The answer by D Stanley gives the basic math. But you asked for specific amounts.
Assuming all gains are kept in the savings account or included in the
CD renewal and rates do not change. Given one year and $1,000 which is
more profitable? What about over 5 years if the pattern continues?
Taking the APY for the CDs and the savings account, and looking at $1,000 for one year and renewing as appropriate.
- One 1 Year CD: 1000 * 0.0210 or 21.00 in interest
- Two 6 month CD: 1000 * 0.0100 or 10.00 in interest
- Four 3 month CD: 1000 * 0.0075 or 7.50 in interest
- Savings Account: 1000 * 0.0180 or 18.00 in interest
If the rates didn't move for 5 years then after 5 years taking into account compounding you would have:
- Five 1 Year CD: 1000 * (1.0210) ^ 5 or 109.50 in interest
- Ten 6 month CD: 1000 * (1.0100) ^ 5 or 51.01 in interest
- Twenty 3 month CD: 1000 * (1.0075) * 5 or 38.07 in interest
- Savings Account : 1000 * (1.0180) * 5 or 93.30 in interest
But that is a big assumption. If rates stay the same that is what you will receive. The Savings account gives the most flexibility because you can get to the money without penalty at any time. It also has risk becasue the bank can change the rate at any time, while for you the CD rates only change at renewal.
If rates rise over the 5 years it is possible that the savings account rates will also go up. If that is the environment then the CD rates will also rise during that time. Having three month CD will allow you to catch the rising rates quicker but the rates would have to go up significantly to catch up with the 1 year CD or the Savings account.
Some people ladder their CDs. They structure them so that they end up with four 1 year CDs staggered by 3 months. They buy shorter term CDs in the beginning or buy them every three months depending on their cash availability and the rates of savings account in their bank. This catches rising rates or can extends better rates when overall rates are dropping. But if you need all the money at once, then you may face penalties that will reduce or eliminate the advantages of the CDs.