume factor of approximately 1.54 is applied — meaning roughly 1.54 cubic metres of dry materials are needed to produce 1 cubic metre of finished wet concrete. This dry volume is then divided according to your selected mix ratio (e.g. M20 = 1:1.5:3, meaning for every 1 part cement, there are 1.5 parts sand and 3 parts aggregate, totalling 5.5 parts). The cement portion is converted to 50kg bags, and sand/aggregate quantities are shown in cubic feet.

Understanding Concrete Grades

M15 (1:2:4) — used for non-structural work like flooring base, pathways and PCC (plain cement concrete) below foundations.
M20 (1:1.5:3) — the most common grade for residential RCC work including slabs, beams, columns and footings in low to mid-rise homes.
M25 (1:1:2) — used for structural members carrying higher loads, such as columns in multi-storey buildings or larger spans.
M30 (1:0.75:1.5) — used in heavily loaded structural elements, water-retaining structures, and where higher durability is required.

Factors That Affect Concrete Material Quantity

Member type and dimensions — slabs, columns, beams and footings have different shapes and volume calculations.
Concrete grade — higher grades (M25, M30) use more cement and less aggregate proportionally compared to lower grades.
Wastage during mixing and transport — site-mixed concrete typically has 5-10% wastage; ready-mix concrete (RMC) reduces this but has minimum order quantities.
Aggregate size and quality — well-graded aggregates (a good mix of particle sizes) need less cement paste to fill voids, slightly improving yield.

How to Use This Calculator

Select the concrete grade (mix) — M15, M20, M25 or M30.
Select the member type — Slab, Column, Beam, Footing, or enter a Custom Volume.
Enter the length and width (for slabs) in metres.
Enter the depth/thickness in metres.
Select a wastage percentage.
Click Calculate to see total concrete volume, cement bags, sand, aggregate quantities and estimated cost.

Site-Mixed vs Ready-Mix Concrete (RMC)

For smaller quantities (typically under 5-10 m³), site mixing using a concrete mixer machine is common and cost-effective, especially in areas where RMC plants are far away. For larger pours like full-floor slabs, Ready-Mix Concrete delivered by transit mixers ensures consistent quality, reduces labour requirements, and allows continuous pouring without cold joints (weak points where old and new concrete meet). RMC typically has a minimum order quantity (often 6 m³ per truck), so plan your pour size accordingly.

Tips to Save Cost and Ensure Quality

Always use the concrete grade specified by your structural engineer — don't substitute a lower grade to save cost on structural members.
Ensure proper curing (keeping concrete moist) for at least 7-14 days after pouring — under-cured concrete doesn't reach its design strength.
For RMC orders, plan your pour to use the full truck capacity to avoid paying for a part-load minimum charge.
Test aggregate for cleanliness — dusty or silty aggregate weakens the cement-aggregate bond.
Schedule pours for cooler parts of the day in summer to reduce rapid moisture loss and surface cracking.

Get Material Prices

Once you have your estimate, compare today's cement and aggregate prices, or find material suppliers near you through BuildHQ for your concrete work.

🏗 Concrete Calculator

Concrete Mix Calculator