Four pours, one idea
Every concrete order starts the same way: work out the volume of the space you are filling, in cubic metres or cubic yards, then convert that into bags or a truck order. This calculator covers the four pours most people actually do at home: a slab for a patio, path or shed base, a footing or trench for a wall or foundation, a column or post hole for a fence or pergola, and a set of concrete steps. Pick the one that matches your project, enter its measurements, and the working is shown alongside the answer.
The maths behind three of the four is straightforward multiplication: a slab and a footing are both just a box, length times width times the third measurement. The post hole brings in a circle, and the steps use a method that is easy to get wrong by guesswork but simple once you see it laid out.
The four formulas
A slab is a flat rectangular box, so its volume is length times width times thickness. The only trap is units: thickness is nearly always given in millimetres or centimetres while length and width are given in metres, so mixing them up without converting is the single most common arithmetic mistake in a concrete order.
A footing or a trench uses exactly the same box formula, just with depth standing in for thickness. Depth is measured from the top of the dug trench straight down to its base.
A column or post hole is a cylinder, so its volume is pi times the radius squared times the height, the same cylinder formula used for tins, tubes and pipes. When a post is being set into the hole, real usable concrete is only what fills the gap around the post, so the post's own volume is subtracted first. A square post's volume is its side squared, multiplied by how deep it sits; a round post uses the same circle formula as the hole itself.
Steps are the odd one out. Picture a solid concrete staircase built up from the ground: the bottom step has concrete underneath it running the full depth of the staircase, the second step sits on a shorter block, and so on up to the top step, which sits on the shortest block of all. Add up those stacked rectangular blocks and the total works out to the rise times the run times the width, multiplied by the sum of 1 up to the number of steps, which has the tidy closed form n(n+1)/2.
Worked through for 3 steps with a 150 mm rise, a 300 mm run and a 1 m width: 1 × 0.3 × 0.15 × (3 × 4 ÷ 2) = 1 × 0.3 × 0.15 × 6 = 0.27 cubic metres. Doubling the number of steps more than doubles the concrete needed, since both the step count and the sum n(n+1)/2 grow together.
How thick should a slab actually be?
Slab thickness depends entirely on what sits on top of it. A path or a patio only ever carries foot traffic, so 75 to 100 mm is the typical range quoted by concrete suppliers and builders' merchants. A driveway or a garage floor has to carry a car's weight, sometimes repeatedly and sometimes a much heavier vehicle, so 100 to 150 mm is the usual range there, with the thicker end favoured for anything heavier than a family car or for ground that is not perfectly firm. These are typical starting figures rather than a specification. Local building rules, the strength of the sub-base underneath, and any reinforcement mesh all shift the right number for a specific job, so treat this calculator's thickness field as a planning figure and check anything load-bearing against local guidance before you dig.
Why you always order over the calculated figure
Ordering exactly the number this calculator shows you, with the waste allowance switched off, is the most common way a concrete pour runs short partway through. Three things eat into the calculated volume every time: the ground underneath is never perfectly flat or perfectly compacted, so the void you are actually filling is always a little bigger than the idealised box or cylinder in the sums; concrete spills, splashes and gets left in the mixer, the wheelbarrow and the pump; and formwork bows slightly under the weight of wet concrete, adding a little more volume than the boards suggested. Ordering 5 to 10% over the calculated figure is standard trade practice for exactly these reasons, and this calculator defaults to the 10% end of that range since running short mid-pour, with concrete already curing in the mixer, is a far worse problem than having a little left over.
Cubic metres, cubic yards, and why both appear
Ready-mix suppliers price and deliver concrete by the cubic metre in the UK and most of the world, and by the cubic yard in the US and Canada. A cubic yard is a fixed, exact conversion: one yard is 0.9144 metres by definition, so one cubic yard is 0.9144 cubed, which comes to 0.764554857984 cubic metres, usually rounded to 0.7646. This calculator shows both figures side by side so the number matches whatever your supplier or your bag's packaging quotes.
