Calculator Methodology

Transparency is the foundation of trust. Every calculator on 1gb.icu is built on a documented formula, sourced from an authoritative reference, with assumptions clearly stated. This page summarizes the methodology behind each category of calculator. For the full methodology of any specific calculator, see the "How it works" section on that calculator's page.

General Principles

Every 1gb.icu calculator follows these principles:

• Use the standard formula. We do not invent our own math. Every formula is sourced from an authoritative reference (IRS, Fannie Mae, standard financial textbooks, peer-reviewed research).
• Implement it accurately. Our JavaScript implementations are tested against known inputs and verified outputs from reputable sources (Bankrate, NerdWallet, official government calculators).
• Document it openly. Every calculator page displays the formula in plain English and explains each variable.
• Disclose all assumptions. When we use a default value (e.g., 0.5% PMI rate, 1% annual maintenance), we state it explicitly.
• Run client-side only. Your inputs never leave your browser. There is no server-side calculation, no logging, no tracking of your numbers.

Real Estate Calculators

Mortgage Payment Calculator

Formula: Standard amortization formula M = P × [r(1+r)^n] / [(1+r)^n − 1]

Where M = monthly payment, P = loan principal, r = monthly interest rate (annual rate ÷ 12 ÷ 100), n = number of payments (years × 12).

Source: Standard financial mathematics; same formula used by Fannie Mae, Freddie Mac, and every major lender.

PMI assumption: When loan-to-value (LTV) exceeds 80%, we apply PMI at 0.52% of loan amount annually for LTV 80–90%, 0.78% for LTV 90–95%, and 1.10% for LTV above 95%. These are industry averages from Mortgage Insurance Companies of America data. Actual PMI rates vary by credit score and lender.

Property tax and insurance: User-input values, divided by 12 for monthly amounts.

Mortgage Pre-Qualifier

Formula: Front-end DTI = (PITI) / monthly income; Back-end DTI = (PITI + all debts) / monthly income. Maximum loan derived by reverse-solving the amortization formula for P given the maximum allowed monthly payment.

Source: Fannie Mae Selling Guide B3-6-02 (Debt-to-Income Ratios); CFPB Ability-to-Repay rules under TILA.

DTI thresholds: Conservative 28/36, Standard 30/43, Aggressive 35/50. The 43% back-end ratio is the CFPB's Qualified Mortgage threshold.

Home Worth Estimator

Method: Sales comparison approach using regional price-per-square-foot baseline, adjusted for property-specific factors.

Source: Standard appraisal methodology from the Uniform Standards of Professional Appraisal Practice (USPAP).

ZIP code baselines: Sourced from recent median sales data. Multipliers: single family = 1.00, condo = 0.78, townhouse = 0.88, multi-family = 1.15 (industry averages from NAR data).

Adjustments: Bedrooms ±3% per bedroom above/below median (3); bathrooms ±4% per full bath above/below median (2); lot size ±0.75% per 1,000 sq ft above/below median (5,500 sq ft); condition +7.5%/baseline/-7.5%/-20% for excellent/good/fair/poor; age +2% for <5 years, -5% for >50 years, -10% for >75 years.

Refinance Calculator

Formula: Same amortization formula as mortgage calculator, applied to both current and new loan. Break-even = closing costs ÷ monthly savings. Total interest = (monthly P&I × n) − principal.

Source: Standard refinance analysis; methodology aligned with CFPB refinance disclosure requirements.

Rent vs Buy Calculator

Method: Year-by-year net cost comparison. Buyer net cost = closing costs + interest + property tax + insurance + PMI + HOA + maintenance − tax savings (mortgage interest deduction, if itemized) − appreciation + selling costs. Renter net cost = total rent (with growth) + renter's insurance − investment growth on down payment and monthly savings.

Source: Standard rent-vs-own analysis methodology; tax treatment per IRS Publication 936 (Home Mortgage Interest Deduction) and the Tax Cuts and Jobs Act of 2017.

Rental Property ROI Calculator

Formulas: NOI = gross rent × (1 − vacancy rate) − operating expenses; Cap rate = NOI / property value; Cash-on-cash return = (NOI − debt service) / cash invested; Total ROI = (cumulative cash flow + principal paid + appreciation − selling costs) / cash invested.

Source: Standard real estate investment analysis; methodology consistent with BiggerPockets and National Real Estate Investors Association guidelines.

Business Calculators

Business Loan Affordability

Formula: Max annual debt service = NOI / required DSCR. Max loan amount derived by reverse-solving amortization formula for P given max monthly payment.

Source: SBA Standard Operating Procedure (SOP) 50 10; commercial bank underwriting standards.

DSCR threshold: 1.25x minimum (SBA standard). Most conventional lenders require 1.25–1.50x.

Break-Even Analysis

Formula: BE units = Fixed Costs / (Price − Variable Cost per unit); BE revenue = BE units × Price; Contribution margin ratio = (Price − VC) / Price.

