A nominal rate needs a compounding convention
If a lender or investment quotes 6% nominal interest compounded monthly, the monthly periodic rate is 6% ÷ 12 = 0.5%. The nominal rate is useful for calculating periodic charges, but it is not the same as the amount a balance would grow over a full year if each month's interest remained in the balance.
Effective annual rate includes compounding
The effective annual rate for a 6% nominal rate compounded monthly is:
That extra 0.1678 percentage point is the effect of earning or charging interest on prior-period interest during the year.
Why the distinction matters when comparing offers
Two products can quote the same nominal rate but compound at different frequencies. The more frequently interest is compounded, the higher the effective annual rate when all other terms are equal. Conversely, two products can have different nominal rates but similar effective rates.
For amortizing loans, also compare fees, payment frequency, term, and total interest. A rate comparison alone does not capture the complete cost.
Nominal and effective rates in an amortization schedule
Most fixed-payment schedules start with a periodic rate derived from the stated annual rate. If the note says 6% nominal with monthly payments, a simple periodic schedule commonly uses 0.5% per month. The loan payment formula then combines that rate with the number of payments.
Date-based loans may use a different convention, so always model the method stated in the agreement.
A practical comparison checklist
- Identify whether the quoted rate is nominal, effective, or an APR disclosure.
- Identify the compounding frequency.
- Convert rates to the same basis before comparing.
- Include fees and term when comparing loan cost.
- Use the actual payment schedule rather than assuming identical timing.
Example: compare monthly and quarterly compounding
Suppose two accounts both quote a 6% nominal annual rate. One compounds monthly and the other quarterly. Monthly compounding produces an effective annual rate of about 6.1678%. Quarterly compounding produces about 6.1364%. The nominal rate is identical, yet the one-year growth differs because the interest is credited at different intervals.
For loans, the same idea is useful but incomplete. A lender may quote a nominal rate, an APR, and a specific payment schedule. The amortization payment depends on the periodic rate and number of payments, while APR disclosures can also reflect certain finance charges. Keep these measures labeled so one is not accidentally substituted for another.
When documenting a comparison, write the rate as a complete phrase—such as “6.00% nominal, compounded monthly”—instead of recording only “6%.” That small habit prevents many spreadsheet and software errors later.
Build the numbers instead of estimating them
Use the free online schedule calculator for a complete amortization table. If you need to edit individual payment dates, amounts, rates, or notes and save the revised loan, use Amortization Pro for Windows.