How this calculator works
Internal container volume sets how much mix is needed before settling. A tapered circular pot is a conical frustum, not a cylinder: both diameters contribute. Leave watering headspace by measuring only to the intended fill line. Displace existing root-ball volume separately if repotting. Reverse depth assumes the entered upper diameter describes the new fill line; it does not reconstruct an unknown taper angle.
Formula
Rectangle: V = L × W × h. Cylinder: V = pi × D2 × h/4. Tapered pot: V = pi × h × (Dtop2 + Dtop × Dbottom + Dbottom2)/12. Multiply by container count.
Worked example
Two planters measuring 50 cm × 20 cm × 30 cm need 60 L. A tapered pot 40 cm across the top, 20 cm across the bottom and 30 cm deep holds about 21.99 L.
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Frequently asked questions
Should I measure outside the pot?
Use inside measurements. Wall thickness and decorative rims can make exterior volume misleading.
Does the count result round down?
The reverse quantity is an equivalent number of containers. Round down to determine how many complete identical pots you can fill.
Can I use a soil root ball already in the pot?
Calculate gross volume and subtract the root ball or other solid displacement. No automatic allowance is made for it.