What is the correct mix ratio for concrete?

There is no single correct ratio — it depends on what the concrete has to do. The most widely used domestic mix is 1:1.5:3 (one part cement, one and a half sand, three aggregate), which is grade M20 and reaches roughly 20 MPa at 28 days. That is appropriate for slabs, beams and columns in ordinary housing. For paths, patios and light foundations, 1:2:4 (M15) is enough and costs less. For heavier reinforced work, 1:1:2 (M25) is the richest nominal mix in normal use.

Why multiply the volume by 1.54?

Because dry materials shrink when you mix them. Loose sand and coarse aggregate contain a great deal of air between the particles. Once water is added, the cement paste and fine sand migrate into the voids between the stones and the whole mass consolidates into a much denser material. The volume you batch is therefore always larger than the volume you place. 1.54 is the standard site figure; the honest range is about 1.52 to 1.57 depending on how well graded the sand is and how damp it is when you measure it.

How many bags of cement are in one cubic metre of concrete?

For M20 at 1:1.5:3, about 8 bags of 50 kg per cubic metre. The arithmetic runs: 1 m³ finished needs 1.54 m³ dry; cement is 1 of 5.5 parts, so 1.54 ÷ 5.5 = 0.28 m³ of cement; at 1440 kg/m³ that is 403.2 kg; divided by 50 gives 8.06 bags. Leaner mixes need fewer bags and richer mixes need more, which is the single biggest driver of what a batch costs.

What water–cement ratio should I use?

Between 0.40 and 0.60 for hand-placed work, with 0.45 to 0.50 being a sensible default. The ratio is the weight of water divided by the weight of cement, so 0.50 with 400 kg of cement means 200 litres of water. Cement only needs about 0.23 to 0.25 by weight to hydrate fully; everything beyond that is there purely to make the mix workable, and it eventually evaporates and leaves a capillary pore behind. That is why adding water to make placing easier reliably costs you strength.

Should I batch by volume or by weight?

Volume batching is what these ratios describe, and it is what happens on most small sites — a gauge box or a bucket used consistently for each material. It is quick and needs no equipment. Weight batching is more accurate because it is immune to bulking and to how loosely a material is heaped, which is why ready-mix plants and any designed mix use it. If you are volume batching, the discipline that matters most is using the same container filled the same way every time.

Does damp sand change how much I need?

Yes, and it is the most underestimated error in volume batching. Damp sand bulks — a film of water around each grain holds the particles apart, and sand at 4 to 6% moisture can occupy 20 to 30% more volume than the same sand dry. If you measure damp sand by the bucket you are therefore delivering substantially less actual sand than you think, and the mix comes out cement-rich and sand-poor. The effect peaks around 5% moisture and disappears again once the sand is fully saturated.

What is the difference between M20 and 1:1.5:3?

They describe the same mix from two different directions. M20 is a performance specification — it means the concrete should reach a characteristic compressive cube strength of 20 megapascals at 28 days. 1:1.5:3 is a recipe that, with reasonable materials and a sensible water content, is expected to achieve that. The grade is the target; the ratio is the traditional way of hitting it without testing anything. Above M25 the relationship stops being reliable enough, and the code requires a designed mix proportioned from materials that have actually been tested.

How much sand and aggregate do I need for one cubic metre?

For M20 at 1:1.5:3, about 0.42 m³ of sand and 0.84 m³ of coarse aggregate per cubic metre of finished concrete. Those come from splitting the 1.54 m³ dry volume in the ratio 1 : 1.5 : 3. By weight that is roughly 672 kg of sand and 1260 kg of aggregate, though weights vary with the density of your local material — river sand and crushed rock differ, and both differ from the figures used here.

How much wastage should I allow?

Five percent is a reasonable default for a straightforward pour, and it is what this calculator uses unless you change it. Wastage covers what sticks inside the mixer and the barrow, spillage during transport, and the extra concrete that disappears into over-excavated formwork or an uneven sub-base. Awkward pours justify more. Running out mid-pour is far more expensive than a little surplus, because a cold joint in the middle of a slab is a permanent weakness rather than an inconvenience.

