Understanding Container Sizes and Their Volumes

July 16, 2005

While trying to find out the minimum size of a container, I ran across some confusing information.

I've been looking on this forum to discover the minimum size container for an indeterminate plant. Whild doing so I came across a post that said a 10" diamter pot was 3 gallons, and a 12" diameter pot is 5 gallons.

Is this correct? Last year I had download a table from the internet re: soil to fill various size pots. I've copied it below in casy anyone would find it useful. According to this table, a 10" pot holds 5 quarts, and a 12" pot holds 12 quarts. Even if these figures allow for not filling the container completely to the top, 12 qts is only 3 gallons and leaving an inch or two at the top of a pot won't explain a 2 gallon difference.

Is this chart wrong? Am I confusing apples with oranges and misunderstanding somehow by thinking in liquid measure?

Any help would be much appreciated. (Hint: that means you, Tapla) LOL

Thanks in advance,


How much soil do I need?

How many times have you wondered about the amount of potting soil you need to fill a flower pot? This should help: Happy Planting!

Pot Diameter in INCHES.....Soil needed in QUARTS

8".........3 QT

10".......5 QT

12".....12 QT

14".....18 QT

16".....22 QT

20".....28 QT

24".....36 QT

30".....72 QT

36".....96 QT

Window Boxes

6' x 6"x 6".....3 cubic feet

4' x 6" x 6"....1 1/2 cubic feet

every 2' of window box: add 1 cubic foot of potting soil

I apologize for not being able to credit the source of this information; usually I try to save the details of where I found helpful things, but I must have been in a hurry when I saved this because the reference isn't there.

Comments (24)

  • Amino_X

    Here's the information I used, I finally found the original file: Container Vegetable Gardening from Iowa State University. (This is a PDF file and requires Adobe Acrobat to read) or View as HTML.

    The information in this bulletin is for standard clay pots, so taller nursery pots could hold more given their hight.

    Best Wishes

  • Amino_X

    Ooops View as HTML

    Best Wishes

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  • DaisyLover

    Join the confused crowd :) First off any list that tells you a 10" pot absolutely holds 5 quarts of soil is just giving you a very loose guideline...it's not gospel. A 10" pot that is 9" deep is called a "trade size 1000" - 2.5 gallons (by one manufacturer)....actual volume is 2.3 gallons and one cubic yard of soil will fill approximately 90 of them. A "#1 Trade Gallon" is a "300" and is 6 3/4" x 7 1/4" and is called a gallon but its actual volume is .75 gallons. A 400 #1 at 7" x 7" is the closest to true gallon volume at .98 gallon capacity.

    To make it even more confusing, some pot charts list the soil capacity in "cubic inches" and others list it as how many pots needed for a cubic yard of soil... and other charts don't even tell you the size of the pot.

    The depth of the pot will increase or decrease the amount of soil it contains. And because a pot is called a 1-gal, 3-gal, or 5-gal doesn't mean that is it's actual size (which I think the government is trying to do something about now). Anyways, a 12" x 11 1/4" nursery pot is a "2000 #5 4-gal" with actual capacity of 3.9 gallons... not 5 gallons.

    I could go on and on and on... and it would change with manufacturers and the type of pot being used. There's a standard, a thinwall, a thinwall with a wider lip, etc etc etc... and they all hold a different amount of soil.

    Now that I have totally confused you even more with useless information, which I am sure tapla could have explained much better, just remember that just because it is called a gallon doesn't mean it holds four quarts and just because a pot is 10" doesn't mean it holds 5 quarts. After all...there are pots that are 10" diameter but only 4" tall and there are pots called 10" pots but they are actually 11" x 8". :)

    Bottom line is... just make sure you have a large bag of soil so you don't run out. ;) And if that "indeterminate plant" you are talking about is a tomato everyone (and all the books) say to use a 5 gallon pot...minimum, but I know people that put them in 3-gallon pots. So.... ?