Source: Standard managerial accounting; consistent with NACM and AICPA cost accounting guidelines.

Profit Margin Calculator

Formulas: Gross margin = (Revenue − COGS) / Revenue × 100; Operating margin = (Revenue − COGS − OpEx) / Revenue × 100; Net margin = Net Income / Revenue × 100.

Source: Standard financial statement analysis; GAAP definitions of gross profit, operating income, and net income.

Business Valuation

Formulas: SDE = Net Profit + Owner Salary + Owner Benefits + Depreciation + Interest + Amortization + One-time expenses; Valuation = SDE × Multiple. EBITDA = Net Income + Interest + Taxes + Depreciation + Amortization; Valuation = EBITDA × Multiple.

Source: Business Brokerage Industry standards; BizBuySell median multiples; IBBA (International Business Brokers Association) methodology.

Multiple ranges: SDE 2.0–4.5x for owner-operated businesses under \$5M revenue; EBITDA 3.0–7.0x for mid-market businesses. Specific multiple depends on industry, growth, recurring revenue, and customer concentration.

Personal Finance Calculators

Loan EMI Calculator

Formula: EMI = P × r × (1+r)^n / [(1+r)^n − 1], where P = principal, r = monthly rate, n = number of months.

Source: Standard amortization formula, identical to mortgage payment formula.

Retirement Calculator

Formulas: Future value of current savings = P × (1+r)^n. Future value of monthly contributions = PMT × [((1+r)^n − 1) / r]. Required nest egg = annual expenses in retirement × 25 (4% rule) or user-specified multiplier. Monthly savings needed derived by reverse-solving the FV-of-annuity formula.

Source: 4% rule from the Trinity Study (Cooley, Hubbard, and Walz, 1998); FV formulas from standard financial mathematics.

Compound Interest Calculator

Formula: A = P(1 + r/n)^(nt), where A = final amount, P = principal, r = annual rate, n = compounding periods per year, t = years. For regular contributions: A = P(1+r/n)^(nt) + PMT × [((1+r/n)^(nt) − 1) / (r/n)].

Source: Standard compound interest formula; same formula used by SEC Investor.gov compound calculator.

Credit Card Payoff

Method: Month-by-month simulation. For each month: accrue interest on each balance at (APR/12), apply minimum payment (typically 1%–3% of balance or \$25, whichever is greater), direct any extra payment to the strategy target (smallest balance for snowball, highest APR for avalanche). Repeat until all balances reach zero.

Source: Standard debt payoff methodology; aligned with FTC and CFPB debt payoff guidance.

Net Worth Calculator

Formula: Net worth = Total Assets − Total Liabilities. Debt-to-asset ratio = Total Liabilities / Total Assets.

Source: Standard personal balance sheet; same methodology used by Federal Reserve Survey of Consumer Finances.

Insurance Calculators

Life Insurance Needs (DIME Method)

Formula: Total need = Debt + Income replacement (present value) + Mortgage payoff + Education fund (per child × number of children) + Final expenses − Existing savings − Existing life insurance. Income replacement = annual income × years needed, discounted to present value using PV-of-annuity formula.

Source: DIME method standard in life insurance industry; consistent with Life Happens and III (Insurance Information Institute) recommendations.

Auto Insurance Estimator

Method: Liability recommendation based on assets: 50/100/50 for assets <\$100k; 100/300/100 for \$100k–\$1M; 250/500/100 for \$1M–\$2M; 250/500/100 + \$2M umbrella for >\$2M. Full coverage recommended if vehicle value >\$4,000.

Source: III and NerdWallet recommendations; industry standards from major carriers.

Immigration Calculators

Visa Eligibility Screener

Method: Rule-based matching of user inputs (nationality, education, work experience, job offer, family ties, investment capacity, extraordinary ability) to visa category requirements. Each visa category has documented criteria sourced from USCIS policy manuals and INA (Immigration and Nationality Act) sections.

Source: USCIS Policy Manual; INA; Department of State Foreign Affairs Manual (9 FAM). DV lottery eligibility per 22 CFR 42.33.

Important disclaimer: This is a pre-screener, not legal advice. Actual eligibility depends on many factors our tool cannot evaluate. Always consult a licensed immigration attorney.

Citizenship Residency Calculator

Method: Counts days physically present in the US during the 5-year (or 3-year if married to US citizen) statutory period. Flags trips >6 months as breaking continuous residence presumption (rebuttable). Flags trips >12 months as breaking continuous residence (unless Form N-470 was filed). Calculates earliest application date as 90 days before 5-year (or 3-year) anniversary.

Source: INA Section 316(a) (naturalization requirements); 8 CFR 316.5 (physical presence); USCIS Policy Manual Volume 12.

