The Concrete Calculator estimates how much concrete a pour actually takes: volume in cubic feet, cubic yards, cubic metres, and litres for slabs, footings, columns, and stairs, the number of pre-mix bags that volume represents, a waste allowance, and a cost estimate at your price per unit. It exists because ordering concrete is the one DIY calculation where being wrong is expensive in both directions.
How It Works
Each shape reduces to a volume: a slab is length x width x depth; a footing adds the trench cross-section over its length; a column is the cylinder formula over its height; stairs are summed as stacked rectangular steps. The volume is converted across units (1 cubic yard = 27 cubic feet), inflated by your waste percentage (10% by default), and divided into bags using the bag yield you select (the default setting assumes 45 standard bags per cubic yard), while cost multiplies the total volume by your per-yard or per-metre price.
Parameters
| Input | Required | Meaning | |---|---|---| | Shape | Yes | Slab, footing, column, or stairs | | Dimensions | Yes | Per the shape; depth in inches or centimetres | | Unit system | Yes | Imperial or metric | | Waste % | No | Default 10% | | Bag size | No | Yield class for the bag count | | Price per unit | No | Enables the cost estimate |
Worked example
A slab 20 ft by 12 ft, 4 inches thick: volume is 80 cubic feet, which is 2.96 cubic yards (about 2.27 cubic metres). With the default 10% waste the order becomes about 3.26 cubic yards, which is the number to give the ready-mix plant (plants sell in quarter or half yard steps, so 3.5 yards in practice). As bags, at the default 45 bags per cubic yard, it is about 147 bags, which is the calculator's way of telling you this pour left bag territory several yards ago: a slab this size is a ready-mix job, and the bag count's real use is on footings, post holes, and small pads where it tells you whether you need a pallet or a pickup load.
Bags versus ready-mix, decided by arithmetic
A bag yields roughly 0.6 cubic feet for the common 80 lb class (smaller bags proportionally less), so one cubic yard is around 45 of them, each of which must be hauled, lifted, mixed, and placed inside the working time of the batch. The crossover most contractors use is around one cubic yard: below it, bags win on simplicity and no short-load fees; above it, ready-mix wins on cost per yard, consistency, and your spine. The calculator's dual output exists for exactly this decision: run your shape, read both numbers, and let the bag count's absurdity (or reasonableness) make the call.
Waste, depth, and the errors that cost money
The 10% default waste covers uneven subgrade, spillage, and forms that bow; a hand-dug footing trench can honestly need 20%, and a laser-levelled slab base on a calm day 5%. Depth is the input with leverage: at 4 inches the example slab needs 2.96 yards, at 4.5 inches (one casual "close enough" with the rake) it needs 3.33, an eighth more concrete bought by a half-inch decision. Measure depth in several places and use the honest average. For stairs, the calculator's step summation assumes uniform rectangular steps; winder or curved stairs leave its geometry behind and belong to a specialist estimate.
Limitations
Volumes are exact for the idealized shapes; real excavations are not. The calculator does not design the mix (strength class, slump, reinforcement, fibre, admixtures), compute rebar, or account for pump minimums, delivery and short-load fees, or local plant rounding. Bag yields vary by brand and class: check the yield printed on your bag against the class you selected before a large purchase.
Frequently Asked Questions
How much extra should I order?
The waste field is that decision: 10% is the standard default, toward 5% for precise formed work on a level base, toward 20% for hand-dug footings in rough ground. Running short mid-pour (a cold joint) is far costlier than a quarter yard returned.
Why does the bag count look so large?
Because bags are small: roughly 45 standard bags make a single cubic yard. If the count looks absurd, the pour is telling you it wants a truck. Conversely, a 6-bag answer means bags are genuinely the right method.
Does the calculator include rebar or mesh?
No, it estimates concrete volume, bags, and cost only. Reinforcement is a design choice by load and code, not a volume fraction, and belongs to your plan or engineer rather than a volume tool.