Compound Interest Calculators: Growth, Rule of 72, DCA

Compound interest is the single most important idea on this site, and it deserves the tools to prove it rather than assert it. The calculators in this category answer five versions of the same question: what does money growing on itself actually do, how fast does it double, is lump-sum or gradual investing better, what does compounding frequency change, and what are reinvested dividends worth. Every one of them prints the formula alongside the answer.

The five questions

The compound interest calculator is the foundation. It projects a balance from a starting principal and a monthly contribution, and — unlike a plain savings projection — it splits the result into the money you put in and the interest it earned, so the shape of the curve becomes visible: flat for years, then steep. The chart draws that curve, and the interest component is the reason every long-term number on this site behaves the way it does.

The Rule of 72 calculator answers the doubling question in a single glance — 72 divided by the rate gives the doubling time — and then does the honest thing: it reports the exact doubling time from the logarithmic formula, which the rule only approximates. The gap between the two is small at normal rates and grows at high ones, which is itself a lesson in why rough rules exist: they trade precision for speed.

The dollar-cost averaging calculator compares the two ways to deploy a lump sum: all at once, or spread out over time. The answer depends on the numbers, and the calculator computes the actual end balance of both paths so the decision is made on a real comparison rather than an anecdote — see dollar-cost averaging vs. lump sum for a full walkthrough of that decision.

The APY vs. APR calculator isolates compounding frequency: the same nominal rate, compounded daily against annually, shown as an effective yield and as the dollar spread it creates. It is the tiebreaker between accounts that quote identical rates but credit interest differently.

The dividend reinvestment calculator makes the DRIP decision explicit: reinvest every payout, or take it as cash. Both paths appreciate at the same rate, so the gap between them is nothing but the compounding effect of the reinvested dividends — the payout becoming principal for the next payout.

What the numbers have in common

These tools share an engine — the standard monthly-compounding recurrence that also powers the 401(k), FIRE, and savings-goal projections elsewhere on this site — and they share a warning. The return input dominates every output, and no calculator on this page can predict it. The honest use of the compounding category is a range: run the projection at 5%, 7%, and 9%, and treat the spread as the uncertainty it is.

Where to go next

The compounding category is the bridge between saving and retiring. The savings category applies the same growth math to dated goals and interest- bearing accounts with lower risk and lower return. The retirement category runs it out to multi-decade horizons where the interest component overwhelms the contributions entirely — which is the point of the FIRE and 401(k) calculators. Start here to see what time does to money, then take that understanding to a goal with a date on it.

Browse the full index of every calculator or the flat sitemap, or explore the other topics below.

The same recurrence powers a dated goal: see the savings goal timeline, emergency fund, inflation-adjusted savings goal, and FIRE calculators for that math with a deadline. For deposits that sit idle, compare a high-yield savings account, a certificate of deposit, or a Mudaraba term deposit — and the net worth tracker ties the compounding balance to the rest of the picture.