What this AER monthly calculator does
If you are comparing savings accounts, fixed-term deposits, or investment projections, you'll often see the rate shown as AER (Annual Equivalent Rate). That annual number is useful for comparing products, but your real balance usually changes month by month. This calculator helps bridge that gap by converting AER into an effective monthly rate and projecting how your money can grow over a chosen number of months.
In plain language: it answers questions like, “If the account pays 5% AER, what does that mean each month?” and “How much will I have after 18 or 24 months if I keep adding money monthly?”
Why convert AER to a monthly rate?
AER already includes the effect of compounding over a year. But most people save monthly, budget monthly, and think in monthly cash flow. Converting to a monthly rate gives clearer planning data.
- It helps estimate realistic monthly balance growth.
- It lets you compare strategies with monthly contributions.
- It prevents common mistakes like simply dividing annual return by 12.
A common shortcut is monthly rate = AER / 12. That’s easy, but it is only an approximation. AER is a compounded annual figure, so the exact monthly equivalent is:
monthly rate = (1 + AER)1/12 - 1
where AER is entered as a decimal (for example, 5% = 0.05).
How the projection is calculated
Step 1: Convert AER to monthly effective rate
The calculator first transforms your annual equivalent rate into a monthly compounded rate. This tells you the percentage growth applied each month.
Step 2: Apply compounding to your starting balance
Your initial balance is compounded over the full period:
Initial Balance × (1 + monthly rate)months
Step 3: Add monthly deposits
Monthly contributions are treated as deposits made at the end of each month. Each deposit has a different amount of time to grow. The calculator uses the standard future value of an annuity formula to include this automatically.
Quick example
Suppose you have:
- 5.00% AER
- $1,000 starting balance
- $200 deposited each month
- 24 months total
Your result will show:
- The exact monthly effective rate implied by 5% AER
- Total amount you contributed
- Estimated ending balance
- Total interest earned from compounding
This gives you a practical planning snapshot without needing a spreadsheet.
AER vs APR (important distinction)
These terms are often mixed up:
- AER: annual rate including compounding effects; useful for savings comparison.
- APR: annual percentage rate often used for borrowing costs; may not represent compounding in the same way.
If you are working with savings products, AER is generally the cleaner comparison number. For loans and credit products, APR is often the headline metric.
Common mistakes to avoid
- Using AER/12 as an exact monthly rate — close, but not exact for compounding.
- Ignoring contribution timing — deposits at start vs end of month can change outcomes.
- Forgetting taxes or fees — real-world returns can be lower than projections.
- Projecting one rate forever — savings rates can change over time.
When this calculator is most useful
Use it when you want to:
- Estimate growth of a savings habit over 6, 12, 24, or 60 months
- Compare two accounts with different AER values
- Set realistic expectations for emergency fund growth
- Model short-to-medium term compounding outcomes
Final note
This AER monthly calculator is designed for clear planning and education. It provides a strong estimate based on fixed rates and regular monthly contributions. In real life, rates may change, and tax treatment depends on your location and account type. Still, if you want a fast and reliable way to understand monthly compounding from an annual rate, this tool is a great starting point.