Tank Volume Calculator

How much does the tank hold — and how much is in it right now, filled to this depth? This calculator answers both for the two tank shapes that hold the world’s water, fuel and chemicals: enter a vertical or horizontal cylinder’s dimensions, or a rectangular tank’s, plus the liquid depth, and it returns the full capacity and the current contents in gallons and litres.

Two Shapes, Three Formulas

Capacity is volume with a customer, and the volumes are old friends. The vertical cylinder is the pipe-volume formula standing up — π r² × height — linear in depth, so its gauge arithmetic is merciful: every foot of depth is the same number of gallons. The rectangular tank is a box: length × width × height, equally linear. The horizontal cylinder is the connoisseur’s case: its cross-section fills as a circular segment, whose area — r²·acos((r−h)/r) − (r−h)·√(2rh − h²) — grows slowly near the bottom, fastest at the middle, and slowly again at the top, because the tank is narrow at its floor and crown and wide at its waist. That single nonlinearity is why horizontal fuel tanks need dipstick charts rather than dipstick rulers, and why this calculator carries the exact segment formula rather than an approximation.

Worked Example: The Four-Foot Tank, Five Feet Tall

The defaults are a standard vertical water tank: 4 ft diameter, 5 ft tall. Capacity = π × 2² × 5 = 62.83 ft³ = 470.0 gallons (about 1,779 litres) — the size class behind countless rural pressure systems and one honest rainwater barrel’s bigger ambition. Read the linearity off it: each foot of depth is 94.0 gallons, each inch about 7.8 — a sight-glass on this tank is a measuring instrument with the scale built in. Turn the same vessel on its side (horizontal, 5 ft long) and capacity is identical when full, but a dipstick reading of 1 ft of depth no longer means a fifth of the tank: the segment formula prices 1 ft of depth in a 4-ft-diameter horizontal tank at about 19.5% of capacity (≈92 gallons), against the naive 25% the vertical intuition supplies — the difference between ordering fuel in time and ordering it in apology.

Gauging: From Depth to Contents

The fill-depth input is the calculator’s working half, because tanks are rarely discussed full — they are gauged. Three gauging disciplines deserve their reputations. Measure the liquid, not the ullage, where you can — depth from the bottom (a dipstick’s native reading) feeds the formula directly; depth from the top (ullage) must be subtracted from the diameter or height first. Inside dimensions rule: wall thickness, internal baffles, sumps and the pump’s unusable bottom few inches (the “dead stock” below the outlet) all separate nominal from working capacity — serious inventory work prices the dead stock once and subtracts it forever. Temperature moves fuel: diesel and gasoline expand measurably with warmth (roughly 0.04–0.07% per °F for diesel, more for gasoline), which is why bulk deliveries are temperature-corrected to 60 °F in commerce — a 470-gallon summer delivery is fewer standard gallons than its gauge proclaims. The calculator prices the geometry exactly; these are the physical corrections the geometry expects you to remember.

Sizing: Working Capacity Is the Honest Number

Buyers meet nameplate capacity; systems live on working capacity. A rainwater tank’s working volume ends at the overflow, begins above the sediment floor, and is spent only down to the pump’s intake — commonly 85–92% of the geometric total. A fuel tank is deliberately never filled past about 95% (thermal expansion needs its headroom — the ullage is a safety feature, not waste). Septic and dosing tanks dose from their working band between floats. The calculator’s percentage readout is the bridge between the two languages: enter the tank’s geometry for the nameplate, then read the contents at the working depths your floats, overflows and intakes actually define, and both numbers exist on one page — the one the brochure prints, and the one the system obeys.

Containment: The Tank’s Second Volume

Regulated tanks sit inside a second, invisible tank: secondary containment sized, in the common rule, at 110% of the largest vessel’s capacity — the full contents plus a margin for rain and displacement. The worked example’s 470-gallon tank therefore wants a berm or double wall holding at least 517 gallons, a volume this calculator prices by the same geometry when the containment’s own dimensions are entered (rectangular berms are its box formula at working depth). It is a fitting closing entry for the page: capacity questions never end at the vessel’s wall, because the environment’s question — where does it all go on the worst day — is also a volume, and also computable.

