Bullet Tank Volume Calculator

This calculator works out the internal volume of a cylindrical tank (often called a bullet tank because of its rounded ends) in gallons or liters. You enter the tank’s diameter, length, and the fill height, and it returns the partial or full volume, plus the total capacity. It uses the standard formula for a horizontal cylinder with flat or dished ends, which is the most common shape for propane, fuel, and water storage tanks.

Formula

For a horizontal cylindrical tank with flat ends, the volume of liquid at a given fill height h is:

V = L × [ r² × acos((r – h) / r) – (r – h) × sqrt(2rh – h²) ]

Where:

• V = liquid volume (cubic inches, then converted to gallons or liters)
• L = length of the cylinder (inches)
• r = radius = diameter / 2 (inches)
• h = height of the liquid from the bottom (inches)
• acos = inverse cosine, in radians
• sqrt = square root

The formula works for any fill height from 0 (empty) to the full diameter (completely full). When h equals the diameter, the acos term becomes acos(0) = π/2 and the second term becomes zero, giving the full cylinder volume πr²L.

To convert cubic inches to gallons, divide by 231 (since 1 US gallon = 231 in³). For liters, multiply the cubic inches by 0.0163871.

Worked Example

Using the default values from the calculator: diameter = 48 inches, length = 120 inches, fill height = 24 inches.

1. Radius r = 48 / 2 = 24 inches.
2. Compute the bracket term: r² = 576.
3. acos((r – h)/r) = acos((24 – 24)/24) = acos(0) = π/2 ≈ 1.5708 radians.
4. (r – h) = 0, so the second term is 0.
5. Bracket = 576 × 1.5708 = 904.78 in².
6. Multiply by length: 904.78 × 120 = 108,573.6 in³.
7. Divide by 231: 108,573.6 / 231 ≈ 470.0 gallons.

The calculator shows 470.0 gallons for the liquid volume. The total capacity is π × 24² × 120 / 231 = 940.0 gallons, so the fill percentage is 50%.

Why Use a Bullet Tank Volume Calculator?

Bullet tanks are horizontal cylinders with rounded or dished ends, commonly used for storing propane, diesel, water, and chemicals. Unlike vertical tanks where volume is a simple cylinder, horizontal tanks have a nonlinear relationship between liquid height and volume. At low fill levels, a small increase in height adds very little volume; near the middle, the same height increase adds much more. This calculator handles that geometry automatically, saving you from manual integration.

You might need this for:

• Checking how much fuel is left in a storage tank when you only have a dipstick reading.
• Sizing a tank for a specific application (e.g., a water tank for a cabin).
• Estimating the volume of liquid in a partially filled tank for inventory or shipping.
• Verifying the calibration of a tank level sensor.

How to Use the Calculator Correctly

Enter the inside diameter and inside length of the tank, not the outside dimensions. The wall thickness reduces the internal volume, so using outside measurements will overestimate capacity. For example, a propane tank with an outside diameter of 30 inches and a wall thickness of 0.25 inches has an inside diameter of 29.5 inches.

The fill height is the vertical distance from the bottom of the tank to the liquid surface. If you are using a dipstick, make sure it rests on the bottom and you read the wetted length. For tanks with dished (curved) ends, this calculator assumes flat ends, which gives a slightly higher volume than actual (typically 3-5% for propane tanks). For a more accurate result, you would need the end shape factor, but for most practical purposes the flat-end approximation is adequate.

Common Mistakes and Misconceptions

• Using outside dimensions: Always use the inside diameter and length. If you only have outside measurements, subtract twice the wall thickness from the diameter and length.
• Mixing units: Keep all measurements in the same unit (e.g., all inches, all feet, all centimeters). The calculator uses inches for the formula, but you can convert to feet by multiplying by 12.
• Assuming linear volume: Many people think that half the height means half the volume. For a horizontal cylinder, half height gives exactly half volume only because the formula is symmetric, but at quarter height you get about 19.5% of the full volume, not 25%.
• Ignoring the ends: Dished ends add volume, but they also change the height-to-volume curve near the ends. The flat-end approximation is usually within 5% for typical tanks.
• Using the wrong gallon: The calculator uses US gallons (231 in³). If you need imperial gallons (277.42 in³), divide the cubic inches by 277.42 instead.

