Coast FIRE Calculator with Spouse

To use the Coast FIRE calculator with your spouse, enter your household's combined assets and spending, then set the monthly contribution to what your household will keep saving after one of you stops. The result shows whether your spouse's ongoing contributions, your shared savings, and their future benefits let you coast now or how soon you can.

1. Enter your Current Age. Use the age of the spouse who plans to coast.
2. Set your Retirement Age. Use your age on the date you both expect to stop work completely.
3. Enter household Annual Spending in Retirement, minus both spouses' benefits. Subtract your own Social Security or state pension and your spouse's. In the US, a spouse with little work history can claim a spousal benefit worth up to 50% of the other spouse's full benefit at full retirement age.
4. Enter Current Invested Assets for both of you. Add your accounts and your spouse's accounts together.
5. Enter the Monthly contribution your household will keep making. This is the key step. If you plan to stop saving but your spouse will keep contributing, enter only your spouse's contribution.
6. Check Advanced Settings. Keep the defaults (7% growth, 3% inflation, 4% withdrawal rate) unless you have a reason to change them.

How to read your results

• Coast FIRE number: the combined amount your household needs invested today.
• Progress: how far your shared savings already go toward that number.
• Coast Age: the age when your household reaches Coast FIRE at your spouse's continuing contribution.
• Cash Freed: the yearly saving you'd stop once the household coasts.
• Growth Projection chart: the "Coast" line shows your savings with no new money; the "Keep saving" line shows the effect of your spouse's ongoing contributions.

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The calculator finds the nest egg your household needs after both spouses' benefits, then discounts it to today using your after-inflation return. It then adds your spouse's ongoing contributions year by year to find when your savings cross the coast line.

The formula

FIRE Nest Egg     = (Household Spending − Both Spouses' Benefits) ÷ Safe Withdrawal Rate
Real Return (r)   = (1 + Growth Rate) ÷ (1 + Inflation Rate) − 1
Coast FIRE Number = FIRE Nest Egg ÷ (1 + r)^Years
Coast Age         = first age where Savings × (1 + r) + (Monthly × 12) ≥ Coast line for that age

Worked example

Jordan is 38 and wants to leave a high-stress job for lower-paid work they enjoy. Jordan's spouse, Riley, is 36 and plans to keep working and saving $1,000 a month. They expect to spend $72,000 a year in retirement and receive $32,000 a year from Social Security combined, a $40,000 gap. They plan to retire fully when Jordan turns 65.

1. FIRE Nest Egg: $40,000 ÷ 0.04 = $1,000,000
2. Real Return: 1.07 ÷ 1.03 − 1 = 3.88%
3. Growth factor over 27 years: 2.797
4. Coast FIRE number: $1,000,000 ÷ 2.797 = $357,473

The couple has $240,000 invested, which is 67% of their number. If both stopped saving today, that $240,000 would grow to about $671,380 by Jordan's 65th birthday, short of $1 million. With Riley's $1,000 a month, the household reaches Coast FIRE when Jordan is 51. Jordan can switch careers now, as long as Jordan's new income covers Jordan's share of today's bills.

Factors that change your result

• Spousal benefits. A lower-earning or non-working spouse may claim a spousal or survivor benefit, which lowers the portfolio you need.
• Health insurance. If the coasting spouse leaves a job with benefits, the household may need coverage through the other spouse's employer.
• Spousal retirement accounts. In the US, a working spouse can fund a spousal IRA for a non-working spouse when filing jointly, which keeps both retirement balances growing.
• Rounding. Results are rounded to whole currency units, contributions are added yearly, and Coast Age is shown in whole years.

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