Concrete is the one material you cannot send back, top up cheaply, or stretch when the forms are already full — so the volume maths has to happen before the pour, not during it. This concrete volume calculator covers the four shapes small jobs actually use — slab, footing, square column and round column — and returns the volume in cubic metres and cubic yards, plus the count of 80, 60 and 40 lb pre-mix bags at the manufacturers’ published yields, with a waste allowance you control. If you have ever watched a mixer driver guess, you know why the bag count matters: it is the number that tells you whether this is a bag job, a bulk job or a truck job.
The only formula, in four shapes
Every option is the same idea — cross-section × length. A slab is length × width × thickness. A footing is length × width × depth. A square column is the same box stood upright. A round column is π × radius² × height, with radius = diameter ÷ 2: a 30 cm tube is not a 30 × 30 box, and treating it as one over-orders by about 27% (the circle is π/4 of its square). The calculator applies the waste percentage after the shape maths, so 10% extra means 10% more concrete everywhere, including the bag counts.
Worked example — the classic patio slab
A slab 4 m × 3 m × 0.10 m is 4 × 3 × 0.1 = 1.2 m³, which is 1.2 × 1.30795 = 1.57 yd³ or 42.4 ft³. Bagged at the published yields that is ceil(42.4 ÷ 0.60) = 71 bags of 80 lb, ceil(42.4 ÷ 0.45) = 95 bags of 60 lb, or 142 bags of 40 lb. Add the 10% waste allowance and the volume rises to 1.32 m³ and the 80 lb count to 78 bags. Numbers like that are why the calculator prints all three bag sizes: somewhere around a cubic metre, the honest answer usually changes from “bags” to “call the ready-mix plant” — and you can see the line instead of discovering it at the till.
Why the waste toggle exists
Ground is never as flat as the drawing. A slab specced at 100 mm over soil that dips 15 mm in the middle takes roughly 10–15% more concrete than the neat product of its dimensions; footings dug by eye take more still. Pros order 5–10% over on formed work over a blinding layer, 10–15% over on ground-bearing pours. The toggle defaults to 10% — set it to 0 only when the forms are measured, rigid and level, and keep a plan for the shortfall anyway (a wheelbarrow mix for the last corner beats a cold joint).
What this calculator does not decide
Volume is not strength: the mix design (cement content, aggregate, water) depends on the exposure and load, which is a specification question, not a geometry one — our Spanish companions cover cement bags per cubic metre by mix ratio and sand-and-gravel quantities if you are batching on site. Reinforcement, expansion joints and curing are likewise outside a volume tool. What it does settle, exactly, is how much material the shape holds — the number every other decision hangs off.
Ordering from the number, not around it
When the volume lands between bag maths and truck maths, one more figure helps: ready-mix is usually sold with a minimum load or a part-load charge, so a 1.2 m³ pour can cost like 2 m³ from some plants and like 1.2 from others — the calculator cannot know your plant, which is why it shows the bag counts even for truck-sized volumes. If you do bag a pour this size, stage it properly: 71 bags is about 2.6 tonnes of material that has to move from kerb to mixer before it moves to the forms, and mixing 1.2 m³ by hand-mixer takes most of a working day with two people. The volume is the easy part of a pour; plan the labour with the same honesty and the number on this page will be the one you actually use.
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Versión en español: Calculadora de Volumen de Hormigón en m³: Losas, Zapatas y Columnas con Sacos de 40, 60 y 80 lb · Brick Wall Calculator · Render & Plaster Mortar Calculator.
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