Tank Volume Calculator

A tank volume calculator returns the full capacity of a cylindrical tank in gallons and cubic feet. It multiplies π by the radius squared by the tank's length, then converts cubic feet to gallons at 7.48052 gallons per cubic foot. The result is the same whether the tank stands upright or lies on its side.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Full Tank Volume

940.0 gal

Breakdown
Full Tank Volume	940.0 gal
Volume	125.66 cu ft

How to Calculate Tank Volume

A cylindrical tank's volume is `π × radius² × length`. It is the same cylinder formula whether the tank stands upright (where "length" is really its height) or lies on its side. Converting the resulting cubic feet to gallons uses the exact 7.48052 gallons-per-cubic-foot factor.

Example: 4 ft diameter, 10 ft long tank

A 4 ft diameter, 10 ft long tank: 1. Cross-sectional area: π × (4 ÷ 2)² = π × 4 = 12.57 sq ft 2. Volume: 12.57 × 10 = 125.66 cu ft 3. Gallons: 125.66 × 7.48052 = 940.0 gal

Tank Volume: Compare Scenarios

Scenario	Full Tank Volume	Volume
Small (2 ft dia × 5 ft, portable tank)	117.5 gal	15.71 cu ft
Medium (4 ft dia × 10 ft, default)	940.0 gal	125.66 cu ft
Large (6 ft dia × 20 ft, bulk storage tank)	4,230.1 gal	565.49 cu ft

What Is Tank Volume?

A tank volume calculator converts a cylindrical tank's diameter and length into its full volume. It is useful for a water storage tank, propane tank, fuel tank, or any similar cylindrical container, whether mounted vertically or horizontally. This calculates the tank's total (full) capacity. A horizontally mounted tank that is only partially filled holds less than this figure, because a partial fill in a horizontal cylinder follows a circular-segment shape rather than a simple fraction of the total.

Tank Volume: Pros and Cons

Pros

Works the same for a vertical or horizontal tank, since a cylinder's full volume doesn't depend on orientation
Uses the exact gallons-per-cubic-foot conversion rather than an approximation
Gives both gallons (the unit most tanks are rated in) and cubic feet

Cons

Calculates full capacity only. A partially filled horizontal tank holds a different, non-linear fraction of that volume based on fill height, which this calculator doesn't compute
Doesn't account for a tank's rounded/domed ends, which add a small amount of extra volume beyond a plain cylinder for some tank designs
Assumes a simple cylindrical shape. A rectangular or irregularly shaped tank needs a different volume formula

Tank Volume: What a Pro Checks

Remember this is full capacity, not current contents. A partially filled horizontal tank follows a circular-segment curve, not a straight percentage of this number.
A horizontal tank filled to exactly half its height holds exactly half its total volume. That's the one fill level where the geometry works out evenly, making it a useful quick check against a gauge reading.
Check for domed or rounded end caps on your tank. They add some volume beyond this calculator's plain-cylinder figure, more noticeably on a tank with pronounced dished ends.

Frequently Asked Questions

How do I calculate the volume of a cylindrical tank?

Multiply π by the radius squared (half the diameter, squared) by the tank's length, then convert to gallons by multiplying by 7.48052. A 4 ft diameter, 10 ft long tank holds about 940 gallons.

Does it matter if the tank is vertical or horizontal?

Not for full volume. A cylinder's total volume is the same regardless of orientation, since it depends only on the cross-sectional area and length. Orientation matters more when the tank is only partially filled, since a horizontal tank's partial volume follows a different curve than a vertical one's.

How much does a partially full horizontal tank hold?

Less than a simple fraction of the full volume. Because a horizontal cylinder's cross-section is a circle, the relationship between fill height and volume is a circular-segment calculation, not a straight-line percentage. A tank filled to half its height, however, does hold exactly half its total volume, since that's the one fill level where the geometry works out evenly.

How many gallons are in a cubic foot?

Exactly 7.48052 US gallons per cubic foot. This is the standard conversion this calculator uses to convert a tank's cylinder volume into gallons.

Does this account for a tank's rounded ends?

No. Some cylindrical tanks have domed or rounded end caps that add a small amount of extra volume beyond a plain cylinder shape. This calculator treats the tank as a simple cylinder with flat ends, which is a close approximation for most tanks but slightly undercounts one with pronounced dished ends.

Plumbing & HVAC