function calculateProration() {
// Clear previous errors
var errorDiv = document.getElementById('errorDisplay');
var resultDiv = document.getElementById('resultDisplay');
errorDiv.style.display = 'none';
errorDiv.innerText = ";
resultDiv.style.display = 'none';
// Get Inputs
var cost = parseFloat(document.getElementById('fullPeriodCost').value);
var cycleStartStr = document.getElementById('cycleStart').value;
var cycleEndStr = document.getElementById('cycleEnd').value;
var usageStartStr = document.getElementById('usageStart').value;
var usageEndStr = document.getElementById('usageEnd').value;
// Validation
if (isNaN(cost) || !cycleStartStr || !cycleEndStr || !usageStartStr || !usageEndStr) {
errorDiv.innerText = "Please fill in all fields correctly.";
errorDiv.style.display = 'block';
return;
}
// Parse Dates
var cycleStart = new Date(cycleStartStr);
var cycleEnd = new Date(cycleEndStr);
var usageStart = new Date(usageStartStr);
var usageEnd = new Date(usageEndStr);
// Normalize time to midnight to avoid timezone offsets affecting day diff
cycleStart.setHours(0,0,0,0);
cycleEnd.setHours(0,0,0,0);
usageStart.setHours(0,0,0,0);
usageEnd.setHours(0,0,0,0);
// Logic Check: Dates Validity
if (cycleEnd < cycleStart) {
errorDiv.innerText = "Billing Cycle End Date cannot be before Start Date.";
errorDiv.style.display = 'block';
return;
}
if (usageEnd cycleStart ? usageStart : cycleStart;
var effectiveEnd = usageEnd effectiveEnd) {
errorDiv.innerText = "The usage dates do not fall within the billing cycle dates provided.";
errorDiv.style.display = 'block';
return;
}
// Constants
var oneDay = 1000 * 60 * 60 * 24;
// Calculate Total Cycle Days (Inclusive)
var totalCycleTime = cycleEnd.getTime() – cycleStart.getTime();
var totalCycleDays = Math.round(totalCycleTime / oneDay) + 1;
// Calculate Billable Days (Inclusive)
var billableTime = effectiveEnd.getTime() – effectiveStart.getTime();
var billableDays = Math.round(billableTime / oneDay) + 1;
// Calculate Rates
var dailyRate = cost / totalCycleDays;
var proratedAmount = dailyRate * billableDays;
// Update UI
document.getElementById('resTotalDays').innerText = totalCycleDays;
document.getElementById('resDailyRate').innerText = currencyFormat(dailyRate);
document.getElementById('resBillableDays').innerText = billableDays;
document.getElementById('resFinalAmount').innerText = currencyFormat(proratedAmount);
resultDiv.style.display = 'block';
}
function currencyFormat(num) {
return '$' + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
What is a Pro Rata Charge?
A pro rata charge (or prorated charge) is a fee calculated proportionately according to the exact time a service or asset was used. Instead of paying for a full billing cycle (like a full month), you only pay for the specific days the service was active.
This is extremely common in subscription services, rent, utilities, and insurance when you start or end a contract in the middle of a billing period.
How the Calculation Works
To calculate a prorated amount, you need to break down the full cost into a daily rate. Once you have the cost per day, you multiply it by the number of days the service was actually used.
The Formula:
1. Daily Rate = Total Cycle Cost ÷ Days in Billing Cycle
2. Prorated Cost = Daily Rate × Days of Active Service
Real-World Example: Rent Proration
Imagine you are moving into an apartment on September 15th, but the standard lease runs from the 1st to the 30th of the month. The monthly rent is $1,500.
Step 1: Determine the days in the cycle. September has 30 days.
Step 2: Calculate the daily rate. $1,500 ÷ 30 = $50 per day.
Step 3: Calculate days occupied. From Sept 15 to Sept 30 is 16 days (inclusive of the move-in day).
Step 4: Calculate the total. $50 × 16 days = $800.
Instead of paying the full $1,500, you would only pay $800 for that partial month.
Why Dates Matter
One common point of confusion is the total number of days in a month. February has 28 (or 29) days, while August has 31. Because the denominator (total days) changes, the "daily rate" of your subscription technically fluctuates slightly from month to month.
Our Pro Rata Charges Calculator handles this automatically by asking for the specific cycle start and end dates, ensuring your calculation is accurate down to the penny based on the exact calendar days involved.
Common Uses for Proration
Rent: Moving in or out mid-month.
Internet & Cable: Canceling service before the billing cycle ends.
Employee Salaries: Hiring an employee in the middle of a pay period.
SaaS Subscriptions: Upgrading a software plan halfway through a month.