Coast FIRE Calculator

1. 1.Turn spending into an FI number

   Divide your yearly retirement spending by your withdrawal rate. At 4% that's 25x spending, and at 3.5% it's about 28.6x.

   fi_number = annual_retirement_spending / withdrawal_rate

2. 2.Count the years you have

   Subtract your current age from your target retirement age. Those are the years your balance gets to compound with no help from you.

   years = retirement_age - current_age

3. 3.Discount it back to today

   Divide the FI number by your growth multiple, which is 1 plus your real return raised to the years left. The result is the coast number. Your portfolio minus that number is your surplus or shortfall.

   coast_number = fi_number / (1 + real_return) ^ years

4. 4.Find the year you cross

   If you still contribute, the tool adds each year's contribution at year end and compares the balance to the coast number for that age. The first age where it clears is your crossing. It doesn't look past your retirement age.

   balance_next = balance x (1 + real_return) + annual_contributions

1. $60,000 of spending at a 4% withdrawal rate gives an FI number of $1,500,000.
2. The runway is 25 years, from 35 to 60. At a 5% real return the growth multiple is 1.05^25, or 3.386.
3. $1,500,000 ÷ 3.386 is a coast number of $442,954, which is 7.4x spending. Your $520,000 is $77,046 above it, 117.4% funded, so the status is Coasting.
4. Left alone for 25 years, $520,000 grows to $1,760,905, which is $260,905 over the FI number. Keep adding $18,000 a year and you reach full FI at 51.

At 35 with 25 years left, about 30% of the full FI number is enough to coast. Stop contributing today and you still land $260,905 over target at 60. Keep saving and you reach full FI at 51.

• One steady return every year. There is no volatility, so a bad early decade would leave you below this projection. Keep a cushion above the coast number.
• Coasting means you never withdraw. Any money you take out during the coast years breaks the math.
• Taxes aren't modeled. Fold your expected retirement tax bill into annual spending, and remember a traditional 401(k) dollar is worth less than a brokerage dollar.
• Social Security, pensions and other fixed income aren't modeled. If you expect any, reduce your retirement spending by what they'll cover.
• Spending stays flat in today's dollars, with no lumpy costs or long-term care shock. Inputs outside the allowed ranges get an error. These are estimates, not financial advice.

Coast FIRE number

The balance you need today to reach your FI number by retirement with no more contributions. The by-age table shows how it climbs, every five years.

Status

Coasting: at or above the coast number. Nearly coasting: within 10% of it. Still climbing: further below, so contributions still matter. Already at your FI number: no coasting needed.

Surplus or shortfall

Your invested portfolio minus the coast number. A surplus is your buffer. A shortfall is what you still need to accumulate.

Coasting portfolio at retirement

Today's balance compounded to your retirement age with no contributions, and how far above or below the FI number it lands.

Coast and full-FI crossing

The first age your portfolio, with contributions, clears the coast number and then the full FI number. Neither is projected past your retirement age.

What moves the number

The coast number recomputed with one assumption changed: retiring five years earlier or later, a withdrawal rate half a point lower, or a real return one point lower.