Rule of 72 for Debt: How Fast Does It Double?
The Rule of 72 is usually pointed at investments, but it works just as well on what you owe. Divide 72 by the interest rate and you get the years for a balance to double if left unpaid: a 22% credit-card balance doubles in about 3.3 years, a 29% balance in about 2.5. And because cards compound daily rather than annually, the real doubling is even faster than the estimate. It is the clearest way to see how quickly high-interest debt works against you.
The short answer
The Rule of 72 estimates how long a balance takes to double: 72 ÷ the interest rate = years to double. Applied to debt left unpaid, at common credit-card APRs:
- 18% — doubles in about 4.0 years
- 22% — about 3.3 years
- 25% — about 2.9 years
- 29% — about 2.5 years
These are the same doubling dynamics that make investing powerful — only here they are working against you, and at rates far higher than most investments earn.
Check any rate: the Rule of 72 calculator gives the doubling time for any interest rate instantly.
How the rule applies to debt
The Rule of 72 does not care whether a balance is an asset or a liability — it is just a shortcut for compound growth. Money compounding at a rate r doubles in roughly 72 ÷ r years, whether that money is a growing investment or a growing debt.
The difference is who benefits. On an investment, doubling is the goal. On a credit-card balance, the same math means the amount you owe can double while you are only making minimum payments — because minimum payments often barely cover the interest, leaving the balance to compound almost untouched.
Doubling time by APR
Here is the Rule of 72 estimate against the exact figure (with yearly compounding) for common card rates:
| APR | Rule of 72 (72 ÷ APR) | Exact doubling time |
|---|---|---|
| 18% | 4.0 years | 4.2 years |
| 22% | 3.3 years | 3.5 years |
| 25% | 2.9 years | 3.1 years |
| 29% | 2.5 years | 2.7 years |
The Rule of 72 runs slightly optimistic at these high rates — it is most accurate near 8% — but it is close enough to make the point: at any typical card rate, an untouched balance doubles in well under five years. The Rule of 72 vs Rule of 70 guide covers why the estimate drifts.
A worked example
Take a $5,000 balance at 22% APR, with no payments and no new charges, compounding yearly:
| After | Balance owed |
|---|---|
| 1 year | $6,100 |
| 2 years | $7,442 |
| 3 years | $9,079 |
| 3.5 years | $10,028 |
| 4 years | $11,077 |
The balance passes $10,000 — double the original — at about the 3.5-year mark, matching the exact doubling time for 22%. Nothing was borrowed after the start; the entire increase is interest compounding on interest.
Why it is actually faster
The figures above use yearly compounding for clarity, but real credit cards compound daily. That raises the effective annual rate above the stated APR — a 22% APR compounds to about 24.6% a year — so the balance doubles a little sooner than the table shows: roughly 3.2 years instead of 3.5.
So the Rule of 72, applied to the stated APR, slightly understates how fast card debt grows. When the estimate errs, it errs in the debt's favour, not yours — another reason to treat high-interest balances as urgent.
The mirror of investing
It is worth seeing the two side by side. At 7%, an investment doubles in about 10.3 years — the case made in double your money at 7%. At 22%, a debt doubles in about 3.3. Same rule, same arithmetic, opposite direction and roughly three times the speed.
That speed gap is why paying down a high-interest balance is often the highest-return move available: clearing a 22% debt is equivalent to earning 22% risk-free, which almost no investment reliably matches. The Rule of 72 explained covers the underlying formula in full.
Assumptions
- No payments and no new charges. The doubling times describe a completely untouched balance; any real payment changes the path.
- Yearly compounding in the tables, for clean comparison — real cards compound daily, which is faster, as noted above.
- A fixed APR. Real card rates are variable and can rise.
- Illustrative figures, not a statement about any particular card or lender.
Frequently asked questions
The bottom line
The Rule of 72 works on debt exactly as it does on investments: divide 72 by the rate for the years to double. At credit-card APRs that is brutally quick — about 3.3 years at 22%, under 3 at 25% — and daily compounding makes it faster still. The same math that builds wealth slowly at 7% destroys it quickly at 22%, which is why clearing high-interest debt is so often the best return you can get.
Check the doubling time for any rate with the Rule of 72 calculator, or see the investing side in the Rule of 72 explained.
Disclaimer: This page is for general educational purposes only and is not financial advice. Doubling times assume an untouched balance at a fixed rate; real card terms, compounding and payments vary. Consider speaking with a qualified financial professional about managing debt.