Coast FIRE Calculator for Couples

How the calculator works

The calculator treats the two of you as one household with one portfolio. Your balances are added together, your contributions are added together, and the years until retirement come from your age and the age you’ll be when you both stop working. Your partner’s age matters for one thing: when their Social Security starts.

household invested     = your invested + partner's invested
household contribution = your monthly contribution + partner's monthly contribution
years to retirement    = retirement age − your age

your benefit starts in      = your claiming age − your age            (years from now)
partner's benefit starts in = partner's claiming age − partner's age  (years from now)

target at retirement = max(spending − your benefit − partner's benefit, 0) ÷ withdrawal rate
                     + your benefit × max(0, your start − years to retirement)
                     + partner's benefit × max(0, partner's start − years to retirement)

real return       = (1 + expected return) ÷ (1 + inflation) − 1
Coast FIRE number = target at retirement ÷ (1 + real return)^(years to retirement)

With both benefits left at zero, the target is just household spending divided by your withdrawal rate. Once you add benefits, each one lowers the long-run target and adds a bridge for every year between your retirement and the day that benefit starts. The bridge is counted in full, as if that money earns nothing after inflation, which keeps the number on the conservative side.

The coast age comes from a month-by-month simulation: the combined balance grows at the real return, both contributions go in each month, and the first month the balance reaches the Coast FIRE number for that point in time is when the household could stop contributing. The calculator shows that moment as your age and your partner’s age.

A worked example

Jess is 38 and Omar is 34. Jess has $220,000 invested and Omar has $90,000. Jess puts in $1,800 a month and Omar $1,200. They expect to spend $90,000 a year together and want to stop working when Jess turns 60, which makes Omar 56.

Jess’s Social Security estimate is $28,000 a year at 67, and Omar’s is $20,000 at 67. Because Omar is younger, his benefit starts 11 years after they retire, while Jess’s starts after 7.

Counting both benefits cuts their Coast FIRE number by about $339K, even with 11 years of Omar’s benefit and 7 of Jess’s set aside as bridge money. At $3,000 a month combined, they could stop contributing when Jess is 49 and Omar is 45.

Now say Omar wants to stop contributing today, maybe to go back to school. With only Jess’s $1,800 a month going in, the household wouldn't reach Coast FIRE before Jess turns 60. If Omar waits until they’ve crossed the line at 45, the same decision costs their retirement nothing. That’s the kind of tradeoff this calculator is good at making concrete.

How do we choose the inputs?

Household spending is what the two of you will spend together, which is usually well under double what one person would spend, because you share housing and most of the bills. It’s also the number couples are most likely to quietly disagree about. Agree on it first.

For balances, count 401(k)s, IRAs, invested HSA money and brokerage accounts in each name. Leave out emergency funds and home equity. Contributions should include any employer match; if you both have one, make sure you’re both getting all of it.

Each partner should pull an estimate from their own my Social Security account and enter the yearly figure for the age they plan to claim. If one of you has a much smaller work record, a spousal benefit of up to half the other’s full-retirement-age benefit may be larger than their own. If you’re more than 15 years from claiming, I’d enter about 75% of each estimate, for the reasons covered in our Social Security guide.

What if we won’t stop working at the same time?

The calculator assumes you both stop on the same day. If the younger partner plans to keep working a few more years, enter the age you’ll be when the first of you stops. The extra paycheck years become margin you haven’t counted, and with retirement money that’s the better mistake to make.

What the calculator leaves out

It doesn’t model survivor benefits, what happens to household spending if one of you dies, or retirements that are years apart. It ignores taxes and pensions, and it assumes steady returns until you retire. If one of you has a pension, subtract what it will pay from household spending, or run your numbers through the Social Security and pension calculator. The couples guide covers age gaps, spousal IRAs and what changes when one of you cuts back.