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Guides
These pages exist so you can read the method before you move a slider. Each guide is written in English, stays on kdp.cool, and includes at least one fully worked numeric example that does not depend on JavaScript. Topics cover mortgage term tradeoffs, prepayments, PITI versus lender disclosures, paycheck withholding models, FICA versus ordinary tax, pretax 401(k) effects, and marginal versus effective rates.
The mortgage, paycheck, and income-tax calculators are on this site. Use a guide when you want the story; use a calculator when you want to change inputs. Neither one is tax, lending, or legal advice.
How monthly mortgage payments are calculated
A plain-English walkthrough of the fully amortizing loan formula, with a static $400,000 example you can check by hand.
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How to read take-home pay and paycheck withholding
What usually comes out of a US paycheck: federal income tax, FICA, state tax, and pretax deductions — plus a worked $72,000 example.
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How US federal income tax brackets work
Why the US system is progressive, not flat, and how to compute tax one bracket at a time using 2026 IRS figures.
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15-year vs 30-year mortgages: what you actually trade
Same loan amount, same rate, two terms. Compare monthly payment, total interest, and how much principal you clear in the first five years.
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Extra principal payments: how payoff shortens
What happens when you add a fixed amount to principal each month, why interest falls, and what a simple online calculator usually leaves out.
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PITI vs the Loan Estimate: tax, insurance, PMI, and fees
How principal and interest sit inside a larger housing payment, what PMI and HOA add, and why a lender’s Loan Estimate can still disagree with a browser form.
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Marginal rate vs effective rate
Why the tax bracket you “are in” is not the share of income you actually pay, with a full 2026 single-filer walkthrough on $72,000.
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Federal income tax vs FICA on one paycheck
Two different stacks on the same wages: progressive ordinary income tax and flat FICA. A side-by-side $72,000 example.
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How a pretax 401(k) changes take-home and taxable income
Defer 10% of a $72,000 salary and watch federal tax, FICA, and net pay move — including what does not change.
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Why “annual tax ÷ 26” is only a paycheck model
How planning calculators spread annual tax across pay periods, how that differs from payroll withholding tables, and when the shortcut is good enough.
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How we write these
- Cite public methods. Mortgage math uses the standard fully amortizing payment formula. Federal tax figures come from IRS Revenue Procedure 2025-32 for tax year 2026.
- Show the arithmetic. A reader should be able to reproduce the example with a calculator and a pencil.
- Label the limits. State tax, credits, PMI, and employer withholding tables are often left out on purpose. We say so.
- Keep tools free. The guides and the calculators stay free. Ads, when they appear, sit in reserved slots and do not gate the explanation.
Looking for a shorter version first? The homepage has two static mini-examples, and About explains who maintains the site.