Nominal and effective interest rates: practical guide

What to prepare

• Have the figures for input annual rate (%) and compounding periods per year ready, from a dated document, with the period (annual, monthly or per transaction) that each input expects.
• Note the date of the transaction, since this model has no year selector and fixed rates may have changed.
• Separate known facts from assumed rates before relying on the headline result.

Understand the inputs

Conversion

Options: Nominal to effective; Effective to nominal.

The calculation method

Effective rate = (1 + nominal rate ÷ periods)^periods − 1; invert for nominal rate.

Worked example

Illustrative inputs and their result.

Example inputs

Input annual rate (%)

6

Compounding periods per year

12

Conversion

Nominal to effective

Calculated example result

Effective Annual Percent

6.1678%

Nominal Annual Percent

6%

How to interpret the result

Equal compounding periods; no fees or tax. Distinguish a tax saving, a tax bill, cash received and money remaining: they are not interchangeable. An estimate does not establish eligibility, filing compliance or when a refund will be paid.

Change one input of nominal and effective interest rates at a time and record the result; keep a record of the year, date and assumptions so a change in the result can be explained.

Common mistakes to avoid

• Do not mix tax years or apply a newly announced measure to an earlier transaction.
• Avoid counting an allowance, credit, contribution or loss twice.
• Check exclusions and personal circumstances before relying on the headline result.

Assumptions and sources

Equal compounding periods; no fees or tax. Nominal interest is not APR.

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