Coast FIRE: Working Out the Math With a Standard FIRE Calculator

Coast FIRE describes a specific milestone: having enough already invested that, left completely alone to compound, it will grow into a full FIRE number by a target retirement age — with no further contributions required. Once someone hits that point, they can theoretically stop saving for retirement entirely and just cover current living costs, letting compounding do the rest.

The Same Calculation, One Input Set to Zero

A standard FIRE projection takes current savings, an annual contribution, an expected return, and a target — and simulates growth until the balance reaches that target. Coast FIRE is the exact same calculation with the contribution set to zero: the question becomes whether current savings alone, growing on their own, reach the FIRE number by retirement age.

A Worked Example

Starting at age 30 with $200,000 already saved, a 7% expected return, and a $1,000,000 FIRE number:

• With zero further contributions (Coast): reaches $1,000,000 at roughly age 53 — pure compound growth over about 23 years.
• Still contributing $10,000/year: reaches the same target at roughly age 47 — about 6 years sooner.

That gap is the actual cost of coasting: about six years of continued contributions traded for six fewer years of required saving, with the same eventual outcome either way.

Why Someone Would Choose This

Reaching Coast FIRE doesn't mean stopping work — it means no longer needing to direct income toward retirement savings specifically, since existing savings are already on track to compound into the full number by the target age. That can free up income for a lower-stress job, a career change, or simply reduced financial pressure, without derailing the eventual retirement target.

Running Your Own Numbers

Since this is the identical FIRE projection with contributions set to zero, any standard FIRE calculator that accepts a contribution amount can answer the Coast FIRE question directly — enter current savings, set the contribution to zero, and check whether the projected balance reaches the target by the intended retirement age.