The CalcxApp blog
Practical guides on the math behind everyday decisions.
- 8 min read
401(k) vs Roth IRA 2026: Contribution Limits Compared
Side-by-side table of all 2026 limits, income phaseouts, tax treatment, and a flowchart to decide which account to fund first.
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BMI Percentile for Children by Age and Gender (Chart + Calculator)
How pediatric BMI percentiles work, what the CDC growth charts mean, and when to talk to a pediatrician about your child's weight.
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Compound vs Simple Interest: A $10,000 Real Example Over 30 Years
See the actual dollar difference between compound and simple interest over 30 years on the same starting investment, and which one banks use.
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Cubic Yards of Concrete for a 10x10 Slab (4', 6', 8' Thickness)
Exactly how much concrete to order for common patio sizes, with 10% extra for waste, plus delivery cost estimates.
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How Much House Can I Afford on a $50k Salary? (Real Numbers)
Specific home prices you can afford at $50k, $60k, and $75k incomes, broken down by down payment and debt level.
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How to Calculate Calories Burned Walking (With MET Formula)
The exact MET formula, why most fitness trackers are off by 15-20%, and how to calculate your true calorie burn per mile.
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How to Calculate Your Debt-to-Income Ratio (and What Lenders Want)
The exact formula lenders use, what DTI ratio you need for a mortgage approval, and 3 strategies to lower yours before applying.
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How to Calculate Your Mortgage Payment with PMI (Step-by-Step)
A detailed walkthrough showing exactly how PMI affects your monthly mortgage payment, when it drops off, and strategies to remove it faster.
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Cap Rate vs Cash-on-Cash Return: Which Should Real Estate Investors Use?
When to use each metric, why cap rate alone is misleading, and a real example comparing both for the same $200k property.
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Rule of 72 vs Rule of 115: Which is More Accurate for Investing?
The mental math shortcuts for doubling your money, when each works, and a comparison table of doubling times at different returns.
Read more - 7 min read
How compound interest really works (and why it's your best friend)
Compound interest is the single most powerful force in personal finance. Here's how it works, how to calculate it, and why starting early matters more than you think.
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Retirement planning by the numbers: how much do you actually need?
Most people guess at their retirement number. Here's how to calculate it properly — from the 4% rule to Social Security, 401(k)s, and Roth IRAs.
Read more - 6 min read
Tax deductions you're probably missing (and how to calculate them)
From home office expenses to student loan interest, here are the most commonly overlooked tax deductions and how to figure out what they're worth.
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What your BMI really means (and what it doesn't)
BMI is everywhere — on doctor's charts, fitness apps, and insurance forms. Here's how to interpret your number correctly and when to look beyond it.
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The math behind buying a house: from down payment to closing costs
Buying a home involves more numbers than most people expect. Here's every cost broken down — down payment, closing costs, mortgage math, and the rent-vs-buy decision.
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How to calculate your mortgage payment from scratch
The math behind a monthly mortgage payment, why principal and interest move differently, and how to use a calculator to plan your home purchase.
Read more - 6 min read
Understanding BMI: what the number actually means
Body Mass Index is a quick screening tool — not a diagnosis. Here's how it's calculated, what the categories mean, and where the formula falls short.
Read more - 5 min read
Five percentage tricks to do in your head
Mental math shortcuts for tips, discounts, and tax — the kind of fast estimation that beats reaching for your phone every time.
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