Methodology & Formula Documentation
Complete transparency into how every EstiNest calculator works. All formulas, assumptions, data sources, and limitations documented.
Covering 44+ calculators across 11 categories · Last updated: February 2026
Our Methodology Principles
Transparent Formulas
Every calculator shows the exact mathematical formula used. No proprietary black boxes.
Cited Sources
Formulas and data sourced from authoritative institutions: IRS, SSA, BLS, CFPB, CDC, NIH, HUD.
Documented Assumptions
Every simplifying assumption is explicitly stated so you can judge applicability.
Known Limitations
We proactively disclose what our calculators cannot model or account for.
Client-Side Execution
All calculations run in your browser. Your financial data is never transmitted to our servers.
Regular Verification
Formulas and data references are reviewed when source agencies publish updates.
Mortgage Calculators
Last verified: February 2026
Mortgage calculations use the standard amortization formula endorsed by the CFPB and Freddie Mac. All rates and terms are user-supplied; no default assumptions about market conditions are baked in.
Formulas
Monthly Payment (Fixed-Rate)
Consumer Financial Protection Bureau ↗M = P × [r(1+r)^n] / [(1+r)^n - 1]Total Interest
Freddie Mac Mortgage Guide ↗Total Interest = (M × n) - PPMI (when applicable)
HUD PMI Guidelines ↗PMI = Loan Amount × PMI Rate / 12Assumptions
- ⚠ Fixed interest rate over the full loan term (adjustable-rate scenarios not modeled)
- ⚠ Monthly compounding and payment frequency
- ⚠ No prepayment penalties assumed
- ⚠ PMI calculated at 0.5–1% of loan amount annually when LTV > 80%
- ⚠ Property taxes, insurance, and HOA not included unless explicitly entered
Known Limitations
- ✘ Does not account for adjustable-rate mortgage (ARM) rate resets
- ✘ Closing costs, origination fees, and discount points not included in base calculation
- ✘ Property tax rates vary significantly by location and are not auto-populated
- ✘ Actual lender offers may differ based on credit score, debt-to-income ratio, and market conditions
Investment & Finance Calculators
Last verified: February 2026
Investment calculators use compound interest formulas and time-value-of-money principles. Historical return assumptions are based on long-term market data but do not predict future performance.
Formulas
Compound Interest (with contributions)
SEC Investor.gov ↗FV = P(1+r/n)^(nt) + PMT × [((1+r/n)^(nt) - 1) / (r/n)]Real Return (inflation-adjusted)
Bureau of Labor Statistics CPI ↗Real Return = ((1 + Nominal) / (1 + Inflation)) - 1Rule of 72 (doubling time)
Federal Reserve Education ↗Years to Double ≈ 72 / Annual Return %Assumptions
- ⚠ Returns compounded monthly or annually (as specified)
- ⚠ Contributions made at end of each period
- ⚠ No taxes on gains during accumulation (pre-tax modeling)
- ⚠ Constant rate of return (no volatility modeling)
- ⚠ Inflation not automatically deducted unless user specifies
Known Limitations
- ✘ Past performance does not guarantee future results
- ✘ Does not model market volatility, sequence-of-returns risk, or drawdown scenarios
- ✘ Tax implications (capital gains, dividend taxes) not included
- ✘ Investment fees and expense ratios not automatically deducted
- ✘ Does not account for contribution limits in tax-advantaged accounts
Tax Calculators
Last verified: February 2026 (2024-2025 tax brackets)
Tax calculations use the current federal income tax brackets published by the IRS. State taxes are not included. These are estimates only — not a substitute for professional tax preparation.
