Accrual vs Cash Accounting, and Why Counting Days Is Harder Than It Looks

The companion calculator computes accrued interest, interest that has been earned or incurred but not yet paid in cash. The very fact that such a thing needs recording points to a fundamental fork in how businesses keep books, and a surprisingly slippery detail: how you count the days in a year is itself a choice, and different choices give different answers.

Two Ways to Keep Score

There are two basic philosophies of accounting, and accrued interest only exists because of one of them.

Under cash accounting, interest would only be recorded when it is paid. Under accrual accounting, interest is recorded as it accumulates, day by day, even before any cash moves, which is exactly what accrued interest captures. Most sizable and all publicly reporting businesses use accrual accounting because it gives a truer picture of performance in each period.

The Matching Principle

The reason accrual accounting bothers to record interest before it is paid is the matching principle: expenses (and revenues) should be recognized in the period they actually relate to, not the period the cash happens to move. A loan accrues interest cost every day it is outstanding, so that cost belongs to each of those days, not lumped into whatever month the payment lands. Recording accrued interest at period-end puts the cost in the right period. Skip it, and a company would understate both its interest expense and the liability it owes, making a period look better than it was, precisely the distortion accrual accounting exists to prevent.

The Hidden Trap: How Many Days in a Year?

Interest is principal times rate times time, and time means counting days, which sounds trivial until you realize there is no single agreed way to do it. Financial markets use several day-count conventions, and they disagree.

These are not rounding quibbles, they produce genuinely different interest amounts. Notably, actual/360 charges actual days but divides by only 360, so it collects slightly more interest than a naive 365-day calculation, a detail that matters on large balances. The convention has to be agreed in the loan or bond terms, and using the wrong one gives a wrong, if plausible-looking, answer.

Where Accrued Interest Shows Up

Beyond period-end adjustments, accrued interest appears prominently when bonds change hands between payment dates: a buyer typically pays the seller the interest that has accrued since the last coupon, because the seller earned it during their holding period. The same day-count conventions govern that calculation, which is why bond markets are precise about which convention applies to which instrument.

Using the Accrued Interest Figure Well

Take the calculator's accrued interest as the amount earned or owed over the period, and record it under accrual accounting to keep the cost in the right period per the matching principle. Just be deliberate about the day-count basis, confirm whether your instrument uses 30/360, actual/365, or actual/360, because the same principal and rate can yield different interest depending on the convention, and matching the calculation to the agreed terms is what makes the figure correct.