How to Calculate Your Coast FIRE Number

Coast FIRE is the point where your current investments will grow into your full retirement number on their own. To know if you are there, you need your Coast FIRE number — the amount you would need invested today.

Step 1: Find your FI number

Start with how much you will spend per year in retirement, then divide by your safe withdrawal rate (commonly 4%).

FI number = annual retirement spending ÷ 0.04

If you will spend $50,000 a year, your FI number is $50,000 ÷ 0.04 = $1,250,000.

Step 2: Discount it back to today

Your Coast FIRE number is your FI number discounted by the growth it will earn between now and retirement:

Coast number = FI number ÷ (1 + r)^n

where r is your expected real (after-inflation) return and n is years until retirement. Using a real return of about 4%:

• 30 years out: $1,250,000 ÷ 1.04^30 ≈ $385,000
• 20 years out: $1,250,000 ÷ 1.04^20 ≈ $570,000
• 10 years out: $1,250,000 ÷ 1.04^10 ≈ $845,000

The further you are from retirement, the smaller your coast number — compounding has more time to work.

Step 3: Compare to what you have

If your invested assets are at or above your coast number, you are Coast FIRE. If not, the gap tells you how much more to invest.

Use real returns, not nominal

The biggest mistake is using a nominal return like 7% without subtracting inflation. Use a real return (nominal minus inflation — about 4% is a reasonable default) so your number lands in today's dollars.

The fast way

You do not have to do this by hand. The Coast FIRE calculator runs all three steps, charts your trajectory, and shows the exact age you hit Coast FIRE and full financial independence. Adjust your savings rate and watch the date move.

Knowing your number turns someday into a target you can aim at.