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Repotting Size Calculator — Next Pot Size Guide
Find the right next pot size when repotting plants.
How We Calculate This
The recommended pot-size step depends on plant type: general houseplants +5 cm, fast growers a bolder step, slow growers and cacti +3 cm.
Whatever the step, the new pot is capped at no more than one third larger than the current one — the RHS limit that prevents over-potting (a large volume of wet compost around a small rootball is the main cause of root rot).
The extra compost is the difference in real capacity between the two pots, read from the standard UK nursery chart (13 cm ≈ 1 L, 15 cm ≈ 1.5 L, 20 cm ≈ 4 L, 25 cm ≈ 7 L). Pots are tapered and roughly 0.8× as tall as wide, so this is well below a naive cylinder estimate. The displaced old rootball is excluded, so the figure is the fresh compost you actually need to add.
Frequently Asked Questions
Signs: roots growing out of drainage holes, water runs straight through, plant is top-heavy, growth has slowed, or soil dries out very quickly.
The RHS advises the new pot should be no more than one third larger than the current one — for pots under 25 cm that means about 2.5-5 cm wider. Vigorous fast-growers can take the larger end of that range, but going beyond the one-third limit leaves a ring of empty compost that stays wet and rots the roots. This calculator caps the recommendation at one third larger for that reason.
Spring is ideal — plants are entering active growth. Avoid repotting in winter when most plants are dormant.
Water the plant the day before repotting. After repotting, water lightly and wait a few days before watering normally.
Signs include: roots growing out of drainage holes, water running straight through, plant becoming top-heavy, slowed growth, or roots circling the rootball when removed from the pot.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.