Formulas
Progressive Tax
IRS Revenue Procedure 2024-40 ↗Tax = ∑ (Income in Bracket × Bracket Rate)Effective Tax Rate
IRS Publication 17 ↗Effective Rate = Total Tax / Gross Income × 100FICA (Social Security + Medicare)
Social Security Administration ↗FICA = Income × 7.65% (up to SS wage base)Assumptions
- ⚠ Filing status and deductions as entered by user
- ⚠ Standard deduction applied unless user specifies itemized
- ⚠ Federal tax only (state and local taxes not included)
- ⚠ 2024/2025 tax brackets and standard deductions
- ⚠ No AMT (Alternative Minimum Tax) calculation
Known Limitations
- ✘ Does not account for all credits (e.g., child tax credit, EITC, education credits)
- ✘ State income taxes vary from 0% to 13.3% and are not modeled
- ✘ Self-employment tax calculated separately
- ✘ Capital gains, dividends, and investment income may have different rates
- ✘ Tax law changes may affect accuracy — always verify with current IRS publications
Health & Fitness Calculators
Last verified: January 2026 (CDC/NIH guidelines)
Health calculators use established medical formulas from CDC, NIH, and WHO guidelines. These are screening tools only and do not constitute medical advice.
Formulas
BMI = Weight(kg) / Height(m)²BMR (Mifflin-St Jeor)
NIH / American Dietetic Association ↗BMR = 10×Weight(kg) + 6.25×Height(cm) - 5×Age - 161 (F) or +5 (M)TDEE = BMR × Activity Multiplier (1.2–1.9)Assumptions
- ⚠ Standard adult formulas (not calibrated for children, pregnant women, or elderly)
- ⚠ Activity multipliers are estimates based on general activity categories
- ⚠ BMI does not distinguish between muscle mass and fat mass
Known Limitations
- ✘ BMI may misclassify athletes, elderly, and certain ethnic groups
- ✘ Calorie calculations are estimates — individual metabolism varies significantly
- ✘ Does not account for medical conditions affecting metabolism
- ✘ Not a substitute for medical advice, diagnosis, or treatment
Business Calculators
Last verified: February 2026
Business calculators use standard financial modeling formulas for break-even analysis, ROI, and profitability projections.
Formulas
Break-Even Point
SBA Business Guide ↗BEP (units) = Fixed Costs / (Selling Price - Variable Cost per Unit)ROI = (Net Profit / Total Investment) × 100Profit Margin
Standard financial accounting ↗Profit Margin = (Revenue - Costs) / Revenue × 100Assumptions
- ⚠ Linear cost and revenue relationships
- ⚠ Fixed costs remain constant within the relevant range
- ⚠ Single product or weighted average for multi-product businesses
Known Limitations
- ✘ Does not model seasonality, market dynamics, or competitive forces
- ✘ Assumes all units produced are sold
- ✘ Does not include time-value-of-money for long-term projections
- ✘ Working capital requirements not modeled
AI Interpretation Methodology
How AI enhances (not replaces) calculator results
Architecture
- • Calculations: 100% deterministic, client-side, using standard mathematical formulas. No AI involved in number computation.
- • AI Layer: OpenAI GPT-4o-mini interprets pre-computed results. AI never performs or modifies calculations.
- • Fallback System: 12 category-specific template-based explanations serve as offline fallback if AI is unavailable.
- • Guardrails: System prompts enforce educational framing, prohibit financial advice language, and require specific numbers from user inputs.
AI Limitations & Disclaimers
- • AI explanations are educational interpretations, not personalized financial advice.
- • The AI model may produce inaccurate, incomplete, or contextually inappropriate analysis.
- • AI responses are not reviewed by financial professionals before display.
- • Optimization suggestions are general educational strategies, not recommendations.
- • What-if scenarios are rough illustrative estimates, not precise projections.
⚠ Important Disclaimer
EstiNest calculators provide estimates for educational and informational purposes only. They do not constitute financial, tax, legal, investment, or medical advice.
Results depend entirely on user-provided inputs and the stated assumptions. Actual outcomes may differ materially due to market conditions, individual circumstances, fees, taxes, regulatory changes, and other factors not modeled.
Always consult qualified professionals (financial advisor, CPA, attorney, physician) before making decisions based on calculator results.