Practical guides
Step-by-step answers to everyday questions. Each guide has the formula, a worked example, practice questions with answers and a link to the calculator that does the arithmetic for you.
Percentages
Percentages, percentage change, discounts and how to avoid the usual mistakes.
- Guide · September 19, 2026How to Calculate a Discount and the Sale PriceFind the final price, the saving and the real percentage off, including stacked discounts that do not simply add up.
- Guide · September 19, 2026How to Calculate a PercentageEvery percentage problem is one of three questions. Here are the formulas, worked examples and a way to check your answer.
- Guide · September 19, 2026How to Calculate Percentage DecreasePercentage decrease compares a drop with the starting value. Learn the formula, the multiplier trick and how to reverse it.
- Guide · September 19, 2026How to Calculate Percentage IncreasePercentage increase compares a change with where you started. Here is the formula, examples and the reverse-calculation trap.
Dates and time
Working out ages, date gaps, hours worked and how the calendar really behaves.
- Guide · September 19, 2026How to Calculate Age From a Date of BirthAge is a count of complete years, then leftover months and days. Here is the method and the leap-day edge case.
- Guide · September 19, 2026How to Calculate Hours Between Two TimesWork out the hours between two clock times, including breaks and shifts that cross midnight, and convert to decimal hours.
- Guide · September 19, 2026How to Calculate the Days Between Two DatesA reliable way to count days between dates by hand, with the inclusive versus exclusive question and working days explained.
Money basics
Interest, loans, tips, salary and savings explained with worked examples.
- Guide · September 19, 2026How to Calculate a Loan PaymentThe standard formula for a fixed-rate instalment loan, worked through with real numbers, plus how interest is split from principal.
- Guide · September 19, 2026How to Calculate a Tip and Split the BillThe tip is the bill times the percentage. Here is the formula, an even-split example and mental shortcuts you can do at the table.
- Guide · September 19, 2026How to Calculate Compound InterestCompound interest earns interest on earlier interest. See the formula, a 20-year example and the Rule of 72.
- Guide · September 19, 2026How to Convert Hourly Pay to an Annual SalaryMultiply the rate by hours and weeks, then adjust for unpaid time. Includes the reverse calculation and its usual pitfalls.
Math and statistics
Averages, ratios, fractions and the statistics ideas behind them, with worked examples you can check by hand.
- Guide · September 19, 2026How to Add, Subtract, Multiply and Divide FractionsThe four fraction operations, each with one clear rule and one worked example, plus how to simplify and convert mixed numbers.
- Guide · September 19, 2026How to Calculate the Average (Mean, Median and Mode)Three different averages, one worked example, and a simple rule for deciding which one describes your data honestly.
- Guide · September 19, 2026How to Simplify a Ratio and Solve ProportionsDivide both sides by their greatest common divisor to simplify, and cross-multiply to solve for a missing value.
Conversions
Units, systems of measurement and how to convert between them correctly, with exact conversion factors.
- Guide · September 19, 2026How to Convert Celsius to Fahrenheit (and Back)Two exact formulas, a mental shortcut that gets you close, and a table of the temperatures people look up most.
- Guide · September 19, 2026How to Convert Units Step by StepMultiply by a fraction equal to one, and let the units cancel. It works for length, weight, volume and everything else.
Productivity
Time-management techniques such as the Pomodoro method, with practical steps for using them in real work.
Study and school
How grades and GPA are calculated, how credit hours weight a result, and practical study-planning methods.