Concrete Calculator — Volume, Bags, Ready-Mix and Cost

A concrete order starts with geometry, not bag weight

A slab, footing, wall and post hole may all use concrete, but they do not use the same measurements. Select the project shape first. The calculator then shows only the dimensions needed for that shape and converts them into one comparable volume.

One short workflow for six common projects

1. Choose the project shape.
2. Enter the dimensions shown for that shape.
3. Choose volume only, bagged mix or ready-mix pricing.
4. Open advanced settings only when you need to change the allowance or add a price.
5. Use the main result as a planning quantity, then confirm product and supplier requirements.

What the result separates

The calculator keeps the measured volume separate from the allowance. This makes it clear whether a large order is caused by the structure itself or by the reserve added for uneven excavation, small spills, form variation and measurement uncertainty.

• calculated geometric volume before allowance;
• additional allowance volume;
• concrete required including allowance;
• approximate fresh-concrete weight for planning;
• bags and surplus only when bagged mix is selected;
• material cost only when a user-entered price is available.

Metric and imperial results without mixing the systems

Metric entries use metres, millimetres, litres and cubic metres. Imperial entries use feet, inches, cubic feet and cubic yards. The calculator converts through one base system, so an inch thickness is not mistakenly treated as a foot and a cubic yard is not treated as a cubic foot.

Worked example: slab volume in both systems

A slab measuring 4 m by 3 m at 100 mm thick has a geometric volume of 1.2 m³. With a 10% allowance, the planning quantity becomes 1.32 m³. The equivalent imperial workflow uses length and width in feet and thickness in inches, then reports the order quantity in cubic yards.

Bagged mix: use product yield, not a generic bag-size rule

For bagged concrete, enter the mixed yield printed for the exact product. Two bags with a similar mass can produce different volumes because lightweight, rapid-setting and standard mixes are not identical. The calculator divides the required concrete volume by the entered yield and rounds the bag count up.

Ready-mix: the calculator does not invent a truck or supplier minimum

When ready-mix is selected, the calculator keeps the order volume unrounded and multiplies it by the entered price per cubic metre or cubic yard. Suppliers may apply a minimum load, short-load charge, delivery zone, washout fee, waiting time, pump charge or order increment, so the displayed cost is material-only unless those items are already included in the entered rate.

The formulas behind each shape

• Rectangular volume = length × width × depth or thickness.
• Round volume = π × radius² × height or hole depth.
• Repeated elements = one-element volume × quantity.
• Allowance volume = geometric volume × allowance percentage.
• Order volume = geometric volume + allowance volume.
• Exact bags = order volume ÷ mixed yield per bag.
• Bags to buy = exact bags rounded up to a whole package.

Allowance is a planning decision, not a structural dimension

A modest allowance can cover minor variation, but it should not be used to hide uncertain measurements. Straight forms over a well-prepared base may need less reserve than augered holes in unstable soil. If the required allowance seems unusually large, remeasure the work before ordering.

Why post-hole diameter changes the answer quickly

Post holes are cylinders, so their volume depends on the square of the radius. Increasing the diameter affects the result much more than increasing it by the same percentage would affect a simple linear measurement. Measure several holes when drilling conditions vary.

Common input errors

• entering slab thickness in feet when the field is showing inches;
• using radius in a diameter field;
• forgetting the quantity of posts or columns;
• counting overlapping footing runs twice;
• using nominal drawing dimensions after excavation has changed;
• copying bag yield from a different product;
• treating the calculated volume as a concrete mix specification.

The approximate weight is only a handling reference

The result uses 2,400 kg/m³ as a preliminary density assumption for ordinary concrete. It is intended to communicate the scale of the material, not to calculate structural dead load, truck payload, lifting capacity or the exact weight of a selected mix.

When the geometry needs professional review

• the concrete supports a building, vehicle, retaining structure or heavy equipment;
• soil conditions, frost action, groundwater or settlement may affect the foundation;
• reinforcement, anchors, joints or cover must be designed;
• the pour is tall, heavily loaded, pumped or difficult to place continuously;
• local permits, building codes, standards or inspections apply;
• the supplier requires a specified strength, slump, exposure class or maximum aggregate size.

Questions before placing the order

Frequently asked questions

Can different footing sizes be combined?

Combine lengths only when width and depth are the same. Calculate different footing sections separately, then add their order volumes.

Does the price include delivery and pumping?

No. The cost is based only on the entered bag price or ready-mix price per cubic unit. Delivery, short-load fees, pumping, waiting time, reinforcement, formwork and labour are separate unless already included in that price.

Why is there no automatic concrete strength recommendation?

Required strength and durability depend on structural loads, exposure, reinforcement, curing, local standards and the complete design. Dimensions alone are not enough to select a safe mix specification.