Concrete Calculator: Estimate Volume, Liters & Bags
Calculate concrete volume, liters, bag count, weight and reserve for slabs, post footings and kerb bedding.
Estimate concrete volume, liters and bags before you buy
The concrete calculator helps you estimate material for slabs, post footings and kerb bedding. It first calculates the volume in cubic meters, then converts it into liters, bag count and approximate total weight.
This is especially useful for bagged dry concrete, where products are sold by weight but yield fresh concrete by volume. A 25 kg bag often yields about 12.5 to 13 liters of fresh concrete, depending on the product. If your bag states a different yield, you can adjust it in the calculator.
The calculation distinguishes slabs, round post holes and concrete bedding for kerbs. A 10 percent reserve is included because excavations are rarely perfectly even, subsoil can compact and some material is lost during mixing or placement.
- Project type and dimensions: Choose slab, post footing or kerb bedding and enter the matching dimensions.
- Liters and bag count: The calculator converts cubic meters into liters and estimates the bags needed from the yield per bag.
- Weight and reserve: The result includes a 10 percent reserve and shows a rough total weight for buying and transport.
Calculator Description
The concrete calculator is a quantity and shopping estimate for simple concrete work. It calculates concrete volume from the entered dimensions, converts it into liters, adds a material reserve and estimates bag count and total weight from that.
Formula for concrete volume, liters and bag count
The concrete calculator works from volume. It calculates cubic meters, converts them into liters, adds a material reserve, divides by the yield per bag and rounds up to full bags.
Slab
Formula: m3 = length x width x thickness
Example
2 m x 2 m x 0.10 m = 0.40 m3, or 0.44 m3 with 10 percent reserve.
0.44 m3 equals 440 liters of concrete.
Post footing
Formula: m3 = pi x radius2 x depth x quantity
Example
A round hole with 25 cm diameter and 80 cm depth has about 39 liters of volume; with several holes, the result is multiplied by the quantity.
If a post stands in the hole, its volume is subtracted from the hole volume.
Bag count
Formula: bags = round up(liters with reserve / liters per bag)
Example
At 13 liters yield per 25 kg bag, 440 liters equals about 34 bags.
The decisive value is the yield stated for the product you use.
- Element volume: Slabs and concrete bedding are calculated as rectangular solids: length x width x thickness.
- Post holes: Round holes use cylinder volume; the post volume is subtracted when a post is entered.
- Bag yield: The liters per bag determine how many bags are needed for the calculated concrete volume.
- Reserve: A 10 percent markup helps cover uneven excavations, compaction and mixing losses.
Calculation basis & transparency
Accuracy The volume calculation is mathematically exact for the entered dimensions. The real material quantity depends on trench width, subsoil, compaction, mixing losses and the exact product yield.
Disclaimer The calculator provides a planning estimate for buying and transport. For load-bearing or safety-relevant elements, it does not replace structural planning.
Formula & logic
Calcolaro calculates the element volume, converts cubic meters into liters, adds a 10 percent reserve and divides the liters with reserve by the entered liter yield per bag. The bag count is rounded up to full bags.
Assumptions
- Cubic meters are converted into liters with the fixed factor 1 m3 = 1,000 liters.
- The material reserve is 10 percent by default.
- The bag count is rounded up to whole bags so the calculated quantity can be bought in practice.
- The yield per bag should be taken from the product or technical data sheet you use.
- For post footings, the post volume is included when matching dimensions are entered.
Known limitations
- The calculator does not account for structural design, reinforcement, frost footings, soil class or required concrete strength class.
- Uneven excavations, collapsing soil, compaction and mixing losses can increase the required quantity.
- Dry concrete yields differ by manufacturer, product, water amount and handling.
- Load-bearing elements, retaining walls, large slabs or projects requiring approval need professional planning.
Typical concrete quantity examples
Small slab
A small slab for a storage box, bin area or garden project should be 2 m x 2 m and 10 cm thick.
Inputs: 2 m x 2 m x 0.10 m, 10 percent reserve, 13 liters per bag
Calculation: 2 x 2 x 0.10 = 0.40 m3 = 400 liters; with reserve 440 liters; 440 / 13 = 33.85
About 34 bags are needed.
The calculation shows before buying whether bagged material is still practical or another delivery form may be more sensible.
Setting fence posts
Several round post holes are being concreted for a fence.
Inputs: 6 holes, 25 cm diameter, 80 cm depth, 13 liters per bag
Calculation: pi x 0.1252 x 0.80 x 6 = about 236 liters; with reserve about 260 liters; 260 / 13 = 20
About 20 bags are needed.
For many post holes, exact calculation is useful because small differences per hole quickly add up to several bags.
Kerb bedding
A narrow concrete bed should stabilize kerbs along a garden path.
Inputs: 15 m length, 20 cm width, 20 cm height, 13 liters per bag
Calculation: 15 x 0.20 x 0.20 = 0.60 m3 = 600 liters; with reserve 660 liters; 660 / 13 = 50.77
About 51 bags are needed.