  • tapla

    Sorry, Spygrrl! I wasn't ignoring you. I didn't see your reference to me in the original post. Truth is, I kind of skipped over it because there's no definitive answer to what you asked, except by mathematics or physically filling, then measuring the container's volume. DaisyLover did a good job of explaining how widely container volumes vary. Some of them are even semi-conical (I think it's just to confuse us.). ;o)

    I know it's no help, but after awhile, you'll be able to look at a container & determine how much soil is required. If you have more than a few containers - know what I'd do? I would invest in the components to build a soil superior to what you can buy in a store & keep an ample supply on hand. I always keep several kinds of bark on hand (I use partially composted pine bark for the floral plantings), a huge (4 cu ft) bag of perlite, and a bale of sphagnum peat. When I need soil for containers, I'll mix up a batch of about 6 cu ft. What I don't use, I put back in the pine bark bags until I need it. By using this system, when I run out of soil, a new batch is only 15 minutes in the making. Usually, I make several batches in spring when I'm doing lots of planting. The leftovers from the last batch I make are usually enough to carry me through the rest of summer & winter.

    Getting back to the volume thing - If you want to do the math, I can give you the formulas to calculate the container's volumes (formulas vary by shape) in cu inches, which can then be easily converted to gallons. Let me know.


  • triple_b

    that's what I figure, tapla. grab a calculator and do the math. Some of those decorative pots are smaller at the bottom, and so on. All are different.

  • bjs496

    If you divide the diameter (d) of the bucket by two you get the radius (r). If you multiply the radius by itself and multiply that number by pi (3.14) you get the area (a) of the circle.

    a = (d/2) * (d/2) * 3.14 or
    a = r * r * 3.14

    If the sides of your pot are sloped, you can take the measurement at the bottom and top and average them.

    Now, when you multiply the area (a) by the height (h) of the pot, you get the volume (v) of the pot.

    v = a * h

    Don't forget to measure to the height you would normally put soil. Also, take all of your measurements using the same scale (ie, inches, feet, centimeter, etc). If you take all your measurements in inches, divide the final number by 1,728 to convert into cubic feet.

    I've long since given up on trying to figure out volume by quarts, liter, or gallons... all the mixins are sold by cubic feet anyway. If anyone wants an excel spreadsheet that will calculate the volume when the diameter and height are entered, email me.

    happy calculating

  • ljrmiller

    Hehe. I just keep at least 5 2-cu. ft sacks of potting soil on hand at all times. If I can get it marked down (Lowe's put my favorite one on clearance--I bought it all--SCORE!!!!), all the better.

    My rule of thumb is that the container should be able to just cover the upper part of the plant--or be at least as wide as a tall plant plus a little extra for "tall". That, and the bigger the container, the better (within reason) for growing things like cut flowers and vegetables. I put my tomatoes and chrysanthemums in the Gardener's Supply Terrazza planters, and they thrive. The containers are easily as big as a footlocker. I have them on casters so I can push them around as needed.

  • poultryguy

    I know this post is "way old" but I thought it might be helpful to clear up an old trade myth. Container pot sizes aren't a measure of volume, more to the point, they should be referred to as sizes. I.E., #1, #2, #5 etc., etc.. Somewhere back in time the trade grabbed on to the gallon designation and it stuck, like a perpetual "old wives tale" that just won't go away.

    Roughly speaking, here are some average volumes for nursery containers;
    Listed by order are the pot size, how many pots can be filled by one cubic yard (27 cubic feet) and how many cubic feet it takes to fill each pot.
    #1 191 .141cuÂ
    #2 96 .281cuÂ
    #3 69 .391cuÂ
    #5 34 .794cuÂ
    #7 26 1.038cuÂ
    #10 19 1.421cuÂ
    #15 14 1.928cuÂ
    #20 10 2.7cuÂ


  • legacy

    Hi Dan,

    I know there is a mathmatical formula in determining the volume of a container, but these measurements you provided are very enlightening at first glance. Upon some reflection, I'm wondering how the units and fractional units as you provided have practical meaning for home gardeners or commercial growers? What am I missing with the utility of containers classified and designated by #1, 2, 3, and etc? Thanks.

  • poultryguy

    Hello Legacy,
    Not precisely sure I understand your question so please forgive me if my answer isn't what you're looking for.

