Methodology and Sources
Last updated
How every ClearPathCalc calculator is built, which published formula and source it relies on, how we check it, and a dated record of what has changed.
Principles
- Published formulas only. Each calculator uses a standard, documented method: loan amortization, the IRS brackets, WHO classifications, peer-reviewed equations. We do not invent scoring systems.
- Show the work. Every calculator page states its formula, explains each input, and walks through a worked example you can check by hand.
- Say what is left out. Each page lists the assumptions behind the result and when you need a professional instead.
- Private by design. Calculations run in your browser. We never receive the numbers you enter. See the Privacy Policy.
Formulas and sources, calculator by calculator
All 34 calculators were last reviewed end to end on September 26, 2026.
Financial
Future value of a lump sum plus regular contributions, compounded at the chosen frequency.
Standard fixed-rate amortization, month by month, with any extra payment applied to principal.
Gross income minus the standard deduction, taxed slice by slice through the federal brackets for your filing status.
Total interest over each debt’s remaining amortization, compared with one new loan at the rate, term and fees you enter.
Break-even months = out-of-pocket closing costs ÷ monthly payment savings. Total savings = current payment × months left on the current loan − (new payment × new term in months + out-of-pocket costs).
Month by month: buyer net worth = home value after selling costs − loan balance; renter net worth = down payment and closing costs invested, plus each month’s difference between owning and renting costs, compounded at your investment return.
Source: Consumer Financial Protection Bureau, buying a home; IRS 2026 standard deduction
Each month: new balance = balance + balance × APR ÷ 12 − payment. Minimums go to every debt; the extra, and every freed-up minimum, goes to the smallest balance (snowball) or highest rate (avalanche).
Target = monthly essential expenses × recommended months (three, plus extra months for your job and household risks); time to target = gap ÷ monthly saving.
Source: Consumer Financial Protection Bureau, essential guide to building an emergency fund
Tax
Deduction = the premium half of time-and-a-half pay, capped at $12,500 ($25,000 joint), reduced by $100 for every $1,000 of MAGI over $150,000 ($300,000 joint); tax saved = tax before − tax after.
Source: IRS, questions and answers on the deduction for qualified overtime compensation
Deduction = qualified tips, capped at $25,000, reduced by $100 for every $1,000 of MAGI over $150,000 ($300,000 joint).
Source: IRS, final regulations listing occupations that customarily receive tips
Interest paid in the year from a standard amortization schedule, capped at $10,000, reduced by $200 for every $1,000 of MAGI over $100,000 ($200,000 joint).
Source: IRS, guidance on the deduction for car loan interest
Deduction = $6,000 per person aged 65 or older, reduced by 6% of MAGI over $75,000 ($150,000 joint).
Source: IRS, eligibility for the enhanced deduction for seniors
SE tax = 92.35% of net profit × (12.4% Social Security up to the $184,500 wage base + 2.9% Medicare), plus 0.9% Additional Medicare Tax over the threshold; half of SE tax and a simplified 20% QBI deduction reduce income tax.
Retirement
Take-home drop per paycheck = contribution − (federal tax saved + state rate × pre-tax contribution) ÷ paychecks. Federal tax uses the 2026 brackets and standard deduction; Social Security and Medicare are unchanged. Contributions stop at the 2026 limit ($24,500, plus catch-ups), and the match is figured each paycheck.
Source: IRS release IR-2025-111 and Notice 2025-67 (2026 limits)
You keep = withdrawal − 10% additional tax (under 59½, no exception) − federal tax (tax with the withdrawal minus tax without it) − state tax. Check = withdrawal × (1 − 20% withholding). Loan limit = lesser of $50,000 or the greater of $10,000 and half the vested balance.
Source: IRS, exceptions to tax on early distributions; rollovers and withholding; plan loan FAQs
FIRE number = annual spending ÷ safe withdrawal rate; years to reach it from your current savings, yearly contribution and expected return.
Source: Bengen, “Determining Withdrawal Rates Using Historical Data”, Journal of Financial Planning, 1994
Claiming early cuts the benefit 5/9 of 1% a month for the first 36 months and 5/12 of 1% after that; delaying past full retirement age adds 2/3 of 1% a month to age 70. Break-even is the age when cumulative benefits cross.
Source: Social Security Administration, benefit reduction and delayed retirement credits
RMD = prior December 31 account balance ÷ the IRS Uniform Lifetime Table factor for your age.
Traditional after-tax value = contribution × growth multiple × (1 − retirement tax rate); Roth = contribution × (1 − current tax rate) × growth multiple.
Future value = yearly contributions compounded at the expected return; tax saved = contribution × (income tax rate, plus 7.65% FICA when made through payroll).
Source: IRS Publication 969 and Rev. Proc. 2025-19 (2026 limits)
Business
ROI = (value of time saved − total cost of ownership) ÷ total cost of ownership, over the period you choose.
ROAS = revenue ÷ ad spend; break-even ROAS = 1 ÷ gross margin.
PERT expected time = (optimistic + 4 × most likely + pessimistic) ÷ 6; standard deviation = (pessimistic − optimistic) ÷ 6.
Source: Program Evaluation and Review Technique (PERT), as taught in project management practice
Real hourly rate = net pay ÷ (paid hours + commute and other unpaid work hours).
Source: Arithmetic from your own inputs
Net hours saved = team size × eligible hours per week × 52 × gain rate × adoption × (1 − oversight share); value = net hours × loaded hourly cost; capacity = net hours ÷ (team size × 2,080).
Source: Arithmetic from your own inputs
Health
BMI = weight (kg) ÷ height (m)², classified with the WHO adult categories.
