Y'all a bunch of jokers here.
In inches: (L x W x H) / 231 = gallons. This is what all manufacturers use for advertised tank size.
Spoiler: outside dims don't equal gallons, and material (glass or acrylic) takes up a lot more volume than most people realize. Then there is rock, sump level, etc...
The best way to calculate actual water volume is to use BRS Alk liquid mix, test the water, dose the amount that "should" raise your Alk by 1.0 dKH, wait 30 minutes and test again.
Reverse the calculation to find out how much water you have using that as the unknown. Ok now I have to actually figure that out sheesh
ok here goes
using Liquid Soda Ash from BRS, 71.43 mL should raise 100g by 1 dKH. Therefore each 0.7143mL should raise 1g by 1dKH.
If you think you have 100g, then add 71.43mL. Let's say that you find your KH increases by 1.2 instead of 1.0. Here's where it seems complicated, but it's actually stupid easy.
Take your estimate of 100g and divide that by the actual increase. In this case, 100/1.2 = 83.33g. That's your actual volume.
Plug 83.33g back into the BRS calculator and enter a differential dHK of 1.2 (as in Current = 7, and Desired = 8.2) and BAM 71.43mL is your dose. Now change Desired to 8 and you get 59.52mL. If you were to dose this amount, you should get a 1.0 dHK increase.
Jumping your dKH by a full point in one shot however might not be a good idea. The good news is you can fractionalize the calculation. Let's say you tried to raise it by 0.3, but it jumped by 0.4.
Just divide actual by desired 0.4 / 0.3 = 1.333333 and divide that into your guesstimated tank volume to get actual volume. 100 / 1.33333333333 = 75 wow you were way off.
As long as you don't vary your sump level or add a significant amount of rock, you should really only need to do this calculation once. However you can use this to cross check your dosing on a regular basis, and you can do it with anything you are dosing.
If you calculated that you needed to raise Magnesium by 150 points, so you dose what you thought was the correct amount for your 134.8 gallon system, but instead it only raised by 145 points, then 145/150 = 0.96666666666667. 134.8/0.96666666666667 = 139.448275862 and that's probably within the error range of your test kit so "good enough for government work"