Round or Rectangular: The Space Ledger

One sizing comparison repays a look: the worked example’s 470 gallons in a box instead of a cylinder. A rectangular tank on the same 4 × 4 ft footprint holds 16 ft³ per foot of height, so it reaches 62.83 ft³ at just 3.93 ft tall — the box packs about 27% more volume into a square footprint of the same width, because the cylinder leaves its corners empty. The cylinder’s compensations are structural (pressure distributes evenly, no corner welds to fail) and hydraulic (no corners for sediment and algae to colonise). Site the shape by what the corner space is worth against those virtues — the calculator prices both honestly, which is all the preference needs to become a decision.

Strapping Tables: The Calculator, Printed

Fuel depots and tank farms keep this page’s output as a laminated table — the strapping chart — generated exactly as the FAQ describes, an inch at a time against the specific vessel’s measured dimensions. The laminated card outlives the instruments around it because it asks nothing of batteries or calibration: depth in, gallons out. Any tank you gauge more than twice deserves one; the calculator is the press that prints it, and the discipline — measured inside dimensions, dead stock noted at the foot of the card — is inherited from a century of dipped tanks.

Related Tools

More from the Construction Calculators hub: the Pipe Volume Calculator for the lines feeding the tank, and the Hole Volume Calculator for the excavation the tank may sit in.

Frequently Asked Questions

How many gallons does a cylinder tank hold?

Gallons = π × radius² × length (or height) in feet × 7.48052. The worked example: a 4 ft diameter, 5 ft tank holds 470 gallons. In inches, the same formula with the cubic-inch result divided by 231 gives US gallons directly. The calculator runs either orientation — vertical for standing tanks, horizontal for the classic fuel-oil and propane-style layout — and reports litres alongside, since tank nameplates now speak both.

Why isn’t my horizontal tank a quarter full at a quarter depth?

Because its cross-section is a circle: the bottom quarter of the depth is the narrowest slice of the circle, holding far less than a quarter of the volume — about 19.5% at exactly quarter depth, while the slice just below the halfway line holds about 30%. The segment formula in this calculator is the exact accounting; any straight-line interpolation between empty and full will mislead you everywhere except half depth, where symmetry makes it accidentally right.

How do I make a dipstick chart for my tank?

Run this calculator at regular depth intervals (every inch or two of depth for a horizontal cylinder, every few inches for a vertical), tabulate depth against gallons, and print the table — that is literally how commercial strapping charts are produced, from the same segment formula. For a vertical or rectangular tank the chart is a straight line (gallons per inch is constant: cross-section area × 7.48 ÷ 12), which the calculator will confirm at two depths.

Does this work for capsule (dish-ended) tanks?

Approximately, and the approximation is the professional one: compute the cylindrical barrel with this page, then add the two end caps’ volume (each dished end holds roughly 60–70% of a same-diameter hemisphere’s volume for standard torispherical heads — manufacturers publish exact cap volumes per head type). For inventory purposes the barrel dominates; for nameplate capacity use the maker’s figure, which accounts for the heads precisely.

What about above-ground pool or cistern sizing?

The same geometry in friendlier units: a round pool is a vertical cylinder (a 15 ft diameter pool at 4 ft of water: π × 7.5² × 4 ≈ 707 ft³ ≈ 5,290 gallons — the number chemical dosing is built from), and a rectangular cistern is the box. Enter water depth rather than wall height where the freeboard matters; capacity-for-dosing is always the water’s volume, not the vessel’s.

How much does a full tank weigh?

Water weighs 8.34 lb per gallon (diesel about 7.1, gasoline about 6.3, heating oil about 7.2): the worked example’s 470 gallons of water is roughly 3,920 lb of contents, before the tank’s own steel. Foundations, stands and floor loads are designed from the full figure — contents from this calculator times the liquid’s per-gallon weight, plus the vessel — which is why this question is asked at installation and never again until the stand starts to lean.