Every result should be inspectable.

Shared calculation principles

• Calculations use decimal arithmetic with 40-digit internal precision; displayed money is rounded for readability.
• Inputs are user-controlled scenarios. The site does not select a suitable return, fee, security or course of action.
• Unsupported or impossible inputs are rejected rather than silently changed; negative-return scenarios remain supported within the stated model limits.
• Taxes are excluded unless a future page explicitly models a named tax rule and date.

Compound interest with contributions

The starting amount grows by (1 + r/m)^(m×t). Contributions use the future-value annuity factor at the effective rate for each contribution interval. Beginning-of-period deposits receive one additional interval of growth. Inflation-adjusted value divides the nominal result by (1 + inflation)^t.

Savings goal

Current savings are grown to the goal date. The remaining target is divided by the monthly future-value annuity factor. If current savings alone meet the entered target under the scenario, the required monthly contribution is zero.

Investment fee impact

The no-fee path uses the gross annual growth factor. The after-fee path multiplies that factor by (1 − annual fee), converts the combined result to a monthly factor and applies the same deposits. Fee drag includes the growth no longer earned on deducted amounts.

Stock and foreign returns

Stock total cost is shares × buy price + buy fee. Net proceeds are shares × sell price + dividends − sell fee. For overseas positions, the rate is always one unit of stock currency expressed in home currency; stock and FX effects are shown separately.

Optional rates come from the European Central Bank. The ECB describes them as informational references and discourages transaction use, so users can overwrite them with broker records.

Examples calculated by the shared engine

Illustrative scenarios, not forecasts or recommended returns. Values are generated by the same functions as the tools; amounts are rounded for readability and shown in USD unless stated otherwise.

• Compound interest: With $10,000 to start, $500 contributed at each month-end, a 7% nominal return for 20 years and 2.5% inflation, the calculated ending balance is about $300,851. Total principal is $130,000; the remainder is modeled growth.
• Savings goal: For a $100,000 goal, $15,000 already saved, a 5% nominal return and eight years, the calculated monthly saving is about $659.
• Fee impact: With $25,000 initially, $500 monthly, 7% gross growth and a 1% annual fee for 25 years, the model ends at about $440,267 after fees versus $527,207 without the modeled fee.
• Stock profit: For 100 shares bought at $42 and valued at $51, with $12 in total fees and $120 in dividends, calculated net profit is $1,008.
• Foreign return: With 25 shares bought at EUR 80 and valued at EUR 105, home-currency rates of 1.09 and 1.16, 16 in home-currency fees and EUR 40 in dividends, calculated net profit is about $895.

Known limitations

• Returns, inflation, fees and exchange rates can change over time.
• Fee modeling excludes tiers, minimum charges, spreads, taxes and product-specific rules.
• Stock calculators do not fetch live prices or assess a security’s quality or suitability.
• Displayed rounding can differ slightly from the internal result.