Last reviewed September 28, 2026
Calculation methodology
Richerly Tools turns established financial formulas into transparent planning scenarios. Calculations run in your browser, retain full precision internally, and round only the displayed result. This page explains what each tool calculates, when cash flows occur, and which real-world factors are not included.
Compound interest with monthly contributions
A principal balance compounded n times per year for t years follows the standard compound-growth equation:
The calculator also models equal monthly contributions. It converts the selected compounding frequency to an equivalent monthly growth factor, applies that growth to the existing balance, and adds each contribution at the end of the month.
This is a constant-rate scenario. It excludes market volatility, taxes, fees, and inflation. Beginning-of-month contributions would produce a slightly higher result. For an independent reference, see the U.S. SEC Investor.gov compound interest calculator.
Rule of 72 and exact doubling time
The Rule of 72 is a mental-math approximation: divide 72 by the annual percentage return. Richerly Tools places that estimate beside the exact logarithmic solution.
Exact years = ln(2) / ln(1 + r)
The shortcut is most useful for ordinary positive rates and becomes less accurate at unusually high rates. It assumes a constant return and no deposits or withdrawals. Investor.gov also describes the Rule of 72 as a quick doubling estimate.
FIRE target and withdrawal rates
The FIRE calculator first subtracts dependable retirement income from annual spending. It then divides the remaining spending gap by the withdrawal rate selected by the user.
A 4% selection produces the familiar 25-times-spending estimate. It is a starting scenario, not a guarantee. The research commonly called the Trinity Study tested historical U.S. stock-and-bond portfolios across different withdrawal rates and payout periods. Its results depend on retirement length, asset allocation, inflation, fees, taxes, and the sequence of market returns.
Primary research: Philip L. Cooley, Carl M. Hubbard, and Daniel T. Walz, A Comparative Analysis of Retirement Portfolio Success Rates. Related paper: Sustainable Withdrawal Rates From Your Retirement Portfolio.
Emergency-fund recommendation
This calculator is intentionally rules-based rather than predictive. It begins with three months of essential expenses, then adds coverage for variable income, dependents, and limited health or income protection, capped at twelve months.
Essential expenses generally include housing, basic food, utilities, insurance, necessary transportation, minimum debt payments, and required care costs. The recommendation is a planning prompt, not a universal financial standard.
Debt snowball and avalanche comparison
The snowball method sends available extra money to the smallest current balance. The avalanche method targets the highest APR. Both approaches continue estimated minimum payments on all other balances and roll freed payment capacity into the next priority debt.
The model estimates each minimum payment as 2% of the starting balance with a $25 floor. Interest is approximated monthly from the entered APR.
Actual lender calculations may use daily interest, statement-specific minimums, promotional rates, fees, or payment-allocation rules. Use lender statements for a binding payoff amount.
HYSA growth
Annual percentage yield already reflects compounding. The calculator converts APY into an equivalent monthly rate before applying end-of-month deposits.
Savings APYs are generally variable. The comparison assumes the entered rates remain unchanged and excludes taxes, account fees, withdrawal restrictions, and inflation. Rates shown on the calculator are user-entered scenarios, not current account offers.
Precision and validation
- Calculations retain JavaScript numeric precision and round only for display.
- Currency results use U.S. dollar formatting; percentages remain user-entered scenarios.
- Every material code change should be checked against independent worked examples and boundary cases.
- Zero values, long time horizons, and unusually high rates should be treated as stress cases rather than typical plans.
Editorial and correction standard
- Formulas and cash-flow timing must be visible to the user.
- Rules of thumb must be labeled as estimates and accompanied by their material limitations.
- Sources and review dates must be updated when assumptions or underlying research change.
- Advertising or affiliate relationships must not change calculator output.
If an output appears inconsistent, reproduce it with the same inputs and contact the site operator with the calculator name, inputs, and expected result. Do not include account numbers or other sensitive information.