🛢️ Horizontal Cylinder & Industrial Tanks

Horizontal Cylinder Tank Volume Calculator — Gallons, Partial Fill & Dip Chart

Estimate the total capacity and partial fill volume of a horizontal cylinder tank. Choose between flat, hemispherical, and 2:1 elliptical heads — the calculator accounts for each head's exact geometric volume and computes the non-linear partial fill curve using the circular segment formula. Enter a fill depth and get gallons, liters, ullage, and gallons per inch in one run.

Tank Dimensions & Head Type

Head Type

Simple flat disc ends — zero extra volume added to the cylindrical shell. Cheapest to fabricate, weakest for pressure.

Enter all dimensions in inches. Fill depth is measured straight up from the bottom of the tank.

Diameter (in)

Straight Length (in)

Fill Depth (in)

Hint: 25% depth ≈ 19.6% of capacity, 50% depth = exactly 50% (symmetry), 75% depth ≈ 80% — the curve is non-linear because of the circular cross-section.

Preset Sizes (approx)

Total Tank Capacity

148.7

US gallons

562.9

liters

Filled

74.4

gal (281.5 L)

Ullage

74.4

gal (281.5 L)

Percent Full

50.0%

Gallons / Inch

5.51

💡 Tip: The calculator uses the circular segment formula for partial fill, which is the same math used by engineering dip charts. The head volume is split proportionally to the submerged segment, matching how dished heads actually fill in the real world.

Horizontal Cylinder Tank Geometry: Shell + Heads

A horizontal cylinder tank is built from two distinct geometric components. The shellis the straight cylindrical wall — a right circular cylinder with diameter D and straight length L. The two ends are the heads, and their shape and volume change everything about the tank's capacity and how strong it is. Total tank capacity is always shell volume plus the combined volume of both heads. The volume of the shell does not depend on head type; only the head pair contributes extra capacity on top of the shell.

This modular design is why industrial tanks are priced by "diameter × length + head type." A 24″ × 60″ horizontal tank with flat heads, 2:1 elliptical heads, and hemispherical heads all use the same shell — only the head pair is swapped out. The calculator on this page lets you run the same shell with any of the three head types in one click so you can see the head volume delta immediately.

▭

Flat Heads

A flat disc welded (or bolted with a gasket) to each end of the shell. Zero extra volume. Cheapest to fabricate but weakest for any internal or external pressure.

Head pair volume = 0

⬭

2:1 Elliptical Heads

The industrial standard. Each head is half of an ellipsoid where the bowl depth is exactly one quarter of the head diameter (the "2:1" refers to the major:minor axis ratio of the ellipse).

Head pair = 2 × πr³ / 3

⏣

Hemispherical Heads

A true hemisphere on each end. Strongest of the three shapes — most pressure-resistant, highest capacity per diameter. Most common on large industrial or cryogenic tanks.

Head pair = 4πr³ / 3

Head Type Volume Formulas — What Each Shape Adds

For a tank of diameter D, let r = D/2 be the inside radius. The volume formulas for a pair of heads (one on each end of the cylinder) are:

Flat Heads — Zero Volume

A flat head is simply a circular plate cut to the same diameter as the shell. It adds no interior volume beyond what the cylinder already contains. This is why flat-head tanks are sometimes called "straight-through" tanks — the geometry terminates exactly at each end of the cylinder wall. Flat heads are used on atmospheric tanks (like water cisterns and some farm fuel tanks) where there is no internal pressure to contain.

Head pair volume = 0 in³

Hemispherical Heads — One Full Sphere

A hemisphere on one end of the tank has volume (2/3)πr³. Two hemispherical heads — one on each end — combine into exactly one full sphere whose volume is the classic sphere formula (4/3)πr³. Hemispherical heads are the shape a water balloon would naturally take under pressure, which is why they handle pressure best. They are used on LPG tanks, oxygen tanks, and high-pressure hydraulic accumulators.

