Start with the space to fill
A 50 × 20 × 20 cm planter has a geometric capacity of 20 L. Roots occupying 2 L leave 18 L for new material. Do not calculate bags from total capacity without considering relevant occupied space.
Check units before adding an allowance. Container dimensions describe geometry, while the bag size must be entered as a volume in litres. Weight in kilograms cannot replace volume without additional information about the material.
Choose the allowance deliberately
A 10% allowance on 18 L adds 1.8 L, bringing the target to 19.8 L. This is a calculation scenario, not a claim about how much every mix will settle or an instruction on how firmly to compact it.
Compare smaller or larger allowances to see how they affect purchasing. Keep the initial volume and added allowance separate. This avoids confusing measurement uncertainty with the extra quantity that comes from buying whole bags.
Round and review the cost
With 10 L bags, dividing 19.8 by 10 requires two whole bags: 20 L purchased. The difference from the target is 0.2 L. Compared with the 18 L space to fill, the extra 2 L includes both allowance and rounding.
Enter a price per bag to calculate that material cost alone. Compare other sizes by repeating the calculation with their actual figures. Check the chosen product and intended use: the tool does not assess composition, fertilisers, plant suitability or delivery charges.
- Enter litres, not kilograms, as bag capacity.
- Separate the allowance from pack rounding.
- Check the chosen product information before purchasing.
Frequently asked questions
Will two bags definitely be enough in the example?
They cover the mathematical target of 19.8 L with 20 L of stated capacity. Measurement accuracy, occupied space and actual material volume still affect the purchase.
Can I combine different bag sizes?
The calculator uses one size per operation. Compare combinations manually by adding litres and prices without applying the allowance again to the purchased amount.