Repot Size Calculator
Enter the current pot's diameter and height. The calculator applies the two-to-three-centimetre rule, works out the litres of fresh potting soil, and lists the root-bound signals that say repot rather than refresh.
Garden centres upsell pots by enthusiasm. This one prices the root physics: too big a pot holds water the roots cannot drink, which is how overwatering happens to careful people - and the upgrade rule keeps the soil-to-root ratio honest.
How to use
- Measure the current pot's rim diameter and height.
- Buy the next size shown and roughly the soil litres - one extra bag if the roots are dense.
- Repot in spring as growth resumes; water lightly for a fortnight while the roots settle.
Frequently asked questions
How much bigger should the new pot be?
Two to three centimetres in diameter - enough fresh soil for a season of root growth without a swamp of unused, ever-damp compost. Jumping several sizes is the classic overwatering trap wearing a generous disguise.
How do I know a plant is root-bound?
Roots circling the soil surface, escaping the drainage hole, or water racing straight through the pot - the root mass now owns the volume. Slowed growth and quick thirst are the quieter versions of the same message.
How much potting soil do I need?
The new pot's cylinder minus the root ball - the calculator gives litres, and one spare bag covers the underestimate. Refresh the top five centimetres yearly even without a full repot; it resets nutrition without moving roots.
When is the best time to repot?
Spring, just as growth resumes - roots colonise fresh soil at their fastest then. Repotting in winter leaves wounded roots sitting in wet, unexplored soil; water lightly for a fortnight after any move.