Turning cubic metres into bags
Bagged, pre-mixed concrete states its yield on the packaging, the volume of finished, mixed concrete one bag makes once water is added, and it is always smaller than the bag's dry weight would suggest since the aggregate settles and compacts. In the UK, a 25 kg bag of ready-mixed concrete typically yields somewhere around 0.011 to 0.012 cubic metres, depending on the brand; this calculator uses 0.0116 m³ as a typical figure. In the US, Quikrete's published yields for their standard concrete mix are 0.30 cubic feet for a 40 lb bag, 0.45 cubic feet for a 60 lb bag and 0.60 cubic feet for an 80 lb bag, and this calculator uses the 60 lb and 80 lb figures alongside the UK bag. Every bag count is rounded up to the next whole bag, since a supplier will not sell you 113.8 bags, and the count is worked out from whichever volume you are currently viewing, with or without the waste allowance switched on.
When bags stop making sense
Bags are the easiest option for small pours, most fence posts, a short section of edging, a small patch. Once a pour needs more than around a cubic metre (roughly a cubic yard) of concrete, mixing enough bags by hand becomes a genuinely long, heavy job, and a ready-mix truck usually works out cheaper and far less effort even after a delivery charge. Above that rough threshold this calculator flags the option, though the exact crossover point in your own case depends on how much of the mixing you are willing to do yourself and what a local ready-mix supplier charges for a small load.
Curing, briefly
Concrete reaches a workable, load-bearing strength after a few days but keeps gaining strength for around 28 days as the chemical reaction with water continues, which is why 28-day strength is the figure quoted on every concrete spec sheet. Keeping the surface damp and out of direct sun and hard frost for the first few days of that period matters more than anything else you can do to help it cure evenly and avoid surface cracking.
When to call a professional
Anything structural, a foundation carrying a building's weight, a retaining wall, a suspended slab, or a pour on sloping or soft ground, is worth getting checked by a structural engineer or a qualified contractor before you order anything. This calculator is built for planning ordinary DIY pours, paths, patios, sheds, fence posts and garden steps, not for sign-off on anything a building inspector will need to approve.
Questions people ask
How much extra concrete should I order?
5 to 10% over the calculated volume is standard trade practice, to cover an uneven sub-base, spillage and formwork loss. This calculator defaults to 10% with a switch to turn it off and see the bare calculated figure instead.
How thick should a concrete slab be?
Paths and patios are typically 75 to 100 mm; driveways and garage floors are typically 100 to 150 mm, since they need to carry a vehicle's weight. These are typical published ranges rather than a fixed rule, so check local building guidance for anything load-bearing.
How many bags of concrete do I need?
Divide the volume you need by a bag's typical yield and round up. This calculator does that for a 25 kg UK bag and 60 lb and 80 lb US bags at once, using published manufacturer yields, so you can compare all three without doing the sums by hand.
Should I use bags or order a ready-mix truck?
Bags suit small pours comfortably. Once a job needs more than roughly a cubic metre (about a cubic yard), mixing enough bags by hand becomes a long, heavy task, and a ready-mix truck usually works out both cheaper and easier, even with a delivery charge added.
Why does the post hole calculator subtract a post's volume?
Concrete only fills the gap around a post, not the space the post itself takes up. Subtracting the post's volume, assuming it runs the full height of the hole, gives the real amount of concrete needed rather than overstating it by the cylinder's full volume.
Why is the steps formula not just rise times run times width?
A solid concrete staircase is not one block, it is a stack of blocks of decreasing depth, one for each step. Summing those stacked blocks gives rise × run × width × n(n+1)/2, where n is the number of steps, rather than a single flat multiplication.
How long does concrete take to cure?
It becomes walkable and load-bearing within a few days but keeps gaining strength for around 28 days, the figure used on every concrete spec sheet. Keeping it damp and protected from direct sun and frost for the first few days helps it cure evenly.
Figures on this page are typical, published ranges for planning an ordinary DIY pour, not a structural specification. For anything load-bearing, a foundation, a retaining wall or a pour on soft or sloping ground, check with a structural engineer or a qualified contractor before you order.