POTTING MIX PLANNER
Know how much mix your pots actually need.
Calculate litres for round pots or rectangular planters, multiply across several containers and convert the total into bags to buy.
Measure the space that will really be filled.
Round containers use cylindrical volume. Rectangular planters use length × width × depth. The fill-level control leaves watering space below the rim, while the optional buffer accounts for settling and small measurement differences.
For strongly tapered pots, large drainage layers or established plants with large root balls, the actual potting-mix requirement can be lower than the simple geometric volume.
Match the formula to the container shape.
A round pot is treated as a cylinder using its internal diameter and usable fill depth. The calculator converts the diameter to a radius, applies π × radius² × depth and then converts the result into litres. Rectangular planters use internal length × width × usable depth.
Those simple shapes work well for many nursery pots, tubs, troughs and balcony planters, but decorative containers can taper sharply or have curved bases. In those cases the geometric answer is best treated as an upper estimate. If the pot is much narrower at the base than at the rim, use an average practical diameter or allow for the shape when buying mix.
Buy enough mix without pretending every litre becomes usable root space.
After calculating container volume, Tiny Mustard Seed multiplies by the number of matching containers and applies any buffer you choose. The total litres are then divided by the selected bag size and rounded upward to a whole bag, because a fraction of a retail bag still means buying the next full bag.
A buffer can be useful when fresh mix settles after watering, when measurements are approximate or when several containers vary slightly in size. Set the buffer to zero if you want the pure geometric quantity. Keep in mind that an established root ball, permanent reservoir, false bottom or coarse drainage layer can reduce the amount of new mix actually required.
The calculator measures quantity, not potting-mix quality.
Different plants need different drainage, aeration, water retention and nutrient conditions. The volume result does not decide whether a compost, peat-free mix, bark-based blend, mineral component or specialist substrate is appropriate for the crop you are growing.
Follow the plant and product guidance that applies to your situation. For large containers, consider the final saturated weight as well as the volume—especially on balconies, shelves or structures where load matters. The calculator is intended to reduce purchasing guesswork, not replace structural or horticultural judgement.