Savings plan calculator
See how regular monthly deposits grow with interest, year by year.
Enter your values and press Calculate. Your inputs stay in the address, so you can switch language or share the link.
Formula
balance(k) = balance(k − 1) × (1 + r) + deposit, r = annual rate ÷ 12
How it works
Each month interest is added to the balance and then the deposit is made, so later interest is earned on earlier deposits and earlier interest.
Worked example
The same calculation with the default values:
- Starting balance 1,000
- Monthly deposit 200
- Annual interest rate 5 %
- Years 10
- r = 5 ÷ 100 ÷ 12 = 0.004167
- n = 10 × 12 = 120
- balance(k) = balance(k − 1) × (1 + 0.004167) + 200
- balance(n = 120) = 32,703.465387
- interest = 32,703.465387 − (1,000 + 200 × 120) = 7,703.465387
Result: Final balance: 32,703.47 USD · Total you paid in: 25,000.00 USD · Interest earned: 7,703.47 USD
Assumes a constant rate. Real returns vary and are not guaranteed; tax and fees are ignored.
Common mistakes
- Entering a one-time lump sum as the "monthly deposit" instead of using the separate initial-balance field for it.
- Assuming the annual rate stays constant for the entire period - real investment returns vary year to year, so this is a steady-rate projection, not a guarantee.
- Forgetting that contributions made later in the period have less time to grow than contributions made early - the order and consistency of deposits matters more than this single-rate model can show.
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