Two tons, quietly
The number that catches people is the density. Water is 8.345 pounds per gallon, so a modest 500-gallon tank is 4,173 pounds of water before the tank, the platform or anything else. A 1,000-gallon tank is over four tons. Spread across a four-foot circle that is more than 330 pounds per square foot under the tank, which is several times what a floor is normally asked to carry.
Divided over four legs it is around 1,100 pounds each, and that is the load that has to reach solid ground. On a 16-inch pad each leg is pressing at roughly 630 pounds per square foot. Whether that is acceptable depends entirely on what the ground is, which is why the allowable bearing value on this page is a field you fill in rather than a number it supplies.
The pressure disappointment
Here is the trade nobody runs before building: 0.433 psi per foot of head. An eight-foot stand under a tank with five feet of water in it, feeding a tap a foot off the ground, gives 8 + 5.3 − 1 = 12.3 feet of head, which is 5.3 psi. Household fixtures generally expect several times that.
So an elevated tank is excellent for filling a stock trough, running a drip line, feeding a hose bib, or supplying a fixture that tolerates low pressure. It is not a substitute for a pump on a flat site, and no realistic stand makes it one — 40 psi would need 92 feet of fall. That is why hillside gravity systems work and flat-site stands end up with a pump downstream anyway. The spring box and gravity flow calculator works the same head arithmetic over a whole pipe run, including what friction takes back once water is moving.
Where the load actually goes
The chain is straightforward and each link is a separate question. Water and tank sit on the deck; the deck spreads it to the legs; each leg carries it to a pad; the pad spreads it into soil. This page does the arithmetic of that chain — total weight, weight per leg, pressure under a pad, and the pad size that matches an allowable bearing value you supply. It does not size a post, a beam, a connection or a footing, and it does not know your frost depth.
The even-division assumption is worth naming too. Four legs sharing exactly one quarter each is what happens on a perfectly level, perfectly rigid stand. Real ones are neither, and a stand that settles slightly on one pad redistributes load in ways a simple division does not capture.
Related arithmetic
For a load hung from a ceiling rather than standing on the ground, the overhead storage platform load calculator is the equivalent. For a slab under the whole thing instead of pads, the concrete slab calculator covers volume and reinforcement quantities. For sizing the storage itself against how the water is used, the cistern sizing calculator works from demand and days of reserve, and the rainwater harvesting calculator works from a roof.
Questions people ask
How much does a full water tank weigh?
Multiply gallons by 8.345 and add the empty weight from the tank data sheet. That single figure surprises nearly everyone: 275 gallons is about 2,300 pounds, 500 is about 4,200, and 1,550 is nearly six and a half tons. The number does not depend on the tank material or the shape, only on the water. It is also why moving a tank is done empty and why a partly full tank on a trailer is a genuinely dangerous load, since the water sloshes and shifts.
How high does the stand need to be for good pressure?
Higher than any stand you would build. Static pressure is 0.433 psi per foot, so 20 psi wants 46 feet of head and 40 psi wants 92. A practical stand of eight or ten feet, plus the water depth in the tank, gives roughly five to seven psi at a low fixture. That is genuinely useful for gravity irrigation, troughs and hose work, and it is not household pressure. If household pressure is the goal on flat ground, the answer is a pump, and the stand is then about storage rather than pressure.
What soil bearing value should I use?
One that came from your site, not from this page. It is either determined by a geotechnical investigation or taken from the presumptive value the building department where you are will accept for the soil class you have, and those figures vary by a large factor between soft clay and dense gravel. Entering an optimistic number produces a small pad on paper and a tilted tank in reality. This page states no value and will apply whatever you type.
Do I need a permit for a water tank stand?
Frequently yes, and it is a question for your building department rather than for a calculator. Elevated storage is a structure carrying a very heavy load, sometimes over a place where people walk, and many jurisdictions treat it accordingly. Some also have rules about tanks near property lines or about water storage specifically. Ask before building; the conversation is far cheaper than the one that follows an inspection of something already standing.
Can I put a tank on my deck or in the attic?
Run the pounds per square foot line on this page and then ask an engineer, because the answer is usually no by a wide margin. A 250-gallon tank concentrates over 2,000 pounds onto a few square feet, which is far beyond what an ordinary residential floor or deck is designed to carry, and the failure is sudden rather than gradual. Structures are designed around distributed loads; a tank is close to a point load. This is exactly the case where the arithmetic tells you to stop and hand the question to someone qualified.