Horizontal tank gauging reference: fill curve, capacity tables, gallons per inch
The one number you can carry between every horizontal tank is the depth-to-fill curve: it depends only on how far up the stick is wet as a fraction of the tank’s diameter, never on the tank’s size. Read your wet mark as a percentage of the diameter, look it up below, and you have the fill percentage for a 200-gallon tank and a 5,000-gallon tank alike. Everything else here, the capacities, gallons per inch, end-cap corrections, and metric, all hangs off that single curve.
Depth-to-fill curve
Wet depth as a fraction of the tank's full diameter, and the fraction of total volume that holds. This is universal: it is the same circular-segment math for any flat-end horizontal cylinder regardless of dimensions.
| Stick depth (% of diameter) | Actual fill (% of volume) | Linear-guess error |
|---|---|---|
| 5% | 1.9% | +3.1 pts |
| 10% | 5.2% | +4.8 pts |
| 15% | 9.4% | +5.6 pts |
| 20% | 14.2% | +5.8 pts |
| 25% | 19.6% | +5.4 pts |
| 30% | 25.2% | +4.8 pts |
| 40% | 37.4% | +2.6 pts |
| 50% | 50.0% | 0 |
| 60% | 62.6% | −2.6 pts |
| 70% | 74.8% | −4.8 pts |
| 75% | 80.4% | −5.4 pts |
| 80% | 85.8% | −5.8 pts |
| 85% | 90.6% | −5.6 pts |
| 90% | 94.8% | −4.8 pts |
| 95% | 98.1% | −3.1 pts |
“Linear-guess error” is what you would be off by if you read the wet mark as a straight percentage. The gap peaks near the quarter and three-quarter marks at almost 6 points and vanishes only at the exact centerline.
Full-cylinder capacity by dimension
Total capacity of a flat-end horizontal cylinder, in US gallons, by diameter (rows) and length (columns).
| Diameter \ Length | 60 in | 96 in | 120 in | 144 in |
|---|---|---|---|---|
| 24 in | 118 | 188 | 235 | 282 |
| 30 in | 184 | 294 | 367 | 441 |
| 36 in | 264 | 423 | 529 | 635 |
| 48 in | 470 | 752 | 940 | 1,128 |
| 64 in | 836 | 1,337 | 1,671 | 2,005 |
| 72 in | 1,058 | 1,692 | 2,115 | 2,538 |
| 96 in | 1,880 | 3,008 | 3,760 | 4,512 |
Nameplate ratings (a “1,000-gallon” tank) rarely match the flat-cylinder figure to the gallon, because manufacturers round to a nominal size and most real tanks add volume in the end caps. Use the table for the barrel; reconcile against the data plate for the true total.
Gallons per inch of stick
How much volume one inch of wet depth represents changes with where you are on the tank. Values below are for a 48 in × 142 in cylinder (about 1,112 gallons full); the pattern is the same shape on any tank, scaled to its size.
| Wet depth | Position | Gallons per inch |
|---|---|---|
| 6 in | 12% (lower flat) | ~19.5 |
| 12 in | 25% (lower quarter) | ~25.6 |
| 24 in | 50% (centerline) | ~29.5 |
| 36 in | 75% (upper quarter) | ~25.6 |
| 42 in | 88% (upper flat) | ~19.5 |
An inch of stick buys the most gallons across the fat middle and the least near the bottom and the brim. Resolution is worst right at the top, where an inch of misread can be the difference between full and an overflow. Round conservatively when you are nearly topped off.
Corrections and conversions
End-cap shape and units are the two adjustments that turn the barrel math into a number you can order against.
| End-cap type | Effect vs flat-end | Typical correction |
|---|---|---|
| Flat (what this tool assumes) | baseline | ×1.00 |
| Dished / flanged | holds more | ×1.02 to ×1.06 |
| Domed / torispherical | holds more | ×1.04 to ×1.10 |
| Hemispherical | holds the most | ×1.08 to ×1.15 |
| Convert | Multiply by |
|---|---|
| US gallons → litres | 3.78541 |
| Litres → US gallons | 0.264172 |
| US gallons → Imperial gallons | 0.832674 |
| Inches → centimetres | 2.54 |
| Cubic inches → US gallons | ÷ 231 |
| Cubic inches → litres | ÷ 61.0237 |
To apply a correction without re-deriving the heads: measure the true full capacity once from the data plate or a known fill, divide it by the flat-cylinder capacity above, and carry that ratio. The non-linear curve inside the barrel still holds; the caps only shift the total.
Related concepts
- Strapping chart: the per-tank depth-to-volume table the trades have used for decades, built by physically measuring a tank under standards like API MPMS §2.2A and ISO 7507. This reference is the same relationship solved closed-form.
- Innage vs outage: innage gauges from the bottom up (the wet stick reading here); outage gauges down from a reference point at the top. Mixing the two is a common custody-transfer error.
- Water draw / bottom water: fuel tanks accumulate water under the product. A bottom inch read as fuel is really water, so check with water-finding paste before trusting the lowest gallons.
- Tank tilt: a cylinder pitched even a couple of degrees pools liquid at the low end; gauge from the centre and treat readings at the ends as biased.