Every calculator. Explained, not just answered.

Draw whole numbers from any range, with or without repeats, from an explicit seed — so the same seed always reproduces the same draw and anyone can check it.

Differentiate a function and see which rule applies at every term — power, product, quotient and chain — plus the value, tangent line and second derivative.

Solve any linear or quadratic equation in one variable with every rearrangement shown — discriminant, exact roots, vertex, factored form and a check.

Days, weeks, months and business days between two dates, or add and subtract days, weeks, months or years from any date — leap years handled exactly.

Turn daily and weekly screen hours into days per year and a share of your waking life, and see what cutting fifteen minutes a day would actually give back.

Test any number up to a trillion for primality, get its full factorisation, the nearest primes either side, and how many primes lie below it.

The Moon's phase, age and illuminated fraction for any date, plus the next new and full moons — from Meeus's periodic series, accurate to about a minute.

Estimate your monthly mortgage payment with taxes, insurance, PMI and HOA, see the full amortization schedule, and how extra payments shorten your loan.

Enter any one of radius, diameter, circumference or area and get the other three — with unit conversion and the π formulas worked step by step.

How long a file takes to transfer at a given connection speed, in bits or bytes, with a real-world overhead allowance and the speed needed to hit a deadline.