The published rate is per interval, not per day
A perpetual swap has no expiry, so funding is the mechanism that keeps its price tethered to spot. Longs pay shorts, or the reverse, every settlement interval. The critical detail is that the rate a venue publishes applies to one interval — and venues do not agree on how long an interval is.
Eight hours is the most common convention, giving three settlements a day. Four hours is widespread on more volatile listings, giving six. Some venues shorten the interval on specific symbols during stress. So the published number answers "what will I pay at the next settlement", and nothing else.
The arithmetic
To compare, scale to a common period. Annualising is the convention because it lines the number up with every other cost of carry you already think about:
Applied to the example above, the two identical-looking rates diverge immediately:
| Venue | Published | Interval | Settlements/year | Annualised |
|---|---|---|---|---|
| Venue A | 0.0100% | 8h | 1,095 | 10.95% |
| Venue B | 0.0100% | 4h | 2,190 | 21.90% |
| Venue C | 0.0150% | 8h | 1,095 | 16.43% |
Venue B costs exactly twice Venue A for the same posted number. And Venue C, which looks the most expensive of the three at a glance, is cheaper to carry than Venue B. If you are choosing where to hold a financed position on the basis of the published column, you will systematically pick wrong on any venue with a short interval.
Three things that make it worse in practice
- Funding is charged on notional, not on margin. A position held at 5× leverage pays the same funding as the same notional held at 1×, so the cost does not scale with the capital you have committed to it.
- The sign flips. A rate that is positive today can be negative next week, which turns a cost into a receipt. A financing book netted to a single number hides which clients and products are on which side of that.
- Intervals are not always aligned to the clock you assume. Settlements at 00:00, 08:00 and 16:00 UTC are common but not universal, and a position opened and closed between two settlements pays nothing at all.
What to do about it
Three practical habits. First, never display a raw published rate next to another venue's raw published rate without the interval beside it — if the interval is not on screen, the comparison is not being made. Second, sort and decide on the annualised column, and treat the published one as a settlement detail. Third, accrue intraday from each venue's own schedule rather than approximating a daily charge, because the approximation is wrong by exactly the ratio above.
For an intermediary there is a fourth: the rate you charge a client and the rate your venue charges you are two different numbers on two different schedules, and the spread between them is the actual business. Netting them at the end of the month tells you whether it worked. Accruing both continuously tells you while you can still change it.