Coast FIRE Calculator for Couples
One household, one number — with your age gap and different retirement dates handled properly, instead of running two single-person calculators and adding them up. Instant, no sign-up, method shown.
Return & inflation assumptions
Quick answer
A couple aged 32 and 30 spending $65,000 a year in retirement needs a household FIRE number of $1,625,000 — and a Coast FIRE number today of $518,152. Saving $2,500.00 a month between them, they reach it in 16.6 years. The household horizon is the later of your two retirement dates — an age gap changes the number.
Your combined portfolio against the household target
Green is what the two of you have invested; indigo is the Coast FIRE line, rising toward your household FIRE number as retirement nears. Where green crosses indigo, you can both stop saving for retirement.
What your age gap does to the answer
Same household, same money, same joint spending — only Partner B's age changes. This is the part a single-person calculator cannot show you.
| Partner B | Age | Household horizon | Coast number today | Reached in |
|---|
What Coast FIRE means for two people
Coast FIRE is the point where what you have already invested will grow into your full retirement number on its own, with no further contributions. You keep working — you just stop saving for retirement, because retirement is already funded.
For a couple, that has to be answered as one household, not two people. You will almost certainly share the money, share the housing and share the spending, so the sensible unit is the joint portfolio against the joint spending figure. Running two single-person calculators and adding the answers gives a number that is wrong in both directions at once: it overstates what you need, and it ignores the thing that actually drives the answer — your two different timelines.
The age gap is the part single-person calculators cannot model
Coast FIRE depends almost entirely on the horizon: how many years the money has to compound before it must be fully grown. For one person that is obvious. For two it is a question, and the answer is not the average.
The portfolio has to still be intact and fully grown when the later of you stops earning. So the household horizon is the longer of the two runways — and that means a younger partner is, in this narrow arithmetic sense, an asset.
Same household throughout — $150,000 invested, $2,500 a month between them, $65,000 joint spending, Partner A aged 32 retiring at 60:
| Partner B | Household horizon | Coast number today | Reached in |
|---|---|---|---|
| 8 years younger (24) | 36 years | $412,266 | 10.7 years |
| 4 years younger (28) | 32 years | $480,135 | 14.3 years |
| same age (32) | 28 years | $559,178 | 19.3 years |
| 4 years older (36) | 28 years | $559,178 | 19.3 years |
| 8 years older (40) | 28 years | $559,178 | 19.3 years |
Two things in that table are worth pausing on.
A younger partner moves the date a long way. Eight years younger takes the wait from 19.3 years down to 10.7 — nearly nine years earlier, on identical money. The extra runway lets compounding do more of the work, so less has to be in the account today.
An older partner does not push it back further. Once Partner B is the same age as A or older, A's retirement date is the later one, so A sets the horizon and B's age stops mattering to the target. The rows for 32, 36 and 40 are identical. That is not a rounding artefact — it is the model correctly saying that the horizon is set by whoever works longest, and nobody else.
A couple is not two singles
This is where adding two individual calculations goes most wrong. A couple's retirement spending is well below two separate people's, because one home, one set of utilities, one internet bill and often one car serve both of you.
| Annual spending | Portfolio needed at 4% | |
|---|---|---|
| Two singles, separately | $45,000 each | $2,250,000 |
| One couple, together | $65,000 joint | $1,625,000 |
| Difference | $625,000 less | |
That $625,000 is the financial value of sharing a household, expressed as capital. It is also why the calculator above asks for your joint spending rather than asking each of you separately and doubling — doubling would hand you a target more than a third too large.
What the second income is worth
Because the household reaches Coast FIRE when the combined portfolio crosses the line, the second partner's contribution moves the date directly. Same household, only Partner B's monthly saving changing:
| Partner B saves | Household total | Coast FIRE reached in |
|---|---|---|
| $0 | $1,400/mo | never, before they stop working |
| $450 | $1,850/mo | never, before they stop working |
| $900 | $2,300/mo | 24.3 years |
| $1,350 | $2,750/mo | 18.1 years |
| $1,800 | $3,200/mo | 14.4 years |
Figures on the original $120,000 starting balance, to show the effect of the contribution alone.
Notice the cliff between $450 and $900. Below a certain combined rate the portfolio never catches the rising Coast FIRE line at all — not because the maths breaks, but because the target grows toward the FIRE number faster than the balance grows toward the target. Couples close to that edge get a large payoff from a small increase, which is worth knowing before assuming you are hopelessly behind.