    The reason I use the designation of numbers instead of gallons is as others have specified. I.E., a one gallon container "should" be able to hold one gallon of soil, which it invariably won't. Although a #1 container will typically hold close to .141 cubic feet of soil. The practical applications for both home gardeners and commercial entities is that they can figure out how much soil they'll need in order to plant up "x" amount of plants when transplanting much quicker by multiplying the number of transplants times the cubic foot capacity of the containers. 20 tomato transplants going in to #1 pots will require apprx. 2.82 cu' of soil, or just shy of a 3 cubic foot bag of potting soil. Or, knowing ahead of time that my bare root shipment will come at a time when planting in the ground isn't plausible, I can get five 2cu' sacks of potting soil for the 12 #5 pots they'll need and have it waiting in the garage for when they arrive.

    I work in the "green industry" and get a lot of confused looks from people who want to know how many "pounds" of soil they'll need, or how many containers a 32qt bag will fill. All I can say is, the green industry really needs to stop confusing folks with measurements that aren't practical, lol. Weight changes according to moisture levels as well as the density of the mix. Qt and gallon measures would be fine, IF, containers were actually manufactured to work more accurately with that type of measure. Some are, but they still use fractional portions of quarts and gallons so you still have to use the math unless you keep a handy-dandy chart with the pre-calculated measures for the containers you use most often.

    I guess the long and the short of it is, I don't use gallon terms unless I have to because it's just not correct and it perpetuates a misnomer.


  • legacy

    I see and I can see how it can be crucial for growers to have sufficient supplies and raw materials on hand without being tied up by costs or excessive unused inventory.

    I just read the same problems recently with respect to how coir has been marketed. Producers, distributors, and retailers use different measurements, and a dutch producer is pushing for uniform certified standards for coir (kind of like certifications and training endorsed by Microsoft in the early days) to ensure quality and making product comparison easier for buyers and consumers. I see perlite having the same problems with standard unit measurement as it is also sold in different volume units.

  • poultryguy

    Yes, unused materials sitting around can be a problem. Not just because the overhead ties up funds, but because soil loses many of it's necessary qualities when it sits around, especially if it dries out.

    It's kind of odd, that a world wide community can be so strict on plant nomenclature can at the same time allow so many inconsistancies in other trade matters to bog it down. I'm just waiting to see a bag labelled with cubic centimeters... that should make for some fun calculations, lol. (I'm still stuck in Standard US measures)


  • franktank232

    Couldn't you seal the bottom of the container, fill with water, and then weigh the water that filled the containers. Couple of calculations might give you volume?????????

  • emgardener

    Even simplier, seal the bottom as suggested (ducktape works great for this temporary seal) then fill up the pot using gallon jugs and count.

  • franktank232

    Duh... i think too much.

  • calistoga_al ca 15 usda 9

    As I understand it, we got into this "gallon" reference when the nurseries boomed at the end of WW2. Our government had contracted with canners to pack millions of one gallon cans of foodstuffs to feed the armed services. At wars end there was a very large surplus of one gallon cans available cheap that were grabbed up by the nursery growers. Since they WERE one gallon cans that was what they called them. When the surplus was gone and new cans were ordered a tapered can that would stack was a lot more practical. The advent of plastic made a tapered can much easier (and cheaper) to produce. The name "gallon" persisted even when the container would no longer hold a gallon of anything. Some time ago the weights and measurement people started looking into "false size claims". Since then the industry has been trying to eliminate any reference to "gallons". Hopefully in the near future the industry will decide on names for sizes that can be standardized and make sense to us all. Al

  • poultryguy

    Now that's an interesting piece of history! That would explain the straight metal cans I had to cut open for customers when I first started working at a small nursery eons ago. There weren't many of them left in the nursery at the time, but the only thing the other employees could tell me about them was that they were an old style container that weren't being used any longer. I didn't realize I had touched a piece of WW2 nostalgia. That's cool!

