Time to Halve Calculator
Losing 15% a year does not halve in seven years — it halves in 4.27. Each year takes 15% of what is left, so the amount lost shrinks year after year. This is the rule of 72 turned around.
Your numbers
Time to halve
4.27yr
Left
19.69%
Formula
Time to halve = ln(0.5) ÷ ln(1 − Drop ÷ 100)
It fits things that shrink in proportion to what remains: depreciation, subscriber churn, radioactive decay. If the same fixed amount goes each year, plain division is the right tool.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Drop (%)How far it dropped from the high point, as a percentage. | 15 | 0 ~ 99 |
| Years (yr)How many years the money stays put; compounding bites harder as this grows. | 10 | 0 ~ 100 |
Step by step
Quick reference table
Results when only Drop (%) changes and everything else stays put.
| Drop (%) | Time to halve (yr) | Left (%) |
|---|---|---|
| 7.5 | 8.89 | 45.86 |
| 11.3 | 5.78 | 30.15 |
| 15 | 4.27 | 19.69 |
| 22.5 | 2.72 | 7.82 |
| 30 | 1.94 | 2.82 |
What each result means
| Result | At default values |
|---|---|
| Time to halve (yr)How long it takes to fall to half of today’s value. | 4.27 |
| Left (%) | 19.69 |
Common mistakes
It fits things that shrink in proportion to what remains: depreciation, subscriber churn, radioactive decay. If the same fixed amount goes each year, plain division is the right tool.
Glossary
- Drop
- How far it dropped from the high point, as a percentage.
- Years
- How many years the money stays put; compounding bites harder as this grows.
- Time to halve
- How long it takes to fall to half of today’s value.
Frequently asked questions
Q. How is Time to Halve Calculator calculated?
Time to halve = ln(0.5) ÷ ln(1 − Drop ÷ 100) — Losing 15% a year does not halve in seven years — it halves in 4.27. Each year takes 15% of what is left, so the amount lost shrinks year after year. This is the rule of 72 turned around.
Q. Can you walk through an example?
With Drop 15%, Years 10yr, the answer is Time to halve 4.27yr.
Q. What do I need to enter?
Enter Drop, Years. 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 Drop (%) moves the answer to Drop (%) 7.5 → Time to halve (yr) 8.89 and Drop (%) 30 → Time to halve (yr) 1.94. 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?
It fits things that shrink in proportion to what remains: depreciation, subscriber churn, radioactive decay. If the same fixed amount goes each year, plain division is the right tool.
Related calculators
Tax rates and interest conventions differ by country and product — check your contract for real transactions.