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Concrete Calculator

Calculators · Added

Two questions, one form. The volume of a pour is pure geometry and comes out the same everywhere. What it takes to fill that volume depends on how you are buying it — cubic yards from a truck, bags from a builders merchant, or cement, sand and stone mixed on site to a nominal ratio. All three are shown, with the method behind each stated rather than assumed.

Units

Volume = length × width × thickness

m
m
mm

Identical elements.

%

Spillage, over-dig, uneven base.

Slabs, beams and columns in small buildings

How to use the concrete calculator

  1. 1Choose metres and millimetres or feet and inches. Thicknesses are asked for in the smaller unit, which is how they are usually specified.
  2. 2Pick what you are pouring — slab, footing, wall, round column or square column — and enter its dimensions.
  3. 3Set how many identical elements there are, so twelve post holes are one calculation rather than twelve.
  4. 4Add a waste allowance. Ten per cent covers spillage and an over-dug or uneven base.
  5. 5Choose a nominal mix grade and a bag size to see both the site-mix quantities and the pre-mixed bag count.

Examples

A garage slab

Input
6 m × 3 m × 150 mm thick
Result
2.70 m³ — 2.97 m³ with 10% waste, about 3.9 cubic yards

Past roughly one cubic metre, ready-mix delivered by truck is normally cheaper and far less work than bags.

Post holes for a fence

Input
12 round holes, 300 mm across, 600 mm deep
Result
0.51 m³ — about 33 bags of 80 lb

The diameter is asked for, not the radius. Holes are also rarely as neat as the number, which is what the waste allowance is for.

The same slab mixed on site at M20

Input
2.97 m³ at 1 : 1.5 : 3
Result
About 24 bags of cement, 1.04 m³ sand, 2.09 m³ aggregate

The wet volume is multiplied by 1.54 to get the dry volume before it is split, which is where most hand calculations go wrong.

About the concrete calculator

Ordering, and why you should order slightly over

Concrete is unforgiving about arriving short. A pour that runs out halfway leaves a cold joint — a plane where fresh concrete meets partly set concrete and never fully bonds — and that joint is a permanent weakness in exactly the place you did not plan one. Running slightly over costs the price of a small amount of waste. Running short costs the integrity of the element.

That is what the waste allowance here is for, and ten per cent is a reasonable default rather than a law. A slab on a well-prepared, level and compacted sub-base with proper edge shuttering might genuinely use five. A footing in a hand-dug trench through soft ground can swallow twenty, because the trench is wider than it was drawn everywhere the sides crumbled. Look at the base before choosing the number.

Ready-mix is ordered in cubic metres or cubic yards and usually sold in increments, so the figure you calculate becomes the next increment up. Below about a cubic metre, bags generally win on both price and practicality; above it, mixing that much by hand is a long day and the consistency between batches becomes its own problem.

What the grades mean

The M numbers are characteristic compressive strengths in megapascals at 28 days — M20 means 20 MPa. The ratios beside them are the nominal volumetric proportions traditionally used to hit approximately that strength: one part cement, some parts sand, some parts coarse aggregate. The relationship between the ratio and the strength is empirical and it holds only when the materials, the water and the compaction are all reasonable.

Choosing higher than you need is not free. Richer mixes cost more, generate more heat as they cure and shrink more, which means more cracking in a large slab rather than less. The grade should follow the job: mass fill and blinding at the weak end, paths and light floors in the middle, structural elements only to whatever the design specifies.

Curing matters at least as much as the mix, and no calculator can help with it. Concrete gains its strength through a chemical reaction with water, not by drying out, so keeping it damp for the first week — covered, sprayed, or under plastic — is what turns the mix you calculated into the strength it was meant to reach. A perfectly proportioned slab left to bake in the sun on its first day will underperform a rougher one that was kept wet.

Frequently asked questions

Why is the dry volume larger than the concrete it makes?
Because the ingredients pack into each other. The sand settles into the gaps between the stones and the cement settles into the gaps between the grains of sand, so the finished concrete takes up noticeably less room than the heap of materials did. The conventional allowance for that is to multiply the wet volume by 1.54 before splitting it by the mix ratio. Different references use anything from 1.52 to 1.57, so the last digit of any of these figures is indicative rather than exact.
How many bags of cement is a cubic metre?
It depends entirely on the grade, which is the point of specifying one. At M20, or 1 : 1.5 : 3, a cubic metre of concrete needs roughly eight 50 kg bags; at M15, or 1 : 2 : 4, roughly six and a half. The conversion underneath is that cement has a bulk density near 1440 kg per cubic metre, which makes a 50 kg bag about 0.0347 cubic metres. Multiply that out by the cement share of the dry volume and you have the bag count.
Why is water not in the list of quantities?
Because water is not proportioned by the volume of the pour, it is proportioned against the cement — the water-cement ratio, typically somewhere between 0.4 and 0.6 by weight depending on what is being built and how it will be placed. Adding water to make the mix easier to work is the single most common way to weaken concrete on a small site, and it is why the ratio is specified against the cement rather than left to the person holding the hose.
Should I trust a nominal mix for something structural?
No. Nominal ratios such as 1 : 1.5 : 3 are volumetric rules of thumb for small works, and they take no account of the actual moisture content, grading or absorption of the aggregate you happen to have. Anything carrying load to a design — a suspended slab, a retaining wall, a foundation to an engineer drawing — should use the mix on those drawings or a designed mix from a batching plant, which is proportioned by weight against tested materials and comes with test cubes to prove it.

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