How to tell how many gallons and fish tank is?

I`m lookng to get a fish and i need to know how many gallon my fish tank is is there and way to tell.
Answers:
Here is the information on tank sizes their gallons and the weight if filled with water. I have it on a land hermit crab site for when we find a tank and not sure of the gallon size.

Small Aquariums
(not recommended for beginners)
Size L x W x H Filled Weight
2 1/2 gallon 12 3/16 x 6 1/8 x 8 1/8 27 lbs
5 gallon 16 3/16 x 8 3/8 x 10 1/2 62 lbs
10 gallon Leader 20 1/4 x 10 1/2 x 12 9/16 111 lbs
15 gallon High 20 1/4 x 10 1/2 x 18 3/4 170 lbs
20 gallon X-High 20 1/4 x 10 1/2 x 23 3/4 232 lbs
10 gallon Long 24 1/4 x 8 1/2 x 12 5/8 116 lbs
15 gallon Show 24 1/4 x 8 1/2 x 16 5/8 170 lbs

Mid-sized Aquariums
Size L x W x H Filled Weight
15 gallon 24 1/4 x 12 1/2 x 12 3/4 170 lbs
20 gallon High 24 1/4 x 12 1/2 x 16 3/4 225 lbs
25 Gallon 24 1/4 x 12 1/2 x 20 3/4 282 lbs
30 X-High 24 1/4 x 12 1/2 x 24 3/4 340 lbs
20 gallon Long 30 1/4 x 12 1/2 x 12 3/4 225 lbs
29 gallon 30 1/4 x 12 1/2 x 18 3/4 330 lbs
37 gallon 30 1/4 x 12 1/2 x 22 3/4 415 lbs
26 Flatback 36 1/4 x 12 1/2 x 16 5/8 300 lbs
23 Long 36 1/4 x 12 5/8 x 13 253 lbs
30 Gallon 36 1/4 x 12 5/8 x 16 3/4 343 lbs
38 Gallon 36 1/4 x 12 5/8 x 19 3/4 427 lbs
45 Gallon 36 1/4 x 12 5/8 x 23 3/4 515 lbs
30 gallon Breeder 36 3/16 x 18 1/4 x 12 15/16 348 lbs
40 gallon Breeder 36 3/16 x 18 1/4 x 16 15/16 458 lbs

Large Aquariums
Size L x W x H Filled Weight
50 gallon 36 7/8 x 19 x 19 5/8 600 lbs
65 gallon 36 7/8 x 19 x 24 5/8 775 lbs
33 gallon Long 48 1/4 x 12 3/4 x 12 7/8 382 lbs
40 gallon Long 48 1/4 x 12 3/4 x 16 7/8 455 lbs
45 gallon Long 48 1/4 x 12 3/4 x 19 510 lbs
55 Gallon 48 1/4 x 12 3/4 x 21 625 lbs
60 Gallon 48 3/8 x 12 7/8 x 23 7/8 710 lbs
80 X-High 48 7/8 x 14 x 30 3/4 990 lbs
75 gallon 48 1/2 x 18 1/2 x 21 3/8 850 lbs
90 gallon 48 1/2 x 18 1/2 x 25 3/8 1050 lbs
110 X-High 48 7/8 x 19 x 30 3/4 1320 lbs
120 Gallon 48 1/2 x 24 1/4 x 25 1/2 1400 lbs
100 Gallon 72 1/2 x 18 1/2 x 19 3/8 1150 lbs
125 gallon 72 1/2 x 18 1/2 x 23 3/8 1400 lbs
150 gallon 72 1/2 x 18 1/2 x 28 1/2 1800 lbs
180 Gallon 72 1/2 x 24 1/2 x 25 5/8 2100 lbs

Hexagon tanks L x W x H Filled Weight
10 Hexagon 14 1/2 x 12 9/16 x 18 3/4 110 lbs
20 Hexagon 18 3/4 x 16 1/4 x 20 5/8 220 lbs
35 Hexagon 23 1/4 x 20 3/16 x 24 3/4 390 lbs
60 Hexagon 27 1/4 x 24 1/8 x 29 1/2 750 lbs

Metric conversions of tank length, to nearest centimeter:
inches centimeters
8 1/4 21
12 3/16 31
13 5/8 35
16 3/16 41
20 1/4 51
24 1/4 62
30 1/4 77
36 7/8 94
48 1/4 123
48 7/8 124
72 1/2 184
you can figure out the cubic footage by measuring and convert it. or pour in gallon by gallon of water. I like option one better myself.
measure the dimensions of the tank, and llook up in the internet "(your measurements) fish tank" and see if any results for water capacity come up.
remember 1st the larger the tank the larger the fish grows. 2nd ask a pet store. 3rd it is written on the tank how many gallons. set your mind as to what tye of fish you prefer and take it from there.
1.Get a piece of paper and a pen or pencil along with a measuring tape.
2. measure the length and record it clearly on your paper
3. measure the width and record it clearly on your paper
4. measure the height and record it clearly on your paper

then search the net for fishtanks as if to purchase when you see one that has your measurements the advert will tell you the gallons (saves you the maths.)
http://answers.yahoo.com/question/index;.
Read the answers.
It depends on the size of the tank ,Like I have a 45 gallon tank
Let me make it really easy on you. Measure the tank height, width, and depth in inches. Let's say we use a tank with the measurements 48x18x20. Multiply the numbers together. (48x18x20=17,280). Now, divide by 231. 17,280 divided by 231= 74.805 which you round up and say you have a 75g tank.

So, LxWxD divided by 231= volume of your fish tank.

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