The $10,000 fork in the road
Park $10,000 at 7% a year for 30 years. Simple interest pays 7% on the original $10,000 every year—like a fixed rent check on year-one principal. Compound pays on whatever the balance has become. At a glance both say “7%,” but simple ends around $31,000 total while compound ends north of $76,000 on the same nominal rate. The percent did not change. The rules did.
Why your brain defaults to the wrong one
Most day-to-day money is linear: hours × wage, miles × mpg. Finance quietly switches to exponential when interest stays in the account and earns again. Ignore that handoff and you underestimate savings, overestimate paydown plans, and treat long horizons like short ones.
Intuition first: two different games
Simple: each period, growth is tied to the original principal only. Compound: growth applies to principal plus interest already credited—so the base grows; the curve bends upward (or, on debt, downward harder).
Simple (illustrative): interest per year ≈ P × r (same r on original P) Compound: each period, new balance = old balance × (1 + period rate)
How it works, step by step
- Fix the principal, the rate, and whether interest is withdrawn or left to sit.
- If withdrawn, balances stay flat—simple behavior is a decent mental model.
- If left to sit, each period’s interest joins the pile—that is compounding.
- Compare apples: match how often interest credits (daily, monthly, annually) before you declare a winner.
Try it yourself
Same rate, different shape
Simple mindset: same dollar interest each year on year-1 principal Compound reality: interest on an expanding balance — “interest on interest” Debt flip: unpaid balances can compound against you — speed to zero matters
Where each actually lives
- Some short-term or simple-interest notes approximate flat growth on principal.
- Most savings and investments that reinvest follow compound logic—timing and fees bend the curve.
- Revolving card balances compound the wrong way: the balance is the base; minimums stretch the tail.
Costly misreads
- Quoting “7%” without matching compounding frequency to the other product.
- Mixing contributions with returns when bragging about performance—money you add is not genius.
- Assuming past compound returns repeat; use ranges, not prophecies.
Use the calculator
FAQ
- Is credit card interest compound?
In practice, usually yes: interest builds on balances you do not pay off. Paying in full cuts the compounding story short.
- What is the rule of 72?
Rough doubling time in years ≈ 72 ÷ growth rate in percent. Fast check only—use tools for real decisions.
- Which is “better,” simple or compound?
For growth you keep, compounding is usually the point. For clarity on some loans or short notes, simple math can be easier—read the contract.