Turning a tank level into gallons
A sight glass, a staff gauge or a dip tape gives you inches. Orders, costs and days left need gallons. Here is the arithmetic for the three shapes of tank most plants have, a table for tanks lying on their side, and a way to check the answer against a delivery. Every figure on this page was worked through on October 16, 2026, with the same arithmetic ChemLevel uses.
For an upright tank with a flat bottom, multiply the level by the tank’s gallons per inch, which comes from its inside diameter. A rectangular tank works the same way from its width and length. A tank on its side does not: every inch holds a different amount, and a straight-line estimate is out by hundreds of gallons near the bottom, where the reorder level usually sits.
If the tank came with a strapping chart, the maker’s table of gallons at each inch, use it. It was worked out for that tank, ends and bottom included, which no general formula can be. This page is for the tanks without one, and for checking the one you have.
1. The short answer
- Upright tank
- Gallons per inch is the inside diameter × the inside diameter × 0.0034. Multiply by the level.
- Tank on its side
- Divide the level by the inside diameter, and take the share of the full tank the table gives for it.
- Rectangular tank
- Gallons per inch is the inside width × the inside length ÷ 231. Multiply by the level.
- Pounds
- Gallons × 8.34 × the specific gravity.
- Checking it
- A delivery’s gallons against the rise it made, plus what was used meanwhile.
2. What to measure
Every formula here works in liquid depth and inside measurements, in inches. Three things put the answer out before any arithmetic starts:
- The nameplate. It gives the whole vessel, to the brim. A sight glass or a level sensor seldom reaches that high, so what the tank holds when the gauge reads full is usually less. Use the depth at the gauge’s full mark, not the height of the tank.
- The outside of the tank. A diameter taken outside a thick plastic or fiberglass wall overstates every figure worked out from it. Measure inside wall to inside wall where you can.
- The gauge’s zero. A sight glass whose zero sits above the floor of the tank never shows the liquid below it. Add that height to each reading for the gallons in the tank, or leave it out for the gallons you can draw.
3. Upright tanks
An upright cylinder with a flat bottom holds the same amount in every inch. A gallon is 231 cubic inches exactly, so one inch of depth holds:
For the common sizes:
| Inside diameter | Gallons per inch | Gallons per foot |
|---|---|---|
| 4 ft (48 in) | 7.83 | 94.0 |
| 5 ft (60 in) | 12.24 | 146.9 |
| 6 ft (72 in) | 17.63 | 211.5 |
| 8 ft (96 in) | 31.33 | 376.0 |
| 10 ft (120 in) | 48.96 | 587.5 |
| 12 ft (144 in) | 70.50 | 846.0 |
| 14 ft (168 in) | 95.96 | 1,151.5 |
An upright tank
A tank 10 ft across, 120 in inside, reads 64 in on its sight glass.
Cone and dished bottoms
A cone holds a third of what a straight wall of the same height and diameter would. With the level measured from the cone’s point, a reading up in the straight wall is:
Inside the cone each inch holds less than the one above it, and the gallons are what the full cone holds × (the reading ÷ the cone’s height), cubed. On a tank 8 ft across with a 24 in cone, at 31.33 gal per inch, the cone holds 31.33 × 24 ÷ 3, or 251 gal. A reading of 72 in is 251 + 31.33 × 48, or 1,755 gal, and one of 12 in, halfway up the cone, is 251 × 0.5 × 0.5 × 0.5, or 31 gal.
Dished bottoms come in several standard profiles, each with its own arithmetic. The maker’s chart is the place to get those gallons.
4. Tanks on their side
A cylinder lying on its side is narrow at the bottom, widest across the middle and narrow again at the top, so every inch holds a different amount. On a tank 8 ft across and 20 ft long, the bottom inch holds about 14 gal and an inch across the middle about 100.
That is why a straight line from empty to full gets it wrong both ways. A quarter of the way up, that tank holds 1,470 gal and the straight line says 1,880: 410 gal that are not there, near the bottom, where the reorder level usually sits. Three quarters of the way up it holds 6,050 and the straight line says 5,640, so the tank looks to have room for 410 gal more than it has, just when a load is being sized.
