Gallons per inch of tank
For straight-walled tanks, one number converts every gauge reading to gallons. Round horizontal tanks vary by depth — use the charts for those.
vertical cylinder: gal/inch = π × (D/2)² ÷ 231
rectangular: gal/inch = L × W ÷ 231
horizontal cylinder: gal/inch varies with depth — see belowAll dimensions in inches. The 231 is the exact definition of the US liquid gallon, so the only approximation in these formulas is your tape measure.What gallons per inch actually means
Gallons per inch is the slope of your tank chart: how much volume one inch of depth represents. For a straight-walled tank it is a single constant that turns any stick reading into gallons with one multiplication. For a round horizontal tank it is not constant at all, and treating it as one is the most common tank-gauging error there is.
How to calculate it
- Measure the tank's internal cross-section, in inches — diameter for a vertical cylinder, length and width for a rectangle.
- Compute the horizontal cross-sectional area in square inches.
- Divide by 231. That is your gallons per inch.
- Multiply by the stick reading to get volume.
A 48-inch-diameter vertical cylinder
- Area: π × 24² = 1,809.56 in²
- 1809.56 ÷ 231 = 7.834 gal/inch
- Stick reading of 37 in → 37 × 7.834 = 289.9 gal
7.83 gallons per inch
Constant from the bottom of the straight side to the top. If the tank has a dished or coned bottom, the first several inches follow a different curve.
A rectangular tank, 96 in × 48 in inside
- Area: 96 × 48 = 4,608 in²
- 4608 ÷ 231 = 19.948 gal/inch
- Stick reading of 22 in → 22 × 19.948 = 438.9 gal
19.95 gallons per inch
A 12-foot-diameter vertical storage tank
- Diameter 144 in → area = π × 72² = 16,286 in²
- 16286 ÷ 231 = 70.50 gal/inch
- One inch of gauge error = 70.5 gallons
70.5 gallons per inch
On a tank this size, an inch of gauge error costs more than a domestic oil tank holds in a week. Gauge resolution matters in proportion to diameter.
Why horizontal cylinders don't have a gallons-per-inch figure
In a round horizontal tank the liquid surface is a chord, and the chord gets wider toward mid-height and narrower at both ends. The volume per inch follows that width.
surface width w = 2 × √( h × (D − h) )
gal per inch = w × L ÷ 231
Example: D = 48 in, L = 120 in
at h = 6 in w = 31.75 in → 16.5 gal/inch
at h = 24 in w = 48.00 in → 24.9 gal/inch
at h = 42 in w = 31.75 in → 16.5 gal/inchThe middle inches hold 51 % more than the inches near the bottom on this tank. Averaging across the range, or interpolating linearly between distant chart rows, produces errors of exactly that size.This is why round horizontal tanks need a chart rather than a factor. Our chart library and dip chart generator compute the partial-volume integral inch by inch instead of interpolating.
Four things that make the calculated number wrong
- Outside dimensions. Steel and polyethylene walls are thick enough to matter. A ½-inch wall on a 48-inch tank removes 3 % of the area.
- Tank tilt. A tank out of level reads long at one end and short at the other. ISO 7507-1 includes explicit corrections for this because it is normal, not exceptional.
- Dead volume below the outlet. Gallons per inch tells you what is in the tank, not what you can get out of it.
- Bottom geometry. Coned, dished and sloped bottoms break the constant for the first several inches — often the inches you care most about.
FAQ
What is gallons per inch?
The volume that one inch of liquid depth represents in a tank. Divide the internal horizontal cross-sectional area in square inches by 231 — the exact number of cubic inches in a US gallon.
What is the gallons per inch of a 48-inch vertical tank?
7.83 gallons per inch. The cross-sectional area is π × 24² = 1,809.56 in², and 1,809.56 ÷ 231 = 7.834.
Why doesn't this work for a horizontal cylinder?
Because the liquid surface is a chord whose width changes with depth. On a 48-inch-diameter tank the mid-height inches hold about 51 % more than the inches near the bottom, so no single figure describes the tank.
What's the gallons per inch of a 275-gallon oil tank?
It varies, because the tank is obround rather than rectangular. Around mid-height it is roughly 7 gallons per inch and it falls away sharply top and bottom — see the 275V chart.
Should I use inside or outside dimensions?
Inside, always. Wall thickness is not negligible: half an inch of wall on a 48-inch tank removes about 3 % of the cross-sectional area, and therefore 3 % of every reading you take.
Sources and standards
Every factor on this page is traced to the document that defines it. Where a value is exact, it is exact because the cited standard makes it so — not because we rounded it well.
- Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-cThe legal-metrology reference for U.S. customary units. Fixes the U.S. liquid gallon at exactly 231 cubic inches and tabulates the full liquid-volume chain (minim → fluid dram → fluid ounce → gill → pint → quart → gallon).
- ISO 7507-1 — Petroleum and liquid petroleum products: Calibration of vertical cylindrical tanks, Part 1: Manual strapping methodInternational Organization for StandardizationPaid standard — cited by designation; obtain from the publisher.Defines how a tank's capacity table (its “strapping table”) is produced from physical measurement, including corrections for plate thickness, shell expansion and tank tilt. It is the reason a real tank's chart rarely matches naive geometry to the last gallon.
- Manual of Petroleum Measurement Standards (MPMS) — Ch. 1 “Vocabulary”, Ch. 11.1 “Temperature and Pressure Correction Factors”, Ch. 12.1 “Calculation of Static Petroleum Quantities”American Petroleum Institute (with ASTM D1250 / ISO 91 volume correction tables)Paid standard — cited by designation; obtain from the publisher.The industry standard for turning a gauged tank volume into a traded quantity: observed volume, gross standard volume at 60 °F, and net standard volume after sediment and water. A unit conversion is only the first step of that chain.
- Special Publication 811, Appendix B.8 — Factors for Units Listed AlphabeticallyNational Institute of Standards and Technologyhttps://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8NIST's master table of conversions into SI. Factors printed in boldface are exact by definition rather than measured or rounded — which is how you tell a defined unit from an approximated one.
Every linked source above was retrieved and checked on 2026-09-02. Factors quoted as exact are exact by definition in the cited document, not rounded by us.