function calculateProRataRent() {
// 1. Get Input Values
var rentInput = document.getElementById("monthlyRent").value;
var dateInput = document.getElementById("moveInDate").value;
var resultBox = document.getElementById("prorata-results");
var dateError = document.getElementById("dateError");
// 2. Validate Inputs
if (!rentInput || isNaN(rentInput) || rentInput < 0) {
alert("Please enter a valid monthly rent amount.");
return;
}
if (!dateInput) {
dateError.style.display = "block";
resultBox.style.display = "none";
return;
} else {
dateError.style.display = "none";
}
// 3. Parse Date Logic
// Note: Adding time component to ensure timezone doesn't shift the date
var moveInDateObj = new Date(dateInput + 'T00:00:00');
var year = moveInDateObj.getFullYear();
var monthIndex = moveInDateObj.getMonth(); // 0-11
var dayOfMonth = moveInDateObj.getDate();
// 4. Calculate Total Days in the specific month
// New Date(year, month + 1, 0) gives the last day of the current month
var daysInMonth = new Date(year, monthIndex + 1, 0).getDate();
// 5. Calculate Billable Days
// Formula: (Total Days – Move In Day) + 1 (because move-in day is usually charged)
var billableDays = daysInMonth – dayOfMonth + 1;
// Edge case: if move in date is invalid for some reason (though HTML5 input prevents this mostly)
if (billableDays < 1) {
billableDays = 0;
}
// 6. Calculate Financials
var rentAmount = parseFloat(rentInput);
var dailyRate = rentAmount / daysInMonth;
var totalProRated = dailyRate * billableDays;
// 7. Format Output Strings
var monthNames = ["January", "February", "March", "April", "May", "June",
"July", "August", "September", "October", "November", "December"
];
var monthString = monthNames[monthIndex] + " " + year;
var currencyFormatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
});
// 8. Update DOM
document.getElementById("resMonth").textContent = monthString;
document.getElementById("resTotalDays").textContent = daysInMonth;
document.getElementById("resBillableDays").textContent = billableDays + " days";
document.getElementById("resDailyRate").textContent = currencyFormatter.format(dailyRate);
document.getElementById("resTotalAmount").textContent = currencyFormatter.format(totalProRated);
// Show results
resultBox.style.display = "block";
}
How to Calculate Pro Rata Rental Costs
When you move into a rental property in the middle of a billing cycle (usually the middle of the month), landlords typically charge "pro-rated rent." This ensures you only pay for the specific number of days you occupy the unit, rather than the full monthly rate.
Understanding this calculation is crucial for budgeting your move-in costs, which often include the security deposit, first month's pro-rated rent, and sometimes the last month's rent.
The Pro-Rated Rent Formula
The most common method used by landlords and property managers involves determining the exact daily rate for the specific month of the move-in. The logic follows these steps:
Step 1: Determine the total number of days in the move-in month (28, 29, 30, or 31).
Step 2: Calculate the Daily Rate by dividing the Monthly Rent by the total days in that month.
Step 3: Count the Billable Days. This is the number of days from your move-in date to the end of the month (inclusive of the move-in day).
Step 4: Multiply the Daily Rate by the Billable Days.
Formula: (Monthly Rent ÷ Days in Month) × (Days Occupied) = Pro-Rated Amount
Example Calculation
Let's say you sign a lease for an apartment with a monthly rent of $1,500. You are scheduled to move in on September 18th.
September has 30 days.
Daily Rate: $1,500 ÷ 30 = $50.00 per day.
Billable Days: From Sept 18 to Sept 30 is 13 days (30 – 18 + 1).
Total Pro-Rated Rent: $50.00 × 13 = $650.00.
Why does the daily rate change?
Because the calendar months vary in length (February has 28 days while March has 31), the "daily cost" of your apartment fluctuates slightly from month to month. A pro-rated move-in during February will have a higher daily rate than one in March, even if the total monthly rent is the same.
The 30-Day "Banker's Month"
While the calculator above uses the exact number of days in the month (the standard legal practice in most jurisdictions), some lease agreements stipulate a "30-day banker's month" or a "365-day year" calculation. Always check your specific lease agreement to see which method your landlord uses.