Pro Rata Calculator for Insurance Endorsements

Pro Rata Insurance Endorsement Calculator

Calculation Summary

Total Days in Term:

Days Remaining:

Pro Rata Factor:

Unearned Premium:

Note: This calculation uses the daily pro rata method. Some insurers may apply "short rate" penalties for mid-term cancellations initiated by the insured.

function calculateProRata() { var premium = parseFloat(document.getElementById('annualPremium').value); var start = new Date(document.getElementById('policyStart').value); var end = new Date(document.getElementById('policyEnd').value); var change = new Date(document.getElementById('endorsementDate').value); if (isNaN(premium) || isNaN(start.getTime()) || isNaN(end.getTime()) || isNaN(change.getTime())) { alert("Please fill in all fields with valid data."); return; } if (end <= start) { alert("Expiration date must be after the start date."); return; } if (change end) { alert("Endorsement date must fall within the policy period."); return; } // Calculate millisecond differences var oneDay = 24 * 60 * 60 * 1000; var totalTermMs = end.getTime() – start.getTime(); var remainingMs = end.getTime() – change.getTime(); // Calculate days var totalDays = Math.round(totalTermMs / oneDay); var daysRemaining = Math.round(remainingMs / oneDay); // Calculate Factor and Amount var factor = daysRemaining / totalDays; var proRataAmount = premium * factor; // Display results document.getElementById('resTotalDays').innerText = totalDays; document.getElementById('resDaysRemaining').innerText = daysRemaining; document.getElementById('resFactor').innerText = (factor * 100).toFixed(4) + "%"; document.getElementById('resAmount').innerText = "$" + proRataAmount.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}); document.getElementById('insuranceResult').style.display = 'block'; }

Understanding Pro Rata in Insurance

A pro rata calculation is the standard method used by insurance underwriters and brokers to determine the refund (return premium) or additional cost due when a policy is modified mid-term. Unlike "short rate" cancellations, which include a penalty fee, pro rata logic assumes a linear consumption of the premium based purely on time.

When to Use This Calculator

• Adding a Vehicle or Driver: If you add coverage halfway through the year, you only owe the premium for the remaining days.
• Removing Coverage: If you remove a piece of scheduled equipment, the insurer owes you back the "unearned" portion of the premium.
• Policy Cancellations: If an insurer cancels a policy, they are legally required to provide a pro-rata refund in most jurisdictions.

Calculation Example

Imagine a policy with a $1,200 annual premium running from January 1st to December 31st (365 days). If you remove a coverage on July 1st:

• 1. Total Days: 365
• 2. Days Remaining: 184 (July 1 to Dec 31)
• 3. Factor: 184 / 365 = 0.5041
• 4. Result: $1,200 × 0.5041 = $604.92

Pro Rata vs. Short Rate

It is important to distinguish between pro rata and short rate. Pro rata is "fair value"—you pay exactly for the time the insurance was in force. Short rate typically involves the insurer keeping an extra 10% of the unearned premium to cover administrative costs, usually applied when the policyholder cancels the policy early for their own convenience.