Practical Applications and Examples

Consider a 500-gallon propane tank. Typical dimensions are 40 inches diameter and 120 inches length. Using the calculator with a fill height of 20 inches (half the diameter) gives exactly 250 gallons, which matches the 50% mark. But if the fill height is 10 inches (25% of diameter), the volume is only about 97 gallons, not 125. That is why a simple dipstick chart is essential for accurate inventory.

For water tanks, a common use is to estimate how much water remains in a horizontal storage tank during a drought. If you measure a fill height of 12 inches on a 30-inch diameter, 60-inch long tank, the calculator gives about 55 gallons out of a total capacity of 183 gallons – about 30% full, not 40% as a linear guess would suggest.

The same formula applies to truck-mounted tanks, fuel trailers, and even some swimming pool filters. In all cases, the key is to measure the fill height accurately, ideally with a calibrated dipstick.

Limitations and When to Use a More Complex Model

The flat-end formula is accurate for tanks with flat heads, but most bullet tanks have elliptical or hemispherical ends. For those, the volume is slightly larger, and the difference becomes significant when the tank is nearly full or nearly empty. If you need extreme accuracy (e.g., for custody transfer of fuel), you should use a tank strapping table provided by the manufacturer, which accounts for the exact end shape and any internal baffles.

Also, this calculator assumes the tank is perfectly horizontal. If the tank is tilted, the liquid surface is not parallel to the bottom, and the volume calculation becomes more complex. For slight tilts (less than 2 degrees), the error is usually negligible.

Finally, the calculator does not account for internal components like dip tubes, baffles, or heating coils that displace liquid. In most tanks, these occupy a small fraction of the volume, but for precise measurements, subtract their displacement if known.

Frequently Asked Questions

How do I measure the fill height of a bullet tank?

Use a clean dipstick or a tape measure with a weight. Lower it vertically into the tank through the fill opening until it touches the bottom, then withdraw and read the wetted length. For pressurized tanks, use a sight glass or a magnetic level gauge if available, as opening the tank can be dangerous.

Why does the volume not increase linearly with height?

Because the cross-sectional area of a horizontal cylinder is not constant. Near the bottom and top, the width of the liquid surface is small, so adding height adds little volume. In the middle, the surface is wide, so the same height increase adds much more volume.

Can I use this calculator for a vertical tank?

No. For a vertical cylinder, the volume is simply πr²h, which is linear. This calculator is specifically for horizontal tanks. For vertical tanks, use a simple cylinder volume calculator.

What is the difference between US gallons and imperial gallons?

US gallon = 231 cubic inches, imperial gallon = 277.42 cubic inches. The calculator outputs US gallons. To convert to imperial, multiply the result by 0.83267. To get liters, multiply US gallons by 3.78541.

How accurate is this calculator for a propane tank with rounded ends?

For typical propane tanks with elliptical ends, the flat-end approximation is usually within 3-5% of the true volume. The error is largest when the tank is nearly full or nearly empty. For exact measurements, use the manufacturer’s tank chart.

Can I use this for a tank that is not exactly horizontal?

If the tank is tilted, the liquid surface is not parallel to the bottom, so the formula does not apply directly. For small tilts (under 2 degrees), the error is usually small. For larger tilts, you would need a more complex model or a calibrated gauge.

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in d | Tank diameter | in | 48 | 0
in L | Tank length | in | 120 | 0
in h | Fill height | in | 24 | 0
out vol | Liquid volume | gallons | (L * (d*d*acos(1-2*h/d)/2 - (d/2-h)*sqrt(h*(d-h))*2) / 231) | 1
out cap | Total capacity | gallons | (pi * (d/2)^2 * L) / 231 | 1
out pct | Fill percentage | % | (vol / cap) * 100 | 1