Repotting Soil & Bottom Layer Bag Calculator

How much potting mix should you buy?

A pot’s outside dimensions do not describe the space available for fresh growing medium.
Its walls, empty space below the rim and retained rootball all affect the amount of new mix required.
This calculator subtracts an optional bottom layer and retained rootball from the filled interior, then pools requirements across pots before rounding to whole bags.

Use it after choosing a suitable pot and growing medium for your plant.
It estimates material quantities, without determining whether a plant needs repotting or prescribing a soil mixture.
Dimensions are centimeters, volumes are liters and all entered prices are KRW, including on this English page.

Supported pots and what to measure

Have a ruler, the labeled bag volumes, an estimate of the retained rootball volume and actual bag prices ready.
Bulging urns, internal reservoirs, double bottoms and large internal projections are outside the model.
A nominal pot size or a measurement across the outside rim should not replace an inside measurement.

Headspace, rootball and bottom layer

Volume formulas

One liter is 1,000 cm³.
For a round pot, B is the bottom inside diameter, T the top inside diameter, H the full inside height and z the height measured from the bottom.
A straight wall means the diameter changes linearly with height.

For a pot that widens toward the top, multiplying full volume by the layer’s fraction of height overestimates the bottom layer.
The calculator uses the diameter at the top of that layer instead.
It then multiplies each per-pot material volume by the group’s count and sums all groups.

Step-by-step purchase planning

1. Select the pot shape and enter its inside dimensions.
   Full height must run from the inside bottom to the rim, before deducting headspace.
   
2. Enter retained rootball volume, optional layer thickness and the count of identical pots.
   Add a group whenever any of these conditions differ.
   
3. Enter the labeled bag volume separately for potting mix and layer material.
   Calculate different mixes in separate runs because this plan pools only identical materials.
   
4. Choose an optional purchase allowance and enter actual bag prices.
   The allowance is your assumption, not an automatic compaction recommendation.
   Leave prices at zero if you only want quantities.
   
5. Calculate, check group subtotals and save the purchase plan as TXT.
   Changing an input hides the previous result; calculate again before saving an updated plan.
   

Worked example: three rectangular pots

Consider three pots measuring 20 × 20 × 25 cm inside, with no headspace, a 5 cm bottom layer and a 2 L retained rootball per pot.
The layer, allowance and prices below are fictional calculation inputs, not plant-care recommendations or market prices.

At entered prices of 5,000 KRW per mix bag and 3,000 KRW per layer bag, total material cost is 16,000 KRW.
The extra allowance is separate from the surplus caused by buying whole bags.
Pooling requirements also avoids buying a full mix bag for every individual pot.

Reading bags, surplus and costs

• Geometric requirement is the volume calculated from the measured shape.
  Purchase target adds your chosen allowance; with zero allowance they are equal.
  
• Bags = ceil(pooled purchase target ÷ labeled bag liters).
  The calculation uses unrounded volumes, while the screen displays up to three decimal places.
  An almost identical displayed volume may therefore require another bag at a boundary.
  
• Bag-rounding surplus = ordered bag volume − purchase target.
  This is not a guarantee of leftovers after planting: settling, spills and measurement errors can change actual use.
  
• A zero price means not entered, rather than free.
  Total cost remains unavailable if a required material has no price.
  A material requiring zero bags costs zero, so an unused layer does not need a price.
  

Shipping and existing stock are excluded.
Compare your saved plan with stock on hand and check the seller’s pack size and final checkout price.
A bag labeled only in kilograms cannot be converted without product-specific density information; obtain its labeled volume instead.

Practical ways to use the plan

Invalid inputs and measurement limits

Headspace must be less than full height, and layer thickness must be less than the remaining fill height.
If the retained rootball is larger than the filled space above the layer, the calculator reports the group number and withholds the purchase result.
It does not silently shrink your rootball input.
Check centimeters versus millimeters, diameter versus radius, and per-pot versus group rootball volumes.

Dimensions and volumes accept two decimal places; pot counts and KRW prices are whole numbers.
You can enter up to 20 groups with up to 100 identical pots per group; field ranges appear below each input.
Volume alone cannot prove that a rootball fits: a broad, shallow rootball and a narrow, tall one can have the same volume.
Compare physical width and height before planting.

Frequently asked questions