How to create a saving calculator.

💰 How to Create a Saving Calculator Function (Installment Savings)

Have you ever wondered:

 “If I deposit $300 every month at 4% interest, how much will I have after one year?”

In this post, we’ll learn how to build a Saving Calculator Function right inside your own programmable calculator app — so you can find the answer instantly without opening a bank app.

🧩 1. Understanding the Saving Formula

The general formula for calculating the future value of monthly savings (ordinary annuity) is:

FV = PMT × ((1 + r)^n - 1)/r

where:

PMT = monthly deposit

r = monthly interest rate (annual rate ÷ 12 ÷ 100)

n = number of months

This version assumes the deposit is made at the end of each month, which is how most banks calculate regular savings plans.

💻 2. Creating the Function (End-of-Month Deposit)

Here’s the SimpleMath code version for your programmable calculator:

def saving(principal, annualRate, months){

    var monthlyRate = annualRate / 12/ 100;

    var total = principal * ((1 + monthlyRate)^months - 1) / monthlyRate;

    return round(total);

}

You can copy above code and execute the code as below by pressing the play button.

📈 3. Example Usage

Input Code Result

$300 per month, 4% annual rate, 12 months saving(300000, 4, 12) 3,666,739

$500 per month, 5% annual rate, 24 months saving(500000, 5, 24) 12,592,960

(Values shown here are in won, but the formula works for any currency — USD, EUR, etc.)

Now you can simply type saving() inside your calculator and get the total instantly.

✨ 4. After-Tax Version

In some countries, interest earnings are taxed (for example, 15.4% in Korea).

You can easily include that:

def savingAfterTax(principal, annualRate, months){

    var monthlyRate = annualRate / 12.0 / 100.0;

    var total = principal * ((pow(1 + monthlyRate, months) - 1) / monthlyRate);

    var interest = total - (principal * months);

    var afterTax = total - (interest * 0.154);

    return round(afterTax);

}

🕐 5. Beginning-of-Month Deposit Version (Annuity Due)

Some savings plans assume the deposit is made at the beginning of each month,

which means each deposit earns one extra month of interest.

The formula becomes:

FV_due = FV_ordinary × (1 + r)

✅ Beginning-of-Month Version

def savingDue(principal, annualRate, months){

    var monthlyRate = annualRate / 12/ 100;

    var factor = ((1 + monthlyRate)^months - 1) / monthlyRate;

    var total = principal * factor * (1 + monthlyRate);

    return round(total);

}

💰 Example

Input Code Result

$300 per month, 4% annual rate, 12 months savingDue(300000, 4, 12) 3,678,961

$500 per month, 5% annual rate, 24 months savingDue(500000, 5, 24) 12,645,431

You can see that the beginning-of-month version gives a slightly higher return,

since every deposit earns interest for one extra month.

💡 After-Tax (Beginning-of-Month)

def savingDueAfterTax(principal, annualRate, months, tax){

    var monthlyRate = annualRate / 12 / 100;

    var factor = ((1 + monthlyRate)^months - 1) / monthlyRate;

    var total = principal * factor * (1 + monthlyRate);

    var interest = total - (principal * months);

    var afterTax = total - (interest * tax);

    return round(afterTax);

}

📱 Final Thoughts

Now your calculator can answer practical questions like:

 “How much will I have after one year if I start saving today?”

“What’s the difference if I deposit at the beginning of each month?”

With just one function, your programmable calculator becomes a powerful personal finance tool.

In the next article, we’ll build a Compound Interest Function to calculate the future value of investments or fixed deposits — perfect for long-term planning! 🚀