Start inside, not outside
Choose the height you plan to fill. Measure inside the container at that line and from the inside base up to that line. This keeps the wall thickness and any empty space above the mix out of the geometry estimate.
Choose the closest supported shape
Use rectangular for straight-sided boxes, cylinder for straight-sided round pots, and tapered round for circular pots with straight sides that narrow toward the base. Curved, bulging or divided containers are not modeled exactly; use a documented usable capacity when available, or treat the geometry result as approximate.
For a tapered pot, measure twice across
Enter the inside diameter at the intended fill line and the inside diameter at the bottom. Both measurements go all the way across, not from the center to the edge. Then enter the filled depth. The method assumes a circular cross-section and a straight taper.
Keep bag units explicit
Use the volume stated on the bag. The tool accepts liters, cubic feet and explicitly identified US liquid gallons. It does not interpret an unspecified gallon or convert dry quarts, Imperial gallons, nursery trade sizes or mass into usable volume. Liters and cubic feet avoid those ambiguities.
- One inch is exactly 25.4 millimeters. NIST: SI Units — Length
- One liter is one cubic decimeter, equal to 1,000 cubic centimeters. NIST: SI Units — Volume
- A US liquid gallon is 231 cubic inches. NIST: How Do You Measure Fuel at the Gas Pump?
Source pages checked 2026-10-02. The shape estimates are mathematical models, not measured product tests.