Your FIRE number is the pot that lets investment returns pay for your life. The arithmetic is honestly simple — spending ÷ withdrawal rate — and most calculators stop there. This one also handles the thing UK planners actually have: guaranteed income (State Pension, a defined-benefit pension) that covers part of the spending later, and needs bridging until it starts. Every step is shown. Nothing you type leaves this page.
Spending, not income. FIRE maths runs on what you spend, not what you earn. If passive income covering your costs is the goal, your expected spending IS that number.
The withdrawal rate is the argument. 4% is the classic US-history starting point; many UK planners prefer 3.25%–3.75% for long retirements. We default to 4% and let you change it — the honest answer is a range, which is why the result below shows one.
Guaranteed income changes the question. Once the State Pension (£12,547.60/year full rate, 2026/27) or a DB pension starts, the pot only funds the rest. Before it starts, the pot funds everything — that's the bridge. The calculator prices both jobs separately, which is the same maths the OwnWorth app runs against your real holdings.
A FIRE number is a destination. The useful questions after it: how far along are you, what direction is your wealth actually moving, and how would different wealth mixes change the arrival year. That's what OwnWorth does with your real pensions, ISAs and property — progress from live values, a projected date from your own trend, and honest ranges instead of promises. £5.99/month after a 30-day free trial, no card, and the tracking approach is explained here.
See it with example data — no signup Free Section 104 CGT calculator →
This calculator is arithmetic on the assumptions you type, not financial advice, and we are not regulated to give any. Withdrawal-rate sustainability is genuinely debated; State Pension figure is the 2026/27 full new State Pension and your entitlement may differ (check your forecast on gov.uk).