One formula, five speeds
A doubling clock answers one question: if nothing is added and nothing is paid, how long until a balance doubles? The exact answer is ln 2 ÷ ln(1 + rate) for a rate that compounds once a year. The Rule of 72 (72 ÷ the rate) gets close for everyday rates. The formula is the same on every clock. Only the rate changes, and the rate sets the speed.
The five clocks from the video
A credit card at 22.15% charges interest every day, so its clock uses daily compounding: ln 2 ÷ ln(1 + 0.2215/365) ÷ 365 ≈ 3.13 years for one lap. A federal undergraduate student loan at 6.52% takes about 10.97 years. A 30-year mortgage rate around 7.28% takes about 9.86 years. A 4% savings example crawls along at about 17.67 years, and a hypothetical 7% return, a common long-run illustration, takes about 10.24 years.
Debt clocks vs savings clocks
Put side by side, the debt clocks spin faster than the savings clocks. A balance left on a 22.15% card would double almost six times as fast as money sitting at 4%. That's why the same "doubling" idea feels so different depending on which side of the rate you're on. Real loans and cards have payments, so balances usually shrink instead of doubling; the clock shows how hard the rate pushes in the other direction.
What this page leaves out
Each clock assumes one constant rate, no payments, no deposits, no fees and no taxes. The card compounds daily because that's how revolving APR usually works; the other four compound once a year for a clean comparison (switch the card to yearly to see the difference). The 4% savings rate and the 7% return are examples, not products or forecasts. Real returns jump around and can be negative.
Where the rates come from
The card rate is the Federal Reserve's G.19 average for accounts assessed interest. The student loan rate is the fixed rate for new federal undergraduate Direct Loans first disbursed from July 1, 2026 to June 30, 2027. The mortgage rate is Freddie Mac's Primary Mortgage Market Survey average for the week ending October 1, 2026; individual quotes differ.
Sources
- Federal Reserve: G.19 Consumer Credit
- Federal Student Aid: interest rates
- Freddie Mac: Primary Mortgage Market Survey
Last reviewed: Oct 9, 2026