How do I know when to repot?

Signs: roots growing out of drainage holes, water runs straight through, plant is top-heavy, growth has slowed, or soil dries out very quickly.

Can I pot up by two sizes at once?

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.

When is the best time to repot?

Spring is ideal — plants are entering active growth. Avoid repotting in winter when most plants are dormant.

Should I water before or after repotting?

Water the plant the day before repotting. After repotting, water lightly and wait a few days before watering normally.

How do I know when a plant needs repotting?

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.

GardenCalcs

Repotting Size Calculator — Next Pot Size Guide

Find the right next pot size when repotting plants.

Current Pot Diameter

Measure the inside diameter across the top of the pot the plant is in now.

Plant TypeVigour sets the step size; the new pot is still capped at one third larger (RHS).

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

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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.