EMF describes a broad family of fields
Electromagnetic fields are associated with electricity and electromagnetic waves. The term covers very different frequencies and sources. A magnetic field near a mains-powered appliance is not the same measurement as radiofrequency energy from a Wi-Fi router. Treating all of them as one number obscures what a meter is actually detecting.
Three measurements you will encounter
Low-frequency electric fields are commonly expressed in volts per metre (V/m). Magnetic flux density is commonly expressed in microtesla (µT) or milligauss (mG). RF instruments may display power density in microwatts or milliwatts per square metre. These are different quantities: converting mG to µT is possible, but converting mG directly to RF power density is not.
Exposure is not a diagnosis
A reading describes the field detected under particular conditions. It does not tell you that a symptom has a particular cause. For cellphone RF, the FDA says the weight of scientific evidence has not linked exposure with health problems. A general statement about cellphones should not be stretched into a claim about every source, exposure level, or medical device.
A useful first step
Write down the question you want to answer. Are you comparing readings before and after moving a device? Checking a meter’s frequency coverage? Understanding a product claim? Start there, then choose a method that can answer that question. You may find that a guide, a manual, or a simple change in setup is more useful than a purchase.