Rogue Turtle: How Big a Water Tank Do I Need?

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If your mathematics is as bad as mine is then you will have a great deal of difficulty finding out how big a water storage tank you need to store water in your Properly Provisioned Pre-Planed Shelter. I'm big on the letter "P". Remember, Proper Prior Planning Prevents Poor Performance? I do.

I hated math in school, and I didn't get any smarter as I got older. For questions like this one, I had to ask others, only they didn't know either. If you look up "tanks" on the internet you get some manufacturer who makes what he thinks is the size you need. I'm not going to go into how to compute how much you need because I've already covered that in another article. What I don't know is how many people you will need water for. You need to know that and how long you plan to be in this location using this stored water before you move on to greener pastures.

I sat down one night with a calculator and some figures for determining how much water could be stored in a water tank of a specific dimension. I assumed that I could pour a tank from concrete and fill it later with drinkable water. As I started writing all the numbers down, it kind of just fell into a graph that actually makes sense to me. I used one foot (and 6 inch) intervals. If you look at the chart, you will see length and width figures for 1', 1'-6", 2', 2'6", etc.

The chart data is for a wall only ONE FOOT TALL. For example, take a tank that is 5'0" in width, and 5'0" in length. Since the chart is for a 1'-0" high wall, then this tank will hold 187.01 gallons. Now YOU have to multiply. If you make your wall 2'-0" high, you will double your capacity and it will hold 374.02 gallons. If you make your tank wall 6'-0" high, then it will hold 1,122.06 gallons. (187.01 x 6 = 1122.06).

I did not compute a graph for round or oval tanks. This is for rectangular tanks only. I don't think I could build a really "round" tank by myself, but I know how to pour square concrete walls. I'm not going to try to tell you how thick a wall to build, but in a pinch, I'm pretty sure that 5" walls with vertical steel reinforcing bars (rebar) spaced about 2' apart should suffice. There should also be steel rebar embedded inside the walls, horizontal to the base. The base will have to be much thicker. If in doubt, consult an architect or an engineer. All this chart is for is to tell you how big a tank you need to build. The chart shows INSIDE DIMENSIONS, not outside dimensions.

I used this chart to design my own (ideal) shelter to come up with a renewable tank that would hold about 12,000 gallons of water. By renewable, I mean I would use water in and water out (of the tank) at the same rate, pumping from my own well. I will be drawing water from the bottom of the tank. The "full" level would never go down unless my pump or generator went out. From that point, I have 12,000 gallons of water usage to find the parts or supplies to fix my problem. My ideal shelter is "permanent", ready for repeated use.

If your floor space for a tank is limited, you can use this chart to estimate how much water can be stored inside the space. For example: If you only have a 3' x 3' space available for a tank, you can see on the chart that a 3' x 3' x 1' tank will hold 67.32 gallons. If you make your tank seven feet high, then it will hold 470.61 gallons of water. (67.32 x 7 = 470.61) If you need to store more water than that, then you need either more floor space (an area greater than 3' x 3'), or a higher tank.

CAUTION: Water weighs 8.345 pounds per gallon. When full, the sample tank above will weigh 3,927.3 pounds. This does NOT include the weight of the concrete tank itself. (470.61 x 8.345 = 3,927.3 pounds) A good, strong base will be needed to hold up this much weight.

Capacity of Rectangular Tanks
Gallons per Foot of Height

Example: 6’ x 6’ x 1’ tank = 269.30 gal.
A 6’ x 6’ x 6’ tank = 1615.8 gallons capacity

W i d t h	Length of Tank
W i d t h	2'-0"	2'-6"	3'-0"	3'-6"	4'-0"	4'-6"	5'-0"	5'-6"	6'-0"	6'-6"	7'-0"	7'-6"	8'-0"	8'-6"	9'-0"	9'-6"	10'-0"	10'-6"	11'-0"	12'0"
2'-0"	29.92	37.40	44.88	52.36	59.84	67.32	74.81	82.29	89.77	97.25	104.73	112.21	119.69	127.07	134.65	142.13	149.61	157.09	164.57	179.53
2'-6"		46.75	56.10	65.45	74.80	84.16	93.51	102.86	112.21	121.56	130.91	140.26	149.61	158.96	168.31	177.66	187.01	196.36	205.71	224.41
3'-0"			67.32	78.54	89.77	100.99	112.21	123.43	134.65	145.87	157.09	168.31	179.53	190.75	201.97	213.19	224.41	235.63	246.86	269.30
3'-6"				91.64	104.73	117.82	130.91	144.00	157.09	170.18	183.27	196.36	209.45	222.54	235.63	248.73	261.83	274.90	288.00	314.18
4'-0"					119.69	134.65	149.61	164.57	179.53	194.49	209.45	224.41	239.37	254.34	269.30	284.26	299.22	314.18	329.14	359.06
4'6"						151.48	168.31	185.14	201.97	218.80	235.63	252.47	269.30	286.13	302.96	319.79	336.62	353.45	370.28	403.94
5'-0"							187.01	205.71	224.41	243.11	261.82	280.52	299.22	317.92	336.62	355.32	374.03	392.72	411.43	448.83
5'6"								226.28	246.86	267.43	288.00	308.57	329.14	349.71	370.28	390.85	411.43	432.00	452.57	493.71
6'-0"									269.30	291.74	314.18	336.62	359.06	381.50	403.94	426.39	448.83	471.27	493.71	538.59
6'6"										316.05	340.36	364.67	388.98	413.30	437.60	461.92	486.23	510.54	534.85	583.47
7'-0"											366.54	392.72	418.91	445.09	471.27	497.45	523.64	549.81	575.99	628.36
7'6"												402.78	448.83	476.88	504.93	532.98	561.04	589.06	617.17	673.24
8'-0"													478.75	508.67	538.59	568.51	598.44	628.36	658.28	718.12
8'-6"