BCBetter Calculators

Savings Interest Calculator

Calculate the interest earned on your savings over time with compound interest.

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Enter your values and click Calculate

How It Works

Compound interest is applied separately to both the initial deposit and to the stream of monthly contributions, then combined. The future value of the initial lump sum uses the standard compound interest formula: FV = P × (1 + r)^n, where r is the monthly interest rate (annual rate ÷ 12) and n is the total number of months. This models the initial deposit growing uninterrupted over the full time period. The future value of monthly contributions uses the ordinary annuity formula: FV = PMT × [(1 + r)^n − 1] / r, which assumes each contribution is made at the end of the month and then earns interest on the remaining months. The two future values are summed to give the total projected balance. Interest earned is the total balance minus the total cash deposited (initial deposit plus all monthly contributions). The interest-as-a-percentage-of-final-balance output shows how much of your end balance was generated by compound growth rather than your own deposits.

Examples

Emergency fund growth
$5,000 initial, $200/month, 4.5%, 10 years.
Result: Balance grows to ~$36,800 on ~$29,000 contributed — earning ~$7,800 in interest.
Long-term wealth building
$10,000 initial, $500/month, 7%, 20 years.
Result: Balance ~$270,000 on ~$130,000 contributed — compound growth adds over $140,000.
Short-term HYSA savings
$2,000 initial, $100/month, 4.8%, 3 years.
Result: Balance ~$5,900 — roughly $500 in interest on $5,600 deposited.

Frequently Asked Questions

What is compound interest?
Compound interest is interest earned on both your original deposit and on the interest you have already accumulated. Unlike simple interest, which only grows on the principal, compound interest causes your balance to grow exponentially over time — the longer the period, the more powerful the effect.
How does APY differ from APR?
APY (Annual Percentage Yield) already accounts for the effect of compounding over a year, while APR (Annual Percentage Rate) does not. When comparing savings accounts, always use APY, as it reflects the actual return you will earn including the compounding effect.
Does the order of deposits affect total growth?
Yes, to a small degree. This calculator uses the ordinary annuity model (contributions made at the end of each month). Some accounts credit interest differently. For most practical savings planning, the difference is minor and this model provides a reliable estimate.

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