Concrete Slab Calculator
Dry materials occupy 54% more than the wet mix.
Work out Concrete Slab. Dry materials occupy 54% more than the wet mix. Counts the thing everyone forgets to count.
1:2:4 is a general-purpose mix; 1:1.5:3 is stronger
Lower is stronger and harder to work
Concrete required
2.1 m³
2.75 cubic yards · 4.8 tonnes when cured
Dry materials occupy more volume than the finished concrete, because the sand fills the voids between the aggregate and the cement fills the voids in the sand — the conventional bulking factor is 1.54, which is applied here. The water-to-cement ratio is what governs strength: more water makes the mix easier to place and weaker once cured, which is the trade every concrete pour makes. At 2.1 m³ this is past the point where mixing on site is practical — ready-mix delivered is usually cheaper and certainly more consistent.
How the Concrete Slab Calculator works
Concrete volume for a slab, broken into cement, sand, aggregate and water for the mix ratio you specify. The bulking factor is applied because dry materials occupy more space than the finished concrete does.
Also known as: how much concrete do i need · cement sand and gravel for a slab · concrete bags for a base · concrete mix ratio quantities
Why dry materials exceed the wet volume
A cubic metre of finished concrete needs roughly 1.54 cubic metres of dry ingredients. The reason is packing: the sand fills the voids between the aggregate, and the cement fills the voids in the sand, so the combined volume shrinks as the mix comes together.
That bulking factor is applied here before the mix ratio is split, which is why the cement figure comes out higher than a straight proportion of the finished volume would suggest. Skipping it is the most common reason a mix comes up short.
Mix ratios are by volume — 1:2:4 means one part cement to two of sand to four of aggregate. The components are reported by weight because that is how they are bought, using conventional bulk densities of about 1,440, 1,600 and 1,500 kg per cubic metre.
Water is the strength control
The water-to-cement ratio governs compressive strength more than any other variable. More water makes the mix easier to place and weaker once cured, and the relationship is steep — moving from 0.45 to 0.6 can cost a third of the strength.
Which is why a mix that pours easily is usually a mix that has been weakened. Workability should be achieved with a plasticiser or better placing technique rather than with extra water, and adding water on site to a ready-mix load is a genuine problem rather than a shortcut.
It also explains why curing is not drying. Concrete hardens by hydration, a chemical reaction that needs water present. Letting a slab dry out too fast stops the reaction and leaves it permanently weaker, which is why slabs are kept damp or covered.
Cracking, joints and when to stop mixing on site
Surface cracking is usually shrinkage as water leaves too fast — sun, wind, or a mix that was too wet. Control joints do not prevent shrinkage; they give it a planned place to happen instead of a random crack across the middle.
Cold weather is the other constraint. Below about 5 °C hydration slows sharply, and below freezing it stops and can damage the concrete permanently as the water expands. Insulating blankets and warmed mix water are the usual measures.
Past roughly a cubic metre, mixing on site becomes impractical and inconsistent — each batch differs slightly and the pour has cold joints between them. Ready-mix is usually cheaper at that volume and certainly more uniform, though small loads carry a substantial surcharge.
Where to go next
The Concrete Slab question rarely arrives on its own. These are the ones that usually come with it:
- Rebar Spacing Calculator — Bars go both ways — this is the count for one axis.
- Sand Calculator — Wet sand weighs 20% more than dry for the same volume.
- Paver Calculator — Bedding sand is screeded, never used to fix levels.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How do I calculate concrete for a slab?
Length times width times thickness gives the wet volume. Multiply by about 1.54 to get the dry volume of materials needed, then split by the mix ratio.
Why is the dry volume larger than the wet volume?
Because the sand fills the voids between the aggregate and the cement fills the voids in the sand. The finished concrete occupies less space than the ingredients did separately.
What mix ratio should I use?
1:2:4 by volume is a general-purpose mix; 1:1.5:3 is stronger and used for structural work. For anything load-bearing the specification should come from an engineer rather than a rule of thumb.
What does the water-cement ratio do?
It governs strength. More water makes the mix easier to place and weaker once cured, and the relationship is steep — going from 0.45 to 0.6 can cost a third of the compressive strength.
When should I use ready-mix instead?
Beyond about a cubic metre, mixing on site becomes impractical and inconsistent. Ready-mix is usually cheaper at that point and certainly more uniform.
How thick should a slab be?
It depends entirely on the load and the ground beneath it, which is why the thickness is an input. A garden path and a garage floor are very different problems, and local codes set minimums for both.
How long does concrete take to cure?
It reaches most of its strength in 28 days and continues gaining slowly for years. Foot traffic is usually fine after 24 to 48 hours and vehicle loads after a week or more.
Why is curing different from drying?
Concrete hardens by hydration, a chemical reaction with water, not by drying out. Letting it dry too fast stops the reaction and leaves it permanently weaker — which is why slabs are kept damp or covered while curing.
Do I need a damp proof membrane?
Under any slab inside a building, yes. Without it, ground moisture travels up through the concrete and into whatever floor covering is laid on top.
What causes surface cracking?
Usually shrinkage as water leaves too fast — from sun, wind or a mix that was too wet. Control joints give the shrinkage somewhere planned to happen instead of a random crack.
Can I pour concrete in cold weather?
Below about 5 °C hydration slows sharply and below freezing it stops and can damage the concrete permanently. Insulating blankets and warm mix water are the usual measures.
How much does concrete cost delivered?
It varies by region, mix strength and volume, and small loads carry a substantial surcharge. Get a quote for your specific volume — the per-cubic-metre rate for a small pour is much higher than the headline figure.
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Related calculators
Rebar Spacing Calculator
Bars go both ways — this is the count for one axis.
OpenSand Calculator
Wet sand weighs 20% more than dry for the same volume.
OpenPaver Calculator
Bedding sand is screeded, never used to fix levels.
OpenRing Size Calculator
Finger measurement to US, UK, EU and Japanese sizes.
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