Substrate volume, repotting & custom blend calculator
Enter each group of plants, the pot it is in and the pot it is going into. You get total litres per mix, the cheapest bag combination, clear pot sizes and a volumetric recipe tuned to your room. Everything updates as you type.
Set relative parts by volume. The formulator normalises them to 100% and estimates air-filled porosity from each component's measured AFP.
- Water in thoroughly until it runs from the slots, then let the pot drain fully.
- Keep out of direct sun and away from radiators for 7 days.
- Do not feed for 14 days; fresh worm castings carry enough nutrients.
- Check moisture through the clear wall; water again only when the lower third lightens.
- Some lower-leaf yellowing in week one is normal root re-establishment.
Pot volumes are measured fill volumes of our clear slotted nursery pots: 6cm 0.12L, 9cm 0.4L, 12cm 0.9L, 14cm 1.4L, 15cm 1.7L, 17cm 2.5L, 19cm 3.5L, 21cm 4.5L, 24cm 6.8L, 27cm 9.5L, 30cm 13.0L.
Plant groups and their blends
| Plant group | Blend | AFP |
|---|---|---|
| Monstera & climbing Philodendron | Monstera & Philodendron Mix | 38% |
| Alocasia & Colocasia | Alocasia & Corm Mix | 26% |
| Anthurium (velvet & epiphytic) | Anthurium Epiphytic Mix | 44% |
| Hoya & Dischidia | Hoya & Epiphyte Mix | 42% |
| Calathea, Maranta & ferns | Calathea & Maranta Mix | 22% |
| Syngonium, Pothos & Scindapsus | Everyday Houseplant Mix | 24% |
| Aglaonema & Dieffenbachia | Everyday Houseplant Mix | 24% |
| Begonia (cane & rex) | Terrarium & Vivarium Mix | 30% |
| Terrarium / closed glass | Terrarium & Vivarium Mix | 30% |
| Rhipsalis & jungle cacti | Hoya & Epiphyte Mix | 42% |
| Cactus, caudex & succulents | Cactus & Succulent Grit Mix | 40% |
How the calculator works
Still unsure? Email care@growtropic.co.uk with a photo of the plant and its roots — the nursery team replies Monday to Thursday.
How much soil do I need to repot from a 12cm to a 15cm pot?
A 12cm nursery pot holds about 0.9L and a 15cm pot about 1.7L. Keeping the root ball, you need the difference, 0.8L, plus 15% for settling: roughly 0.95L. If you are removing all the old substrate because of root rot or dense peat, you need the full 1.7L plus 15%, about 1.95L.
Why does the calculator add 15%?
Chunky bark-and-mineral mixes settle by around 10–15% in the first fortnight as particles nest together and the plant is watered in. Filling to the rim with loose mix and then topping up later disturbs the roots twice. The allowance lets you fill once, tap the pot down and finish at the right level.
What do the humidity and watering settings change?
They shift the recipe, not the volume. If you water heavily or keep plants in a humid cabinet, the drainage fraction (bark, perlite, pumice, charcoal) goes up by up to 12 percentage points. If you are a forgetful waterer or your room is dry and heated, the moisture fraction (coir, castings, vermiculite) goes up instead. The AFP figure updates to match.
Should I step up more than one pot size?
For most aroids, no. Jumping from 12cm to 21cm surrounds a small root ball with a large volume of wet substrate that the roots cannot reach, which is how root rot starts. Step up 2–3cm at a time when roots circle the clear pot wall. The exception is a bare-root reset of a large plant whose roots were lost to rot.
How does trade mode differ?
Trade mode prices bags at wholesale ex-VAT rates, allows up to 500 plants per line and measures progress against the £250 ex-VAT carriage-paid threshold instead of the £60 retail free-delivery threshold. Opening trade orders start at £150 ex-VAT.