The tank volume calculator above returns two numbers for six tank shapes: the total capacity from the tank's dimensions, and the volume of liquid actually sitting in it at the depth you measured. It handles vertical and horizontal cylinders, capsule tanks with rounded or domed ends, cone-bottom process tanks and plain rectangular tanks, and it reports the answer in US gallons, imperial gallons, litres, cubic feet or cubic metres.
Arb Digital publishes this as part of a free construction and site estimating set. The reason it exists as a separate page is the partial-fill case. Total capacity is easy arithmetic. Working out how much diesel is in a horizontal tank dipped to 14 inches is not, because the cross-section of the liquid is a circular segment, and the volume it holds is nowhere near proportional to the depth on the dipstick.
What This Tank Volume Calculator Does
It computes the filled volume properly for each geometry rather than scaling the total capacity by a depth ratio. For a vertical cylinder or a rectangular tank those two approaches happen to agree, because the cross-section is constant all the way up. For every other shape here they do not, and the difference is largest exactly where tanks are usually read — around a quarter and three quarters full.
The boundary against the adjacent tools is worth stating. Our cylinder volume calculator gives the volume of a full upright cylinder and nothing else; there is no partial fill, no horizontal orientation and no dished ends. The pipe volume calculator handles pipe runs, where the geometry is a full bore over a length. The pool volume calculator deals with pool shapes and sloping floors. This page is specifically about vessels holding a measured depth of liquid, which is a different integral.
How to Use It
- Pick the shape. The field labels change to match — a rectangular tank asks for width, a cone-bottom tank asks for the cone height.
- Choose your units independently. You can enter dimensions in inches and read the answer in litres; nothing has to match.
- Enter internal dimensions. Measure inside the shell where you can. On a steel tank the wall thickness is small; on a plastic or lined tank it is not.
- Enter the liquid depth from the lowest point. If you measured from the top down, subtract that reading from the internal height first.
- Set the specific gravity if you want the weight of the contents rather than just the volume.
The Formula and How It's Calculated
A vertical cylinder is π r² h and a rectangular tank is L × W × h; both scale linearly with depth. A horizontal cylinder is the interesting one. The liquid cross-section is a circular segment, whose area is A = r² · arccos((r − h) ÷ r) − (r − h) · √(2rh − h²), and the volume is that area multiplied by the tank length. That segment formula is standard geometry — Wolfram MathWorld's circular segment entry gives it in exactly this form, as R² cos⁻¹((R−h)/R) − (R−h)√(2Rh − h²).
Capsule tanks add hemispherical ends. A partially filled sphere of radius r to depth h is a spherical cap, V = π h² (3r − h) ÷ 3, and the two hemispherical ends of a horizontal capsule together make exactly one sphere, so the cap formula covers both at once. A vertical capsule is handled in three bands: bottom dome, straight side, then total minus the empty cap at the top. A cone-bottom tank uses the cone V = ⅓ π r² H below the transition, scaling the radius with height, and adds straight cylinder above it.
A worked example: a horizontal cylinder 48 inches in diameter and 120 inches long has a total capacity of π × 24² × 120 = 217,146 cubic inches, which is 940.0 US gallons at 231 cubic inches to the gallon. Dipped to exactly 24 inches it is half full by both measures — 470.0 US gallons. Dip it to 12 inches, though, and the answer is not 235 gallons but about 184, because the bottom quarter of the depth holds far less than a quarter of the volume.
Why a Dipstick Lies on a Round Tank
This is the single most useful thing on the page. On a horizontal cylinder, volume as a function of depth is an S-curve. At 10 percent of the diameter you have about 5.2 percent of the volume. At 25 percent depth you have roughly 19.6 percent. At 50 percent, the two agree exactly at 50 percent. At 75 percent depth you have about 80.4 percent, and at 90 percent depth about 94.8 percent. The error is worst near the ends and vanishes only in the middle.
The practical consequences run in both directions. A tank read as a quarter full on a linear stick actually holds a fifth, so you have less fuel than you think and will run dry earlier than the reading suggests. A tank read as three quarters full holds four fifths, so there is less room for the delivery than you planned, and overfilling a fuel tank is not a rounding error. This is why properly made dipsticks for round tanks have unevenly spaced graduations, and why calibration charts — strapping tables — exist as a separate document for commercial tanks.
Nominal Capacity Is Not Usable Capacity
The number stamped on a tank is a geometric capacity, and there are at least three reasons you can never put that much into it. Ullage, the deliberate empty space at the top, exists so that thermal expansion has somewhere to go — liquids expand measurably with temperature, and a tank filled solid on a cold morning can vent or deform by afternoon. Fill limits imposed by regulation or by the delivery equipment cut it again. And a tank that is not perfectly level holds an effective volume that depends on which end the dip is taken.
