Guides · 42 articles
Guides
In-depth explainers for every Calcatalog calculator — how the math works, what the numbers mean, and how to use your results.
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How Mortgage Payoff Works: Extra Payments, Interest Savings, and the Math Behind Them
A plain-English guide to how extra mortgage principal payments accelerate payoff, reduce interest, and change your loan's end date — with a worked example.
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How Social Security Benefits Are Calculated: PIA, AIME, and Bend Points
Social Security benefits are calculated from your earnings record using AIME and bend points. Learn how the PIA formula works and what it means for your check.
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Keto Macros for Beginners: How to Calculate Your Daily Fat, Protein, and Carb Targets
A beginner's guide to calculating keto macros from your daily calorie goal, understanding the standard 5/25/70 split, and adjusting for your needs.
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Self-Employment Tax Explained: SE Tax, Deductions, and 2026 Rates
Self-employed workers pay both sides of FICA. Learn how SE tax is calculated, what the above-the-line deduction is, and how to plan for quarterly estimates.
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Understanding Bond Yields: Current Yield, YTM, and What They Mean for Investors
Learn what bond yields really measure, how current yield and yield to maturity differ, and how price and yield move in opposite directions.
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What Is a Required Minimum Distribution? RMDs Explained
After age 73, the IRS requires annual withdrawals from traditional IRAs and 401(k)s. Learn how RMDs are calculated, why they exist, and how to plan around them.
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How Capital Gains Tax Works: Short-Term vs. Long-Term Rates Explained
Capital gains tax applies when you sell an asset for a profit. Learn short-term vs. long-term rates, when NIIT applies, and how to calculate your federal tax.
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How Home Equity Works: Building It, Measuring It, and Using It
Home equity is your home's value minus what you owe. Learn how it builds, how LTV works, and how much you can access through a HELOC or cash-out refinance.
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How to Save for College: 529 Plans, Monthly Contributions, and the Math Behind the Goal
College saving starts with a target and a timeline. Learn how to calculate monthly contributions, how 529 plans work, and why starting early matters so much.
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How to Use Z-Scores: Standard Scores, Percentiles, and the Normal Distribution
A z-score measures how many standard deviations a value sits from the mean. Learn the formula, how to find percentiles from z-scores, and when to use them.
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Understanding Heart Rate Zones: Training Smarter with the 5-Zone System
Heart rate zones divide your max HR into five intensity bands. Learn what each zone does, how to find your max HR, and how to use zone training effectively.
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APY vs APR: What's the Difference and When Does It Matter?
APY and APR are two ways to express interest rates — one accounts for compounding, the other doesn't. Learn which to use for savings accounts, CDs, and loans.
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How Big Should My Emergency Fund Be?
The 3-to-6-month rule is a starting point, not a finish line. Learn how to size your emergency fund for your income, expenses, and risk profile.
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How HSA Contributions and Tax Savings Work
An HSA lets you save pre-tax dollars for medical costs. Learn the 2026 contribution limits, the triple tax advantage, and how to project your HSA balance.
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How Much House Can I Afford? The 28/36 DTI Rule Explained
The 28/36 DTI rule is the standard lender benchmark for home affordability. Learn how front-end and back-end ratios work and how to calculate your limit.
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How Much Sleep Debt Do You Have?
Sleep debt accumulates quietly — and most people underestimate it. Learn how to calculate your sleep deficit, what it costs you, and how long recovery takes.
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How Roth IRA Contribution Limits Work: MAGI Phase-Out Explained
Roth IRA contributions phase out at higher incomes. Learn the 2026 limits, how the MAGI phase-out reduces your allowed contribution, and the IRS rounding rule.
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How to Calculate Payback Period for Home Upgrades
Learn how to use simple and discounted payback period to evaluate home improvement investments — from HVAC to insulation to solar panels.
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How to Set a Calorie Deficit for Weight Loss: Formula, Pace, and Common Mistakes
A calorie deficit means eating less than you burn. Learn how to calculate your TDEE, choose a safe deficit size, and estimate a realistic weight-loss pace.
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How to Use Significant Figures: Rules, Counting, and Rounding
Significant figures express the precision of a measurement. Learn the four counting rules, how to round to N sig figs, and when to use scientific notation.
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The True Cost of an Early 401(k) Withdrawal: Penalty, Taxes, and Opportunity Cost
Cashing out a 401(k) early triggers a 10% penalty plus ordinary income taxes. Learn how to calculate the total cost and weigh your real options.
