Repotting calculator
Whether your plant actually needs repotting, what size pot to move it to, and exactly how much of each component to mix — measured in cups, for your pot.
Yes — repot it
Yes — roots are coming out the drainage holes, and roots are circling the bottom of the root ball. That's a genuinely root-bound plant.
8" → 10" · about 5.15 qt of extra mix
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Why "one size up" matters more than it sounds
The instinct when repotting is to give the plant room to grow. It backfires. Roots can only draw water from soil they actually occupy, so a small root ball dropped into a large pot leaves most of that soil sitting wet for days. That is the ordinary route to root rot — not neglect, but a well-meant oversized pot.
Moving one size at a time keeps the ratio of roots to soil close enough that the pot dries at a rate the plant controls.
What goes in the mix, and what to buy
Bagged potting soil on its own is too dense for most houseplants in a pot — it holds water at the expense of air, and roots need both. Amending it is the whole job.
Chunky aroid mix
Aroids grow on tree bark in the wild — they want air around the roots, not dense soil.
4 parts bark · 3 parts perlite · 2 parts potting mix · 1 parts coir
Standard houseplant mix
Bagged potting soil alone stays too wet in an indoor pot; perlite fixes the drainage.
7 parts potting mix · 3 parts perlite
Fast-draining succulent mix
Succulents and dracaenas rot in moisture-retentive mixes. Grit is the whole point.
4 parts potting mix · 4 parts pumice · 2 parts sand
Palm mix
Palms want moisture retention with enough structure that the roots still breathe.
6 parts potting mix · 2 parts perlite · 2 parts bark
Orchid bark (no soil)
Phalaenopsis are epiphytes. Potting soil suffocates and rots their roots.
8 parts bark · 1 parts charcoal · 1 parts perlite
The components
| Orchid bark (fir bark) | Medium grade suits most aroids. Fine grade compacts too fast. buy → |
|---|---|
| Perlite | Coarse grade. The fine dusty stuff compacts and defeats the point. buy → |
| Pumice | Heavier than perlite, so it does not float to the surface when watering. buy → |
| Potting mix | Any decent peat- or coir-based mix. This is the part you amend, not the whole answer. buy → |
| Coco coir | Bricks are far cheaper than loose bags and expand about 5×. buy → |
| Coarse sand | Horticultural or builders sand — never play sand, which packs solid. buy → |
How this calculator decides
| Go up one pot size only — about 1–2" in diameter under 10", 2–3" above. | Excess soil around a small root ball stays saturated because there are not enough roots to draw it down, which causes rot. This is the most widely agreed rule in the literature. |
|---|---|
| Repot in response to evidence, not on a schedule. | The reliable signals are roots emerging from drainage holes, roots circling the root ball, water running straight through, or a mix that has broken down and no longer drains. |
| A nursery plant usually should not be repotted on arrival. | It is already in an appropriately sized pot and is simultaneously adjusting to new light and humidity. Two shocks at once is worse than one. |
| Some species prefer to be pot-bound. | Hoya, Phalaenopsis orchids and several succulents flower or offset better when snug; routine upsizing delays flowering. |
| Spring and early summer are the right window. | The plant is actively growing and colonises fresh mix quickly. In winter, fresh soil sits wet around dormant roots. |
| Never add a gravel layer at the bottom. | A coarse layer under finer soil raises the perched water table, keeping the root zone wetter, not drier. This is well demonstrated in soil physics and still widely mis-taught. |
| Plant at the original soil line. | Burying the stem deeper than it grew encourages stem rot. |
Method last reviewed 2026-08-20.
Questions
How much bigger should the new pot be?
One size up: about 1–2 inches wider for pots under 10 inches, 2–3 inches for larger ones. Going bigger is the single most common repotting mistake — a small root ball surrounded by a large volume of wet soil cannot draw that water down, and the soil stays saturated long enough to rot the roots.
Should I repot a plant as soon as I bring it home?
Usually no. A nursery plant is already in a pot sized for its roots, and it is simultaneously adjusting to new light, humidity and handling. Repotting immediately stacks a second shock on the first. Wait two to four weeks, then check whether roots are circling or coming out the drainage holes.
When is the best time of year to repot?
Spring and early summer, while the plant is actively growing and will colonise the fresh mix quickly. What actually matters is day length rather than the calendar — below roughly 11 hours of daylight most houseplants idle, and fresh soil then just sits wet around roots that are not drinking.
Should I put gravel in the bottom of the pot for drainage?
No. This is the most persistent piece of bad gardening advice there is. A coarse layer underneath finer soil raises the perched water table, so the root zone stays wetter, not drier. A working drainage hole is what matters.
How do I know if a plant is actually root-bound?
Slide it out of the pot and look. Roots circling the bottom or spiralling around the outside of the root ball, roots pushing out of the drainage holes, or water running straight through without being absorbed all mean the roots have displaced most of the soil. Stalled growth alone is not enough — that has many other causes.
How deep should the plant sit in the new pot?
At the same soil line it grew at before. Burying the stem deeper invites stem rot, and planting too high leaves roots exposed to drying out.
General horticultural guidance. Pot sizes and mixes are starting points — check your own plant's roots before repotting, and adjust for how your home actually behaves.