Determining your retirement number — the amount you’ll actually need to retire comfortably — can be unpleasant, but it’s a necessary process. Unfortunately, a recent survey found only 42% of Americans have actually crunched the numbers.

Luckily, finding your retirement number isn’t that difficult. Any number of online calculators and tools are available to help, though they often rely on your input of unpredictable estimates. Here are a few assumptions you’ll have to make and some reasonable estimates to keep in mind:

– Inflation assumption: 4.5% (higher than where we are today, but most economists believe that inflation is headed up in the coming years).

– Rate of investment return both before and after retirement: Consider your risk tolerance and err on the side of being conservative. If you’re stuck, use 4 or 5%.

– Life expectancy: If you are younger than 50, use 95; if you’re older than 50, use 90. If you want a closer estimate, go to www.livingto100.com and use their Life Expectancy Calculator.

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