Rebase to an Index of 100
Divide the current value by the base and multiply by 100. An index of 115 means 15% above base, which lets you overlay series with completely different units on one chart.
Your numbers
Index
115
Change
15%
Difference
1,875
Formula
Index = Actual ÷ Base × 100
Changing the base period leaves the shape untouched but every number moves. Always quote the base year alongside an index.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| BaseThe value everything is measured against — the denominator. | 12,500 | 1 and up |
| ActualWhat actually happened, as opposed to the plan. | 14,375 | 0 and up |
Step by step
Quick reference table
Results when only Base changes and everything else stays put.
| Base | Index | Change (%) | Difference |
|---|---|---|---|
| 6,250 | 230 | 130 | 8,125 |
| 9,375 | 153.33 | 53.33 | 5,000 |
| 12,500 | 115 | 15 | 1,875 |
| 18,750 | 76.67 | -23.33 | -4,375 |
| 25,000 | 57.5 | -42.5 | -10,625 |
What each result means
| Result | At default values |
|---|---|
| IndexThe value with the base set to 100; 115 means 15% above. | 115 |
| Change (%)How far it moved, measured against the before value. | 15 |
| DifferenceThe gap between two values — a quantity, not a rate. | 1,875 |
Common mistakes
Changing the base period leaves the shape untouched but every number moves. Always quote the base year alongside an index.
Glossary
- Base
- The value everything is measured against — the denominator.
- Actual
- What actually happened, as opposed to the plan.
- Index
- The value with the base set to 100; 115 means 15% above.
- Change
- How far it moved, measured against the before value.
- Difference
- The gap between two values — a quantity, not a rate.
Frequently asked questions
Q. How is Rebase to an Index of 100 calculated?
Index = Actual ÷ Base × 100 — Divide the current value by the base and multiply by 100. An index of 115 means 15% above base, which lets you overlay series with completely different units on one chart.
Q. Can you walk through an example?
With Base 12,500, Actual 14,375, the answer is Index 115.
Q. What do I need to enter?
Enter Base, Actual. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Base moves the answer to Base 6,250 → Index 230 and Base 25,000 → Index 57.5. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
Changing the base period leaves the shape untouched but every number moves. Always quote the base year alongside an index.
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