How CostSignals Calculates Finance Costs

Key Assumptions

• The interest rate remains fixed for the entire loan term (adjustable-rate mortgages will have different payment trajectories)
• Property taxes and insurance premiums are estimated using national averages and may differ significantly in your area
• PMI rates are approximated based on down payment percentage and typical lender requirements; actual rates depend on credit score and loan-to-value ratio
• Closing costs are either paid upfront or rolled into the new loan balance (rolling in costs increases the principal and reduces net savings)
• The homeowner will remain in the property long enough to reach the break-even point
• The comparison assumes fixed-rate terms for both the current and new mortgage; ARMs introduce variable payment uncertainty

Sample Calculation Approach

Calculates the monthly mortgage payment using the standard amortization formula: M = P[r(1+r)^n] / [(1+r)^n - 1], where P is the loan principal, r is the monthly interest rate, and n is the total number of payments. The tool then adds estimated property tax, homeowner's insurance, and private mortgage insurance (PMI) when the down payment is below 20%.

Key Assumptions

• Closing costs are estimated as a percentage of the loan amount; actual fees vary by lender, location, and loan type
• Title insurance rates vary significantly by state; some states have regulated rates while others are competitive
• Prepaid items (taxes, insurance) depend on closing date and local tax assessment schedules
• PMI rate is based on the original LTV ratio; rates may adjust if home value changes significantly
• PMI can be canceled when the loan balance reaches 80% of the original appraised value (Homeowners Protection Act)
• Borrower-paid monthly PMI is assumed; lender-paid PMI baked into the rate is not modeled here

Sample Calculation Approach

Estimates total closing costs as a percentage of the loan amount (typically 2-5%), broken down into lender fees (origination, underwriting, appraisal), third-party fees (title insurance, escrow, survey), and prepaid items (property tax escrow, homeowner's insurance, prepaid interest).

Key Assumptions

• Essential expenses include fixed costs that cannot be quickly reduced (rent/mortgage, insurance, minimum debt payments)
• The recommended coverage period is 3 months for dual-income households with stable employment and 6 months for single-income or variable-income households
• Emergency funds are held in liquid, low-risk accounts (savings, money market) rather than invested
• The interest rate on your savings remains constant over the savings period
• Monthly contributions are made at the beginning of each month
• Interest compounds monthly; actual compounding frequency may vary by institution

Sample Calculation Approach

Calculates the recommended emergency fund size based on monthly essential expenses (housing, food, insurance, transportation, debt payments) multiplied by a coverage period of 3 to 6 months. Projects the time needed to build the fund based on monthly savings capacity.

Key Assumptions

• Contributions are made per-paycheck throughout the year; lump-sum contributions would yield slightly different results
• Employer match vests according to the specified schedule (immediate, 3-year cliff, or 6-year graded)
• Investment returns are constant at the modeled rate; actual annual returns fluctuate significantly
• Portfolio returns use historical averages by asset allocation (stocks/bonds mix)
• Inflation is modeled at 2.5% annually (20-year average CPI); healthcare inflation runs 5–7%
• Social Security and pension income is inflation-adjusted via COLA

Sample Calculation Approach

Projects 401(k) account growth using compound growth with employer matching: models annual contributions up to the IRS limit ($23,500 in 2025, $31,000 with catch-up for 50+), employer match percentage and vesting schedule, expected investment return by asset allocation, and tax-deferred growth. Shows projected balance at retirement age under conservative, moderate, and aggressive allocation scenarios.

Key Assumptions

• Net self-employment income is calculated after all Schedule C deductions
• The 92.35% factor represents the employer-equivalent portion of FICA that reduces the tax base
• The deductible half of SE tax is factored into adjusted gross income on Form 1040
• Triple tax advantage assumes contributions are deductible, growth is tax-deferred, and withdrawals for medical expenses are tax-free
• Investment return projections use historical diversified portfolio averages
• State tax savings excluded for CA, NJ, and NH which do not recognize federal HSA deductions

Sample Calculation Approach

Calculates self-employment tax (Social Security + Medicare) on net self-employment income after the 92.35% adjustment. Applies current FICA rates: 12.4% Social Security (up to the wage base limit) + 2.9% Medicare (no cap) + 0.9% Additional Medicare Tax above $200,000.