The example makes visible that longer concrete beds can need a lot of material despite a small width.
Bagged concrete or ready-mix?
Whether bagged concrete makes sense depends mainly on volume, transport distance, mixing effort and the desired consistency.
For small slabs, individual post footings and short kerb sections, bagged concrete is convenient because it is easy to dose and quickly available.
For larger quantities, bagged concrete quickly becomes heavy, time-consuming and expensive. Ready-mix concrete, a larger mixer or delivery may then be more economical and more consistent.
Typical mistakes
- Not converting centimeters into meters even though the volume formula works in meters.
- Setting the slab thickness too low and ignoring compaction or the base layer.
- Planning no reserve even though excavations are rarely perfectly even.
- Calculating with a generic bag yield instead of checking the value on the actual product.
- Planning post footings without enough depth or without a professional view of frost and loads.
Typical concrete quantities and bag counts
Planning values without additional reserve. Basis: about 12.5 liters of fresh concrete per 25 kg bag.
| Project | Example size | Volume approx. | Liters approx. | Bags approx. |
|---|---|---|---|---|
| Small concrete pad | 1 m x 1 m x 10 cm | 0.100 m3 | 100 liters | 8 bags |
| Small slab | 2 m x 2 m x 10 cm | 0.400 m3 | 400 liters | 32 bags |
| Strip footing | 2 m x 1 m x 20 cm | 0.400 m3 | 400 liters | 32 bags |
| 4 post holes | 30 cm diameter x 80 cm | 0.226 m3 | 226 liters | 19 bags |
| 6 post holes | 25 cm diameter x 80 cm | 0.236 m3 | 236 liters | 19 bags |
| Kerb bedding | 10 m x 20 cm x 10 cm | 0.200 m3 | 200 liters | 16 bags |
The actual bag count depends on reserve, yield and handling.
Frequently Asked Questions About Concrete Quantity
How do I calculate concrete for a slab?
Multiply the slab length, width and thickness. Make sure all dimensions are converted to meters. A slab that is 2 m long, 2 m wide and 10 cm thick is 2 x 2 x 0.10 = 0.40 m3 of concrete.
How many liters are in 1 m3 of concrete?
One cubic meter is exactly 1,000 liters. If the calculated volume is 0.2 m3, that equals 200 liters.
How many bags of concrete do I need for 0.1 m3?
0.1 m3 equals 100 liters. If one 25 kg bag yields about 12.5 liters of fresh concrete, you need 100 / 12.5 = 8 bags. With reserve, one extra bag can be sensible.
How does the calculator handle post footings?
For post footings, the round hole is calculated as a cylinder. If you enter a post diameter, the post volume is subtracted from the hole volume. This gives a more realistic estimate than a flat amount per hole.
Why does the calculator subtract the post volume?
The post takes up space inside the footing. If it is not subtracted, the concrete quantity will be slightly overestimated. The difference is small for one narrow post but noticeable across several larger holes.
How much concrete do I need for one post hole?
A round hole with 30 cm diameter and 80 cm depth has about 56 liters of volume before subtracting any post. Depending on bag yield, that is roughly 5 bags of 25 kg concrete.
How much reserve should I add for concrete?
For typical DIY projects, 5 to 10 percent reserve is useful. This calculator uses 10 percent because excavations often become wider than planned, soil can compact and material can be lost while mixing or placing.
Which yield should I enter for dry concrete?
Use the value printed on the bag or product data sheet. Many 25 kg dry concrete bags yield around 12.5 to 13 liters of fresh concrete. If your product says 12 or 14 liters, enter that value.
When is bagged concrete enough, and when is ready-mix better?
Bagged concrete is practical for small slabs, single post footings and kerb bedding. For larger volumes, it can become expensive and physically hard to move. Ready-mix concrete or a larger mixer may be more efficient and consistent.
Can I use this calculator for load-bearing structures?
No. It is a quantity and shopping estimate for simple projects. Load-bearing elements, retaining walls, garage slabs, reinforced concrete and permit-relevant work require professional planning for structure, reinforcement, subsoil and concrete strength class.
Sources, Standards and Notes
Standards and context
- EN 206 / national concrete standards
European concrete standard for specification, performance, production and conformity. It provides technical context, but this calculator is not a structural design tool. - DIN EN 206 and DIN 1045-2
German concrete rules are relevant for the original German page and for projects in Germany. Always follow the standards and regulations that apply in your location.
Practical assumptions
- Dry concrete bag yield
Many 25 kg dry concrete bags yield roughly 12.5 to 13 liters of fresh concrete, but the product data sheet or bag label is decisive. - Frost depth for small post footings
For outdoor post footings, a depth around 80 cm is commonly used as a practical reference in Germany. Local frost depth and soil conditions may differ.
Safety note
- Load-bearing work
For load-bearing or safety-relevant elements, structural design, reinforcement, concrete strength class and subsoil conditions must be checked professionally.