Add-On CD Calculator
Add-on CDs let you keep depositing at the rate you originally locked. Model the opening deposit together with everything you add later.
Locking today’s rate for money you have not saved yet
The add-on CD’s trick is applying today’s rate to deposits you make later. Open with $5,000 at 4.25%, add $500 a month, and every contribution earns 4.25% from the day it arrives — whatever rates have done in the meantime. In a falling-rate environment that is quietly excellent: come month 18 you might be depositing at a locked 4.25% while new CDs pay 3.50%. In a rising environment the same lock works against you, which is why banks mostly offer these when they expect rates to hold or fall.
The math above separates your maturity balance into what the opening deposit earned and what the stream of contributions earned — handy for showing that, with regular deposits, the stream usually does most of the work.
If you cannot find one
Add-on CDs are a credit-union specialty, and inventory comes and goes. The workable substitute is a ladder of small standard CDs — open a fresh $1,500 certificate every quarter. You give up the single locked rate but keep the discipline, and in a rising market the quarterly re-pricing turns into an advantage.