For anything regulated, the codes matter more than the geometry. The US Environmental Protection Agency's page on underground storage tank laws and regulations sets out the federal framework for USTs and notes that the applicable design, installation and closure requirements come from industry codes published by bodies such as API, ASTM, NFPA and UL. This calculator is geometry, not compliance — it tells you what a vessel of those dimensions holds, and nothing about whether the installation meets any code.
Measuring a Tank You Cannot See Inside
Most real measurements are taken from outside, and the errors that follow are systematic rather than random. Measure the outside diameter of a plastic tank and you overstate capacity, because a 5 mm wall on each side removes 10 mm of internal diameter, and volume goes with the square of the radius — a 1,000 mm outside diameter tank with 5 mm walls has 2 percent less internal volume than the outside figure suggests, which is 20 litres in a 1,000 litre tank.
Three more field notes. Many horizontal tanks have dished rather than flat ends, which adds capacity the plain cylinder formula misses; the capsule shape is a reasonable approximation for deeply dished heads and an over-estimate for shallow ones. Internal structures — baffles, heating coils, sump pipes — subtract volume that no external measurement can see. And a horizontal tank installed with deliberate fall toward a drain point holds slightly more than the level calculation gives, with the dip reading depending entirely on where along the length you dipped.
Turning Volume Into Weight
The weight output exists because volume alone does not tell you whether a base, a trailer or a floor can carry the tank. Water is the reference at 1,000 kilograms per cubic metre, so a 1,000 litre tank of water is a tonne. That is a load which matters: a full 275 gallon tote holds around 1,040 litres, a little over a tonne of water, concentrated on a footprint of roughly 1.2 by 1.0 metres.
Specific gravity scales that directly. Diesel at around 0.84 makes the same volume about 16 percent lighter; a concentrated brine or a caustic solution is heavier than water. Take the figure from the product's safety data sheet rather than from memory, and note that it is quoted at a reference temperature — density falls as liquids warm, which is one of the reasons fuel is sold with temperature compensation. If you need to move between volume units for a delivery note, the volume converter handles that, and the cubic yard calculator covers bulk earthworks quantities around the installation.
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Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Scaling capacity by depth on a round tank — half depth is half volume, but quarter depth is about a fifth, and three-quarter depth is about four fifths.
- Using outside dimensions — wall thickness is squared into the radius term, so it costs more capacity than it looks like it should.
- Confusing dip with ullage — one is measured up from the bottom and the other down from the top, and swapping them inverts the answer.
- Treating nominal capacity as usable — thermal expansion space, fill limits and internal fittings all reduce what you can actually put in.
- Assuming specific gravity is one — it is only true for water, and the weight of a full tank is a structural question.
Related Free Tools From Arb Digital
Use the cylinder volume calculator for a simple full cylinder, the pipe volume calculator for pipe runs, the pool volume calculator for swimming pools, the volume converter for unit changes and the sphere volume calculator for spherical vessels. The full free online tools hub lists every calculator we publish.
Frequently Asked Questions
The liquid cross-section is a circular segment, with area r² arccos((r − h) ÷ r) − (r − h) √(2rh − h²), and the volume is that area multiplied by the tank length. Scaling total capacity by depth gives the wrong answer everywhere except exactly half full.
A horizontal cylinder 48 inches in diameter and 120 inches long holds π × 24² × 120 = 217,146 cubic inches, which is 940.0 US gallons or about 3,558 litres. At a 24 inch dip it is exactly half full at 470.0 US gallons.
Because a round tank is widest in the middle. At 25 percent depth a horizontal cylinder holds about 19.6 percent of its volume, and at 75 percent depth about 80.4 percent. Only the halfway point matches, which is why calibrated sticks for round tanks have uneven graduations.
Inside wherever you can reach. Volume depends on the square of the internal radius, so a wall thickness that looks trivial removes real capacity — 5 mm walls on a 1,000 mm outside diameter tank cost about 2 percent of the volume.
Not exactly. The capsule shapes model hemispherical ends, which is a fair approximation for deeply dished heads and an over-estimate for shallow ones. For a certified figure on a pressure vessel, use the manufacturer's capacity table or a strapping chart.
Volume in cubic metres multiplied by 1,000 kilograms gives the weight for water. Multiply by the specific gravity for anything else — around 0.84 for diesel. A 1,000 litre tote of water is roughly one tonne on a footprint of about 1.2 by 1.0 metres.
No. Tanks need ullage space for thermal expansion, fill limits are often set by regulation or by the delivery equipment, and internal fittings take up volume. Order against usable capacity and the current level, not against the nominal figure.
This tool computes geometric volumes only. Tank installation, fill limits, secondary containment and regulatory compliance are governed by local codes and industry standards, and must be confirmed by a qualified professional.
