Savings Growth Calculator

Interest is applied monthly using a rate derived from the APY, and deposits land at the end of each month. Solving for a goal uses the same relationship rearranged for the deposit.

• Monthly rate r = (1 + APY ÷ 100)^(1/12) − 1
• Projection — each month: balance = balance × (1 + r) + deposit
• Goal mode: shortfall = goal − current savings × (1 + r)^months
• Annuity factor = ((1 + r)^months − 1) ÷ r, or the month count when r = 0
• Deposit needed = shortfall ÷ annuity factor (zero if the shortfall is already covered)

In projection mode the balance is what your deposits plus interest reach. In goal mode the deposit needed is the monthly amount that closes the gap between your current savings, grown forward, and the target.

Common mistakes and assumptions

• Entering an interest rate rather than the APY your account quotes.
• Assuming deposits land at the start of the month — they are applied at month end here, which is slightly conservative.
• Setting a goal date so close that the required deposit is unrealistic; lengthen the term and compare.

Saving for a $30,000 down payment in 5 years with $4,000 already set aside in a 4.5% account.

1. 1Monthly rate = 1.045^(1/12) − 1 = 0.0036748.
2. 2The $4,000 grows to $4,000 × 1.0036748^60 = $4,984.73.
3. 3Shortfall = $30,000 − $4,984.73 = $25,015.27.
4. 4Annuity factor = (1.0036748^60 − 1) ÷ 0.0036748 = 66.99.
5. 5Deposit needed = $25,015.27 ÷ 66.99 = $373.41 a month, of which interest supplies roughly $3,600 of the goal.

What is the difference between APY and interest rate?

APY already includes the effect of compounding, which is what banks advertise on savings accounts. This calculator expects APY and converts it to an exact monthly rate, so no double-compounding occurs.

Why is the deposit needed smaller than the goal divided by months?

Because interest covers part of the goal. The Interest doing the work line shows exactly how much of the target you do not have to deposit yourself.

Does it account for tax on interest?

No. Interest in a taxable account is generally taxable income, so your after-tax balance will be lower. Rates in tax-advantaged accounts can be used as entered.

What if my rate changes?

Savings rates move. Run the projection at your current rate and again at a lower one to see how sensitive the plan is; deposits usually matter more than the rate over short periods.