How the household number is calculated
- Joint spending — what the two of you spend together in a retirement year, in today's dollars
- SWR — safe withdrawal rate, commonly 4%
- r — the real return: (1 + nominal) ÷ (1 + inflation) − 1, which is 3.88% at 7% and 3%
- t — years until the later of your two retirement dates
The calculator then runs your combined contributions forward month by month to find when the balance first crosses the line — and each partner's contributions stop at their own retirement date, not the household one.
See the method with your own numbers
These update live from the calculator inputs above.
Example: a couple at 32 and 30
$150,000 invested between them, saving $1,400 and $1,100 a month, spending $65,000 a year in retirement, both retiring at 60, 4% withdrawal rate.
- Household FIRE number: $1,625,000.
- Coast FIRE number today: $518,152 — about 32% of the full number, because there are 30 years of compounding available.
- Reached in 16.6 years, at ages 49 and 47. From then on, retirement saving is optional.
The horizon here is 30 years rather than 28, because Partner B is younger and retires later in calendar terms. That two-year difference is worth about $41,000 off the number they need today.
What this deliberately does not model
- Social Security or pensions. Any guaranteed retirement income reduces what the portfolio must cover — subtract it from joint spending before entering it.
- Separate finances. The model assumes one shared pot. If you keep money genuinely separate, run each side individually with its own spending share.
- One partner dying first. Real plans account for a survivor's spending and any survivor benefit; this is a simplification.
- Separation. Uncomfortable but real: a joint plan assumes the household stays a household.
- Sequence-of-returns risk. Growth is applied smoothly; markets are not smooth, and a bad first decade of withdrawals hurts more than a bad later one.
Frequently asked questions
How do you calculate Coast FIRE for a couple?
Treat it as one household. Divide your joint annual retirement spending by your withdrawal rate to get a household FIRE number, then discount it back to today at your real return over the years until the later of your two retirement dates. For a couple aged 32 and 30 spending $65,000 a year at 4%, that is a $1,625,000 FIRE number and about $518,152 needed today.
Should a couple calculate Coast FIRE separately or together?
Together, if you will share the money in retirement. Two separate calculations overstate what you need, because a couple's joint spending is far below two individuals'. Two singles at $45,000 each need $2,250,000 between them at 4%; one couple at $65,000 joint needs $1,625,000 — $625,000 less.
How does an age gap change Coast FIRE?
It changes the horizon, and the horizon drives everything. The portfolio must be fully grown by the later of your two retirement dates, so a younger partner lengthens the runway and lowers what you need today. In the scenario above, a partner eight years younger cuts the wait from 19.3 years to 10.7. An older partner does not push it back further, because then the other partner is already setting the horizon.
Does it matter which partner earns more?
For the number itself, no — only the combined monthly contribution and the joint spending figure enter the calculation. What does matter is how long each of you keeps contributing, because contributions stop at that partner's own retirement date. An earlier retirement costs the household those payments even when it does not change the horizon.
What if one partner wants to retire earlier?
Set different retirement ages above. Two things then happen: that partner's contributions stop sooner, and if they were the one setting the household horizon, the horizon shortens — which raises the amount you need today. If the other partner was already retiring later, the horizon is unchanged and only the lost contributions cost you.
Does this include Social Security or pensions?
No. The model assumes the portfolio funds all joint spending, which is the conservative assumption. If you expect guaranteed income in retirement, subtract it from your joint spending figure before entering it — the portfolio only has to cover the gap.
Method & sources
- Calculation: household FIRE number = joint annual spending ÷ safe withdrawal rate; Coast number = FIRE ÷ (1 + real return)years to the later retirement date; combined contributions simulated monthly in real dollars, each partner's stopping at their own retirement. Verified against an independent test suite (18 checks).
- The 4% safe withdrawal rate comes from the widely cited Trinity study; it is a planning guide, not a guarantee, and a long joint retirement gives it more chances to fail.
- Reviewed: · Assumptions reviewed quarterly.
Educational estimate, not financial advice. Investment returns vary and can be negative.
Run your own numbers
Put in both ages, both retirement dates and what you each save — and see the one household number, plus how much your age gap is actually worth.
Back to the calculator ↑