Coast FIRE is the amount you need invested today so that, with no further contributions, compound growth alone reaches your retirement nest egg by your target age. After hitting it, you only need to cover current expenses.

How is the Coast FIRE number calculated?

Take your retirement nest egg target and discount it back to today using your expected real (inflation-adjusted) return over the years until retirement. The result is what you need invested now.

Does the Coast FIRE number change with age?

Yes, dramatically. The younger you are, the smaller the number, because you have more years of compounding. Waiting even a decade can roughly double what you need invested today.

Retirement

Coast FIRE: the number that lets you stop saving

Maya is 32, with a good job, a good salary and a quiet daydream about leaving it for something that pays less and makes her happier. She has $120,000 invested, and the question keeping her up is whether there’s a point where she could stop saving for retirement and let what she has do the rest. There is. It’s called Coast FIRE, and for someone her age the number is smaller than you might expect.

What Coast FIRE means

Normal retirement math asks how much you need saved by 65. Coast FIRE asks how much you need invested today so that compounding alone, with no more contributions, carries you to that number. Once you’re there, your paycheck only has to cover the life you’re living now.

A worked example

Here’s Maya’s plan: age 32, retiring at 65, spending $50,000 a year in retirement (a $1,250,000 nest egg at a 4% withdrawal rate), with 7% returns and 3% inflation:

FI target (nest egg needed): $1,250,000
Coast FIRE number (invested today): $355,528
That amount, untouched, grows to your full nest egg by 65

So $355,528 invested now, with nothing added, grows to $1.25M by 65 because it has 33 years to compound. Growth supplies about $894,000 of that, more than 70% of the target. Maya is about $235,000 away. Once she gets there, that lower-paying job stops being a crazy idea, because her paycheck only has to cover the life she’s living now.

Find your Coast FIRE number →Free Coast FIRE calculator, no sign-up. Enter your age, spending and assumptions.

Why age matters so much

The Coast FIRE number is very sensitive to age, because it depends on how many years of compounding are left. At 32, Maya needs about $355k. The same plan at 45 needs about $583,000, roughly 1.6 times as much, and at 55 about $854,000, more than two-thirds of the whole target. A dollar invested young does several times the work of a dollar invested later, which is the best argument for getting there early.

Where this estimate can be off

This assumes your money stays invested and earns something close to your expected return, and markets don’t move in a straight line. Reaching Coast FIRE means you can stop saving more. You still have to work to cover today’s expenses. Use a lower withdrawal rate if you want more cushion, and check the number again every few years.

Figures from TheSharpDollar’s Coast FIRE calculator using its sample scenario. Your result depends on your age, spending and return assumptions.