Buco Concrete Calculator: How to Use It

The Buco concrete calculator is a free online tool that estimates how much ready-mix concrete you need for a project. You enter the dimensions of the area to be filled (length, width, and depth) and the calculator returns the volume in cubic metres, along with the number of 20 kg or 40 kg bags of pre-mixed concrete required. This article explains how the calculator works, how to use it correctly, and what to watch out for so you order the right amount.

Formula

The core formula for concrete volume is simple:

Volume (m³) = Length (m) × Width (m) × Depth (m)

Because the calculator asks for depth in centimetres, you must convert it to metres first:

Depth (m) = Depth (cm) ÷ 100

So the full formula becomes:

Volume (m³) = Length (m) × Width (m) × (Depth (cm) ÷ 100)

To estimate the number of bags, you need to know the yield of a bag. A standard 20 kg bag of pre-mixed concrete typically yields about 0.01 m³ (10 litres) of wet concrete. A 40 kg bag yields about 0.02 m³ (20 litres). Therefore:

Number of 20 kg bags = Volume (m³) ÷ 0.01 = Volume (m³) × 100

Number of 40 kg bags = Volume (m³) ÷ 0.02 = Volume (m³) × 50

In the calculator above, the bag counts are rounded up to the nearest whole bag, because you cannot buy a fraction of a bag. The formula uses 2000 kg as the approximate density of concrete (1 m³ weighs about 2000 kg). So the number of 20 kg bags is (Volume × 2000) ÷ 20 = Volume × 100.

Worked Example

Using the default values from the calculator: length = 3 m, width = 2 m, depth = 10 cm.

1. Convert depth to metres: 10 cm ÷ 100 = 0.1 m
2. Calculate volume: 3 × 2 × 0.1 = 0.6 m³
3. For 20 kg bags: 0.6 m³ × 100 = 60 bags (rounded up, already a whole number)
4. For 40 kg bags: 0.6 m³ × 50 = 30 bags

So for a slab 3 m long, 2 m wide, and 10 cm thick, you need 0.6 cubic metres of concrete, which is 60 bags of 20 kg mix or 30 bags of 40 kg mix.

When to Use the Buco Concrete Calculator

The Buco concrete calculator is ideal for small to medium DIY projects where you mix concrete from bags rather than ordering a ready-mix truck. Common projects include:

• Setting fence posts
• Making a small garden path or patio slab
• Building a shed base
• Repairing a concrete step or driveway patch

For larger projects (more than about 1 m³), it is usually cheaper and more practical to order ready-mix concrete delivered by a truck. The calculator can still help you estimate the volume to get a quote, but you will not need the bag count.

How to Read the Results

The first result is the volume in cubic metres. This is the amount of space your concrete will fill. The bag counts tell you how many pre-mixed bags to buy if you are mixing by hand or with a small mixer.

Remember that the bag counts are estimates. The actual yield of a bag can vary slightly by brand and by how much water you add. Also, if you are filling an irregular shape (like a post hole), you should calculate the volume of the hole minus the volume of the post that sits inside it.

For a post hole, the volume of concrete needed is:

Volume = (hole volume) – (post volume)

For a round post hole, the hole volume is π × radius² × depth. The post volume is length × width × depth (or π × radius² × depth for a round post).

Common Mistakes and How to Avoid Them

• Mixing units: The calculator uses metres for length and width, and centimetres for depth. If you measure in feet or inches, convert to metres first. 1 foot = 0.3048 m, 1 inch = 2.54 cm.
• Forgetting to subtract the post: When setting a post, the concrete only fills the space around the post, not the whole hole. Subtract the post volume.
• Not accounting for waste: Concrete can spill, stick to the mixer, or be slightly overmixed. Add 5-10% to your order to be safe.
• Rounding down bag counts: Always round up to the nearest whole bag. You cannot buy a half bag, and running short mid-project is a hassle.
• Assuming 1 m³ = 1000 kg: The density of concrete is about 2000 kg/m³, not 1000 kg/m³. The calculator uses 2000 kg, which is correct for typical concrete.

Limits of the Formula

The volume formula (length × width × depth) assumes a perfect rectangular prism. Real-world slabs may have slight slopes, irregular edges, or reinforcements (rebar) that displace some concrete. The bag yield also varies: a 20 kg bag of some mixes may yield slightly more or less than 0.01 m³ depending on the aggregate size and mix design. Always check the bag label for the exact yield.

The calculator does not account for the concrete being compressed or settling. Fresh concrete is often compacted with a vibrator or tamped, which reduces its volume slightly. In practice, you will use about the calculated volume, plus a small waste factor.

Alternatives to the Buco Calculator

If you do not have access to the Buco calculator, you can use any concrete volume calculator or do the maths yourself with the formula above. Many hardware stores and concrete manufacturers provide similar online calculators. You can also use a simple spreadsheet or even a phone calculator.

For ready-mix concrete, you can call a supplier and give them the volume in cubic metres; they will quote you a price per cubic metre. For bagged concrete, the bag count is the key number.

Frequently Asked Questions

How many 20 kg bags of concrete do I need for 1 cubic metre?

You need about 100 bags of 20 kg mix for 1 cubic metre, because each 20 kg bag yields about 0.01 m³. For 40 kg bags, you need 50 bags. This is based on the typical density of concrete (2000 kg/m³).

Can I use the calculator for a round post hole?

No, the Buco calculator is for rectangular slabs. For a round post hole, use the formula: Volume = π × radius² × depth (in metres). Subtract the volume of the post. For example, a hole 0.3 m in diameter (radius 0.15 m) and 0.6 m deep has a volume of π × 0.15² × 0.6 ≈ 0.042 m³.

What does m³ mean in concrete?

m³ stands for cubic metre, a unit of volume. One cubic metre is a cube that is 1 metre long, 1 metre wide, and 1 metre high. It is the standard unit for ordering concrete.

How much concrete do I need for a 10×10 slab that is 4 inches thick?

First convert inches to metres: 4 inches = 10.16 cm = 0.1016 m. Then multiply: 10 ft × 10 ft = 100 sq ft. Convert to metres: 100 sq ft ≈ 9.29 m². Volume = 9.29 × 0.1016 ≈ 0.944 m³. So you need about 0.94 cubic metres, or about 94 bags of 20 kg mix.

Is it cheaper to mix my own concrete or order ready-mix?

For small projects (under about 1 m³), bagged concrete is usually cheaper because ready-mix trucks have a minimum order fee. For larger projects, ready-mix is more cost-effective and saves labour. The Buco calculator helps you compare by giving both the volume and the bag count.

Why does the calculator ask for depth in centimetres, not metres?

Because most DIY projects have depths in the range of 5–30 cm, entering centimetres avoids decimals like 0.15 m. The calculator converts automatically, but you must be careful to enter the number correctly (e.g., 10 for 10 cm, not 0.1).

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in l | Length | m | 3 | 0
in w | Width | m | 2 | 0
in d | Depth | cm | 10 | 0
out vol | Volume | m³ | l * w * (d / 100) | 2
out bags20 | 20 kg bags | bags | ceil(vol * 2000 / 20) | 0
out bags40 | 40 kg bags | bags | ceil(vol * 2000 / 40) | 0