    Thanks Al,

  • donn_

    I find it convenient, for potting mix calculations, to keep NSI's (Nursery Supplies Inc.) website bookmarked. They're one of the larger manufacturers of nursery pots, and have a chart for each of their lines on the website. The chart lists, by their item # (which is stamped on the bottom of every pot), all the salient dimensions and capacities for each pot.

    Here is a link that might be useful: NSI Grip-Lip Pots

  • nattydoll

    I found DaisyLover's post very helpful.

    I have been in the same confused boat, and they mentioned the exact size I was wondering about! (10" x 9")

    Here is a formula I found:

    Volume in inches: pi x radius squared x height
    1 Gallon = 231 cubic inches, so divide by 231.

    Source: http://thehotpepper.com/topic/37942-need-to-convert-inches-to-gallons/

    BEFORE that, I tried these 2 websites, but it shows different measurements for DRY and WET volume. I didn't know there was a difference...
    Find cubic inches:
    Cubic inches to gallons:

  • RichieD

    I think I can clear up some confusion here...and I hope not add to it.

    In the older English system of measurement, which became the "American" system early in US history, there were two different gallons...the liquid gallon and the dry gallon. They are NOT the same.

    That's why a number 20 pot doesn't hold 20 liquid gallons...obviously soil is not a liquid, so you have to measure it in dry gallons, not liquid gallons.

    This is the current system of measurement being used in the US, yet very few people know how to use it properly. I guess it's not taught in schools anymore. So watch your step, there are at least four different gallons...the Imperial gallon, the US dry gallon, US liquid gallon, etc.

    I personally think it's insanity that my country hasn't yet converted to the metric system, which would avoid ALL of this confusion. A liter is a liter is a liter.

    If you want to know the conversion ratios, just google the word gallon. My Webster's Collegiate Dictionary has a table of weights and measures that covers this with all the conversions.

  • drew51 SE MI Z5b/6a

    I personally think it's insanity that my country hasn't yet converted to the metric system, which would avoid ALL of this confusion

    I listen to two BBC garden broadcasts, and they avoid the metric system like the plague. I was surprised. They use the American system, at least for gardening. Maybe because of buying so many products from the states? Not sure?

    From the way I filled out my handle here you can tell what zone and even state I'm in. I have no idea what country you're even talking about? As you have none of that info.

  • RichieD

    The "American System" of measurement is not really American, it's an obsolete British system of measurement. It was in use in pre-Revolutionary times because the colonies were part of the British Commonwealth. Thomas Jefferson and others tried to switch to the metric system after the Revolution and the struggle has continued to the present day. There's a federal law on the books which "encourages" the switch, but it was never mandated as it was in other countries.

    I live in the US, which is the only country in the industrialized world which hasn't finished converting to the metric system. The older British system is impractical in the modern world, particularly since so many things are now manufactured in the US with metric parts in order to make them marketable in other countries. So we need double sets of wrenches to fix things now. That's just stupid.

    I doubt if many American supporters of the old British system even know how it works. I never find anyone who knows the difference between dry measure and liquid measure...or how to measure things in rods and furlongs.

    There's a political movement in Britain against metrification, euro currency, and membership in the EU. I don't know much about that.

    I do know that resistance to metrification is economic suicide in our global world.

  • drew51 SE MI Z5b/6a

    I think your last statement is way over the top. We can sell a lot of tools if we stick with the current system. It would be foolish to switch. Well the brits are going to the old ways. I hated the system myself, forced to use it for work. Give me a grand imperial quart anyday!

    As far as size of pots what about going to a numerical system like #1, #2 etc, oh wait we already did!
    Gardener's Question Time from the BBC is playing on my mp3 player right now as i type this, and not a mention of metrics. Seems to me we are winning them over.

    Can you imagine football in meters? No way!

    I myself hate all reference to global anything. I really don't care what the world is doing. They can figure it out or go elsewhere. I don't wish to lower out standards. No thank you.

  • randyrossvermont

    1 gallon container holds 0.734 gallons

    2 gallon container holds 1.66 gallons

    3 gallon container holds 3.00 gallons

    5 gallon container holds 3.843 gallons

    7 gallon container holds 6.085 gallons

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