The first thing you need to know when planning for retirement is how much capital you can get at your chosen retirement date. For now, we will assume that you invest your savings every month at a constant rate. To simplify things, we will ignore the effects of inflation, hence the capital calculated will be in today's money and the interest rate will be calculated as real rate = nominal interest rate - inflation rate.
We intentionally do not mention any currency in the rest of this chapter. You can consider that the amounts are in USD, EUR, or any other currency. It will not change the results as long as all the amounts are expressed in the same currency.