Most calculators either suggest your required total retirement income (based solely on how much you make now) or ask you to enter a constant amount yourself. But after everything we said above, how can you possibly guess this and is it really constant over your years in retirement?
Your required retirement income will likely change over time as your circumstances change (your kids finish college, your mortgage gets paid off, Medicare kicks in, your tax situation changes, different inflation rates apply to different aspects of your life, etc.).
National studies, telling you how much you will need, are of little help too. These studies are only intended for large groups, for example when creating a retirement plan for large corporations or an entire country. They are not meant for individuals because everyone's situation is so unique.
A better question is how much free monthly cash you will need in retirement (in today's prices, after taxes, home-related expenses, healthcare, child-related expenses, and special events) - your disposable cash for personal and optional expenses each month.
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