Concrete Mix Ratio Calculator

A concrete mix ratio is the recipe that decides how strong your concrete ends up and how much it costs you. It is written as three numbers — cement, sand and coarse aggregate — measured by volume rather than weight, so 1:1.5:3 means one part cement to one and a half parts sand to three parts stone. ...

CEMENT : SAND : AGGREGATE

CONCRETE NEEDED (FINISHED VOLUME)

NOMINAL GRADE

MIX PARTS BY VOLUME

cementsandaggregate

That is M20 — slabs, beams, columns in housing.

WATER–CEMENT RATIO

A ratio in the 0.40 to 0.60 band is the usual working range for hand-placed concrete.

CEMENT TO BUY

9 bags

8.47 exact · 50 kg each · 423.4 kg of cement

SAND

0.441 m³

AGGREGATE

0.882 m³

WATER

211.7 L

Dry batch volume

1.617 m³

Wet volume + wastage

1.050 m³

Cement weight

423.4 kg

Sand weight

706 kg

Aggregate weight

1,323 kg

Total dry weight

2,452 kg

Water

211.7 L

Yield per bag

0.124 m³

BATCHING IT

Order 9 bags of cement, 0.441 m³ of sand and 0.882 m³ of coarse aggregate.

Batch by volume in the ratio 1 : 1.5 : 3. One bag of cement is about 0.0347 m³, so use that as your gauge box.

Mix the cement and sand dry until the colour is even, then add the aggregate and turn it through.

Add water gradually to 211.7 litres in total. Stop at a workable slump — the last litre is the one that costs you strength.

THE BATCH

Why you batch 1.54 times the volume you want. Loose sand and aggregate are full of air. Once water is added, the cement paste and fine sand migrate into the gaps between the stones and the whole mass consolidates. Batching 1.617 m³ of dry material yields about 1.050 m³ of placed concrete. Skip this step and you finish roughly a third short.

Heap height is drawn to the cube root of volume, the way a real stockpile grows — a pile twice as tall holds eight times as much, which is why sand deliveries always look smaller than the number suggests.

HOW RICH IS THIS MIX?

Live calculation · updates as you type

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HOW TO USE

1
Enter the finished volume of concrete you need. That is the volume of the slab, footing or column once it is poured and compacted — not the volume of dry material, which the calculator works out for you.
2
Pick a nominal grade, or type your own ratio into the three boxes. M20 (1:1.5:3) suits most domestic slabs and beams; M15 (1:2:4) is fine for paths and light foundations; M25 (1:1:2) is for heavier reinforced work.
3
Set the water–cement ratio. Between 0.40 and 0.60 is the usual working range for hand-placed concrete. Lower is stronger but stiffer to place; higher is easier to work but measurably weaker.
4
Read off the cement bags, sand, aggregate and water. Open site adjustments if you need to change the wastage allowance, the dry volume factor, or the cement bag size to match what your supplier actually sells.

THE FORMULAS

Dry batch volumeV_dry = V_wet × 1.54

Cement volumeV_dry × c / (c+s+a)

Sand volumeV_dry × s / (c+s+a)

Aggregate volumeV_dry × a / (c+s+a)

Cement weightV_cement × 1440 kg/m³

Bags of cementweight / bag size

Waterw/c × cement weight

Volume of one bag50 / 1440 = 0.0347 m³

NOMINAL MIX REFERENCE

Grade	Ratio	Approx. strength	Typical use
M5	1:5:10	~725 psi	Blinding, levelling under footings
M7.5	1:4:8	~1090 psi	Mass fill, non-structural bedding
M10	1:3:6	~1450 psi	Kerbs, floor blinding, fence posts
M15	1:2:4	~2175 psi	Paths, patios, light foundations
M20	1:1.5:3	~2900 psi	Slabs, beams, columns in housing
M25	1:1:2	~3625 psi	RCC work, heavier structural members

Grade names give the characteristic cube strength in megapascals at 28 days — M20 means 20 MPa. The psi figures are the approximate imperial equivalents. Nominal volumetric mixes stop at M25; anything stronger needs a designed mix proportioned from tested materials.

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Last updated: July 29, 2026 · Formula verified · Eagle-eyed accuracy for every calculation.