How to Calculate Prepaid Expense Amortization: Formulas and Examples

Getting the amortization calculation right is fundamental to accurate financial reporting. A small error in the formula compounds across 12 or 24 months and can result in a meaningful misstatement on the balance sheet. This guide covers the three main methods with full worked examples.

Method 1: Straight-Line Monthly

This is the most widely used method. The original prepayment amount is divided equally by the number of months in the coverage period. The same amount is expensed every month regardless of how many days are in the month.

Example: You pay £18,000 on 1 April for a 12-month software licence (1 April to 31 March). Monthly amortization = £18,000 ÷ 12 = £1,500 per month. After 6 months (end of September), the remaining prepaid balance is £18,000 − (6 × £1,500) = £9,000.

Method 2: Straight-Line Daily

The daily method calculates a rate per day and multiplies by the number of days in each period. This gives different amounts for months of different lengths (January has 31 days; February has 28 or 29). It is more precise and required when coverage periods start or end mid-month.

Example: You pay £6,000 on 15 March for a 6-month insurance policy covering 15 March to 14 September (184 days). Daily rate = £6,000 ÷ 184 = £32.61 per day. March amortization covers 17 days (15–31 March): 17 × £32.61 = £554.35. April amortization covers 30 days: 30 × £32.61 = £978.26.

Method 3: Partial Month at Start and End

A simpler variant of the daily method: count the number of complete months and add partial months at the start and end. This is faster to calculate manually than the full daily method and accurate enough for most purposes.

Example continued: The policy runs from 15 March to 14 September. That is 5 complete months (April–August) plus a partial month at the start (half of March) and a partial month at the end (half of September). Full month amount = £6,000 ÷ 6 = £1,000. March = £1,000 × (17/31) = £548.39. September = £1,000 × (14/30) = £466.67. April–August = £1,000 each.

Reconciling to Zero

Whichever method you use, the sum of all monthly amortization amounts must equal the original prepayment amount exactly. If there is a rounding difference (common with the daily method), adjust the final month's entry to close out the remaining balance precisely. Never leave a residual balance after the coverage period has ended.

Handling Mid-Year Adjustments

If a prepayment is cancelled or a refund is received part-way through the coverage period, the remaining balance changes. The new remaining balance is: (Original amount − Total amortization posted to date − Refund amount). The new monthly rate for remaining months is: New remaining balance ÷ Remaining months. Do not recalculate historical months — only adjust the future schedule.

Verification Check

• Sum of all amortization entries equals the original prepayment amount
• No amortization is posted before the coverage period starts
• No amortization is posted after the coverage period ends
• The prepaid balance is zero at the end of the coverage period (or reflects a refund/write-off)
• Each monthly entry matches your amortization schedule — post the amount the schedule says, not an estimated round number