Potting Mix and Planter Volume Calculator
The potting mix and planter volume calculator estimates the fill required for rectangular troughs, planter boxes, round pots, cylinders and tapered flower pots. Enter the dimensions, shape, wall thickness, quantity, below-rim allowance and bag size to calculate litres per planter, total litres, cubic metres and approximate bag counts.
The result is a planning estimate, not a structural or load calculation. Root balls, drainage material, false bases, wall thickness and the final soil level can reduce the potting mix required. For balconies, rooftops and suspended slabs, completed planter loads must be assessed by an appropriately qualified professional.
Calculator features
- Rectangular trough and planter-box volume
- Round pot and cylinder volume
- Round or square tapered-planter volume
- Centimetre and millimetre inputs
- Internal or external dimensions with wall-thickness adjustment
- Quantity calculation for matching planters
- Below-rim allowance for root balls, drainage setups, false bases and planting depth
- Litres per planter and total litres
- Cubic metres for larger projects
- Approximate 25 litre, 30 litre and 50 litre bag counts
Measuring the planter
Internal measurements provide the closest estimate. For a rectangular trough, measure internal length, width and intended planting depth. For a round pot or cylinder, measure internal diameter and planting depth. For a tapered planter, measure the top opening, base, height and whether the opening is round or square.
If only external product dimensions are available, select external dimensions and enter the wall thickness. The calculator will reduce the measurements to estimate the internal volume.
Allowances and completed weight
The below-rim allowance accounts for space that will not be filled with potting mix. This can include a root ball, false base, drainage setup or the gap between the soil surface and planter rim.
Volume does not determine completed weight on its own. Planter material, potting mix, water content, drainage material, plant size and any false base all contribute to the final load.
Long trough planters and rectangular planter boxes and oversize planters can require substantial fill volumes. Material, drainage, exposure and placement also affect the final specification for garden pots and planters.
Frequently asked questions
How is potting mix volume calculated for a rectangular planter?
Multiply internal length by internal width by planting depth in centimetres, then divide by 1,000 to convert cubic centimetres to litres.
How much potting mix fits in a 100cm x 40cm x 40cm trough?
The full internal volume is 160 litres. A 10% allowance reduces the estimate to 144 litres, which is approximately three 50 litre bags after rounding up.
Should I measure inside or outside the pot?
Internal measurements provide the closest estimate. External dimensions can be used when wall thickness is entered so the calculator can estimate the internal space.
What is the difference between litres and cubic metres?
One cubic metre equals 1,000 litres. Litres are convenient for bagged potting mix; cubic metres are useful for larger planter and project quantities.
Can the calculator account for a root ball or false base?
Yes. Use the below-rim allowance to reduce the fill estimate for root balls, drainage setups, false bases or space below the planter rim.
Does the calculator estimate completed planter weight?
No. It calculates volume only. Completed weight depends on planter material, potting mix, water content, drainage material, plant size and any false base.