You’re 34, with about $45,000 invested and a baby due. At some balance ahead of you, you could stop saving for retirement completely and still retire comfortably at 67. That balance is your Coast FIRE number, and for a lot of people at your stage it’s around $140,000 in today’s dollars. With steady saving, you’d reach it in your early forties.
The baby changes that date, though not the way most people guess.
What coasting actually means
Coast FIRE doesn’t mean you quit. You keep working and you keep paying today’s bills. You just stop feeding the retirement accounts, because the money already in them will grow into enough on its own.
What you get is choice. A job you like more that pays less. Part-time years while the kid is small. A raise that goes to the house, or to a 529, instead of the 401(k).
The math is one line:
Coast number = what you need at retirement ÷ (1 + real return)^(years until retirement)
That’s the whole formula. The inputs are where people go wrong.
Start with the number you actually need
Most Coast FIRE calculators ask for your retirement spending, multiply by 25, and stop there. That skips the biggest input a regular earner has, which is Social Security.
Say your household wants to spend $50,000 a year in retirement, in today’s dollars. Your Social Security statement gives your actual estimate. For this example, assume $22,000 a year combined, starting at full retirement age.
That leaves your portfolio covering $28,000 a year. At a 4% withdrawal rate, a standard assumption for a 30-year retirement, you need $700,000.
Leave Social Security out and the same household needs $1.25 million. Same lifestyle, same person, and nearly double the target. A calculator that skips Social Security tells you you’re a decade further away than you are.
The honest caveat: Social Security’s trust fund faces a projected shortfall. If you want margin, cut the estimate by 20%. That takes you to $17,600 a year from Social Security, $32,400 from the portfolio, and an $810,000 target. Still well short of $1.25 million.
This piece uses $700,000. Swap in your own.
The Coast FIRE calculator, by hand
Use real returns, meaning returns after inflation, so every number stays in today’s dollars. Long-run U.S. stock returns after inflation have landed in the mid-single digits. The rates below are assumptions, not promises. Markets don’t owe you 5%.
You’re 34 and the target age is 67, so the money has 33 years to grow.
| Real return assumption | Growth factor over 33 years | Coast number today |
|---|---|---|
| 4% | 3.65x | about $192,000 |
| 5% | 5.00x | about $140,000 |
| 6% | 6.84x | about $102,000 |
At 5%, you need about $140,000 invested today to coast. You have $45,000.
You’re not there. Almost nobody is at 34.
How long until you get there
Now add saving. Say you put in $13,000 a year across your Roth IRA and your 401(k) contributions plus match. That’s about 15% of an $85,000 salary.
Two things move at once. Your balance grows from contributions and returns. Your Coast number also drops every year, because each year closer to 67 means fewer years of growth still needed. At some age, the two lines cross.
At 5% real, they cross at about 43. By then your balance is roughly $220,000, and with 24 years left until 67, your Coast number is also about $220,000. From that point, you could contribute nothing and still reach $700,000 at 67.
| Real return assumption | Age you reach Coast, saving $13,000/yr |
|---|---|
| 4% | about 49 |
| 5% | about 43 |
| 6% | about 39 |
The spread is the lesson. Return assumptions swing the answer by a decade, so use the middle row for planning and treat the top row as the one that has to still be okay.
What the baby does to the math
Childcare is expensive, often a five-figure annual bill. Something usually gives, and retirement savings is the easiest thing to cut.
Say you drop from $13,000 a year to $8,000 for the first five years, then go back to $13,000.
| Plan | Total contributed in the baby years | Coast age at 5% |
|---|---|---|
| $13,000 every year | $65,000 | about 43 |
| $8,000 for five years, then $13,000 | $40,000 | about 46 |
You saved $25,000 less. It cost you roughly three years of full-rate saving on the back end.
That’s the uncomfortable part of the math. Early dollars do the most work. At 5% real, a dollar invested at 34 grows to about $5 by 67. A dollar invested at 50 grows to about $2.30. Cutting contributions in your thirties is the most expensive time to cut, and it happens to be the exact moment the baby shows up.
So flex something else first if you can: the car, the house budget, the lifestyle bump from your next raise. If retirement savings has to give, never drop below the full employer match. A dollar-for-dollar match is a 100% return before the market does anything, and no tradeoff in the baby years beats that.
The move to make
Log into your Social Security account this week and pull your benefit estimate. Take your target retirement spending in today’s dollars, subtract that estimate, and divide the gap by 0.04. That’s your number at 67. Divide it by 5.0, the 5% growth factor over 33 years, and you have your Coast number today. If you’re not 34, recalculate the factor as 1.05 raised to your years left until 67.
Write two figures on one line: your Coast number and the age you’ll hit it at your current savings rate. Then protect that savings rate through the baby years, the full match at minimum. Re-run it every year on your kid’s birthday.
When your balance crosses the line, the next raise is yours to spend however you want. That’s the point of all this.
Sources
- Social Security Administration, “my Social Security” account (personalized benefit estimates). https://www.ssa.gov/myaccount
- Social Security Administration, Retirement Age Calculator (full retirement age by birth year). https://www.ssa.gov/benefits/retirement/planner/ageincrease.html
- Social Security Administration, Board of Trustees, 2026 Annual Report , summary. https://www.ssa.gov/oact/trsum/
- Social Security Administration, Board of Trustees, 2026 Annual Report , full report (PDF). https://www.ssa.gov/oact/TR/2026/tr2026.pdf
- Internal Revenue Service, “401(k) limit increases to $24,500 for 2026, IRA limit increases to $7,500,” IR-2025-111, Nov. 13, 2025. https://www.irs.gov/newsroom/401k-limit-increases-to-24500-for-2026-ira-limit-increases-to-7500
- William P. Bengen, “Determining Withdrawal Rates Using Historical Data,” Journal of Financial Planning , October 1994 (FPA reprint, PDF). https://www.financialplanningassociation.org/sites/default/files/2021-04/MAR04%20Determining%20Withdrawal%20Rates%20Using%20Historical%20Data.pdf
- Aswath Damodaran, NYU Stern School of Business, “Historical Returns on Stocks, Bonds and Bills.” https://pages.stern.nyu.edu/~adamodar/New_Home_Page/datafile/histretSP.html
- U.S. Department of Labor, Women’s Bureau, National Database of Childcare Prices. https://www.dol.gov/agencies/wb/topics/featured-childcare
Last reviewed: September 29, 2026. Contribution limits and Social Security projections change annually; this page is flagged for review each year.
The post Your Coast FIRE number, with a Kid on the Way appeared first on Thousandaire.
