Investment Growth Calculator

Every long-term investment projection ultimately reduces to the same two moving parts: how much money is already working (the starting balance) and how much new money keeps arriving (contributions) — both compounding at whatever rate the underlying investment actually earns. Understanding how these two pieces combine is more useful than memorizing any single projected number, since the actual return will always differ somewhat from any single assumed rate.

The starting balance compounds on its own, growing by the same factor every period regardless of what happens with contributions — a lump sum invested once and left alone follows the familiar compound-interest curve. Monthly contributions add a second stream: each new deposit starts compounding from the moment it arrives, so a contribution made in year one has decades to grow, while one made in year nineteen has barely gotten started. This is exactly why the earliest contributions in a long horizon matter disproportionately more than later ones of the same size — a dollar invested at 25 does meaningfully more work than a dollar invested at 45, purely because it compounds for longer.

Over long horizons, the growth component — not the contributions themselves — usually ends up being the majority of the final balance. This is a genuinely counterintuitive result for many people: it can feel like a retirement account's size mostly reflects how much was put in, when for a multi-decade horizon at a reasonable return, growth on growth typically outweighs the sum of contributions by a wide margin. Seeing this split explicitly, rather than only seeing a single final number, is one of the more useful things a projection like this can show.

It's worth treating any single assumed return rate as a planning estimate, not a promise. Real markets don't compound at a smooth, constant rate — they move in a genuinely uneven sequence of good years and bad years that happens to average out to something like the assumed rate over a long enough horizon. Running the same projection at a few different rate assumptions — a base case and a more conservative one — builds a more honest sense of the range of plausible outcomes than trusting a single number too precisely. It's also worth revisiting the projection periodically rather than treating it as a one-time exercise — as actual contributions, market conditions and goals evolve over a multi-decade horizon, updating the inputs keeps the projection genuinely useful instead of a stale snapshot from years earlier.