Guides
Every calculator on SolvexCalc is built on a formula someone eventually has to trust. These guides open that formula up — where it comes from, what it assumes, and where it quietly breaks down — so you're reading a number you actually understand, not just a number you were given. Each guide is written by working through the math from scratch, with a worked example you can check by hand.
How compound interest actually works (with examples)
Why starting early matters more than the amount you invest, explained with real numbers.
6 min read Read the guide → FinancialSimple interest vs compound interest: the difference that costs (or earns) you thousands
Why the gap between the two grows dramatically over time, and which one applies to your loan or account.
6 min read Read the guide → FinancialHow loan amortization works (and why your early payments are mostly interest)
Why early payments barely touch the balance, and how extra payments and shorter terms change that.
7 min read Read the guide → FinancialUnderstanding US tax brackets: why your marginal rate isn't your real tax rate
How the progressive tax system works, and a full worked example.
7 min read Read the guide → MathHow percentage discounts actually work (and why "30% off" isn't always what it seems)
Why stacked discounts don't add up the way you'd expect, and how to reverse-calculate an original price.
5 min read Read the guide → MathHow GPA is actually calculated (and why two "B students" can have different GPAs)
Why GPA is a credit-weighted average, not a simple average, and what that means for your transcript.
5 min read Read the guide → ConversionWhy unit conversion trips people up (and the one trick that prevents mistakes)
The core reason metric-to-imperial conversions cause errors, and why temperature works differently.
5 min read Read the guide → HealthUnderstanding BMI: what it measures, and what it doesn't
How the BMI formula works, its known limitations, and when to look at other health markers.
6 min read Read the guide → HealthUnderstanding daily calorie needs: BMR, TDEE, and activity levels explained
What BMR and TDEE actually measure, how activity level factors in, and how to use them for weight goals.
7 min read Read the guide → HealthHow much water do you really need each day?
The science behind hydration calculators, and why "eight glasses a day" isn't the full picture.
7 min read Read the guide → HealthBody fat percentage: how it's estimated, and why it beats BMI for some people
The US Navy formula explained, with a worked example and its real limits.
6 min read Read the guide →How these guides are written
Each guide starts from the underlying formula, not from the calculator's output. We derive the math step by step, run at least one worked example by hand to confirm the calculator's result matches, and call out the most common mistake people make with that formula. Guides are updated when a formula, tax bracket, or standard reference value changes — see the date on the "Frequently asked questions" section of each calculator page for the underlying figures currently in use.