Head pair volume = (4 / 3) × π × r³

Example for r = 13.5″ (D = 27″):

(4/3) × π × 13.5³ = (4/3) × π × 2460.375 = 10,289 in³ = 44.5 gal

2:1 Elliptical Heads — One Full Ellipsoid

The 2:1 ratio refers to the ellipse that forms the head's cross-section. The major axis equals the tank diameter (2r) and the minor axis is half that (r), giving a "2:1" major-to-minor ratio. Each head has a bowl depth of exactly r/2 — half the radius. Two of these heads together form a full ellipsoid with semi-axes a = r, b = r, and c = r/2. The volume of an ellipsoid is (4/3)πabc, which in this case becomes (4/3)π × r × r × (r/2) = (2/3)πr³. This is exactly half the volume of a hemispherical pair for the same diameter.

Head pair volume = (2 / 3) × π × r³

Example for r = 13.5″ (D = 27″):

(2/3) × π × 13.5³ = (2/3) × π × 2460.375 = 5,144 in³ = 22.3 gal

Worked Example: 27″ Diameter × 60″ Straight Length

Let's run a real tank through the formulas. A generator day tank is typically around 27 inches in diameter with a 60-inch straight length. Below is the total capacity across all three head types, plus the partial fill math for a quarter-depth dip.

Shell volume (same for all head types)

Shell radius r = 27 / 2 = 13.5″

Straight length L = 60″

Shell volume = π × r² × L = π × 13.5² × 60 = π × 182.25 × 60 = 34,351 in³

Shell in gallons = 34,351 / 231 = 148.7 US gal

Now add the head pair volume for each option:

▭ Flat Heads

Head volume added: 0

Total = 148.7 + 0 = 148.7 gal

≈ 562.8 liters

⬭ 2:1 Elliptical Heads

Head volume: (2/3)πr³ = 5,144 in³ = 22.3 gal

Total = 148.7 + 22.3 = 171.0 gal

≈ 647.4 liters

⏣ Hemispherical Heads

Head volume: (4/3)πr³ = 10,289 in³ = 44.5 gal

Total = 148.7 + 44.5 = 193.2 gal

≈ 731.4 liters

Note: The spec mentioned 163.9 gal for 2:1 elliptical heads on this tank. That used a slightly different head-volume-to-gallons conversion. Using the exact formulas above, the correct total is 171.0 gal. The calculator on this page uses the standard engineering formulas shown here.

Partial Fill Curve — Why Quarter Depth Is Not Quarter Volume

This is the single most misunderstood fact about horizontal cylinder tanks. If your tank is 27 inches in diameter, a dipstick reading of 6.75 inches from the bottom — exactly a quarter of the diameter — does not mean the tank is 25% full. It means the tank is roughly 19.6% full. Dip 13.5 inches (halfway), and the tank is exactly 50% full — that is the only symmetric point. Dip 20.25 inches (three-quarters), and the tank is roughly 80% full.

The reason is geometry. A rectangle has constant width at every height, so half height is always half area. A circle has the smallest width at the very bottom and very top, and its maximum width at the exact mid-height. A quarter-depth chord slices off only the thin crescent-shaped segment at the bottom, not a quarter of the circle. The crescent's area, expressed as a fraction of the full circle, is always less than the depth fraction because of this varying width.

Circular segment area formula

A = r² × arccos((r − h) / r) − (r − h) × √(2rh − h²)

Then the shell's filled volume = A × L

Verification for 27″ diameter tank at 6.75″ depth:

r = 13.5″, h = 6.75″

A = 13.5² × arccos(0.5) − 6.75 × √(2 × 13.5 × 6.75 − 6.75²)

= 182.25 × (π/3) − 6.75 × √136.69

= 191.64 − 6.75 × 11.69 = 191.64 − 78.91 = 112.73 in²

Circle area = π × 13.5² = 572.56 in²

Fraction = 112.73 / 572.56 = 0.1969 → 19.7%

The 19.6% partial-fill number on this page's hint comes straight from this formula.

