Formulas and sources
Each formula this calculator uses, where it comes from, and what it leaves out. "How it works" shows the same formula with your own numbers.
Annual percentage yield
Simple, Growth and Compare modes
- APY
- annual percentage yield
- APR
- the stated yearly interest rate, before compounding
- n
- compounding periods per year: 365, 12, 4, 2 or 1
For one rate, with interest of APR ÷ n added n times over a 365-day year (for daily, 1/365 of the rate each day) and all interest left in the account, this gives the same APY as Regulation DD. Compare mode works it out for each offer.
Source: Based on Regulation DD's APY formula, Appendix A, Part I.A [1]. Regulation DD has banks round the APY to two decimals [2]; this tool shows four.
Growth projection
Growth mode
- Future Value
- the balance at the end
- Principal
- the amount you deposit
- APY
- annual percentage yield, from the formula above
- Years
- the projection horizon; part years count
Assumes the rate and compounding stay the same and all interest stays in the account.
Source: Based on Regulation DD's APY formula [1], rearranged to give the balance after a term.
Sources
- Consumer Financial Protection Bureau. 12 CFR part 1030 (Regulation DD), Appendix A: Annual Percentage Yield Calculation, Part I. eCFR, current as of October 1, 2026.
- Consumer Financial Protection Bureau. 12 CFR 1030.3 (Regulation DD): General disclosure requirements, paragraph (f)(1), rounding. eCFR, current as of October 1, 2026.
What this calculator does not cover
- This converts one stated rate into an APY for a 365-day year. It does not handle rates that change by balance tier or on set dates, introductory rates, account fees, bonuses or tax on interest.
Sources checked on .