Pot size & potting mix calculator
Measure the pot your plant is in now. You get the next size up, a drawing of both pots to scale, and how much potting mix to buy, in litres, US quarts and cups.
Next pot
15 cm (5.9 in) across
2–5 cm wider than now: 14 cm to 17 cm. Common sizes in that range: 14 cm, 15 cm, 17 cm.
| Pot | To the rim | Mix, filled to 2.5 cm below the rim |
|---|---|---|
| Now, 12 cm | 960 ml (0.87 US dry qt, 4.05 cups) | 690 ml (0.63 US dry qt, 2.92 cups) |
| Next, 15 cm × 13.8 cm deep | 1.87 L (1.7 US dry qt, 7.92 cups) | 1.45 L (1.32 US dry qt, 6.13 cups) |
Mix to buy for repotting: about 760 ml (0.69 US dry qt, 3.21 cups)
That is the next pot's fill minus the old root ball, which takes up roughly the volume of mix in the current pot. Round up: fresh mix settles after the first watering. The smallest common bag covers it: one 5 L bag or, in the US, one 4 qt bag.
Filling the next pot from empty (a new plant or a full change of mix) takes 1.45 L (1.32 US dry qt, 6.13 cups): one 5 L bag or one 4 qt bag.
How to measure and what the numbers mean
- Measure across the top inside the rim, the way pots are sold: a "12 cm pot" is about 12 cm across the top.
- Measure the base inside, and the depth from the rim to the floor of the pot. Tapered pots are narrower at the base; for straight sides, enter the same width twice.
- Read the suggestion, or type the width of a pot you already own into the last box.
Next pot. Clemson University's Home & Garden Information Center advises a pot 1 or 2 in larger in diameter than the current one, which is 2 to 5 cm. A much bigger pot holds a lot of mix the roots cannot reach, and it stays wet for longer. The calculator picks a common size inside that range and keeps your pot's proportions for the depth.
Volumes. Tapered pots are worked out as a cone with its tip cut off (a frustum), square ones as a truncated pyramid. "Mix to buy" is the new pot's fill minus the root ball you move across, which roughly fills the old pot. US bags of potting mix are labelled in dry quarts (1.10 L) or cubic feet (28.3 L), not the liquid quarts of a measuring jug.
Worked example
A 12 cm round nursery pot with a 9 cm base, 11 cm deep, holds about 0.96 L, or 0.69 L of mix filled to 2.5 cm below the rim. The next pot is 15 cm (range 14 to 17 cm), 13.75 cm deep in the same proportions, and takes 1.45 L. Repotting therefore needs about 0.76 L of fresh mix, a little over 3 US cups.
Drainage: what actually helps
- Use a pot with a hole. Water that cannot leave the pot collects at the bottom and rots roots. If the pretty pot has no hole, keep the plant in a plastic pot with holes inside it, and lift it out to water and drain.
- Skip the gravel layer. Clemson HGIC says a layer of gravel or pot shards in the bottom "actually slows water movement through the pot". Fill the whole pot with potting mix instead; a piece of broken pot or mesh over the hole is enough to keep the mix in.
- Empty the saucer. After watering, tip away what has drained through within about half an hour; Illinois Extension warns against leaving pots standing in water.
- Match mix to plant. Cacti and succulents want a gritty, fast-draining mix; orchids grow in bark, not compost.
Sources
- Clemson Cooperative Extension, Home & Garden Information Center, Indoor Plants: Transplanting & Repotting (HGIC 1458): a pot 1 or 2 in larger in diameter, no gravel layer, root ball within 1 in of the container top.
- University of Illinois Extension, Houseplants: Watering: let excess water drain and don't leave the pot standing in the saucer.
- NIST, Guide to the SI, Appendix B.8: 1 US dry quart = 1.101 L, 1 US liquid quart = 0.946 L, 1 US cup = 236.6 ml, 1 cubic foot = 28.32 L.
Questions
How much bigger should the new pot be?
About 2 to 5 cm (1 to 2 in) wider across the top than the current pot. That gives the roots a fresh band of mix to grow into without leaving a large volume of soil that stays wet. Plants that like being snug, such as snake plants, hoyas and kentia palms, are happiest at the small end.
Can I put a small plant straight into a big pot?
It is better not to. Mix the roots cannot reach dries slowly, so the pot stays wet for days and the roots are more likely to rot. Moving up one size at a time, when roots fill the pot, keeps watering simple.
Should I put gravel in the bottom of the pot for drainage?
No. Clemson HGIC notes that a gravel or shard layer slows water movement through the pot. Water sits in the mix just above the gravel instead of draining away. Use a pot with a hole and fill it with potting mix all the way down.
How many quarts of potting mix fill a 6 inch pot?
A typical tapered 6 in pot, about 4.5 in across the base and 5.5 in deep, holds about 1.8 US dry quarts (2 litres) to the rim and about 1.4 quarts filled to 1 in below it. Enter your pot's own size above, as depths vary a lot between makers.
What is the difference between US dry and liquid quarts?
A US dry quart is 1.101 litres and a liquid quart 0.946 litres. Bags of potting mix sold in quarts use dry quarts, so an 8 qt bag holds about 8.8 litres. Larger bags are usually labelled in cubic feet: 1 cu ft is 28.3 litres.