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What Savings Rate Do I Need to Retire Early?
Your savings rate — not your income — is the biggest lever in early retirement math. Learn the formula, key benchmarks, and how to project your own timeline.
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How CD Interest and APY Work: A Plain-Language Guide
APY vs APR, compounding frequency, and CD maturity math explained clearly. Understand what your certificate of deposit will actually earn.
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How Credit Card Minimum Payments Work (and Why They Cost You So Much)
Minimum payments keep you current but barely touch your balance. Learn the formula, see a worked example, and find out how much you are really paying.
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How to Calculate Net Worth: Assets, Liabilities, and the Full Picture
Net worth is assets minus liabilities — but knowing what to count (and what not to) matters. Learn the formula, categories, and how to track progress over time.
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How to Calculate Percentage Change: Formula, Examples, and Common Mistakes
Percentage change measures how much a value grew or shrank relative to its starting point. Learn the formula, avoid common confusions, and see worked examples.
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How to Calculate Rental Yield: Gross Yield, Net Yield, Cap Rate, and Cash-on-Cash
Rental yield measures property income return. Learn gross yield, net yield, cap rate, and cash-on-cash return — plus how to calculate each metric correctly.
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How to Estimate Your One-Rep Max (1RM): Formulas, Science, and Practical Use
Estimate your one-rep max from a submaximal set. Covers Brzycki, Epley, and four other validated formulas, accuracy limits, and percentage-based programming.
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What Is a Good Debt-to-Income Ratio? DTI Explained for Borrowers
Lenders use DTI to decide if you can afford new debt. Learn the front-end and back-end DTI formulas, what ratios lenders consider good, and how to lower yours.
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When to Take Social Security: FRA, Early Reduction, Delayed Credits, and Breakeven
Claiming Social Security early means smaller monthly checks; waiting means larger ones. Learn FRA, the early reduction, delayed credits, and breakeven math.
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BMI Explained: What Body Mass Index Measures and Its Limitations
Learn what BMI is, how it's calculated, what the WHO categories mean, and where BMI falls short for athletes, older adults, and diverse populations.
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The Power of Compound Interest: How It Grows (and Costs) Your Money
Compound interest grows money exponentially by earning returns on prior returns. See the formula, a worked example, and how it affects savings and debt.
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Debt Avalanche vs. Debt Snowball: Which Payoff Strategy Saves More?
A plain-math comparison of the two most proven debt payoff methods, with a worked example showing exactly how much each strategy costs in time and interest.
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How Due Dates Are Calculated: Naegele's Rule, Ultrasound, and Why It's an Estimate
Learn how pregnancy due dates are calculated using Naegele's Rule, why ultrasound revises the estimate, and what 'full term' actually means for timing a birth.
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How Mortgage Payments Work: Principal, Interest & Amortization Explained
A plain-English breakdown of the mortgage payment formula, how amortization shifts your monthly costs over time, and a worked example on a $400,000 loan.
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How Much Should You Save for Retirement? A 401(k) Planning Guide
How much to save for retirement: benchmarks by age, 401(k) contribution rules, employer match, and a worked projection for your retirement balance.
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How Is Your Paycheck Calculated? Federal Tax, FICA & Deductions Explained
A step-by-step walkthrough of the withholding math behind gross-to-net pay, covering federal tax brackets, Social Security, Medicare, and pre-tax deductions.
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How to Calculate Macros: Protein, Carbs, and Fat Targets Explained
Learn how macronutrient gram targets are calculated from a calorie goal, what each macro does, and how to choose a split for your nutrition goals.
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What Is Inflation? How It Erodes Purchasing Power Over Time
What is inflation? How the BLS CPI-U measures it, how purchasing power is calculated, and what decades of compounding price growth means for your money.
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Renting vs. Buying a Home: How to Actually Do the Math
A methodical rent-vs-buy comparison covering opportunity cost, break-even year, and a worked 10-year scenario using real calculator outputs.
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Sleep Cycles Explained: What Happens During the Night and When to Wake Up
Learn how 90-minute sleep cycles work, what each stage does for your body, why waking mid-cycle feels terrible, and how to plan bedtimes around full cycles.
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TDEE Explained: Total Daily Energy Expenditure, BMR, and Calorie Targets
Understand how TDEE is calculated from BMR and activity level, how it bridges to macros and weight goals, and what the numbers mean in practice.