Guides
Deep-dive explanations and how-tos paired with our calculators.
BMI Limits: When BMI Misleads and What to Use Instead
BMI is a population screening metric, not a personal diagnosis. Learn when the number misclassifies people, how to read calculator output responsibly, and which measures add context.
Percentage Pitfalls: Common Mistakes and How to Fix Them
Wrong denominators, mixing “percent” with “percentage points,” and stacked discounts confuse even careful people. Here is how to spot the error and cross-check with our calculators.
Sales Tax vs VAT: Key Differences and Practical Implications
Sales tax and VAT both raise government revenue on consumption, but the collection chain, invoices, and who claims credits differ. Use this to pick the right calculator mindset — not to file returns without local advice.
Debt-to-Income (DTI): What Lenders Look At Beyond the Payment
Understand how monthly debt payments compare to income, why DTI matters for mortgages and loans, and how it differs from a simple payment calculator result.
US Sales Tax: Why State Base Rates Are Not the Whole Story
Learn how state, county, city, and special district taxes combine, and where to look up the total rate that applies to your purchase.
How Your Loan Payment Is Actually Calculated (Step by Step)
See how a fixed monthly payment splits into interest vs principal each month, why early payments are interest-heavy, and how the standard amortization formula fits together.
APR vs APY in Plain English
APR is the simple yearly quote you hear for loans; APY tells you what you effectively earn or pay when compounding is included. Learn when each label shows up and how to compare products fairly.
Percentage Points vs Percent Change: Don’t Mix Them Up
Moving from 3% to 5% is a 2 percentage point increase, but a roughly 66.7% relative increase. Mixing these ideas causes bad headlines and bad business decisions.
How to Estimate a Realistic Break-Even Point
Textbook break-even divides fixed costs by contribution margin. In practice, capacity, seasonality, discounts, and semi-variable costs distort the line — here is how to stress-test.
Stacked Discounts: Why 30% + 20% Off Is Not 50% Off
Sequential percent-off promotions multiply remaining price, they do not add. Learn the correct order, how to compute an effective single discount, and common retail wording traps.
Understanding Compound Interest (APR, APY, Compounding Frequency)
Learn how compound interest grows your money, how APR differs from APY, and why compounding frequency matters less than time and rate.
Loan Repayment Methods: Equal Payment vs. Equal Principal
Compare amortization methods, interest impact, and monthly payment profiles to pick a plan that fits your cash flow.
Mortgage Total Cost: Beyond Principal and Interest
Understand the full monthly cost of home ownership: taxes, insurance, PMI, and how they affect affordability.
VAT Basics: Add or Remove VAT Correctly
Learn the correct formulas to add VAT to net prices or remove VAT from gross prices, and common pitfalls to avoid.
After-Tax Return: Estimating Real-World Investment Performance
Understand how taxes reduce nominal returns and how to approximate after-tax performance for planning purposes.
Early Repayment Effects: Save Interest and Shorten Term
See how extra principal payments reduce total interest and loan term, and when prepayment penalties apply.
Compounding Frequency: When It Matters (and When It Doesn’t)
Monthly vs daily compounding provides small differences compared to time and rate. Know when the frequency choice is material.
Savings Goal Planning: How Much Per Month to Reach Your Target
Translate a future savings target into a realistic monthly contribution using compound growth assumptions.
Variable Rate Risks: What Changes When Rates Move
Understand how variable-rate loans can increase payments and total cost, and ways to stress-test scenarios.
Loan Comparison Checklist: Apples-to-Apples Evaluation
A simple list to compare loans fairly: APR, fees, term, compounding, and early repayment terms.
Rounding Standards: Be Consistent Across Calculations
Avoid discrepancies by choosing rounding rules up-front: bank rounding vs half-up, monetary decimals, and display vs storage rules.
Unit Conversion Mistakes: Common Errors and Fixes
Avoid frequent unit mix-ups: metric vs imperial, area vs length squared, volume vs weight in recipes, and temperature conversion pitfalls.
Tipping by Country: What’s Expected Around the World
Learn typical tipping expectations for restaurants, hotels, and delivery in major regions to avoid over- or under-tipping.
Inflation and Real Return: See Your Money in Today’s Terms
Nominal returns ignore inflation. Estimate purchasing-power growth by subtracting expected inflation to get a real rate.
Currency Effects: Exchange Rates and Cross-Border Prices
Understand how currency moves affect international purchases and investments, and when to account for fees and spreads.
Price With vs Without Tax: Work Forwards and Backwards
Learn the correct forward formula to add tax and the reverse formula to remove tax from a total price.
Find Local Regulations: Where to Check Current Tax Rules
Rates and rules change. Learn where to find authoritative, current VAT and sales tax information for your location.