How to Calculate Savings Growth With Monthly Deposits: Formula and Free Online Calculator (2027)

Want to know exactly how your savings will grow? This free 2027 guide explains what savings growth is, why it is worth calculating, how to do it with monthly deposits, when the result can differ from reality, and the good and bad of relying on projections. Check every figure with our free online savings calculator.

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What is savings growth?

Savings growth is the increase in your balance over time from two sources: the money you deposit and the interest that money earns. Splitting the two shows how much of your final balance came from you and how much from interest. That split is the heart of any savings projection.

Why calculate it?

• It shows whether your plan reaches your goal on time.
• It lets you compare choices, such as a higher deposit versus a higher rate.
• It gives you a benchmark to check your real account against.
• It is motivating: seeing interest add up encourages regular saving.

How to calculate savings growth with monthly deposits

Use this formula for deposits made at the end of each month:

Balance = P × (1 + i)^n + D × ((1 + i)^n − 1) ÷ i

where P is the starting balance, D is the monthly deposit, i is the monthly interest rate (the annual rate divided by 12) and n is the number of months. The first part grows your starting balance. The second part adds up the growth of every deposit, each earning interest for a different length of time.

A worked example

Start with $2,000, add $150 a month, at 4.8% a year compounded monthly, for 5 years. The monthly rate is 0.4% (0.004) and n is 60.

1. (1.004)^60 is about 1.2706.
2. Starting balance grows to about $2,541.
3. Deposits grow to about 150 × 67.66 = $10,149.
4. Total: about $12,690.

You put in $11,000 ($2,000 plus $9,000 of deposits), so interest earned is about $1,690, or roughly 15% on your contributions. Along the way, the balance is about $3,938 after year 1 and $8,105 after year 3. A spreadsheet FV function gives the same answer, which is a handy cross-check.

Deposit timing: start or end of the month?

Our calculators assume deposits at the end of each period. If you deposit at the start of each month, each deposit earns one extra month of interest. In the example above that adds about $40, making the total about $12,730. The difference is small, but it is always worth knowing which assumption a calculator uses.

Different deposit frequencies

For weekly, biweekly or daily deposits, the same idea applies: convert the annual rate to the growth for one deposit period and repeat. Our free online savings calculator handles weekly, biweekly, monthly, quarterly and annual deposits, with your choice of daily, monthly, quarterly, semi-annual or annual compounding.

When can real results differ?

• Your rate changes, as many savings rates are variable.
• You skip, increase or reduce deposits.
• Fees apply, or the bank compounds or credits interest differently.
• Taxes are due on the interest.

Treat the output as a guide and recalculate when something changes.

Good or bad: how much should you trust a projection?

The good: it turns a vague hope into a concrete number, and it makes trade-offs visible.

The bad: a projection looks precise but rests on assumptions. Rates move, and life changes deposits. It is safest to run a few versions, such as a lower rate or a smaller deposit, and check that the plan still works.

Sanity checks

• With 0% interest, the balance should equal the deposits.
• Doubling the deposit should roughly double the deposit-growth part.
• Interest earned should rise faster in later years than in early ones.

How much does compounding frequency change the result?

For $5,000 starting balance plus $200 a month at 5% over 10 years, the compounding frequency makes only a modest difference: about $39,017 compounded annually, $39,240 quarterly, $39,292 monthly, and $39,317 daily. The gap between annual and daily compounding here is only about $300 on a nearly $39,000 balance, confirming that the deposit amount and time in the market matter far more than how often interest is added.

Cross-checking with a spreadsheet

Most spreadsheet programs have a built-in FV (future value) function that can double-check a calculator’s output. For the monthly-compounding example above, the formula would be entered as =FV(5%/12, 120, -200, -5000), where 5%/12 is the monthly rate, 120 is the number of months, -200 is the monthly payment (negative because it is money going out of your pocket) and -5000 is the starting balance (also negative for the same reason). This should return a value matching the $39,292 figure above, which is a reliable way to confirm any online calculator is set up the way you expect.

Key takeaways

• The core formula grows the starting balance and every deposit separately, then adds them together.
• Contributions are usually assumed to be made at the end of each period unless stated otherwise.
• Compounding frequency changes the result only slightly compared with the effect of the deposit amount and time.
• A spreadsheet FV function is a reliable way to double-check any calculator’s output.
• Testing 0% interest is a quick way to confirm a calculator is set up correctly, since the result should equal your deposits.

Once you are comfortable with the formula and have checked it against a spreadsheet, the fastest way to explore different plans day to day is our free online Savings Calculator, which applies the same maths shown here instantly as you change any input, including starting balance, contribution, rate and compounding frequency.

Frequently asked questions

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Educational content, not personal financial advice. See our disclaimer.