How we calculate and review every number

Finturify publishes calculators and analysis on investing, retirement, housing and personal budgeting. This page explains exactly what our tools do, which assumptions they apply, where the data comes from and what we do when we find a mistake. If a result looks odd, everything you need to check it yourself is here.

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All calculators run on your device. The figures you enter (income, savings, net worth) are not sent to any server or stored in any database. We only use aggregated visit analytics if you accept it in the cookie notice.

Calculator formulas and assumptions

• Compound interest and financial freedom. We convert the annual return you enter into an equivalent monthly rate, (1 + r)^1/12 − 1, and add your contribution at the end of each month. A 7% annual return therefore compounds to exactly 7% over twelve months. Enter returns net of fees (TER) and, if you want results in today's money, net of inflation as well.
• 4% rule. The target capital is annual spending divided by the chosen withdrawal rate (at 4%, annual spending multiplied by 25). The “historical success rate” is calculated with real data: the share of 30-year periods since 1928 in which a 60/40 portfolio with inflation-adjusted withdrawals did not run out.
• Retirement simulator. Replays your plan in every historical period since 1928 (backtest) and in 5,000 Monte Carlo scenarios that randomly combine complete historical years (stocks, bonds and inflation of the same year). Spending is withdrawn at the start of each year, the portfolio is rebalanced yearly and fees are deducted; everything is in today's money. Optionally, it adds a state pension from a chosen year, a guardrails rule (spending is cut or raised by 10% when the withdrawal rate drifts more than 20% from the initial one, never below a floor you set) and a cash buffer held in 3-month Treasury bills. Try it in the retirement simulator.
• House savings. The capital required is the down payment (20% of the price by default, since banks typically lend up to 80%) plus purchase taxes and costs (10% by default, adjustable). To estimate the timeline we assume your savings earn 4% a year and the property price stays constant.
• 50/30/20 budget. Splits net income into 50% needs, 30% personal spending and 20% saving and investing. It is a starting point, not a fixed rule.

The tables in our articles use the same formulas, which we show next to each table so you can recompute any figure.

Sources we use

• Historical market data: Aswath Damodaran's (NYU Stern) annual series 1928-2025 for the S&P 500 with dividends, 10-year US Treasury bonds and inflation. Reference academic work: the Trinity Study on withdrawal rates.
• Official tax law in each country: Spain's Personal Income Tax Act (Ley 35/2006, e.g. article 94 on fund transfers), Germany's Investment Tax Act (InvStG) and the French rules for the PEA, assurance-vie and PER.
• Supervisors and public bodies: CNMV, Banco de España and the Deposit Guarantee Fund, BaFin, AMF and the European Central Bank.

How we write and review

Every article has a named author shown at the top. Before publishing we recompute every table with the stated formula and check legal references against the official source. When regulation or market data change, we update the article and its last-updated date.

Corrections

If you spot a mistake, write to us from the Contact page with the page and the figure. We review it, correct the content where needed and update the review date.

Independence and how we are funded

Finturify is free. Some links to platforms may be affiliate links: if you open an account through them we may receive a commission at no cost to you. It does not change our calculations or conclusions, and we only cover platforms supervised by top-tier regulators.

What we are not

We are not an investment firm and we do not provide personalised financial advice. Our content is educational and informational. Past returns do not guarantee future returns, and taxation depends on your personal circumstances and may change. For specific decisions, consult a licensed professional.