I've been with my company for 18yrs and finally got up the nerve to speak to HR regarding my salary and they suggested, I write up a Fair Market Value in order for my salary to be re-evaluated. What should be put in this letter?
At my first job I asked for a salary increase and they dismissed it saying that my salary was competitive with the current market in the area. I was however asked to have a look around myself to ensure this fact was correct. I was young and so asked what they meant by that and simply put they suggested looking on job websites and comparing the salary provided for the same role advertised in the same area.
It was competitive according to that but I still felt underpaid for what I did and for my potential. I moved on and gained an increase in my salary in a slightly different role. As time went on I still felt I wasn't being paid enough and no I wasn't being greedy. As you get older, life gets more expensive; children, mortgage, bills etc. I asked for a salary increase and again was refused as it was competitive. I was lucky however and had a recruiter contact me for a local job. I took it. It was a £10k pay rise putting me apparently £15k more expensive than the going rate in the area. It's not really, it's fair. Before leaving my employer wanted to see if they could match. When I told them the offer they nearly fell off their seat. They couldn't believe the wage I was being offered and were in no way prepared to match it. I now feel I'm getting the wage I deserve for the job I'm doing in the area I'm in.
If you want to write to a letter, you'll have to show your research. The best research will be job sites which are offering the same job in the same area. This will give you a good indication of what you can ask for at your current job.
TL:DR your best bet is to move jobs. This I've found is the easiest way to get a pay increase.