function calculateLeave() {
// Reset output
document.getElementById('resultBox').style.display = 'none';
document.getElementById('dateError').style.display = 'none';
// Get Inputs
var entitlementInput = document.getElementById('leaveEntitlement').value;
var startInput = document.getElementById('periodStart').value;
var endInput = document.getElementById('periodEnd').value;
// Validation
if (!entitlementInput || !startInput || !endInput) {
alert("Please fill in all fields correctly.");
return;
}
var entitlement = parseFloat(entitlementInput);
var startDate = new Date(startInput);
var endDate = new Date(endInput);
// Validate Dates
if (endDate < startDate) {
document.getElementById('dateError').style.display = 'block';
return;
}
// Calculate Days Employed (Inclusive of start and end date)
// Set times to noon to avoid daylight saving issues causing day shifts
startDate.setHours(12, 0, 0, 0);
endDate.setHours(12, 0, 0, 0);
var timeDiff = endDate.getTime() – startDate.getTime();
// Convert ms to days. Add 1 to make it inclusive (e.g. Jan 1 to Jan 1 is 1 day, not 0)
var daysEmployed = Math.round(timeDiff / (1000 * 3600 * 24)) + 1;
// Determine days in the year based on the start date year
var year = startDate.getFullYear();
// Leap year logic: divisible by 4, not by 100 unless divisible by 400
var isLeap = ((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0);
var daysInYear = isLeap ? 366 : 365;
// Core Calculation
var proRataRatio = daysEmployed / daysInYear;
var calculatedLeave = entitlement * proRataRatio;
// Formatting result (Rounding to 1 or 2 decimal places is standard for leave)
var finalResult = Math.round(calculatedLeave * 100) / 100;
// Display Logic
var resultBox = document.getElementById('resultBox');
var finalValue = document.getElementById('finalLeave');
var breakdown = document.getElementById('calculationBreakdown');
finalValue.innerHTML = finalResult + " Days";
breakdown.innerHTML = "Based on " + daysEmployed + " days employed in a " + daysInYear + "-day year." +
"Formula: (" + daysEmployed + " ÷ " + daysInYear + ") × " + entitlement;
resultBox.style.display = 'block';
}
How to Calculate Pro-Rated Annual Leave
Calculating annual leave entitlements can become complex when an employee starts or leaves partway through the holiday year. This process, known as pro-rating, ensures that employees receive a fair amount of holiday based on the exact proportion of the year they were employed. Whether you are an HR professional, a business owner, or an employee checking your payslip, understanding the pro-rata formula is essential.
Why is Pro-Rating Necessary?
Standard annual leave entitlements assume an employee works a full 12-month period. However, in the dynamic world of employment, staff turnover is common. You need to calculate pro-rated leave when:
New Starters: An employee joins the company after the start of the holiday year.
Leavers: An employee resigns or is dismissed before the end of the holiday year.
Fixed-Term Contracts: Staff hired for a specific project duration less than a year.
The Pro-Rata Leave Formula
The calculation relies on the ratio of days employed versus the total days in the calendar year. While some companies calculate by months worked, calculating by days is generally considered the most accurate and fair method.
The standard formula used in our calculator above is:
(Days Employed ÷ Days in Year) × Full Annual Entitlement
Key Components:
Days Employed: The total number of calendar days the employee is under contract, including weekends and public holidays, from their start date to their end date.
Days in Year: Usually 365, but 366 during a leap year. It is crucial to use the correct denominator to avoid slight miscalculations.
Full Annual Entitlement: The total number of days an employee would receive if they worked the full year (e.g., 28 days including bank holidays).
Example Calculation
Let's look at a practical example. Imagine an employee named Sarah.
Full Entitlement: 28 Days
Start Date: 1st July
End Date: 31st December
Year: Standard (365 days)
First, we calculate the days employed. From July 1st to December 31st is 184 days.
Next, we find the ratio: 184 ÷ 365 = 0.5041
Finally, multiply by the entitlement: 0.5041 × 28 = 14.11 Days.
Most employers will round this figure, often up to the nearest half-day or full day depending on company policy, though statutory requirements may vary by jurisdiction.
Common Pitfalls to Avoid
When calculating pro-rated leave, ensure you check if the year is a leap year. A common mistake is dividing by 365 in a year that actually has 366 days, which slightly inflates the entitlement. Additionally, clarify if the "Full Annual Entitlement" figure includes or excludes public holidays, as this changes the base number for your calculation.
Use the calculator above to get an instant, accurate figure based on your specific dates.