You can find the volume of a balloon using a regular tape measure and the mathematical calculation for this process.
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Decide whether your gas-filled balloon is spherical in shape or is more of a cylinder shape. This will affect the formula you need to use to find the volume of the balloon. If your balloon resembles a perfect circle, it is a sphere. If your balloon is taller than it is wide, it is a cylinder.
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Measure the length of the balloon from the opening at the bottom to the top using a regular tape measure (if your balloon is a cylinder). Lay the balloon on a flat surface and measure the length from beside the balloon. Do not place the tape measure on the actual balloon, as this will give you an incorrect measurement.
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Measure the radius of your balloon (for both cylinder- and sphere-shaped balloons). Place the tape measure in the center of the balloon and measure to the outer edge. Record this measurement.
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Cube the radius of your balloon (for sphere-shaped balloons). For example, if the radius was 6 inches, 6 x 6 x 6 would be 216. Multiply this number by Pi and then again by 4/3. For example, 216 x 3.14 x 4/3 would be 904.32. There are 904.32 cubic inches of gas in the balloon.
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Square the radius of your balloon (for cylinder-shaped balloons). For example, if the radius was 6 inches, 6 x 6 = 36. Multiply this number by Pi and then again by the length of the balloon. For example, 36 x 3.14 x a length of 5 inches would be 565.2. There are 565.2 cubic inches of gas in the balloon.