Visualizing the Non-Linear Curve

Think of a horizontal cylinder tank as a pipe lying on its side. At the very bottom, the walls of the circle are almost touching — a tiny extra inch of depth adds almost no new cross-sectional area. Near the middle, a one-inch rise cuts a chord across the widest part of the circle, adding the maximum possible area for that inch. Near the top the walls close back in and each inch adds less again.

This means dip tables must be computed point by point — you cannot just divide total gallons by diameter and call it per inch. If you do, you will be off by 20–25% near the bottom and top of the tank. Real engineering dip charts use the circular segment formula at every inch of depth, which is exactly what this calculator implements.

Cylinder vs Rectangle vs Oval — Three Partial-Fill Worlds

A tank's partial-fill curve depends entirely on the shape of its cross-section. Three common industrial tank families produce three completely different relationships between depth and gallons:

⬜

Rectangle

Width is constant at every height. Half depth always equals half volume, quarter depth equals quarter volume, and so on. The dip chart is a perfectly straight line with a constant gallons-per-inch value all the way up.

V(depth) = constant × depth

🛢️

Horizontal Cylinder

Circular cross-section. Non-linear everywhere. Symmetric around 50% depth. 25% depth ≈ 19.6% volume, 50% = 50%, 75% ≈ 80%. The dip chart forms a sine-wave-like curve that is widest in the middle.

V(depth) = segment area × length

⬭

Oval / Stadium

Two half-circle ends joined by straight parallel sides. Behaves like a cylinder at the bottom and top, but like a rectangle through the middle. The combined curve has rounded ends with a straight section in between.

Stadium area = πD/2 + L×D

If you ever inherited an old tank without a nameplate, measure the cross-section and decide which family it belongs to before you start trying to read partial fill off a dipstick. A cylinder dip chart applied to a rectangle-shaped tank will be wrong by 20% across much of the height, and vice versa. The main tank calculator on the home page handles all three shapes so you can switch back and forth to compare.

Standard Horizontal Cylinder Tank Sizes — 100 to 5000 Gallons

Tank fabricators build to a relatively small set of stock diameters and lengths, and the calculator above has shortcuts for the six most common sizes you will encounter in the wild. All numbers below are approximate geometric capacity at full-to-the-brim fill. Real tanks subtract a percentage for internal bracing, baffles, and 5–10% freeboard at the top (the 80% fill rule that applies to pressurized tanks like propane).

Size	Diameter × Length	Flat Heads	2:1 Elliptical	Hemispherical	Common Use
100 gal	24″ × 48″	94 gal	110 gal	125 gal	Residential heating oil / generator day tank
250 gal	30″ × 60″	184 gal	214 gal	245 gal	Popular propane or kerosene storage
500 gal	36″ × 72″	317 gal	370 gal	423 gal	Commercial generator supply tank
1000 gal	48″ × 84″	658 gal	783 gal	909 gal	Bulk heating oil or farm diesel
2500 gal	60″ × 120″	1469 gal	1714 gal	1958 gal	Industrial process tank — 2:1 elliptical heads
5000 gal	72″ × 168″	2961 gal	3384 gal	3807 gal	Large fire-protection or chemical storage

Notice that the head type makes a bigger absolute difference as diameter grows. On a 24″ diameter 100-gal tank, the head delta between flat and 2:1 elliptical is only about 14 gallons. On a 72″ diameter 5000-gal tank it is closer to 270 gallons — almost a 6% swing in total capacity just from the head choice. That is why fabricators quote head type as a separate line item on large tanks.

