How Monthly Contributions Compound

By Nathan Hays · Updated July 31, 2026

A lump sum compounds along one curve. Regular deposits stack a new curve on top every month, each one starting its own clock. That is why the balance in a savings plan with steady contributions looks nothing like the balance in a plan without them, and why the shape of the growth chart changes so much when you type a number into the monthly field on the compound interest calculator. Every figure below comes from that same engine.

The convention, stated exactly

Contribution math has two possible timings, and calculators that do not say which one they use are impossible to check. This one walks forward one month at a time. In each month it first credits interest if a compounding period has closed, then adds your deposit to the balance. Your very first deposit lands at the end of month one, so it earns interest for the remaining term minus one month, and your final deposit lands on the last day and earns nothing at all.

The alternative convention, an annuity due, deposits at the start of each period and produces a slightly larger result for the same inputs. Neither is wrong. What matters is knowing which you are reading, because a comparison between two calculators using different conventions is not a comparison at all.

One more detail worth naming: deposits are always monthly, even when interest is credited less often. If you set compounding to annual, the engine still adds your money every month; it simply waits until the twelfth month to credit interest. That mirrors how a real account with automatic transfers and annual crediting behaves.

The formula

When the deposit frequency and the compounding frequency both equal twelve, the month-by-month walk is exactly the closed-form future value of a lump sum plus the future value of an ordinary annuity:

FV = P × (1 + r/n)^nt + PMT × [((1 + r/n)^nt − 1) ÷ (r/n)]

P is the starting amount, PMT the monthly deposit, r the annual rate as a decimal, n the compounding periods per year, and t the years. The left term is the original money growing. The right term is the sum of every deposit, each grown for however many periods remained after it arrived. The main guide checks this formula against the calculator on the site's reference case: $10,000 plus $500 a month at 7% compounded monthly for 20 years gives $300,850.72.

What deposits actually add

The table below starts with $10,000 at an illustrative 6%, compounded monthly, and runs it twice: once untouched, once with $250 added every month.

The thirty-year row is the one worth sitting with. Adding $250 a month for thirty years means putting in $90,000 of your own money. The balance is $311,355, of which $211,355 is interest. The same starting $10,000 left alone for the same thirty years reaches $60,226. Deposits did not just add $90,000 to the outcome; they added $251,129.

There is a second way to read the same table that is easy to miss. Look at the interest column on its own: $5,931 at five years, $19,164 at ten, $78,612 at twenty, $211,355 at thirty. Interest roughly tripled between year five and year ten, quadrupled between ten and twenty, and nearly tripled again between twenty and thirty, while the deposit stayed at the same flat $250 a month throughout. Nothing about the plan got more aggressive. The base simply kept getting larger, and a fixed percentage of a larger base is a larger number.

Contribution size, rate, and time

Three levers move the final number, and they do not move it equally. Start from a clean base case: no starting balance, $200 a month, an illustrative 6%, monthly compounding, 20 years. That produces $92,408. Now change one lever at a time.

Raising the deposit by 25% adds twice as much as raising the rate by a full percentage point. Three extra years of the same $200 a month adds more than either. Five extra years adds twice what the bigger deposit did. This ordering is not universal, and it flips at long horizons where the rate has more compounding periods to work through, but at twenty years the two levers you actually control, how much and how long, are doing most of the work.

Deposits dominate early, compounding dominates late

The honest way to describe a contribution plan is that it has two eras. Early on, the balance is almost entirely your own money. Later, it is mostly interest. Take the $10,000 plus $250 a month at 6% again and watch the share of the balance that came from interest rather than deposits:

The crossover, the first year in which cumulative interest exceeds every dollar you have put in, arrives in year 19: $70,074 of interest against $67,000 of your own money. Before that year the plan is mostly a savings habit. After it, the balance is mostly the product of arithmetic you are no longer feeding.

The same shift is visible inside a single year. In year one of that plan, $3,000 of deposits earned $701 of interest. In year thirty, the identical $3,000 of deposits sat alongside $17,993 of interest. The deposit never changed. The base it landed on did. That is also the reason when you start moves the outcome more than most people expect.

Frequently asked questions

Related reading: How compound interest works · Simple vs compound interest · Starting early vs starting late · How fees compound · Glossary · FAQ

All rates on this page are illustrative and assumed constant so the arithmetic is comparable. Real account rates change and returns vary. Educational only, not investment, tax, or financial advice.