Key Assumptions

• Lease terms assume standard mileage allowance (12,000 miles/year); excess mileage penalties apply at $0.15-$0.25/mile
• The buy scenario includes resale value at the end of the analysis period based on depreciation curves
• Tax treatment assumes no sales tax advantage for leasing; some states tax lease payments differently
• Lease terms of 24-36 months with 10,000-15,000 mile/year allowance. Excess mileage: $0.15-$0.30/mile
• Vehicle depreciation averages 20% in year 1, 15% in year 2, and 10-12% per year thereafter
• Sales tax varies by state — some tax full purchase price, others only monthly lease payments

Sample Calculation Approach

Compares the total cost of leasing versus buying a vehicle over a specified period. Leasing costs include monthly payments, upfront fees, and mileage penalties. Buying costs include loan payments, depreciation, maintenance, and resale value. The net cost difference shows which option is cheaper over the analysis period.

Key Assumptions

• The current federal estate tax exemption is approximately $13.6 million per individual ($27.2 million for married couples with portability)
• State estate taxes are not included; 12 states and DC impose their own estate taxes with lower exemption thresholds
• Estate value includes all assets: real estate, investments, retirement accounts, life insurance death benefits, and business interests
• Gift valuations are at fair market value on the date of the gift
• The unified credit (lifetime exemption) has not been previously used unless specified
• Gifts to spouses who are US citizens are unlimited (marital deduction) and not taxable

Sample Calculation Approach

Estimates the federal estate tax liability by subtracting the applicable exclusion amount from the gross estate value, then applying the graduated estate tax rates (18-40%) to the taxable estate. Accounts for the unlimited marital deduction and charitable deductions.

Key Assumptions

• Vacancy rate is estimated at 5-8% of gross rent; actual rates vary by market and property quality
• Operating expenses (maintenance, insurance, management) are estimated at 35-50% of gross rent
• Property appreciation uses a conservative 3% annual rate; actual appreciation varies significantly by location
• After-repair value (ARV) is estimated from comparable sales within 0.5 miles and 6 months
• Refinance LTV is 75% of ARV for conventional loans or 80% for DSCR loans
• Operating expenses follow the 50% rule (50% of gross rent covers taxes, insurance, maintenance, vacancy, and management)

Sample Calculation Approach

Calculates cash-on-cash return, cap rate, and total ROI for rental properties. Cash-on-cash = Annual Cash Flow / Total Cash Invested. Cap Rate = Net Operating Income / Property Value. Total ROI includes appreciation, equity buildup through mortgage paydown, and tax benefits.

Key Assumptions

• Tuition inflation rate of 5% per year (historical average); actual increases vary by institution
• Room and board costs assume on-campus housing for years 1-2 and off-campus thereafter
• Financial aid packages are estimated using national averages; actual aid depends on institutional policy and family finances
• Refinance rates for excellent credit (750+): 4-6% fixed, 3-5% variable (2026 market conditions)
• Refinancing federal loans into private eliminates access to IDR plans, PSLF, and federal forbearance
• Origination fees are rare in student loan refinancing — most lenders charge no origination or prepayment fees

Sample Calculation Approach

Projects total college costs by combining current tuition, room and board, books, and personal expenses, then applying an annual inflation rate (historically 5-7% for higher education). Factors in financial aid, scholarships, and 529 plan savings to estimate the net out-of-pocket cost.

Key Assumptions

• Interest rates are based on creditworthiness tier (excellent, good, fair) and loan type
• SBA loan rates are capped at SBA base rate + maximum spread (currently WSJ Prime + 2.25% for loans over $50K)
• Origination fees and guarantee fees are included in the total cost of borrowing
• Utilization rate (billable vs. total hours) varies by industry: consulting ~65%, design ~55%, development ~60%
• Self-employment tax is estimated at 15.3% on net earnings; actual SE tax includes the 50% deduction for the employer portion
• Vacation, sick days, and holidays are subtracted from total available working days (standard: 10 vacation + 10 holidays + 5 sick = 25 days)

Sample Calculation Approach

Calculates business loan payments and total cost across loan types: term loans (fixed monthly payments), SBA loans (7(a) and 504 programs with government-backed rates), lines of credit (interest-only on drawn balance), and equipment financing. Models different repayment structures including interest-only periods, balloon payments, and amortizing schedules.

