Many Veterans become confused when they have different medical problems that the VA rated them for and may not understand how to calculate multiple VA ratings.  Some veterans wrongly presume that the VA simply adds together the separate VA ratings to get an overall rating.  If you have multiple VA ratings you want to consult the VA ratings table located here.  So in the example they give a Veteran may have an 60% disability, a 20% disability, and may think the overall rating is not 80%.  Instead, look at the ratings with the highest rating on the left side of the chart and lower rating on the top of the chart.  The 60% rating will be located on the left side and the 20% on the top.  Then you look at the number where the two numbers intersect–and in this case the number is 68.  Ratings are always divisible by 10 and so you round to the nearest number that is divisible by 10, which in this case would be 70.
If you have more than two ratable disabilities you do the same thing and line up the disabilities from highest to lowest.  For example if you have a three rated disabilities 60, 40, and 20.  You first take the 60 on the left side of the column and the 40 on the top they meet at the number 76 which is then found on the left side of the column and the 20 at the top.  These two numbers meet at 81 which is rounded to 80.  80 is the overall rating for this example.  Even easier than doing this is utilizing a calculator developed by the Vietnam Veterans of America.  This calculator can be found here.
If your curious as to how much each VA rating pays per month go to the charts here.  The amount of compensation you can see varies depending on if your married or if you have dependent children.  One thing that’s very important to do if you are the recipient of VA compensation is to tell the VA if your married status changes or if your dependent children status changes.  The reason this is so important is because if your situation changes but you do not tell the VA then you will keep receiving an amount of money that you are not entitled to and this will create the dreaded overpayment situation where you owe the government money.  Alternatively, if you get married or have children then you will be missing out on money that you are entitled to.  Like virtually all VA situations there is a form to give VA notice of these change in status.  Go to this form.