How is concrete volume calculated?

Volume = length × width × thickness (or, for a column, π × radius² × height). The calculator does the geometry for slabs, footings and columns and converts the result to cubic metres, cubic yards and the number of bags.

How much does a 25 kg bag of concrete cover?

A 25 kg bag of pre-mixed concrete yields approximately 0.011 cubic metres (about 0.39 cubic feet). It takes roughly 90 bags to make 1 m³.

How many 80 lb bags of concrete in a yard?

An 80 lb bag yields about 0.022 m³ (0.6 ft³). It takes about 45 bags of 80 lb to make 1 cubic yard. The calculator shows 25 kg, 60 lb and 80 lb bag counts side by side.

What waste factor should I use?

Most contractors order 5–10% extra to allow for spillage, settling and any over-excavation. The default is 10%; you can change it in the input field.

Should I use bags or order ready-mix?

For volumes up to about 0.5 m³ (≈ 0.65 cubic yards) bags are usually cheaper and easier. Above this, a ready-mix delivery is faster and often more economical — most suppliers have a minimum order of around 1 m³.

🧱 Concrete Calculator

Volume of concrete needed for a slab, footing, column or post hole — in cubic metres, cubic yards and bag count.

Shape

Input units

Length (m)

Width (m)

Thickness (cm)

Length (m)

Width (m)

Depth (m)

Diameter (cm)

Depth / Height (m)

Number of identical posts

Waste factor (%)

Concrete type

ℹ️ Tip. Bag yields vary slightly by manufacturer (typically ±5%). Always check the bag label for the exact yield, and order 5–10% extra to cover spillage and settlement.

About the Concrete Calculator

The Concrete Calculator tells you exactly how much concrete you need for a DIY or trade project — and how that translates into ready-mix volume or pre-mixed bags. It supports the three most common pour shapes: slabs, strip footings and round columns or post holes.

Formulas used

Slab: Volume = length × width × thickness
Footing: Volume = length × width × depth (rectangular trench)
Column / post hole: Volume = π × (diameter / 2)² × height, multiplied by the number of posts

Bag yields used

25 kg pre-mix bag: ≈ 0.011 m³ (≈ 0.39 ft³) — about 90 bags per m³
60 lb (US) bag: ≈ 0.0167 m³ (0.59 ft³) — about 60 bags per m³
80 lb (US) bag: ≈ 0.0222 m³ (0.78 ft³) — about 45 bags per m³

Yields vary slightly by manufacturer — check the bag for the exact yield. The calculator rounds bag counts up to the nearest whole bag because no merchant sells half-bags.

Waste factor

A waste factor allows for spillage, over-excavation, settlement and the small amount of concrete that always sticks to the mixer or wheelbarrow. Most contractors add 5–10%. The default is 10%; for very small jobs (single post hole) drop it to 5%, and for messy or windy site conditions use 15%.

When to use bags vs ready-mix

Up to ~0.5 m³ (≈ 0.65 yd³): bags are usually cheapest and easiest. You can mix in a wheelbarrow or hire a small mixer.
0.5–1 m³: hire a mini-mix supplier (mobile mixer trucks delivering small loads).
Above ~1 m³: a full ready-mix delivery is fastest and often the cheapest per cubic metre.

Common uses

Garden patios and paths
Shed and greenhouse bases
Fence post holes (typically 0.6–0.9 m deep)
Driveways and parking pads (C25 grade)
House extension foundations (C30 structural)

Frequently Asked Questions

How many 25 kg bags in a cubic metre? Approximately 90 bags, before any waste factor. The calculator displays the exact count rounded up to the nearest bag.

What grade of concrete should I use? Paths and minor footings: C20. Driveways and standard slabs: C25. Structural foundations and load-bearing work: C30 or higher. The grade selector adjusts the recommended density slightly.

Why is my volume coming out so small? Check that thickness is in centimetres, not metres — a typical slab is only 10 cm (0.1 m) thick. The unit selector controls this automatically.

How long does fresh concrete take to set? Initial set 2–4 hours, walking-firm by 24 hours, full design strength at 28 days. Keep cured slabs damp for the first 7 days for maximum strength.

Is my data shared anywhere? No — every calculation runs locally in your browser.

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Important Note: This tool is intended to provide estimates and should not be used as a substitute for professional advice. Information generated by these calculators may be incomplete and does not account for all individual circumstances. Always seek the counsel of a certified expert (such as a financial advisor, healthcare provider, or licensed engineer) before taking action based on these results.

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