What Is the Propane Tank Volume Calculator?
This calculator estimates the water capacity and usable propane capacity of a horizontal cylindrical (capsule-shaped) propane tank from its diameter and overall length. Propane tanks are typically filled to about 80% of their water capacity to leave room for vapor expansion, so this tool reports both the full water volume and the usable propane volume.
How to Use It
Measure the tank's outside diameter in inches and the overall length end-to-end in inches. Enter your fill percentage (80% is the industry-standard safe maximum). The calculator returns the internal volume in cubic inches, the water capacity in US gallons, and the usable propane gallons at your chosen fill level.
The Formula Explained
A propane tank is modeled as a capsule: a cylinder with a hemispherical cap on each end. With radius \(r = \text{diameter} / 2\), the two hemispheres form one full sphere and the cylindrical middle has length \((L - 2r)\). So:
$$V = \pi \cdot r^{2} \cdot (L - \text{diameter}) + \frac{4}{3} \cdot \pi \cdot r^{3}$$
Convert cubic inches to US gallons by dividing by 231 (1 US gallon = 231 in³). Usable gallons = water gallons × fill%.
Worked Example
For a tank 30 in in diameter and 68 in long at 80% fill: \(r = 15\), cylinder length = \(68 - 30 = 38\).
$$V = \pi \cdot 15^{2} \cdot 38 + \frac{4}{3} \cdot \pi \cdot 15^{3} = \pi \cdot 8550 + \pi \cdot 4500 = 13050\pi \approx 41{,}000.4 \text{ in}^{3}$$Water gallons = \(41{,}000.4 / 231 \approx 177.49\). Usable = \(177.49 \times 0.80 \approx 141.99\) gallons.
FAQ
Why only 80%? Liquid propane expands with temperature; the 20% headspace prevents dangerous over-pressure, so tanks are filled to 80% maximum.
Are real tank ends true hemispheres? Many tanks use shallower dished or elliptical heads, so this capsule model is a close approximation, not exact.
What is a US gallon in cubic inches? Exactly 231 cubic inches.
