Finding a measurement of the amount of gas in a balloon involves finding the balloon's volume. One thing to keep in mind is that the balloon's volume will increase or decrease as gas such as helium is added or subtracted. This fact will also affect the shape of the balloon, which in turn affects the volume. You can find the volume of a balloon using a regular tape measure and the mathematical calculation for this process.
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.
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.
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.
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.
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.