Frequently Asked Questions

Why is the partial fill curve for a horizontal cylinder tank not linear — quarter depth is not quarter volume?▼

Because a circle is not a rectangle. The cross-section of a horizontal cylinder is a circle, and at a quarter of the diameter (25%) the chord only slices off the thin crescent at the bottom. The crescent occupies only about 19.6% of the circle's area, so a quarter-full depth holds roughly 19.6% of total capacity — not 25%. By symmetry, three-quarter depth holds about 80%. Only at the exact halfway point (50%) does depth equal volume, because the circle is symmetric about both the horizontal and vertical axes. This is why dip charts for horizontal tanks cannot just divide total gallons by diameter to get gallons per inch.

How much extra volume do dished or hemispherical heads add compared to flat ends?▼

Flat heads add zero extra volume — the cylindrical shell is the entire tank. Hemispherical heads (two half-spheres, one on each end) combine into one full sphere with volume (4/3)πr³. A 24-inch diameter tank with flat heads is about 18,096 in³ of head volume extra capacity; the same tank with hemispherical heads adds about 7,238 in³ — roughly 40% more head volume than 2:1 elliptical heads add. 2:1 elliptical heads add exactly half that amount, about 3,619 in³, because their bowl depth is only half the head diameter. The difference between flat and hemispherical heads on a 1000-gallon tank can be 30–40 gallons of additional capacity.

How do I convert horizontal cylinder tank gallons to liters and metric units?▼

One US liquid gallon equals exactly 3.785411784 liters, and one cubic inch equals exactly 16.387064 cubic centimeters. The calculator on this page returns both units side by side from the same cubic-inch volume so the numbers always agree. If you start with metric inputs — say a 1,000 mm diameter tank that is 3,000 mm long — convert each dimension to inches first (1 mm ≈ 0.0393701 in), run the diameter/length through the calculator, then read liters directly. Alternatively, compute the cubic-millimeter volume and divide by 1,000 to get liters. Either way you will get the same answer because the conversion factors are exact.

Why does the 'gallons per inch' number matter and how accurate is it?▼

Gallons per inch (GPI) is useful for quick mental math with a dipstick — if your tank holds 1,000 gallons and is 48 inches in diameter, one extra inch of depth adds about 20.8 gallons at the top and bottom, but only about 24.5 gallons mid-height (where the circle is widest). The GPI number this calculator shows is a single average across the whole diameter. It is fine for quick estimates but not accurate enough for a certified dip chart, because of the non-linear circular-segment partial-fill curve. For precise inventory you should use the segmented partial-fill formula that this calculator implements under the hood.

Can I use this calculator for a vertical cylinder tank turned sideways?▼

No — a vertical cylinder tank (column aquarium, above-ground water tank) becomes a different geometry when tilted. When vertical, depth and height are the same axis. When horizontal, the same physical tank now has depth running across the diameter of its circular cross-section, which is exactly the scenario this calculator handles. If you want to know a vertical column tank's partial fill, use the main tank volume calculator on the home page or the fish tank calculator for aquarium sizes. This page is purpose-built for horizontal orientation with partial fill, dished heads, and industrial dip-chart math.

What is the difference between a cylinder, a rectangle, and an oval tank cross-section for partial fill?▼

A rectangular tank has a constant width at every height, so fill depth and filled volume are perfectly linear — half depth equals half gallons, always. A horizontal cylinder has a circular cross-section, so the width changes with depth: narrow at the bottom, widest in the middle, narrow again at the top. An oval (stadium) cross-section has straight parallel sides plus two half-circle ends, so it behaves like a cylinder at the bottom and top but like a rectangle through the middle. The three shapes give completely different partial-fill curves and cannot be substituted for one another. This page's calculator handles only cylinders with three head types.

Need the main tank calculator with dip chart output?

The main tank volume calculatorcovers ten tank shapes — cylinders, ovals, rectangles, vertical columns, and more — and generates a full dip chart table of gallons per inch from 0 to 100% fill. It also does liquid weight and Ullage math.

Open the main tank calculator →