The nursery aisle stops you cold: All those pots, all about the same shape, but labeled differently, five gallons here, twelve inches there, and over there it’s thirty centimeters. It’s a confusion of units, which leads most of us to simply reach for what appears to be large enough. That is a big no-no if you want to keep plant alive.
Here’s why: The size of the diameter doesn’t translate linear into the volume (or amount of soil held) inside the container. Double the diameter, but only have a little more? This is reflected in the chart which plots the rim diameters on one axis and corresponding trade gallons on the other. You can see that jumping to next size up (e.g., 4 inches for the pots, or 6) is not a big deal in terms of gallons, while the giant leap upward (going to an 18 inch tub) are huge.
Why Pot Volume Is More Important Than Width
So: look at this graph and realize that you’re purchasing space for your roots. The higher the bar, the larger the area your roots gets to go into. If your plant’s roots run out of room, they will begin to circle and that is bad for healthy growth. Therefore: size matter. Volume matters; not width.
For perspective, those pot sizes translate into some numbers to help you keep it all straight. Always keep the number twelve inches in your mind. That’s the standard five-gallon pot we all grow our tomatoes in. So once you get that one down, remembering that it’s about five gallons and about thirty centimeters across, you’ve got a reference point. All the others are relative to that.
Ten inches isn’t half the size of 12; far from it. Why? Because it’s more like the cube of the diameter (which is the square of the radius), something we’re all missing when we do volume calculations. We imagine linear things relate to each other linear, but geometry says that is not the case.
The basics of converting to metric are easy: multiply the rim diameter times 2.5 and voila! Centimeters. Okay, so it’s approximate, but good enough when purchasing bags of soil. Then follow the gallons with the liters, which works out to roughly three and three-quarters liters per US trade gallon. No need to worry about accuracy to the decimal point; this isn’t a chemical compound you’re mixing up, its just dirt to put in a pot.
You want to order plenty…but not so much that you waste money, or run out mid-planting.
People also don’t realize or think about the taper when it comes to pot size. Yes, a wide-rimmed pot makes it appear larger, but it’s really all show. It’s height of the wall and the narrowest spot on the bottom that tells you how much useful space remains. And typically, the greater the size of the pot, the more dramatic it tapers, meaning more of its stated capacity is wasted due to form instead of content. That’s why an empty twenty-five gallon tub seem to weigh less than you’d imagine from its girth.
Don’t take outside measurements… Measure the interior of the rim instead. That lip flare doesn’t count when it comes to your root ball. The nursery label says “gallon” somewhere on there; believe that mark. That’s the industry measurement for the container’s volume in gallons. Imperial gallons (bigger!) are not what we mean here; don’t get them confused, or you’ll be out of soil.
Height isn’t all that counts when judging a pot, either. Tall and skinny is a recipe for trouble, usually, as there won’t be enough width for lateral root growth.
Ultimately, pot sizing is as much art as science, letting your plant have space to stretch its roots so they don’t start circling. So begin with the right size for the crop you’re going to grow in there, not what was cute sitting on the shelf. Use the conversion logic to check your gut. If your garden becomes one where you know what volume you’ve got instead of just guessing, then it’s no longer a gamble.
Your plants that have been saved from cramped quarters will reward you with most vibrant blooms. It all comes down to providing the roots with the actual space they need, rather than just the space that you think they need.