6 minute read · Published by Swhift Stainless Fabrication, building stainless tanks in Sandpoint, Idaho since 1980
Sizing a tank is a negotiation between three things that rarely agree: the volume you need, the space you have, and what a fabricator can economically build. Here is how to work through it in an order that avoids expensive surprises.
Start with usable volume, not tank volume
The number you need is how much liquid the tank must hold in service. The tank itself has to be bigger than that, because almost nothing runs 100% full.
- Storage tanks need freeboard so they do not overflow on a surge
- Hot water tanks need room for thermal expansion
- Softener vessels need space above the resin bed for it to lift during backwash
- Any tank with an internal coil loses the volume the coil occupies
- Baffles, distributors and dip tubes all take up space
Then work out the geometry
For a cylinder, volume is π × radius² × length. For a rectangular tank it is width × height × length. One US liquid gallon is 231 cubic inches. Our tank volume calculator does both, in inches, feet, centimetres or metres.
One number worth knowing is gallons-per-inch — how much volume each inch of length adds. Our published pressure tank chart lists it for every diameter, and it is genuinely useful: at 24 inches diameter each inch of length adds 1.95 gallons, so adding a foot of length adds about 23 gallons. That tells you instantly whether you should be adding length or increasing diameter.
| Diameter | Gallons per inch | Gallons per foot of length |
|---|---|---|
| 14-5/8 in. | 0.727 | ≈ 8.7 |
| 18 in. | 1.100 | ≈ 13.2 |
| 24 in. | 1.950 | ≈ 23.4 |
| 30 in. | 3.060 | ≈ 36.7 |
| 36 in. | 4.406 | ≈ 52.9 |
| 48 in. | 7.836 | ≈ 94.0 |
Now check it against the building
This is the step that gets skipped, and it is the one that turns a delivered tank into a problem. Measure the whole route, not just the room.
- Ceiling height in the final position — and remember a vertical tank has to be tipped upright, which needs more headroom than the tank is tall.
- Every doorway, stair turn, corridor width and elevator on the route in.
- Whether the tank goes in before or after the walls. Retrofits are much more constrained than new builds.
- Access for a forklift, pallet jack or crane at the delivery point.
- Floor loading. Water is 8.34 lb per gallon, so a 1,000 gallon tank is over four tons of water plus the tank itself.
Check the stock charts before going custom
Once you have a target volume and an envelope, look at our published size charts. A stock size is normally faster and less expensive than a custom build, and our charts list gallons and approximate weight for every one.
Cylindrical stock runs from 9-5/8 in. through 60 in. in diameter and from about 8 gallons up to roughly 1,913 gallons. Rectangular stock covers 55 to 3,231 gallons. If a stock size is within a few percent of your target, taking it is usually the right call.
Multiple tanks versus one large one
Worth considering when a single tank is awkward. Two smaller tanks can get through a door that one large one cannot, allow one to be taken offline for service without shutting the process down, and sometimes cost less to ship. Against that, they need more floor area, more pipework and more connections to leak.