Container sizes are trade names, not volumes
A container sold as a three gallon pot is a trade size. What it actually holds depends on the manufacturer, the mould and the taper, and figures between 75 and 90 percent of the stated gallons are entirely ordinary. Buy media on the assumption that twenty three gallon pots need sixty gallons and you will finish with bags left over; work from the true volume and the count comes out right. Fabric pots tend to be closer to honest than rigid nursery pots, but there is no rule, and the only way to be sure is to fill one from a measuring jug.
If you can measure the container, the geometry modes are exact. A straight-sided cylinder is pi times the radius squared times the height. A tapered pot is a frustum, and its volume is pi times the height divided by three, times the sum of the top radius squared, the bottom radius squared, and the two radii multiplied together. That taper term is why a pot that looks generous at the rim holds noticeably less than a straight-sided one of the same top diameter and depth.
The unit conversions everything else rests on: one US gallon is 231 cubic inches, which is 0.13368 cubic feet or 3.785 litres. A cubic foot is 1,728 cubic inches or 7.48 US gallons. A cubic yard is 27 cubic feet.
Wet weight is the number that breaks things
Dry potting media is light. A coir or peat based mix runs somewhere around 5 to 10 pounds per cubic foot dry, which is why a large bag is easy to carry and why people consistently underestimate what happens next. Water weighs 62.4 pounds per cubic foot, and a well-made container mix holds a great deal of it: at container capacity, after watering and free drainage, 40 to 50 percent of the container volume being water is normal for a fine mix.
Run those numbers. A mix at 8 pounds per cubic foot dry, holding 45 percent water by volume, weighs 8 plus 0.45 times 62.4, which is 36 pounds per cubic foot wet. That is four and a half times the dry weight. Twenty containers of half a cubic foot each go from 80 pounds dry to 360 pounds watered, and the transition happens in the ten minutes it takes to water them.
| Media | Dry, lb/cu ft | At 45% water | Ratio |
|---|---|---|---|
| Light soilless, heavy perlite | 5 | 33 | 6.6x |
| Coir based mix | 7 | 35 | 5.0x |
| Peat based potting mix | 9 | 37 | 4.1x |
| Bagged garden soil blend | 22 | 50 | 2.3x |
The lighter the dry mix, the more dramatic the multiplier, which is the opposite of what intuition suggests. A light mix is not a light shelf load; it is a light shelf load until Tuesday.
Shelving is where this goes wrong
Domestic shelving ratings are quoted for an evenly distributed static load, on a level floor, with the unit assembled as designed and not partly on carpet. Containers of watered media are none of those things: the load is concentrated at pot footprints, it arrives suddenly during watering, and it is at its heaviest exactly when someone is leaning over the shelf with a hose. Work out the watered load per level before buying the shelving, not after.
Two failure modes are worth naming. The first is the shelf itself deflecting, which the shelf sag calculator covers directly for wooden and sheet-good shelves, and which becomes visible well before anything breaks. The second is the whole unit racking sideways, which is what happens to light steel and wire shelving under a load it was not braced for, and which fails without much warning. For a platform carrying weight overhead rather than a shelf, the overhead storage platform load calculator is the right page.
Runoff belongs in the same calculation. Trays holding drained water add their own weight, and a tray that does not drain adds it permanently. If there is a reservoir under the shelf, that is more weight again, and water on a shelf will find the lowest point of the floor eventually.
Buying the media
Bags are sold by expanded volume for loose mixes and by compressed volume for bales, and the two are not comparable. A compressed bale expands substantially when wetted, and the expanded yield is on the label as a nominal figure that varies with how thoroughly you hydrate it. Use the expanded yield in the bag size field here, and buy a margin over the calculated requirement, because media settles as it is watered and the first top-up is not optional.
Settling is worth planning for. A freshly filled container settles by a noticeable fraction over the first few waterings as the mix consolidates and air pockets close. Filling to the rim to compensate simply means the water runs off the top instead of soaking in, so the usual approach is to fill to around ninety percent, water it in, and top up once it has settled.
Related pages
For beds rather than containers, the raised bed soil calculator works from bed dimensions and handles the topsoil, compost and aeration split. Bulk material by the yard is the topsoil calculator and the mulch calculator, and if you are blending your own amendments the compost amendment calculator covers the ratios. Once the containers are filled and running, the reservoir top-up and drift calculator and the nutrient solution EC and ppm calculator deal with what goes into them.
Questions people ask
How much does a 5 gallon pot of wet media weigh?
Work it from volume rather than from the pot name. A pot sold as 5 gallons typically holds around 4 to 4.5 actual gallons, which filled to ninety percent is roughly 0.5 to 0.55 cubic feet of media. At a wet density of 35 to 37 pounds per cubic foot for a coir or peat mix at container capacity, that is 18 to 20 pounds of watered media, plus the container, plus the plant, plus anything in the tray. Dry, the same pot is 4 or 5 pounds. The multiplier between the two is what catches people out, and it is largest for the lightest mixes.
Why does a 3 gallon pot not hold 3 gallons?
Because the number is a trade size that describes a class of pot rather than a measured capacity, and it has been that way for a long time in the nursery trade. Actual volumes commonly land between 75 and 90 percent of the nominal figure, varying by manufacturer, by mould generation and by how much taper the design has. Fabric pots are generally closer to their stated volume than rigid nursery pots because they are made to a dimension rather than to a trade class. If the count matters for a large order, measure one pot with water and use the measured mode here.
How much water does growing media hold?
The figure that matters is container capacity: what remains after watering thoroughly and letting it drain freely. For a fine coir or peat based mix that is commonly 40 to 50 percent of the container volume as water, and a coarse mix with a lot of perlite or bark holds less, perhaps 25 to 35 percent. It also depends on the container height, because a short wide container drains less completely than a tall narrow one of the same volume, thanks to the perched water table that sits at the bottom of any container. If you want your own number, weigh a container dry, water it thoroughly, let it drain fully, and weigh it again: the difference in pounds divided by 8.345 is the gallons of water it held.
Can my shelving take this?
This calculator gives you the load; the rating has to come from the shelving manufacturer for the actual span you are using. Two things are worth checking beyond the headline number. First, whether the rating is per level or for the whole unit, because those get confused constantly and the difference is a factor of four or five. Second, whether the rating assumes an evenly distributed load, because containers concentrate weight at their footprints and a heavy pot at mid-span is a worse case than the same weight spread out. Add the trays, any reservoir, and the plants, and treat the published figure as a maximum rather than a target.
How many bags do I need for a batch of pots?
Total media volume divided by the expanded yield per bag, rounded up, which is what this page does. The mistake to avoid is using the compressed size of a bale rather than its expanded yield, since a compressed bale swells several times over when it is hydrated and the two figures differ enormously. Buy a margin above the calculated figure regardless, for two reasons: the expanded yield printed on a bale is nominal and depends on how thoroughly you wet it, and every container settles over its first few waterings and needs topping up. Ten to fifteen percent over is a reasonable allowance.