Is your pot a true cylinder from top to bottom or is the bottom diameter of the pot less than the top? Are the sides of the pot linear or curved? Using your measurements of 31cm diameter and 22cm height for a true cylinder I calculate a volume of 4.38654 gal. For the volume of a " cone" or "curved" type pot you would have to use an integral mathematical formulation. In such a cases, I would suggest that you just take your bags of soil, fill the pots to the top and Bob's your uncle! Welcome to GOY and Seasons Greetings To You!

Agree. Make sure the soil you are using is correct for the type of plant.

I make each pot hold 1.5 gallons. This sounds about right as your dimensions relatae to a 12inch 3/4 depth pot. Remember that the root ball of the plant takes up quite a lot of volume.

Where are you in the world? When you talk about quarts, it makes me think that you are in the USA? A US gallon is smaller than an imperial gallon by about 20%.

This volume bit isn't rocket science. No calculators needed, no mathematical formulas. You just get a measuring jug, you know, the ones used in cooking, or a graduated bucket, fill with compost, then add the compost to your container. All you need to be able to do is then add up how much you've added.
The reason you have loads left is because you aren't compressing as you fill.

Bulba has a valid point, you are probably not allowing for the root ball.

Most of us just fill the pot half way, pop the plant in and top up. Tap the pot to settle the compost and be prepared to top it up again after its been rained on or watered. I never ever thought of doing calculations, just do it! If you run out of compost buy some more, if you get some left over use it for something else.

And as it sounds as if you are new to this, stand the pot on some little feet if you can so the water can drain out - yoo only need to raise if a small amount..

Agree with Stera

Thanks for all the replies. Very helpful

I've never thought of plant pot sizes like that - usually buy a bag of compost and use it till it runs out!

You might find this useful... http://www.hardytropicals.org/blog/entry/pot_sizes_inches_to_gallon_conversion

Gardening like life is an art not a science. In science 2+2 will always equal 4 no matter how you add it up. In art 2+2 can equal 4 or 5 or 10, results that you never expected to get even though you did everything right. That is the fun and magic of gardening. Enjoy!

One things for sure, I need to stop over analysing everything. Thanks once again to everyone who replied. As many of you could probably tell I don't have much experience, but I'm getting there....slowly lol

Mike

Welcome back any time Mike! Happy gardening. If you are going to be joining us it would be a good idea to put your location on your profile - the county will do, as it can affect the advice you get.

compost is very airy but your measurments arnt . if you look at a ton of lead and a ton of feathers you will understand what I mean .

Good thinking that man!

thank you sg

a glass is always full even if its half full of beer lol .

I wouldn't buy a pint from your local then ;o)

not of beer know but half air and half beer is a full glass lol . its never empty or thered be a void and there called black holes .

Good point NP

lol mg we agree on a lot lately