Tank Volume Calculator: Gallons Per Inch

This calculator works out the number of gallons in a tank for each inch of liquid depth, based on the tank’s shape and dimensions. You enter the tank’s length, width, and height (or diameter for a cylinder), and it returns the volume per inch in gallons, along with the total capacity. The result helps you estimate fill rates, dosage amounts, or remaining capacity from a dipstick reading.

Formula

For a rectangular tank, the volume per inch of depth is simply the area of the base (length × width) divided by 231 (the number of cubic inches in one US gallon).

Gallons per inch = (Length × Width × 1 inch) / 231

Where:

• Length = interior length of the tank, in inches
• Width = interior width of the tank, in inches
• 1 inch = the depth increment you are measuring
• 231 = cubic inches in one US liquid gallon

For a cylindrical tank (horizontal or vertical), the formula changes because the cross-section is a circle. For a vertical cylinder, gallons per inch = (π × radius² × 1) / 231. For a horizontal cylinder, the volume per inch is not constant—it follows a segment-of-a-circle formula, which is more complex.

Worked Example

Using the default values from the calculator: a rectangular tank that is 36 inches long, 18 inches wide, and 24 inches high.

1. Calculate the base area: 36 × 18 = 648 square inches.
2. Multiply by the 1-inch depth: 648 × 1 = 648 cubic inches.
3. Divide by 231: 648 / 231 = 2.805 gallons per inch.
4. For total capacity: 36 × 18 × 24 = 15,552 cubic inches; 15,552 / 231 = 67.32 gallons.

The calculator shows 2.805 gallons per inch (rounded to 3 decimals) and 67.32 total gallons. This means that for every inch the liquid rises, you add roughly 2.8 gallons.

Why Gallons Per Inch Matters

Knowing the volume per inch is useful in many real situations:

• Dosing chemicals – Pool or water treatment chemicals are often measured per gallon; knowing how many gallons per inch helps you add the right amount when filling.
• Fuel or water tanks – A dipstick reading in inches can be converted to gallons without a full tank chart.
• Aquarium water changes – If you remove 2 inches of water, you know exactly how many gallons you are replacing.
• Rain barrels or storage tanks – Track how much water you have collected based on depth.

This measurement is especially handy when the tank is irregularly filled or when you need to estimate remaining capacity without a gauge.

How to Choose the Right Calculation

The shape of your tank determines which formula to use. Here is a quick guide:

• Rectangular tank – Use length × width ÷ 231. This is the simplest and most common for aquariums, sumps, and rectangular storage tanks.
• Vertical cylinder – Use π × radius² ÷ 231. The volume per inch is constant because the cross-section is the same at every height.
• Horizontal cylinder – The volume per inch is not constant. It starts at zero at the bottom, increases to a maximum at the middle, then decreases again. You need a segment-of-a-circle calculation or a tank chart.
• Spherical or elliptical tanks – These have even more complex formulas; a simple per-inch value is rarely accurate across the whole depth.

For the horizontal cylinder, a common approximation is to use the average of the bottom and top areas, but that is only rough. For precise work, use a tank chart or a calculator that integrates the segment area.

Common Mistakes and How to Avoid Them

Several errors can throw off your gallons-per-inch calculation:

• Using outside dimensions instead of inside – Tank walls take up space. Always measure interior dimensions, especially for thick-walled tanks.
• Mixing units – If you measure in feet, convert to inches first (1 foot = 12 inches). The formula requires consistent units.
• Forgetting that 231 is US gallons – The UK (imperial) gallon is 277.42 cubic inches. If you use imperial gallons, the result will be about 20% lower.
• Assuming a horizontal cylinder is linear – Many people make this mistake. The volume per inch changes with depth, so a single number is only valid for a small range around that depth.
• Rounding too early – If you round the base area before dividing, you may lose accuracy. Keep full precision until the final step.

Also, remember that the liquid may not fill the tank perfectly level, especially in moving vehicles or sloped installations. Measure depth at the center or at a consistent point.

Limitations of the Simple Formula

The formula above assumes a perfectly rectangular tank with straight, vertical sides and a flat bottom. Real tanks often have sloped bottoms (like many fuel tanks), rounded corners, or internal baffles. These features change the volume per inch, especially at the bottom and top of the tank.

For tanks with sloped or dished bottoms, the volume per inch is not constant. You may need to calculate the volume of the bottom section separately and then add the constant per-inch volume for the straight-walled section above.

Also, the formula gives the theoretical volume based on interior dimensions. Actual capacity may be slightly less due to fittings, sensors, or sediment at the bottom. For safety-critical applications (like fuel tanks), always calibrate with a known quantity of liquid.

Frequently Asked Questions

Does this calculator work for round tanks?

No, this calculator is for rectangular tanks only. For a vertical cylindrical tank, use the formula: gallons per inch = (π × radius²) / 231. For a horizontal cylinder, you need a more complex calculation because the volume per inch varies with depth.

What is the difference between US gallons and imperial gallons?

A US gallon is 231 cubic inches, while an imperial (UK) gallon is 277.42 cubic inches. That is about 20% larger. If you are using imperial gallons, divide by 277.42 instead of 231. The calculator uses US gallons by default.

How do I convert gallons per inch to liters per centimeter?

One US gallon = 3.78541 liters, and one inch = 2.54 cm. So multiply gallons per inch by 3.78541 and divide by 2.54 to get liters per centimeter. For example, 2.8 gal/in × 3.78541 / 2.54 ≈ 4.17 L/cm.

Can I use this for a tank that is not full?

Yes, that is exactly what this measurement is for. If your tank is rectangular with straight sides, the volume per inch is constant, so you can multiply the depth in inches by the gallons-per-inch value to get the current volume. For example, if you have 10 inches of liquid and the tank gives 2.8 gal/in, you have 28 gallons.

Why is the volume per inch not constant in a horizontal cylinder?

Because the cross-section of a horizontal cylinder is a circle, and the width of the liquid at a given depth changes as the depth changes. Near the bottom, the width is small; at the middle, it is the full diameter. So each inch of depth adds a different area, and therefore a different volume.

How accurate is this for an aquarium?

For a standard rectangular aquarium with flat glass, the formula is very accurate if you use interior dimensions. However, if the aquarium has a plastic frame or curved corners, the actual volume may be slightly less. Subtract about 5% for a conservative estimate.

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in l | Length | in | 36 | 0
in w | Width | in | 18 | 0
in h | Height | in | 24 | 0
out gal_per_in | Gallons per inch | gal/in | (l * w * 1) / 231 | 3
out total_gal | Total capacity | gal | (l * w * h) / 231 | 2