Knowing your FIRE number — the amount you need invested to live without working — is one of the most clarifying things in personal finance. It converts "I hope to retire someday" into a specific date and a specific target.
Our FIRE Calculator is designed to get you to that number in under two minutes. This guide walks you through every input, explains the math, and helps you interpret your results accurately.
Why Accuracy Matters
Your FIRE number is only as reliable as the data you put in. A £200/month error in your reported expenses translates into a 2-3 year error in your projected FIRE date. Take five minutes to gather your actual bank statements before you start.
Step 1: Your Current Situation
Current Age
The starting point for all compound interest calculations. The earlier you begin calculating, the more aggressively you can plan.Monthly Income (After Tax)
Enter your total take-home pay, including side income if it is consistent. The calculator uses this to determine your savings capacity.Monthly Expenses
This is the most critical input. Enter what you actually spend — not what you think you spend. Include:Current Savings / Investments
The total value of liquid, investable assets you already have. This includes brokerage accounts, ISAs, pensions, 401ks. Do not include the equity in your primary home unless you plan to sell and rent in retirement.Step 2: Your Goals and Assumptions
Target Retirement Age
Enter when you would *like* to stop working. The calculator will tell you if your current trajectory meets that goal — and by how much you need to change if it doesn't.Expected Monthly Expenses in Retirement
Will you spend more or less than now? Many people expect to spend less (mortgage paid off, no commuting costs). Others plan to travel more. Be honest — a gap here compounds over decades.Expected Annual Investment Return
Historical average for a diversified global index portfolio (post-inflation): approximately 5-7%. We suggest:Safe Withdrawal Rate
The 4% Rule is the standard. It means a portfolio of 25× your annual expenses has historically survived 30-year retirements in >95% of scenarios (Trinity Study, 1998). For FIRE investors planning 40-50 year retirements, 3.5% is more conservative and appropriate.Understanding Your Results
The FIRE Age
The age at which your portfolio reaches 25× (or your chosen multiple) of your annual retirement expenses. This is when you are financially free.The FIRE Number
Your required portfolio size. With a 4% withdrawal rate: Annual Expenses ÷ 0.04 = FIRE Number.If you plan to spend £30,000/year in retirement: £30,000 ÷ 0.04 = £750,000
Progress Percentage
How close you are to your FIRE Number today. Even 10% progress is meaningful — it represents years of compound growth already in motion.The What-If Slider
This is the most powerful feature. Adjust your monthly savings amount by ±£500 increments and watch your FIRE date move in real time. You will often discover that an extra £200/month moves your freedom date forward by 2-3 years.Your FIRE Date Is Further Away Than You Hoped — Now What?
This is the most common reaction. Here is the framework:
1. Check Your Savings Rate First
Your savings rate is the single biggest lever. Even going from 15% to 20% can move your FIRE date forward by 4-5 years. Read our guide on how the 50/30/20 rule works to systematically raise your savings rate.2. Attack High-Interest Debt
Debt with an interest rate above 6-7% is effectively a guaranteed negative return on your investment. Paying it off is like investing at that rate. Our Debt Payoff Calculator — coming soon — will show you exactly how different payoff strategies accelerate your timeline.3. Use Compound Interest to Your Advantage
Even small additional contributions early in your journey have outsized impact. Check our Compound Interest Calculator to see what happens when you invest an extra £100/month for the next 20 years.4. Reconsider Your Retirement Expenses
Many people overestimate what they will need. If you plan to move to a lower cost-of-living area or pay off your mortgage before retiring, your actual withdrawal need may be significantly lower than your current expenses.The Full FIRE Report
After calculating, scroll down to the Full FIRE Report section. It shows:
Conclusion
The FIRE Calculator is a clarity tool. It converts an abstract desire for "financial freedom someday" into a specific number, a specific date, and a specific monthly action.
The most important thing you can do is start — even if your current numbers are discouraging. Every percentage point improvement in your savings rate, every month earlier you start, and every unnecessary expense you eliminate compounds in your favour.
Calculate your FIRE number now →