Guides
Each calculator on CalcOak is built on a rule, a formula or a standard. These guides explain those rules in plain English, with worked examples, so you can check a result by hand and understand what the number means before you rely on it.
Dates & Time
Finance
How compound interest works and how to calculate it yourself with the formulaHow compound interest works, the A = P(1 + r/n)^(nt) formula, what compounding frequency and monthly deposits add, and the rule of 72, with worked numbers.Sep 27, 2026How to set a freelance hourly rate from the income you want, not the salary you hadWhy salary divided by 2,080 understates a freelance rate, how tax, expenses, unbillable hours and holidays shrink billable time, and a worked example to a rate.Sep 27, 2026How to read a loan amortization schedule and check the numbers yourselfHow a fixed loan payment is calculated, why early payments are mostly interest, what an extra payment saves, and how to check a lender's schedule line by line.Sep 27, 2026How VAT, GST and sales tax work, and how to add or remove tax from any priceWhat separates VAT, GST and sales tax, how to add tax to a net price or back it out of a gross one, standard rates as of 2026 and what a tax invoice needs.Sep 27, 2026
Shipping & Ecommerce
How carriers calculate dimensional weight and how to stop paying for empty spaceHow DHL, FedEx, UPS and USPS turn box size into a billable weight, which divisor each one uses, and how to shrink the charge on light, bulky parcels.Sep 27, 2026Margin versus markup, why the two numbers differ and how fees eat into what you keepMargin and markup describe the same profit from two directions. Learn both formulas, convert between them, and see what fees do to your net margin.Sep 27, 2026How to measure a parcel, read carrier size limits and avoid oversize surchargesLearn how to measure length and girth, what size limits USPS, UPS, FedEx and DHL Express apply, and which box shapes trigger oversize or handling fees.Sep 27, 2026
Developer
How to measure password strength in bits and choose a length that holds upWhat password entropy means, how to calculate it from pool size and length, how long each strength takes to crack, and why length beats complexity rules.Sep 27, 2026How Unix time works and how to turn a timestamp into a readable dateWhat a Unix timestamp is, how to tell seconds from milliseconds apart, why it carries no time zone, and how to convert one in JavaScript, Python and SQL.Sep 27, 2026
Everyday Life
How to calculate percentages, percentage change and reverse percentages the right wayHow to work out X percent of a number, what percent one number is of another and percentage change, plus the traps of stacked discounts and reverse percentages.Sep 27, 2026How to compare products by unit price so you know which package size is really cheaperLearn how to compare products by price per 100 g, per litre or per use, convert ounces and pounds to grams first, and spot when the bigger pack is a worse deal.Sep 27, 2026