To read one, take the level as a share of the inside diameter, measured from the very bottom of the tank, find that share in the table, and take the share beside it of the full tank. Full, a tank with flat ends holds:
| Level, share of the diameter | Share of the full tank |
|---|---|
| 5% | 1.87% |
| 10% | 5.20% |
| 15% | 9.41% |
| 20% | 14.24% |
| 25% | 19.55% |
| 30% | 25.23% |
| 35% | 31.19% |
| 40% | 37.35% |
| 45% | 43.64% |
| 50% | 50.00% |
| 55% | 56.36% |
| 60% | 62.65% |
| 65% | 68.81% |
| 70% | 74.77% |
| 75% | 80.45% |
| 80% | 85.76% |
| 85% | 90.59% |
| 90% | 94.80% |
| 95% | 98.13% |
Between two rows, take a share in proportion between their figures. Anywhere in the table, that lands within 0.3% of the full tank of the exact answer.
A tank on its side
A tank 8 ft across (96 in) and 20 ft long (240 in), with flat ends, reads 24 in.
The exact formula
For a level h, on a tank whose inside radius (half its inside diameter) is r and whose length is L, all in inches:
In a spreadsheet, with the level in A2, the radius in B2 and the length in C2: =C2*(B2^2*ACOS((B2-A2)/B2)-(B2-A2)*SQRT(2*B2*A2-A2^2))/231. Excel and Google Sheets both give ACOS in radians.
The formula and the table are for flat ends. Dished or rounded ends hold more than the formula counts, and a maker’s strapping chart includes them.
5. Rectangular tanks and totes
A tank with straight sides holds the same in every inch: the inside width × the inside length ÷ 231 gallons. A tank 48 in by 96 in inside holds 19.95 gal per inch, so a reading of 30 in is 598 gal.
A tote’s bottle has rounded corners and a floor that slopes to the drain, so the same arithmetic reads a little high, and most in the bottom inches. Many totes have a gallon scale molded into the bottle, which is the better guide.
6. Reading down from the top
A dip tape lowered through a hatch, or a sensor that reports the distance down to the surface, measures the empty space above the liquid, so its number rises as the tank empties. Subtract it from the distance between the same point and the bottom of the tank, and use the result as the level. A tape that meets the surface 40 in below a hatch 130 in above the floor means 90 in of liquid.
7. Gallons to pounds
Doses and some deliveries are in pounds. Water weighs 8.34 lb a gallon, and a chemical’s specific gravity says how many times heavier it is, so:
Take the specific gravity from your own product’s safety data sheet or the supplier’s certificate, not a general table: it changes with the product’s strength and with temperature. 1,000 gal at a specific gravity of 1.50 weighs 1,000 × 8.34 × 1.50, or 12,510 lb. Where the strength is given as a percentage by weight, that share of it is the chemical itself: at 50%, 6,255 lb.
8. Checking it against a delivery
A delivery is the one time a known amount goes into the tank, which makes it the best check on everything above. Read the level shortly before and shortly after, and add what the plant used in between:
If the ticket says more arrived, the tank holds more than you have it set up for; if less, it holds less. On a tank on its side, turn both levels into gallons with the table first, since its gallons per inch change with the level.
A tank set up from its nameplate
6,000 gal on the nameplate, over a 66 in sight glass, is 90.9 gal per inch. A 3,000 gal load took the tank from 20 in to 56 in, over a day in which the plant used 120 gal.
One load can be short, or not fully pumped off, so look for the same answer from three deliveries before changing anything. Readings taken close to either side of the delivery keep the estimate of what was used small, and the room for error with it.
9. Keeping track of it
ChemLevel does this arithmetic on every reading. A supervisor sets each tank up once, on the chemical’s Setup view: what it holds at its full reading, its shape (vertical cylinder, horizontal cylinder, rectangular or tote, or other), what the gauge reads in (inches, feet or gallons), and whether it is measured from the bottom or the top. Readings are then logged as they come off the gauge, and ChemLevel turns each one into gallons.
- No figure for what the tank holds? Work it out from measurements instead, under that field, takes the inside measurements this page uses and gives the gallons.
- A tank on its side needs its diameter and length. With them, ChemLevel works out how full it is with the formula above. Without them it reads the tank as though it filled evenly, and its Setup view says so.
- A reading above the full reading is not saved, since it is nearly always a slipped decimal point.
- After deliveries: once three of them agree that the tank holds 8% or more above or below what it is set up for, its Setup view says so, with the figure they point to, to accept or turn down.
- A specific gravity lets ChemLevel convert between pounds and gallons where loads are priced or sized by weight.
- There is no cone-bottom shape. Set up as a vertical cylinder, a cone-bottomed tank reads high below full, by the most around the top of the cone. If that matters for yours, read it in gallons from its chart.
The guide shows where each of these sits on screen, How the numbers work carries the gallons on to days left and order dates, and the storage rules by state say how many days a full tank has to last.