Knowledge Base: Practical Calculation Guides
Understand the method behind the result. These guides explain formulas, measurements, rounding and common mistakes, with examples you can check in our free calculators.
Age & Dates
How to Calculate Age from Date of Birth
Count completed calendar years and months, then the remaining days. Understand why days divided by 365 is not calendar age.
Days Between Dates: Elapsed Days vs Inclusive Counting
Decide whether you need the gaps between dates or every calendar date in an interval, including both endpoints.
Geometry
How to Calculate Volume: Five Common 3D Shapes
Choose the right measurements and formula for five solids, then check cubic units, rounding and container assumptions.
Everyday Percentages
How to Calculate a Restaurant Tip and Total Bill
Separate the amount you tip on from tax and service charges, and avoid adding charges twice when finding the final bill.
Percentage Increase and Decrease: Formula and Common Mistakes
Choose the correct baseline, interpret the sign and distinguish percentage changes from percentage points.
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