ISA Bridge Apex Calculator

Retiring before your pension is accessible (58, or 68 for the State Pension) means one thing has to fund the gap: your ISA. This calculator finds your ISA apex — the point where your ISA has grown large enough to carry you, through withdrawals, from the day you stop work to the day your pension takes over.

Enter your current ISA balance, what you’re contributing each year, and what you expect to withdraw each year in the bridge phase. Choose whether you already have a target FIRE age in mind, or let the calculator solve for the earliest age at which the numbers work. All figures are shown in today’s money (real terms) — see the note on the calculator for how that assumption is built in.

This tool accompanies The Road to FIRE: Building the ISA Bridge to 58 and Finding the Apex, which walks through the reasoning and worked example behind it.

Current Age:

Current ISA Balance (£):

Annual ISA Contribution (£):

Annual Bridge Withdrawal (£, today's money):

Bridge Ends At Age (pension/other income starts):

Assumed Real Growth Rate: 5%

Real growth = market return minus inflation and fees. e.g. 8% market − 3% inflation/fees ≈ 5% real. All figures are shown in today's money; a flat withdrawal amount is used for simplicity, but in reality you would increase it in line with inflation.

I know my target FIRE age Solve for my earliest FIRE age

Target FIRE Age:

ISA Bridge Apex Calculator

Bridge Summary

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