How do you measure how much a headache hurts?

The standard way to measure how much a headache hurts is to ask the one with the headache, "on a scale of 1 to 10, how much does it hurt?".

Unfortunately, this is completely subjective. Currently we have no way of knowing whether a different person experiencing exactly the same amount of pain would then give the same numerical rating.

Normally, we settle for mere descriptions, rather than measurements. Headaches can be described as mild, serious, agonizing, or disabling. If the headache causes vomiting as a side effect, it is extremely bad.

If you had to scientifically measure the severity of a headache, you could try looking at pictures of related brain activity by means of a functional MRI.

However, as of 2010 no one on earth can tell the difference between a "headache fMRI picture" and a "no headache fMRI picture", much less distinguish between various levels of pain.

Perhaps someday someone will have looked at enough fMRI pictures to know what to look for and be able to tell the difference.