Tax Calculators

Income Tax Calculator

Formula: Taxable income = AGI − deductions (standard or itemized). Tax computed using progressive marginal rates: each bracket taxed at its own rate. Federal tax brackets for 2024:

• Single: 10% (\$0–\$11,600), 12% (\$11,601–\$47,150), 22% (\$47,151–\$100,525), 24% (\$100,526–\$191,950), 32% (\$191,951–\$243,725), 35% (\$243,726–\$609,350), 37% (\$609,351+)
• Married Filing Jointly: 10% (\$0–\$23,200), 12% (\$23,201–\$94,300), 22% (\$94,301–\$201,050), 24% (\$201,051–\$383,900), 32% (\$383,901–\$487,450), 35% (\$487,451–\$731,200), 37% (\$731,201+)
• Head of Household: 10% (\$0–\$16,550), 12% (\$16,551–\$63,100), 22% (\$63,101–\$100,500), 24% (\$100,501–\$191,950), 32% (\$191,951–\$243,700), 35% (\$243,701–\$609,350), 37% (\$609,351+)

Standard deduction 2024: Single \$14,600, MFJ \$29,200, HoH \$21,900.

Source: IRS Revenue Procedure 2023-34 (annual inflation adjustments); IRS Publication 505 (Tax Withholding and Estimated Tax).

Sales Tax Calculator

Method: Combined state + average local sales tax rates for each state. Sales tax amount = pre-tax amount × combined rate. Total = pre-tax + sales tax.

Source: State-by-state rates from Tax Foundation's annual state and local sales tax rates report; verified against each state's Department of Revenue.

Capital Gains Calculator

Formulas: Gain = sale price − purchase price − selling costs. Long-term capital gains rates (2024): 0% (income up to \$47,025 single / \$94,050 MFJ), 15% (up to \$518,900 single / \$583,750 MFJ), 20% (above). Short-term gains taxed at ordinary income rates. Net Investment Income Tax (NIIT) of 3.8% applies to investment income above \$200,000 single / \$250,000 MFJ.

Source: IRS Publication 544 (Sales and Other Dispositions of Assets); IRC Section 1(h) (capital gains rates); IRC Section 1411 (NIIT).

Health Calculators

BMI Calculator

Formula: Metric: BMI = weight(kg) / height(m)². Imperial: BMI = 703 × weight(lbs) / height(in)².

Categories: Underweight <18.5, Normal 18.5–24.9, Overweight 25.0–29.9, Obese ≥30.0.

Source: WHO (World Health Organization) BMI classification; CDC BMI calculator methodology.

BMR & TDEE Calculator

Formula (Mifflin-St Jeor): BMR men = (10 × kg) + (6.25 × cm) − (5 × age) + 5. BMR women = (10 × kg) + (6.25 × cm) − (5 × age) − 161. TDEE = BMR × activity multiplier (1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 very active, 1.9 extra active).

Source: Mifflin MD, St Jeor ST, et al. "A new predictive equation for resting metabolic rate in healthy individuals." American Journal of Clinical Nutrition, 1990. Activity multipliers from Harris-Benedict revision.

Macro Calculator

Method: Calculate TDEE using Mifflin-St Jeor. Apply goal multiplier (cut −20%, maintain, lean bulk +10%, bulk +15%). Set protein at 1.6–2.2 g/kg bodyweight (user-adjustable). Set fat at 25% of total calories. Carbohydrates = remaining calories ÷ 4.

Source: Protein recommendations from International Society of Sports Nutrition Position Stand (2017); macronutrient framework consistent with Academy of Nutrition and Dietetics.

Investment Calculators

Investment Return Calculator

Formula: Future value with contributions = P(1+r)^n + PMT × [((1+r)^n − 1) / r], where P = initial investment, PMT = monthly contribution, r = monthly rate, n = months. CAGR = (Final/Initial)^(1/years) − 1.

Source: Standard financial mathematics; same formula used by SEC Investor.gov and major brokerage calculators.

Dividend Yield Calculator

Formulas: Current yield = annual dividend / current price × 100. Yield on cost = annual dividend / purchase price × 100. Annual income = annual dividend × shares. DRIP projection: FV of reinvested dividends using compound interest formula.

Source: Standard dividend analysis methodology; consistent with methodology used by Seeking Alpha, Morningstar, and S&P.

Limitations and Disclaimers

Despite our rigorous methodology, every calculator on this site has limitations:

• Simplified models: Real-world financial situations are complex. Our calculators use standard formulas that may not capture every nuance of your situation.
• Rule changes: Tax laws, immigration policies, and insurance regulations change frequently. We update content regularly but cannot guarantee real-time accuracy.
• Input dependence: Calculator results are only as accurate as the inputs you provide. Verify your inputs carefully.
• Not professional advice: Our calculators are educational tools, not substitutes for advice from qualified professionals.

Always verify critical calculations with the relevant authority (IRS, USCIS, your lender, your insurance agent, your financial advisor) before making major decisions. See our Disclaimer for full details.

Questions About Our Methodology?

If you have questions about how any calculator works, want to verify a formula, or believe you've found an error, please contact us at contact@1gb.icu. We welcome scrutiny—it makes our content better.