Future Value Calculator: The Two Components Behind the Number

When a future value includes both a starting balance and regular contributions, the total isn't one formula — it's two separate calculations added together: what the starting balance grows into on its own, plus what the stream of contributions grows into separately.

Component 1: The Lump Sum

Lump Sum Future Value = Principal × (1 + Monthly Rate)^Total Months

This is standard compound growth on whatever you started with, completely independent of any contributions added afterward.

Component 2: The Contributions (Annuity)

Contributions Future Value = Monthly Contribution × [((1 + Monthly Rate)^Total Months − 1) ÷ Monthly Rate]

This is the standard future-value-of-an-annuity formula. It assumes each contribution is made at the end of its month — a common convention sometimes called an "ordinary annuity" — meaning the first deposit doesn't start earning interest until the following month.

Adding Them Together

On a $10,000 starting balance, $500 monthly contributions, a 6% annual rate, over 10 years:

• Lump sum component: $10,000 × (1.005)¹²⁰ = $18,193.97
• Contributions component: $500 × [((1.005)¹²⁰ − 1) ÷ 0.005] = $81,939.67
• Total future value: $100,133.64

Notice the contributions component is actually larger than the lump sum component here, even though the starting balance is bigger than any single monthly deposit — 120 monthly deposits, each compounding for a different length of time, add up to more than one static starting amount compounding the whole way.

Why Splitting It This Way Matters

Seeing the two components separately makes it clear which lever actually moves the number more for your situation. Someone with a large starting balance and small ongoing contributions is mostly relying on the lump-sum component; someone starting from zero is entirely dependent on the annuity component — and knowing which one applies changes what's worth optimizing first.