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If you’re taking gas (like helium) from a tank, place the lip of the balloon tightly over the nozzle and fill it up. Or, if you want to measure air, just blow up the balloon using your mouth. Tie the end of the balloon in a tight knot when you’re done. Wrap a cloth tape measure around the center of the balloon to check its circumference. To get the radius of your inflated balloon, place a ruler on each side of the center of the balloon, measure the distance between them, and divide it in half. For instance, you place your balloon between two rulers and find that the distance is 10 centimetres (3.9 in). Divide this in half to get the radius, 5 centimetres (2.0 in). Plug in your measurements into the basic formula for the volume (V) of a sphere, V = 43πr3{\displaystyle {\frac {4}{3}}\pi r^{3}}. Plug the radius into the formula to get your sum. For instance,: V = 43π53{\displaystyle {\frac {4}{3}}\pi 5^{3}}, or about 523.33 cm3{\displaystyle ^{3}} or 206.04 in3{\displaystyle ^{3}}. Place the lip of a balloon over a faucet and carefully run water into it. Stop when the balloon is the same size as the balloon filled with gas. If you accidentally put too much water in, carefully pour some out.  It will be difficult to get the balloons to exactly the same size. To be more exact, wrap a cloth tape measure around the center of the water balloon to check its circumference. Adjust until it's the same as the gas-filled balloon's. Have someone help you measure the circumference of the water balloon while you hold it. Empty the balloon into a large measuring cup, beaker, or something else that can measure volume. Record the amount of water that was in the balloon. The volume of water will equal the volume of gas in the other balloon. If your water-filled balloon is the same size as the gas-filled balloon that you determined (via V = 43πr3{\displaystyle {\frac {4}{3}}\pi r^{3}}) had a volume of 523.33 cm3{\displaystyle ^{3}}, then you should measure about 523 milliliters (18  fl oz) of water poured out of it (1 ml = 1 cm3{\displaystyle ^{3}}).

Summary:
Inflate a balloon with the gas you want to measure. Measure the balloon's circumference and radius. Use the volume of a sphere formula. Fill up another balloon with water to compare your results. Pour the water out and measure it.