METHOD CHECK
Same numbers. Different questions.
The right calculation starts with knowing what your inputs represent.
Match the method to the question
Start by identifying the base, the unit, or the endpoint convention. These examples are illustrative, not prices or current rates.
| Question | Method | Illustrative result | Watch for |
|---|---|---|---|
| What is 20% of 80? | Base × percent ÷ 100 | 16 | Name the base |
| 16 is what percent of 80? | Part ÷ whole × 100 | 20% | Whole cannot be zero |
| How much did 80 grow to 100? | (New − old) ÷ old × 100 | 25% | Positive original value |
| What is 80 after 20% off? | Price × (1 − discount ÷ 100) | 64 | Tax and fees excluded |
| Days from Jan 1 to Jan 8, 2026? | End − start | 7 days | Excludes start |
| Count including both endpoints? | End − start + 1 | 8 dates | Same date counts once |
| Weekdays in that elapsed interval? | Remove Saturday and Sunday | 5 weekdays | Holidays still count |
| 12 inches in centimeters? | Multiply by 2.54 | 30.48 cm | Length, not area |
| 68 °F in °C? | (°F − 32) × 5/9 | 20 °C | Offset as well as scale |
| One US liquid gallon in liters? | Multiply by 3.785411784 | 3.785411784 L | Not an Imperial gallon |
Cost and tool choice
The three calculators on this site are free in this local version. A paid tool is not inherently more accurate. Consider whether you need batch work, an audit trail, specialized units, holiday calendars, or contractual rounding rules. We do not compare provider prices or claim that these basic tools replace professional software.
When to use another source
Use the organization responsible for a deadline to confirm its holiday calendar and cutoff time. Use the label, drawing, or specification that defines your unit. For money, confirm the agreement's rounding and tax rules. For scientific work, preserve measurement uncertainty and use a method appropriate to the instrument. Sources and conventions explains what these tools implement.