The title might sound ridiculous, but I recently discovered that all 1 gallon pots are not created equally. In fact, there is not even an industry standard for how big a 1 gallon pot MUST be!
Shortcut to the end : Most pots advertised as 1 gallon are about 75% of that volume.
The story:
The other day I opened a package of 1 gallon pots from Amazon in order to up-pot some plants from solo cups. I measured and mixed my soil and amendments for X amount of pots, but when I was done up-potting I still had lots of soil left over. I wrote it off as being late at night and after having a smoke I had likely made a mistake measuring. However, the next day I was on another project and was left with excess soil again! I could accept making a mistake once, but the same mistake again the very next day? I needed to do some digging.
There are some very handy calculators online that will calculate the volume of a tapered pot, so I used those to start my investigation. I searched around online for various suppliers to compare different 1 gallon pots and found that most were only approximately 75% of 1 gallon.
I further discovered that the pots I had received from Amazon were actually only half a gallon by volume, even though they were advertised as 1 gallon. Additionally the measurements given on the Amazon site were not accurate and all dimensions had been exaggerated!
Now, this is not a rant about Amazon and their false advertising and fake reviews (over 4.5k positive reviews and no one noticed they got half of what they paid for?!?! Unlikely!).
Also, my research revealed that most people in the “industry” (gardening centres and landscapers mainly) are fully aware of this discrepancy as it has something to do with measuring when the plants are being planted in the ground.
That said, I believe these discrepancies are relevant to us because pot size can greatly affect performance of a plant, depending on the plant size. There will potentially be issues with feeding and mixing nutrients if you assume you have 1 gallon of medium but only actually have 50-75% of that volume. In that case, your soil will be nutrient rich or “hot”, which could cause issues growing. You would also have more frequent waterings than expected as the medium would dry out more quickly.
Additionally, if you are attempting to compete with other growers or do comparison grows, or simply imitate someone else’s grow style, your results will not be an accurate reflection of the plant’s abilities, or your abilities as a grower if you are unknowingly using different size pots.
My personal issue was this: I wanted to flower out some small plants in 1 gallon pots but did not realize my pots were only half gallon in reality. I have every confidence the small plants would’ve finished in one gallon pots, but zero confidence that they could have finished properly in half gallon pots.That definitely would have led to root bound issues which would have reduced yields and quality of the finished product.
I was finally able to source some 1 gallon pots (industry standard so technically only 3/4 of a gallon 
) but the plants have grown a little bigger and I have to put them through another transplant, so not ideal but oh well.
Ultimately my soil was not hot as I had measured it out using my 5 gallon buckets, which are true 5 gallons. The nutrients were measured with measuring cups and spoons, so the concentration of the soil was ultimately correct, there was just more than the half gallon pots could hold. The soil would have been hot if I had been mixing individual pots or if I had been using those pots to measure it out.
Moving forward, assume that 1 gallon means 3/4 of a gallon when it comes to plant pots and calculate your amendments and watering schedules accordingly.
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