Source: World Health Organization
Mifflin-St Jeor resting energy × activity multiplier, adjusted for your goal, then split into protein, carbohydrate and fat.
Source: Mifflin MD, St Jeor ST, et al. American Journal of Clinical Nutrition, 1990
U.S. Navy circumference equations using height, neck and waist (plus hip for women), with log₁₀ terms.
Source: Hodgdon JA, Beckett MB. Naval Health Research Center, 1984
Waist-to-hip ratio = waist ÷ hip, compared with the WHO risk thresholds (0.90 men, 0.85 women).
Source: WHO Expert Consultation on Waist Circumference and Waist-Hip Ratio, 2008
Waist-to-height ratio = waist ÷ height (same units): 0.40–0.49 healthy, 0.50–0.59 increased, 0.60 or more high central adiposity.
Source: NICE guideline NG246, overweight and obesity management
Sustainability
Annual fuel cost = miles ÷ efficiency × energy price, for the gas car (mpg, $/gal) and the EV (mi/kWh, $/kWh).
Simple payback = net installed cost ÷ yearly electricity savings.
Source: U.S. Department of Energy, homeowner’s guide to going solar
Emissions = Σ activity × emission factor, across electricity, driving, flights and diet.
Daily need (kWh) = Σ watts × hours per day ÷ 1,000; usable capacity = rated kWh × depth of discharge × inverter efficiency; backup days = usable capacity ÷ daily need.
Source: Watt-hour arithmetic from your appliance list; federal credit status from the IRS
How calculators are checked
- Automated tests. Each calculator has tests that pin its results for known inputs, including every worked example quoted on its page, so a code change cannot silently alter an answer.
- Hand checks. Worked examples are recomputed independently of the calculator code during each review.
- Rule changes. Tax brackets, deductions and federal credits are re-checked against the IRS and the relevant law each tax year and whenever the law changes.
How the guides are written and updated
Guides in the Journal explain the math behind a calculator with a worked example. Figures are taken from primary sources where possible (the IRS, CFPB, WHO, CDC, EPA and Department of Energy) and linked in the text. Each guide shows the date it was first published and the date it was last updated. When a law or figure changes, affected guides are revised and their updated date moves; we do not re-date articles that have not changed.
Found something wrong? Email hello@clearpathcalc.com. Confirmed corrections are applied to the live page and listed below.
Update log
September 26, 2026
- Added two new sections. Tax: no tax on overtime, no tax on tips, car loan interest deduction, senior deduction and self-employment tax. Retirement: FIRE number, Social Security break-even, required minimum distributions, Roth vs traditional and HSA growth.
- Added the 401(k) Paycheck calculator (take-home change, employer match, the percent to max out, and the paycheck where you reach the limit) and the 401(k) Early Withdrawal calculator (penalty, tax, withholding, rule of 55 and 401(k) loans), with four guides: 2026 401(k) limits, employer matches, balance by age, and the cost of cashing out early.
- Added Rent vs Buy, Debt Snowball vs Avalanche, Emergency Fund, Waist-to-Height Ratio and Home Battery Backup calculators.
- Published seven new guides: the 2026 overtime, tips and senior deductions, quarterly self-employment tax, Social Security claiming age, RMD rules, and the Roth vs traditional choice.
- Full accuracy review of the original 17 calculators: formulas re-derived, worked examples re-checked, and a test added for each result the page quotes.
- Fixed results that were wrong: debt consolidation overstated savings, refinance break-even double-counted closing costs rolled into the loan, refinance total savings multiplied the monthly saving by the new term (it now compares all remaining payments on the old loan with the new loan plus fees), the AI productivity capacity figure was inflated, and ROAS profit ignored your profit margin.
- Formulas now render as plain, readable text instead of raw math markup.
- Number boxes now accept a typed value digit by digit (for example a $195,000 salary) instead of jumping to the minimum on the first keystroke. Inch entries on the body fat and waist-to-hip calculators now keep their half inches. Raised limits that were too low: other income on the overtime and tips calculators (the married phase-out runs to $550,000), housing on the emergency fund calculator, and home price and rent on rent vs buy.
- Solar and EV pages updated for the end of the federal residential clean energy credit (Section 25D, after December 31, 2025) and the clean vehicle credit (Section 30D, after September 30, 2025).
- Rewrote the 36 original guides. Each now answers the question in the first paragraph, works a numeric example with the matching calculator, links its primary sources, and says what it does not cover. Titles were changed to say plainly what each guide answers.
- Corrected guide claims that overstated the evidence, including diet breaks, the leptin effect of short sleep, fiber and appetite, and a protein figure of 1 gram per pound.
- Corrected how the 2026 tips, overtime, car loan interest and senior deductions are described: they are claimed on Schedule 1-A and lower taxable income, not adjusted gross income.
- Waist measuring instructions now follow NICE and WHO (midway between the lowest rib and the top of the hip bone) instead of "at the navel".
- Every guide in the Journal re-checked against 2026 figures; each now shows when it was last updated.
- Added Terms of Use, a financial and health Disclaimer, and this Methodology page. Expanded the Privacy Policy.
September 1, 2026
- Added the Body Fat (U.S. Navy method) and Waist-to-Hip Ratio calculators.
- Rewrote every calculator page to lead with the answer, then the formula, then the explanation.
- Published six new guides, including debt avalanche vs. snowball, the HSA as a retirement account, and waist-to-height ratio vs. BMI.
- Fixed horizontal scrolling on phones across all calculator pages.
June 24, 2026
- Rebuilt the site so every calculator and guide has its own page and address.
- Upgraded the site framework to resolve published security advisories.
January 21, 2026
- ClearPathCalc launched with financial, business, health and sustainability calculators.