What you need to know
- Convert thickness from centimetres to metres.
- Add a margin for level variation and handling.
- For bagged products, use the stated yield or density rather than a generic guess when available.
Use your own measurements
Enter the slab dimensions, thickness, allowance and product data to estimate volume, mass and illustrative bags.
Calculate geometric volume
For a rectangular slab, multiply length × width × thickness in metres. A 10 cm layer is 0.10 m, not 10 m.
Add a realistic margin
Sub-bases are rarely perfectly flat. A small allowance helps cover local low spots and unavoidable loss.
Bags or ready-mix delivery
Large quantities can mean hundreds of kilograms or many bags. Compare delivery, mixing time, storage, access and labour—not just the bag price.
Structural work needs design
This calculator estimates quantity. Reinforcement, bearing capacity, concrete grade, joints and structural design require project-specific technical judgement.
How much concrete for a 4 × 3 m slab?
Suppose you are estimating a 4 m × 3 m area with a planned concrete layer 10 cm thick. To illustrate the calculator, use an 8% allowance, an assumed wet-mix density of 2,200 kg/m³, and a hypothetical 25 kg bag at €5. These inputs are examples, not a specification or live product prices.
- Convert thickness: 10 cm ÷ 100 = 0.10 m.
- Find geometric volume: 4 × 3 × 0.10 = 1.20 m³.
- Add the margin: 1.20 × 1.08 = 1.296 m³, or 1,296 litres of finished volume.
- Convert using assumed density: 1.296 × 2,200 = 2,851.2 kg of theoretical wet-mix mass.
- Illustrate the calculator’s bag model: 2,851.2 ÷ 25 = 114.048, rounded up to 115 bags; at €5 each, the simple model gives €575.
Important: the mass-based bag calculation is only a model. A 25 kg sack of dry concrete mix is not necessarily 25 kg of finished wet concrete. For actual ordering, use the manufacturer’s stated finished volume per bag and follow the product’s mixing and application instructions.
Why average thickness matters so much
For the same 12 m² footprint, 8% allowance, assumed density and illustrative €5 bag price:
| Thickness | Volume incl. margin | Theoretical mass | 25 kg bag model | Model cost |
|---|---|---|---|---|
| 8 cm | 1.037 m³ | 2,280.96 kg | 92 | €460.00 |
| 10 cm | 1.296 m³ | 2,851.2 kg | 115 | €575.00 |
| 12 cm | 1.555 m³ | 3,421.44 kg | 137 | €685.00 |
A change from 8 cm to 12 cm increases the required volume by 50%. Check actual layer depth in several places rather than assuming the base is perfectly flat.
Repeat the estimate with your measurements in the concrete calculator. For a structural or load-bearing slab, thickness, reinforcement, concrete grade and design must be specified by a qualified professional; this quantity guide is not structural design advice.
How to translate volume into actual bags or delivery
If a dry-mix bag states a finished yield in litres, divide the required finished volume by that yield and round up. For example, only if the chosen product explicitly states 12 litres per 25 kg bag, a 1,296-litre estimate would call for 108 bags (1,296 ÷ 12). This hypothetical product-yield method differs from the 115-bag mass estimate above, which is why the technical data sheet must take priority.
- Bagged mix: confirm finished yield, bag weight, usable temperature range, working time, water ratio and the intended application.
- Ready-mix: compare the order volume, minimum delivery quantity, access for the truck or pump, discharge time and any delivery surcharges.
- Both options: plan safe handling and placing time. A large slab can involve several tonnes of material; the cheapest theoretical sack price is not necessarily the cheapest or safest solution.
- Structural use: ask the appropriate designer or engineer to determine load capacity, reinforcement, joints, concrete specification and site requirements.
Final check
Confirm the actual base levels, the job specification and the material yield before buying. Re-check the calculated volume if the intended average thickness changes.
- Check thickness at several points.
- Use product density or yield from the data sheet.
- Plan delivery and mixing logistics.
- Get technical advice for structural work.
Keep a small job sheet
Record length, width, measured depths, volume before and after the allowance, the chosen concrete specification, product data-sheet yield and delivery method. Keep a supplier reference, required delivery date and any access restrictions so that a revised estimate is easy to check.
Turn the guide into a decision
For small non-structural work, follow the concrete product’s permitted uses and mixing instructions. For structural slabs, foundations, driveways with design requirements or uncertain supporting ground, seek project-specific technical advice before ordering or placing concrete.