Useful when planning a pad, path panel or other slab of uniform thickness. Measure the footprint once, then enter how many identical slabs you need.
- Plan dimensions
- Measure the concrete outline inside the form: length and width for a rectangle; diameter, not radius, for a circle.
- Thickness and quantity
- The full concrete depth and number of identical slabs. Extra is added to their total volume.
Rectangular mode multiplies length by width. Circular mode uses π × diameter² / 4. Both support quantity and an extra percentage.
- Area and perimeter for one slab and for the whole quantity.
- Net volume, extra volume and required volume for supply planning.
One rectangular slab, with a deliberately chosen 10% allowance.
| Quantity | Value |
|---|---|
| Length × width × thickness | 3 m × 2 m × 0.10 m |
| Quantity; extra | 1; 10 % |
| Area; perimeter | 6 m²; 10 m |
| Net; extra; required | 0.600; 0.060; 0.660 m³ |
Area = length × width. Net volume = area × thickness × quantity; required volume = net volume + chosen allowance.
For a circular slab of diameter 2 m and thickness 0.10 m, net volume is π × 1² × 0.10 = 0.314 m³; with the same 10% allowance, 0.346 m³.
Ready-mix planning uses the required volume and a truck capacity you enter. Bagged mix uses the finished yield of one bag; bag weight alone does not determine it. Whole bags and loads round up. Optional cost uses your price per volume or per bag.
Bag-count example for a separate slab: 0.660 m³ divided by an assumed yield of 0.020 m³ per bag gives 33 bags. This yield is an example input, not a specification for every 40 kg or 50 kg bag. Check the selected product.
Metric and imperial entry are supported. In these examples, lengths are converted to metres before multiplication; displayed volumes are rounded to three decimals. The arithmetic retains the unrounded value.
The extra percentage is a chosen allowance for this estimate. It is not a required building-code value. Geometry estimates quantity; it does not select concrete strength, foundation depth or reinforcement.
A thickened edge or a sloping base is not represented by one uniform thickness. Break the geometry into non-overlapping volumes.
Is thickness the compacted base depth?
No. Enter the concrete thickness. Estimate gravel or soil layers separately.
Can I calculate several pads at once?
Use quantity for identical pads. Different dimensions need separate calculations.
- Is thickness the compacted base depth?
- No. Enter the concrete thickness. Estimate gravel or soil layers separately.
- Can I calculate several pads at once?
- Use quantity for identical pads. Different dimensions need separate calculations.
Change only the allowance
Both variants keep the length, width, thickness and quantity from the worked example above. Net volume stays the same.
| Variant | Allowance | Required volume |
|---|---|---|
| A | 0 % | 0.600 m³ |
| B | 10 % | 0.660 m³ |
Variant B adds 0.060 m³ to the net volume. This follows from the chosen percentage; it is not a recommendation for an optimal allowance.
Read a mix specification
Teaching card for an imaginary mix. This is not a product, an order or a mixing recommendation.
- Net weight · 25 kg
- Dry contents of the bag. Mass does not tell you the mixed volume.
- Yield · 12.5 L
- Illustrative mixed volume per bag. Yield is the quantity used to turn a required volume into bags.
- Coverage · 0.5 m² at 25 mm
- Area needs an accompanying depth. Here, area × depth equals the stated yield.
- Units and conditions
- Check whether the value applies per bag, per litre or per coat. For a real material, read its water, substrate, compaction and permitted-depth requirements.
Measurements, units and material terms
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