About the Concrete Calculator
Ordering concrete is one of those jobs where a small arithmetic slip becomes an expensive problem. Order too little and you are left with a half-poured slab and a weak joint where the next load meets the first. Order too much and you pay for concrete you have to dispose of.
The geometry is simple. The mistakes are almost always in the units — mixing inches and feet, forgetting that a cubic yard is 27 cubic feet — and in forgetting to allow for waste. This concrete calculator handles the three most common pour shapes, does the unit conversions properly, adds a waste allowance, and gives you the answer in the forms you actually buy: a ready-mix order rounded to what suppliers sell, or a count of premix bags.
How to Use the Concrete Calculator
Choose feet and inches or metres and millimetres. Lengths are entered in the large unit (feet or metres) and thicknesses, diameters and step sizes in the small unit (inches or millimetres), which is how they are usually measured.
Pick a shape:
Slab, wall or rectangle — length, width and thickness. A wall is simply a slab standing on its edge.
Round column or post — diameter and height. Use this for sonotube columns and round piers.
Steps — the width of the flight, the number of steps, and the rise and run of each.
Enter how many of the shape you are pouring and a waste allowance.
How the Volume Is Worked Out
slab: length × width × thickness
column: π × (diameter ÷ 2)² × height
steps: n(n + 1) ÷ 2 × rise × run × width
then: × how many × (1 + waste %)Everything is converted to cubic metres first and back to cubic yards at the end, so there is no chance of mixing units part-way through.
Step-by-Step Example
A 10 by 10 foot slab, 4 inches thick, with 10% waste.
Thickness: 4 in ÷ 12 = 0.333 ft
Volume: 10 × 10 × 0.333 = 33.3 ft³
In yards: 33.3 ÷ 27 = 1.23 yd³
With waste: 1.23 × 1.10 = 1.36 yd³
Order: 1.50 yd³ of ready-mix
Or: 62 bags of 80 lb premixThe order rounds up to the next quarter yard because that is how most suppliers sell ready-mix.
Why Steps Take More Than You Think
A flight of steps is the shape people most often underestimate. Each step does not just sit on the ground; it sits on top of the steps below it. So a solid flight of four steps contains four blocks in the bottom row, three in the next, then two, then one:
4 + 3 + 2 + 1 = 10 step-sized blocks = 4 × 5 ÷ 2Counting four blocks instead of ten would under-order by more than half. In practice many flights are built over compacted fill or on a sloping base slab, which uses less concrete; the calculator gives the solid figure as the safe upper bound.
Bags or Ready-Mix?
Premix bags are practical for small jobs: fence posts, repairs, small pads. An 80 lb bag makes about 0.6 cubic feet, so a cubic yard takes about 45 of them, and a cubic metre about 86 bags of 25 kg. The calculator derives the metric bag yield from the same figure per kilogram as the US bags state, so the two agree.
Ready-mix makes sense from about a cubic metre (1.3 cubic yards) upward. It is usually cheaper per unit at that scale, arrives consistently mixed, and saves hours of hard labour. Suppliers typically sell in quarter-yard or quarter-metre steps, and many charge extra for small loads.
Why Allow for Waste
Some concrete is always lost or absorbed. Ground is uneven, formwork bows outward under the weight, some is spilled, and some is left in the mixer or chute. 5 to 10% is the standard allowance.
It is worth being generous. Running short mid-pour leaves a cold joint where fresh concrete meets concrete that has already begun to set, and that joint is weaker and more prone to cracking than the rest.
Measuring Before You Order
A good order starts with good measurements, and a few habits prevent most surprises.
Measure inside the formwork, not the planned outline, once the forms are set. Forms are rarely placed exactly where the drawing says, and the concrete fills what is actually there.
Check depth in several places. Excavations are seldom level. Measure the depth at the corners and the middle, and use the average rather than the shallowest point.
Split odd shapes into rectangles and circles. An L-shaped slab is two rectangles; a curved edge can be approximated by a rectangle plus part of a circle. Work each out separately and add them.
Keep the units consistent. If the drawing is in millimetres, stay in millimetres; converting halfway through is where errors creep in.
Understanding Your Result
Concrete needed is the total including waste.
Volume gives the total in cubic yards and cubic feet, or cubic metres.
Ready-mix order rounds up to the nearest quarter, as you would order it.
Premix bags gives the number of bags in two common sizes.
Worth knowing covers steps, when ready-mix makes sense, and mixing bags.
When Should You Use This Calculator?
Before ordering ready-mix, to know what to ask for.
When buying bags, to avoid a second trip to the builders' merchant.
For columns and posts, where the round volume is easy to get wrong.
For steps, where counting blocks by hand usually undercounts.
Common Mistakes
Entering thickness in feet as 0.4 for 4 inches. It is 0.333.
Forgetting to divide cubic feet by 27. That overstates the order 27 times.
Ordering the exact volume. Always add waste.
Counting steps as separate blocks. Each sits on those below.
Adding too much water to bagged mix. It makes placing easier and the concrete weaker.