Coast FIRE Calculator With Social Security

How the calculator works

A regular Coast FIRE number assumes your portfolio pays for all of retirement. If Social Security or a pension will cover part of it, the target shrinks. The one complication is timing: if you stop working before that income starts, your portfolio has to carry the full load until it does.

counted Social Security = yearly benefit × share to count

retirement target = max(spending − counted Social Security − pension, 0) ÷ withdrawal rate
                  + counted Social Security × max(0, claiming age − retirement age)
                  + pension × max(0, pension start age − retirement age)

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

The first line of the target is what your portfolio needs once all your income is flowing. The next two add the bridge: every year between your retirement and the start of a benefit, the portfolio has to replace that benefit in full.

Bridge money is treated conservatively. The calculator assumes whatever you set aside for those years earns nothing after inflation, so seven years of a $26,000 benefit adds seven times $26,000 to the target. In reality that money would probably earn something, so your true number is likely a bit lower.

Alongside the adjusted target and Coast FIRE number, the calculator shows the Coast FIRE number you’d get ignoring Social Security and pensions entirely, so you can see what they’re worth. The coast age comes from the same month-by-month simulation as the main calculator: your balance grows at the real return, your contribution goes in each month, and the first month you’re at or above the Coast FIRE number for your age is when you could stop.

A worked example

Kevin is 35, has $200,000 invested and adds $2,000 a month. He wants to stop working at 60 and expects to spend $70,000 a year. His Social Security statement estimates $26,000 a year if he claims at 67.

Ignoring Social Security, Kevin never reaches Coast FIRE before he plans to retire. Counting all of it, he could stop contributing at 51. Counting 75%, he gets there at 55. Counting the full benefit cuts his Coast FIRE number by $180,544, even after paying for seven bridge years.

How do I pick the Social Security inputs?

Start with a real estimate from your my Social Security account. The statement assumes you keep earning roughly what you earn now until you claim. If you plan to stop working entirely in your 40s, your benefit will likely come in lower, since it’s based on your 35 highest-earning years.

Enter the benefit that matches your claiming age. Claiming before full retirement age shrinks your check permanently: someone whose full retirement age is 67 gets 30% less by claiming at 62. Waiting past full retirement age adds delayed retirement credits of 8% a year until 70.

That tradeoff shows up clearly in Kevin’s numbers. Claiming at 62, at 70% of his estimate, puts his target at $1,331,400 and his Coast FIRE number at $513,623. Claiming at 70, at 124% of it, means a ten-year bridge but a bigger check for life: a target of $1,266,400 and a Coast FIRE number of $488,548. Waiting comes out ahead here, even with the conservative bridge, because the bigger check covers more of his spending for the rest of his life. That won’t hold for everyone, so run both ages with your own estimates.

For the share to count, 100% is reasonable if you’re within 10 or 15 years of claiming. If you’re younger, I’d use 75%. The trustees’ latest summary projects the retirement trust fund can pay full benefits until late 2032, then about 78% of scheduled benefits from incoming taxes. Changes are likely before then, but you don’t need to bet on which ones.

What about a pension?

Enter what it pays each year in today’s dollars and the age it starts. If it starts after you retire, the bridge works the same way as with Social Security.

The catch is inflation. Social Security is adjusted for inflation every year, and many private pensions aren’t. A pension that pays the same dollar amount forever buys less every year. Over a 30-year retirement at 3% inflation, its average buying power is 67% of the face amount, so counting about two-thirds is a fair starting point. If Kevin also had a flat $12,000 pension from age 65, he’d enter about $8,075, and his Coast FIRE number with full Social Security would fall to $432,260.

What the calculator leaves out

It treats Social Security as rising with inflation, which matches how the program works today. It doesn’t handle spousal benefits; if you’re planning with a partner, the couples calculator takes a benefit for each of you. It doesn’t model taxes on benefits or withdrawals, and it assumes steady returns until you retire.

For more on all of this, including how stopping work early can shrink your benefit, read how to factor Social Security and pensions into Coast FIRE.