Repotting Pot Size Calculator

Quick answer

Enter the current pot diameter to get a suggested next size and a sensible range. Then enter the new pot's top diameter, bottom diameter and height to see its volume in litres and how much soil to buy, with about 10% extra to allow for settling.

How the size suggestion works

The suggestion follows the widely used "go up a little" guideline:

For example, a plant in a 15 cm pot gets a suggestion of 19 cm, with a range of 17 to 20 cm.

Why not go straight to a big pot?

It's tempting to skip a few sizes so you don't have to repot again soon. The problem is the soil the roots can't reach yet. It stays wet long after the root zone has dried, and that wet, airless soil is where root rot starts. Moving up gradually keeps soil and roots in balance. Fast-growing plants in bright light, such as a vigorous monstera, can sometimes handle a bigger jump. Slow growers in low light shouldn't get one.

How the volume is calculated

Most pots taper, so they aren't simple cylinders. The calculator treats the pot as a frustum, a cone with its top cut off:

Volume = π × h × (R² + R·r + r²) ÷ 3

Here R is the top radius, r is the bottom radius and h is the height. The result is converted from cm³ to litres (÷ 1,000).

Worked example

New pot: 19 cm across the top, 14 cm across the bottom, 17 cm tall.

• R = 9.5 cm, r = 7 cm
• R² + R·r + r² = 90.25 + 66.5 + 49 = 205.75
• Volume = π × 17 × 205.75 ÷ 3 ≈ 3,663 cm³ ≈ 3.66 L
• Buy about 4.0 L of mix to allow for settling.

You won't fill the whole pot with new soil, because the root ball takes up some of the space. But the figure keeps you from buying a 20 L bag for a small plant, or from running out halfway through.

Repotting step by step

1. Water a day before. Slightly moist roots slide out more easily and suffer less damage.
2. Prepare the mix. The soil mix calculator scales common recipes to your volume.
3. Remove the plant. Squeeze or tap a plastic pot, or run a knife round the inside edge of a rigid one.
4. Check the roots. Healthy roots are firm and pale. Mushy, dark or bad-smelling roots point to rot, so trim them with clean scissors.
5. Loosen circling roots gently with your fingers.
6. Set the depth. Add mix to the bottom so the top of the root ball sits about 2 cm below the rim.
7. Fill around the sides and firm lightly. Don't pack it hard.
8. Water thoroughly and let it drain.
9. Keep it out of harsh sun for a week while it settles in.

Pot material

Whatever the material, use a pot with a drainage hole. Decorative pots without holes work as outer covers. Keep the plant in a plastic nursery pot inside one, and empty any water that collects.

After repotting

Expect the plant to pause for a couple of weeks. Hold off on fertilizer for about a month, because fresh potting mix usually contains some nutrients. Rerun the watering schedule builder with the new pot size, because larger pots dry more slowly.

Next pot size for every common pot

These are the exact suggestions the calculator gives, from tiny nursery pots to large floor planters:

Two things to notice. Pots under 15 cm get a 2.5 cm step and a maximum of 3 cm, because a small root ball is easily swamped by extra soil. From 25 cm upwards the suggestion and the maximum are the same, +5 cm. A 40 cm planter's next step is 45 cm. Beyond that, most people switch to top-dressing (replacing the top few centimetres of soil each spring) rather than repotting, because pots get very heavy.

Volume of common pot shapes

Here's what the volume calculator returns for a range of typical tapered pots, with the 10% "soil to buy" allowance:

Pot sizes in shops are usually the top diameter, and the bottom is often about three quarters of that. Height varies more. If you only know the top diameter, measure the other two with a ruler or tape. It takes a minute and the volume will be much closer than a guess.

How shape changes the volume

The same top diameter can hold very different amounts. Three 20 cm-tall pots, all 20 cm across the top:

A straight-sided pot holds three times as much soil as a cone of the same height and top width. That's why decorative tapered planters often "feel" smaller than their label suggests, and why straight-sided pots stay wet longer.

Worked example: monstera from 20 cm to the next size

Your monstera's roots are circling a 20 cm nursery pot.

1. Enter Current pot diameter: 20. The calculator suggests 24 cm (range 22–25 cm).
2. At the garden centre you find a 24 cm pot that's 18 cm across the bottom and 22 cm tall.
3. Enter 24, 18 and 22. The calculator shows 7.67 L and soil to buy ≈ 8.4 L.
4. The old root ball will fill part of that. An 8–10 L bag of chunky aroid mix is plenty. Our soil mix calculator splits 8 L into 3.2 L soil, 2.4 L bark, 1.6 L perlite and 0.8 L charcoal.

Our monstera deliciosa care guide explains the first repot in more detail, including adding a moss pole while the plant is out of its pot.

Worked example: a small pothos

A pothos in a 12 cm pot gets a suggestion of 14.5 cm (range 14–15 cm). A 14.5 cm pot, 11 cm at the base and 13 cm tall, holds about 1.67 L, so buy about 1.8 L of mix. For small plants like this, a 2–3 L bag of general mix with perlite is the most economical choice.

Signs it's time to use the calculator

• Roots growing out of the drainage hole
• Roots circling the inside of the pot
• Water running straight through without soaking in
• The plant drying out within a couple of days of watering
• The plant becoming top-heavy and tipping over

If you see these, repot in spring if possible. Our step-by-step how to repot a houseplant guide covers the whole job.

Plants that like a snug pot

Some plants grow better slightly pot-bound and should be repotted less often, using the smaller end of the range:

What the calculator doesn't do

• It doesn't know your plant's growth rate. A vigorous monstera in bright light can sometimes handle a bigger jump. A snake plant in a dim hallway shouldn't get one.
• It assumes a round pot. For square pots, the frustum formula doesn't apply. Multiply length × width × height in cm and divide by 1,000 for a straight-sided square pot.
• It doesn't subtract the root ball. That's why "soil to buy" is an upper bound for a repot and an accurate figure for a new planting.
• Pot labels can be in inches. Multiply by 2.54 to get centimetres. A 6-inch pot is about 15 cm, so the calculator suggests 19 cm, roughly 7.5 inches.

After repotting, your watering routine changes. A bigger pot holds more water. If the new pot moves you into a larger band, the watering schedule builder will lengthen its check interval accordingly.

Converting inches and pot labels

Many shops label pots in inches or by a nursery number. To use the calculator, convert to centimetres (1 inch = 2.54 cm):

These use the calculator's own steps on the converted diameter. In practice, round to the nearest size the shop sells.

Frequently asked about pot size

Can I put a small plant in a big decorative pot? Yes, as long as the plant stays in its own right-sized nursery pot inside the bigger one. That gives you the look without the wet soil.

What about wide, shallow pots? Fine for shallow-rooted plants such as haworthia and other small succulents. Measure the top, bottom and height as usual. The volume will be smaller than a deep pot of the same width.

How soon can I repot again? Most plants need a year or more to fill a pot one step up. Fast growers such as pothos and spider plants may be ready again the following spring. Slow growers such as snake plants and ZZ plants often wait two to four years.

Should the new pot be the same material? Not necessarily, but if you switch from plastic to terracotta, the soil will dry faster, so check it more often.