Are you talking about plastic nursery pots?

I just measured a one-gallon black plastic nursery pot, and the top diameter is 6.5 inches.

Josh

It's difficult to assign a volume based on pot diameter because both the depth and the shape of the container play heavily into the determination. E.g., a truncated cone has less volume than a cylinder of the same (opening) diameter ht., and a 12" deep container with a given diameter will hold twice as much soil as a 6" deep container with the same diameter.

Al

Measuring straight sided tapering pots in inches: the area (pi or 3.142 x radius squared) of the top + the area of the bottom divided by 2 and then multiplied by the height gives you the volume in cubic inches. Divided by 1728 gives cubic feet. I cubic foot = 7.48 US gallons.

A quicker way: (Diameter top + Diameter bottom divided by 4) squared,...then multiply by the height by 0.0136 = volume in US gallons.
(For volume in liters multiply height by 0.051)

I should have said roughly speaking...Like a 16" pot is like 3 gallons? Now that is a off the top of my head guess. Probably all wrong. But I'm just looking for ball park sizes. And recognizing that shape has a lot to do with it. Anyway thanks all.

Take 1/2 of the diameter and multiply it by itself, then multiply that answer by 3. In this case half of 16 is 8 - 8x8=64 64x3=192. This roughly gives you the surface area of the container opening. To get the volume, multiply by the ht of the container. Let's say the container is 10" deep. 192x10= 1,920 cubic inches for the volume. To convert cubic inches to (liquid measure) gallons, divide by 230. so 1,920 divided by 230= 8-1/3 gallons. That would be for a container with straight sides, For tapered nursery containers, deduct about 25% of the volume of containers with straight sides, so a tapered container would hold closer to 6 gallons.

Al

Thanks Al. I will make my own table for various common sizes.

Next I need to consult with rose people on their experiences with potting different types of roses.

Be careful making comparisons with nursery containers and volumes. For reasons known only to the nursery industry, labled volumes do not necessarily match actual volume. For example, a standard #1 round black nursery pot, known as a "trade" gallon, holds only 3 quarts of soil.

Here is a link that might be useful: common nursery containers and their volumes

Thanks gardengal, math makes me dizzy. For those of us who can remember the pre-plastic days with the straight sided metal cans that every can had to be cut down the side to get the plant out, who longs for the good old days? A one gallon metal can was able to hold one gallon liquid and so they were referred as "one gallon containers". When Plastic pots were made with a taper that allowed the plant to slip out, the pots to be able to stack together, we were all so grateful, who cared that they no longer held a gallon. Someone filed a complaint a few years ago with the weight and measures people that they were paying for a one gallon measure that was not "one gallon". Now you will notice they refer to them as "trade one gallon", thus avoiding the claim of "not containing one gallon". Al

Yep, my "one gallon" plastic pots are labeled .71 gallon.
However, I use the term "one gallon" since most folks are familiar with the container size.

Josh