Key Assumptions

• Target replacement ratio is 60-70% of pre-disability gross income to maintain essential living expenses
• SSDI benefits average $1,500-$1,800/month but approval takes 3-6 months and denial rate exceeds 60%
• Employer group LTD typically replaces 60% of base salary only, excluding bonuses and commissions
• Term premiums locked for the selected term length; renewal rates post-term increase 5-10x
• Whole life cash value growth averages 1-3% annually (dividend-paying mutual companies)
• Buy-term-invest-difference assumes 7% average annual market return with 15% capital gains tax on withdrawals

Sample Calculation Approach

Calculates recommended disability insurance coverage using current gross income, monthly essential expenses, existing employer group disability benefits, SSDI eligibility estimate, liquid savings runway, and selected benefit period. Produces gap analysis showing uninsured monthly shortfall.

Key Assumptions

• The interest rate is fixed for the entire loan term
• No origination fee is included in the payment calculation; some lenders charge 1-8% upfront
• Payments are made monthly without prepayment or late fees

Sample Calculation Approach

Calculates the monthly payment and total interest on a fixed-rate personal loan using the standard amortization formula. Compares multiple term lengths (24, 36, 48, 60 months) to show the trade-off between monthly payment size and total interest paid.

Key Assumptions

• Withholding is calculated based on W-4 selections; actual withholding may vary by employer payroll system
• State tax calculations use standard rates; local taxes (city, county) are not included
• Pre-tax deductions are applied before federal and state tax calculations as permitted by law

Sample Calculation Approach

Estimates net take-home pay by subtracting federal income tax (using current IRS brackets), state income tax, Social Security (6.2% up to wage base), Medicare (1.45% + 0.9% additional above $200k), and pre-tax deductions (401k, health insurance) from gross pay.

Key Assumptions

• College cost inflation averages 5-6% annually (historically higher than general CPI)
• Investment returns are before-fee averages based on typical age-based portfolio glide paths
• State tax deductions are estimated using the highest marginal state income tax bracket; actual benefit depends on total income

Sample Calculation Approach

Projects 529 education savings plan growth using compound interest with regular contributions, then compares the projected balance against expected total college costs. Models the tax-free growth benefit of 529 plans vs. taxable accounts to quantify the advantage. Accounts for contribution timing, expected return rates based on the plan's asset allocation, and state tax deduction benefits where applicable.

Key Assumptions

• Investment returns are compounded annually at the specified average rate; actual returns vary year to year
• The FIRE number is calculated as 25x target annual spending (based on 4% safe withdrawal rate)
• No additional contributions after reaching Coast FIRE; the portfolio grows purely through market returns

Sample Calculation Approach

Determines the Coast FIRE number — the portfolio value at which you can stop contributing and let compound growth carry you to your full FIRE number by your target retirement age. Formula: Coast FIRE Number = FIRE Number / (1 + Expected Return)^(Years Until Retirement). Once your portfolio reaches this threshold, all new income can go toward current living expenses.

Key Assumptions

• All crypto-to-crypto trades are taxable events (per IRS guidance); cost basis is determined at the time of each trade
• Long-term capital gains rates (0%, 15%, 20%) apply to assets held over 1 year; short-term gains are taxed as ordinary income
• The 3.8% Net Investment Income Tax (NIIT) applies to individuals with MAGI above $200,000 ($250,000 married filing jointly)

Sample Calculation Approach

Calculates capital gains tax liability on cryptocurrency transactions using FIFO (First In, First Out), LIFO (Last In, First Out), or Specific Identification cost basis methods. Determines short-term vs. long-term capital gains based on holding period (>1 year = long-term), applies the appropriate federal tax bracket, and estimates state tax where applicable. Handles multiple transactions across different crypto assets.