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Aquarium Arithmetic
Measuring and Calculating Volumes

This page contains information and advice on how to do some measurements and calculations that you may need.

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1. How much is 20% of my Aquarium's Water?
Click here to read about changing 20% of the water in your aquarium or pond. How do you calculate 20%? Here are three examples.

If you have a 10 gallon aquarium. Calculate 20% of 10 gallons = 0.20 x 10 gallons = 2 gallons. Multiply the 2 gallons times 16 (cups/gallon), to get 2 x 16 = 32 cups. Use a clean measuring cup to remove 32 cups of water from your aquarium. Now mark the level of water in your aquarium with a permanent marking pen or a piece of tape. In the future, drain the water in the aquarium down to the mark and replace it with fresh tap water.

You know your pond holds 50 gallon of water. Calculate 20% of 50 gallons = 0.20 x 50 gallons = 10 gallons. Use a clean bucket with a mark at one gallon to remove 10 gallons from your pond. Put a clean stick in a certain spot in your pond and carefully mark the stick at the water line. In the future put the stick back in the same spot in the pond and pump the water from your pond down to the mark on the stick then refill the pond with fresh tap water.

You don't know how many gallons of water your aquarium holds, but you can see that your aquarium has vertical sides. That means straight up and down. Measure the water depth and multiply by 20%. For example many small aquariums are about 12 inches tall with water about 10 inches deep. Then 20% of 10 inches is  0.20 x 10 inches = 2  inches. Measure 2" below the surface of the water and use a permanent marker to put a mark on the aquarium glass. You still don't know how many gallons of water your aquarium holds, but you do know how far to drain the water to replace approximately 20%.

Remember that removing 20% of the water and replacing it with tap water from the faucet is usually safe even without water conditioner, but replacing more than 20% can be risky even with water conditioner. So twice a week remove 20% of the water and replace it with tap water.
2. How much Water Does my Aquarium Hold?
The here to read the Recommended Treatment, which includes adding one drop of Quick Cure for each gallon of water and one Tablespoon of Aquarium Salt for each five gallons of water.  To give your fish the Recommended Treatment you'll need to know how many gallons of water your aquarium holds. Here's how to measure and calculate the volume of your aquarium.
  Measure your aquarium from the left edge to the right edge. For example, my aquarium measures 18 inches.
  Measure your aquarium from the bottom edge to the top edge. My aquarium is 12 inches.
  Measure the aquarium along the side from the front edge to the back edge. Mine is 12 inches.

Multiply the three measurements. I got my small calculator and carefully entered 18 x 12 x 12 = 2592 cubic inches.

FINALLY multiply that number by 0.00433 to convert from cubic inches to gallons. I got 11.2 gallons of water in my aquarium.

To get an even more accurate answer I realized I should have measured inside the aquarium. Measuring inside I get 17.75 x 11.75 x 11.75 = 2450.6, multiplied by 0.00433 is 10.6 gallons. So measuring inside the aquarium instead of outside reduces the volume from 11.2 to 10.6 gallons of water inside my aquarium.

3. A Slight Correction
But my aquarium also has a big rock and some gravel that reduce the amount of water in my aquarium. I'm not sure of the rock's volume in gallons, but I measured it by doing the following steps.
  Got a big clean bucket.
  Put the rock in the bucket.
  Added enough water to cover the rock.
  My assistant held a finger just touching the water.
  Removed the rock.
  Used a measuring cup to add water to the bucket.
  Until the water just touched my assistant's finger.
  The total was 38 oz. added to the bucket.
  Divided by 128 (oz./gallon) to get 0.3 gallon.

So this measurement shows the rock's volume is 0.3 gallons. I was surprised by this answer. I thought the rock had more volume like maybe a gallon, so I repeated the measurement, and the second time I put my finger just above the waterline while my assistant added the water. We got 37 oz. or 37/128 = 0.29 gallons. We decided 0.3 gallons is close enough and my estimate of about a gallon was wrong.

I realized that the rock had a hollow cave inside and lots of other small holes. So it had fooled me into thinking it had more volume than it did. I took note of my mistake, so that next time I'd take the fact that a rock or something else is partially hollow into my estimate.


For more accuracy we subtracted the 0.3 gallons for the water displaced by the rock from the 10.6 gallons to get 10.3 gallons of water in my aquarium.

There is still the gravel in my aquarium with an estimated thickness of 1/4 inch. The bottom of the aquarium measured inside is 11.75 inches times 17.75 inches = 208.6 square inches. At 1/4 = 0.25 inches thick or 208.6 x 025 = 52.15 cubic inches of gravel on the bottom of my aquarium.

Next multiply this number by the same conversion factor, 0.00433, to convert the answer to gallons: 52.15 x 0.00433 which is about 0.23 gallons. But the layer of gravel is not solid gravel, it is, say, 50% rock and 50% water.

So take half of 0.23 gallons to get 0.11 gallons. Of course, this estimate of the gravel might be a little inaccurate. I'd say it might be half as much or twice as much. So it's likely between 0.05 and 0.20 gallons.

I could put all the gravel in the bucket and measure its volume just like I measured the volume of the rock. But for now I'll use the calculated value of 0.11 and subtract the 0.11 gallons from the 10.3 gallons to get about 10.2 gallons. Not much of a difference, so I can probably ignore the inaccuracy in the volume of the gravel.

There's about a pint of water in the filter that hangs on the back of my aquarium. A pint is 1/8 of a gallon, or about 0.13 gallon. Add that to the 10.2 gallons to get back to 10.3 gallons. I think this number is pretty close.

The first easy measurement and calculation gave 11.2 gallons of water in the aquarium. A more refined measurement and calculation, that took a lot more time, gave 10.3 gallons. So I'd put 10 drops of Quick Cure instead of 11 drops. This is one drop less and probably wouldn't matter anyway. But it's good exercise to make these calculations and to find out that the original number 11.2 was probably close enough. The more accurate number, 10.3 gallons, is more accurate than we needed.


We could refine our methods and get a more accurate answer, maybe something like 10.27 gallons. But this is more accuracy than we need. We're just trying to find the answer to the nearest gallon here, so we can add one drop of Quick Cure to each gallon of water. The point here is that we can be very sure the volume is between 10 and 11 gallons, and is probably closer to 10 than 11.

If you make these kinds of measurements and calculations for a few years, you will develop a feel for what matters and what doesn't. But even after 30 years of doing these calculations, every so often my confidence wavers, and then I make a calculation and a measurement just to see if the two numbers are about the same. If they aren't the same, I think about it, make some more calculations, and some more measurements.

The dimensions you measure on your aquarium will probably be different from the ones that I measured on my aquarium, but the formula is the same. Here is the formula for any normal rectangular aquarium

V = L x W x D x 0.00433

where V is the symbol for the Volume in gallons, L for the Length, W for the Width, D for the Depth, and the 0.00433 is a so-called conversion factor that converts the answer from cubic inches to gallons.

Maybe this is more than you wanted to know about aquarium arithmetic. Notice that we measured some things and used those measurements to calculate other things. Measuring and calculating is fun and good exercise for the brain once in awhile.

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This page was updated on August 26, 2014.