Houseplant Repotting and Soil Mix Calculator

What this estimate does and does not do

This calculator helps you plan a houseplant repot before you open bags of soil. Enter the current pot size, choose the closest plant group, and describe the root condition. The calculator estimates a next pot diameter, the new pot’s approximate capacity, the fresh mix to prepare, and a soil recipe by volume.

It does not diagnose plant problems or guarantee that repotting is the right choice. Plants can wilt from underwatering, overwatering, low light, heat, cold, root loss, pests, salts, or disease. If roots are mushy, smelly, blackened, or collapsing, treat it as a plant-health problem rather than a routine size-up.

Formula / method used

Pot volume

The calculator approximates a pot as a tapered cylinder, then subtracts headspace below the rim:

Where:

• taper factor = 0.9, because many nursery pots are narrower at the bottom
• fill factor = 1 - headspace percent

This is a practical estimate. Real pots vary by shape, drainage holes, inset bottoms, inner rim, and how firmly you pack the mix.

Next pot size

The calculator uses common houseplant repotting rules:

Succulents, cacti, and orchids are capped to smaller step-ups because excess wet media is especially risky for them.

Fresh mix to prepare

For a healthy repot, not all new-pot volume is fresh mix because the retained rootball still occupies space:

For a soil refresh in the same pot, the calculator assumes you replace about 25% of the current soil volume. For root-rot recovery, it assumes you prepare enough fresh mix for the whole recommended pot because old wet or infected mix is usually discarded.

Soil mix blender

Ingredient volumes are calculated from the selected recipe percentages:

The recipe table shows litres, quarts, and cups so you can use a scoop, jug, or nursery pot as a “part” measure.

Worked example

For a 6 inch wide × 6 inch high rootbound tropical houseplant in a general foliage mix:

For the general tropical recipe, roughly 50% base, 33% perlite/pumice, and 17% bark:

How to use the result

• Measure the inside diameter and usable height of the grow pot.
• Choose root condition after sliding the plant out and inspecting the rootball.
• Keep the plant at roughly the same soil depth unless the plant-specific guidance says otherwise.
• Use a pot with drainage holes; a cachepot alone is not a drainage system.
• Pre-moisten dry coir, bark, or peat-based mixes before potting.
• Do not compact the mix hard; firm gently enough to remove large air gaps.
• Watering after repotting depends on plant type and root health. Root-rot recovery may need a drier reset than routine repotting.

Assumptions and limitations

• Pot volume uses a simple tapered-cylinder estimate.
• The new pot height is estimated from your current pot’s height-to-diameter ratio.
• Mix recipes are starting points, not universal formulas.
• Low light, cool rooms, decorative cachepots, and overwatering may require more drainage material.
• Very dry homes, terracotta pots, high light, and frequent underwatering may need more moisture retention.
• Orchids and semi-hydro setups often need genus-specific guidance beyond this calculator.
• Fertiliser, pH, salts, pests, and root diseases are not modelled.

For other maker and plant-adjacent planning tools, see the Wood Movement Predictor and the Miniature Painting Dilution and Airbrush PSI Calculator.