Estimate a container top-up from its actual empty layer, using internal dimensions and the correct diameters for straight or tapered containers.
Measure the additional layer
Do not refill from the bottom on paper when usable mix already occupies most of the pot. Measure only the depth you plan to add and leave appropriate watering room above the final surface. Check whether topping up is suitable for the specific plant.
Use the filled section’s shape
For a straight round pot, use its internal diameter and the added layer depth. For a tapered pot, use the diameters at the bottom and top of that layer, not automatically the entire container’s base diameter.
Check a small example
Two straight pots with 12-inch internal diameters and an additional 2-inch layer need about 7.4 L before extra, or 8.2 L with a chosen 10% allowance. One 25 L bag would leave substantial material. The example is arithmetic, not an instruction to bury a stem or root collar.
Record what the estimate means
Specify top-up depth and bag volume on the list. Avoid raising soil against stems or crowns without plant-specific guidance. If existing mix or roots need assessment, resolve that before choosing a layer just to use up a bag.
Iowa State University Extension: selecting plant containers · USDA Plant Hardiness Zone Map · University of Minnesota Extension
Measure container volume · Build a specification list · Contact local help
General guidance: adapt tasks to your local climate, plants and soil. Confirm product specifications with your supplier.