How it works
Most savings plans are built around two things: money you already have and money you add on a schedule. This calculator combines both. You enter an initial deposit, a monthly deposit, an annual interest rate, and a number of years. It then projects the ending balance using monthly compounding, which is how most savings accounts and high-yield accounts credit interest in practice.
The tool is designed for short-to-medium-term savings goals—building an emergency fund, setting aside cash for a car, or accumulating a travel buffer. It is not a retirement projector, and it does not model contributions that change over time. It assumes your monthly deposit stays constant and that you make it at the end of each month.
One useful way to use this calculator is in reverse: pick a target balance, then adjust the monthly deposit and years until the projected balance hits your goal. That tells you how much you need to save each month—or how long you need to keep going—to get where you want to be.
The formula
B = P(1 + r/12)^(12t) + D [((1 + r/12)^(12t) - 1) / (r/12)]
Where:
- B = ending balance
- P = initial deposit
- D = monthly deposit
- r = annual interest rate (as a decimal, so 4% = 0.04)
- t = number of years
- The expression
r/12is the monthly interest rate - The exponent
12tconverts years to months
The first term grows your initial deposit with compound interest. The second term is the future value of a series of equal monthly deposits—each deposit compounds for the remaining months until the end of the term.
Worked example
Suppose you want to build a $10,000 emergency fund. You start with $500 in a high-yield savings account paying 4% APY, and you commit to depositing $200 at the end of every month. You want to know how long it will take to reach the $10,000 target.
Because the calculator solves directly for ending balance given a set number of years, you find the answer by testing a few year values until the projected balance crosses $10,000. Here is the progression:
| Years | Ending Balance |
|---|---|
| 2 | $5,852 |
| 3 | $8,518 |
| 3.5 | $9,890 |
| 3.6 | $10,139 |
At 3.5 years the balance is just shy of the goal. At 3.6 years it exceeds $10,000, so the target is reached roughly 3 years and 7 months in.
To see the mechanics, here is the full calculation for the 3.6-year result:
- Monthly rate:
0.04 / 12 = 0.003333 - Total months:
12 × 3.6 = 43.2 - Growth factor:
1.003333^43.2 ≈ 1.1543 - Initial deposit portion:
$500 × 1.1543 ≈ $577.15 - Monthly deposit portion:
$200 × [(1.1543 - 1) / 0.003333] ≈ $200 × 46.29 ≈ $9,258.00 - Ending balance:
$577.15 + $9,258.00 ≈ $9,835.15
The small difference from the $10,139 figure above is due to rounding in the displayed steps; the calculator uses full precision internally.
Things to watch
- Deposit timing matters. This calculator assumes end-of-month deposits. If you deposit at the start of each month, each contribution earns one extra month of interest, and your final balance will be slightly higher.
- Rate quotes vary. Banks sometimes quote APY (which already includes compounding) and sometimes APR (a nominal rate). If your account quotes APY, use that number directly; the monthly compounding assumption will slightly overstate growth, but the difference over a few years is usually small.
- Inflation erodes real value. A $10,000 balance in four years buys less than $10,000 today. For planning purposes, consider whether your goal should be adjusted upward for inflation.
- Keep the rate realistic. Use the rate your account actually pays, not a promotional rate that expires after a few months. Promotional rates can distort projections significantly over longer horizons.
This calculator produces an estimate, not professional financial advice. Actual account terms, fee structures, and rate changes will affect your real-world results.