Compared

30/360 vs Actual/365: How the Day-Count Basis Changes Your Refund (2026)

By Miguel Lin · Last updated June 30, 2026

You worked out your insurance refund, the insurer quoted a slightly different number, and neither of you is obviously wrong. Nine times out of ten the culprit is the day-count basis — how many days the year is treated as having, and which endpoints get counted. It moves the figure quietly. Open the insurance refund calculator, switch the basis, and watch the number shift; here’s what’s actually going on underneath.

How does the day-count basis change a refund? A pro rata refund is (total premium ÷ total days in the term) × unused days. The “total days” denominator depends on the basis: a real calendar year is 365 days (366 when the term spans Feb 29), while a 360-day basis treats the year as 360. A different denominator means a different per-day cost — so the same premium and the same unused days yield a slightly different refund.

On your own numbers, the day-count calculator shows the day count and the per-day math and lets you flip between bases — which is usually how you find the convention behind a quote without arguing over a few dollars.

The Denominator: 360 vs 365 vs 366

Every pro rata refund divides the premium by the total days in the term to get a daily cost. That denominator is where the bases diverge:

• Actual/365 — uses a real 365-day year. The most common consumer-facing basis.
• Actual/366 — applies when the policy term spans Feb 29 in a leap year, so the real day count is 366.
• 30/360 (a 360-day basis) — treats every month as 30 days and the year as 360, a convention borrowed from banking and bond math. The shorter denominator makes each day cost a touch more.

Same premium, same unused days, different denominator: the per-day rate changes, and the refund moves with it. The shift is usually small in percentage terms, but on a large premium it can be enough to make your figure and the insurer’s disagree.

Inclusive vs Exclusive Endpoints

The denominator isn’t the only lever. The other is whether the endpoints are counted — specifically, the cancellation day.

Many insurers treat the cancellation day as used, so it falls on the earned side and the unused-day count starts the next day. That keeps used days plus unused days adding up to the full term, with nothing double-counted. Count it the other way — as still unused — and you add a day to the refundable side. One day rarely swings the total far, but combined with a different denominator, two “reasonable” calculations can land a few dollars apart.

Because conventions differ, this is a check your policy matter, not a fixed rule. The point is simply to know that the endpoint choice exists, so a small gap doesn’t look like an error.

Why Your Refund Doesn’t Match the Quote

Put the two levers together and you have the usual explanation for a mismatch:

1. A different denominator — your insurer used 360 where you assumed 365, or 366 where a leap day applies.
2. A different endpoint rule — the cancellation day landed on the used side in one calculation and the unused side in the other.

Neither is anyone cheating. They’re just different defaults. The fix isn’t to debate which is “correct” — it’s to reproduce the insurer’s basis and confirm the math is consistent. That’s exactly what switching the basis lets you do: flip from 365 to 360 to 366, see the day count update, and find the combination that reproduces the quoted figure.

Reading the Bases Against Your Quote

The way to identify which convention your insurer used is comparison, not memorization. Each basis produces its own per-day cost and its own day count, so the same dates can yield three close-but-distinct figures across actual/365, actual/366, and a 360-day basis. Whichever one lands on the number you were quoted is the convention behind it — and that match also tells you the figure is internally consistent rather than wrong. The refund calculator is built for exactly this read: it surfaces the days it counted alongside each basis, so the comparison is visible instead of a guess.

If no basis matches, the gap probably isn’t the day count at all. It may be a refund applied to an outstanding balance instead of paid out, or an early-cancellation penalty — a short-rate cancellation, which is its own calculation (a dedicated short-rate calculator is in progress). Everything here is deliberately the straight, no-penalty pro rata view, which is what makes it a reliable reference point. Need the rest of the toolset? The pro rata calculator handles salary, rent, and bonus splits too.