Savings Account Monthly Interest Calculator

Formatting only. This does not change the calculation.
The lump sum already invested or deposited today. Use 0 if you are starting from nothing.
Banks advertise an APY, lenders a nominal rate. Say which one you typed in the field below.
Choosing APY converts the quoted yield back to the nominal rate its compounding schedule implies, so a full year returns exactly the APY you typed.
Amount added every contribution period. Leave at 0 for a pure lump sum.
Used only to restate the final balance in today's purchasing power.
Applied to the interest earned to show an after-tax balance. Use 0 for a tax-sheltered account.
Average interest a month
33.33
Compound Interest Calculator
Future value
10,400.00
Total you put in
10,000.00
Of which contributions
0.00
Compound interest earned
400.00
Effective annual yield (APY)
4.0000 %
Equivalent nominal annual rate
3.928488 %
Multiple of what you put in
1.0400
Balance after tax on interest
10,400.00
Future value in today's money
10,400.00
Years for the balance to double
17.67
Average interest a year
400.00
Worked out from Compound interest formula A = P(1 + r/n)^(nt), and the future value of an annuity FV = PMT[((1+i)^N - 1)/i] and 3 other sources.

How this is worked out

7 steps
  1. The 4% quoted is an APY, so the nominal rate behind it is 12 x ((1 + 4%)^(1/12) - 1) = 3.928488%.
  2. Rate per compounding period = 3.928488% / 12 compounding monthly (12 a year).
  3. Effective annual yield = (1 + r/n)^n - 1 = 4%.
  4. No recurring contributions, so the balance grows from the 10000 starting amount alone.
  5. Future value after 1 years = 10400, of which 400 is compound interest on 10000 paid in.
  6. Average interest a month = 400 / (1 years x 12) = 33.33.
  7. At this yield the balance doubles every ln(2)/ln(1+i) = 17.67 years.
Year-by-year balancetable
yearcontributionsinterestbalance
1040010,400

Results are general information, not financial, tax, legal, medical or engineering advice.