Compound Interest for Retirement — How Much Do You Need?

How Compound Interest Builds Retirement Wealth

Retirement planning is really just a compound interest problem: what monthly contribution, at what rate, for how many years, reaches your goal?

The formula works in your favor when you have decades. A 25-year-old investing $400/month at 7% reaches $1,048,000 by age 65 — contributing only $192,000 out of pocket.

Monthly Investment Needed to Reach $1,000,000

Assuming 7% annual returns, compounded monthly:

Starting Age	Years to 65	Monthly Needed	Total Contributed	Interest Earned
25	40	$381	$182,880	$817,120
30	35	$556	$233,520	$766,480
35	30	$820	$295,200	$704,800
40	25	$1,234	$370,200	$629,800
45	20	$1,920	$460,800	$539,200
50	15	$3,155	$567,900	$432,100

Notice: starting at 25 vs 45 means needing 5x less per month.

The 4% Withdrawal Rule

A common retirement guideline: you can safely withdraw 4% of your portfolio annually. This means:

• $500K portfolio → $20,000/year spending
• $1M portfolio → $40,000/year spending
• $2M portfolio → $80,000/year spending

Work backward from your desired annual spending to find your target number.

Inflation-Adjusted Planning

Inflation erodes purchasing power. $1 million in 30 years won’t buy what it does today. At 3% inflation, you’d need about $2.4 million in 30 years to have today’s equivalent of $1 million.

Solution: use a real return rate (nominal rate minus inflation). If you expect 10% returns and 3% inflation, plan with 7%.

Three Levers You Control

1. Start earlier — the biggest lever by far
2. Contribute more — increase by 1% of salary per year
3. Reduce fees — a 1% fund fee vs 0.1% costs you hundreds of thousands over 40 years

Plan Your Retirement Now

Use our compound interest calculator with the Goal Planner to find exactly how much you need to invest monthly to reach your retirement target.