What a Perpetual Future Is

You already know a future: a contract to buy or sell an asset at a fixed price on a fixed future date. Its whole personality comes from that date — as expiry approaches, the futures price is dragged toward the spot price, because on the last day the contract simply becomes the asset. That convergence is automatic and free; the calendar does the work.

Now delete the date. What you’re left with is a perpetual future (a “perp”), the single most-traded instrument in all of crypto — and a puzzle. With no expiry to force convergence, what stops a perp’s price from drifting away from spot forever? The answer is a small, elegant payment mechanism that this whole course is built around. But first, let’s be precise about what a perp actually is.

A future with the calendar torn off

Analogy. A dated future is a hotel reservation: it names a specific night, and as that night arrives the reservation becomes the room — its value and the room’s value converge because they’re about to be the same thing. A perpetual future is a month-to-month tenancy with no end date. You keep the place as long as you like. Nothing forces a “settlement night” — so nothing automatically keeps the rent you pay aligned with what the place is actually worth. Some other mechanism has to do that job.

Definition. A perpetual future (perpetual swap, “perp”) is a derivative contract that mimics a futures position — leveraged, long or short exposure to an underlying price — but has no expiry date and no delivery. You hold it until you choose to close it (or until you’re liquidated). It’s cash-settled continuously against a price feed rather than delivering the actual asset.

Three properties define it:

1. No expiry. Unlike a dated future, there’s no calendar pulling the price to spot. Convergence has to be manufactured.
2. Leverage by default. You post margin (collateral) and control a much larger notional position. We’ll quantify this two lessons from now.
3. Cash-settled, continuously. Your profit and loss (PnL) is marked against a price feed tick by tick; there’s no physical hand-over of the underlying.

Why crypto fell in love with the perp

Perps weren’t the first crypto derivative, but they took over for a stack of practical reasons that all point the same way: convenience for a leveraged speculator.

• No expiry to manage. A dated-futures trader has to roll — close the expiring contract and reopen the next one — paying spreads and babysitting the calendar. A perp never expires, so a directional bet is fire-and-forget.
• One deep, fungible market. Dated futures fragment liquidity across every expiry (March, June, September…). A perp pools all of that interest into a single contract, so the book is deeper and the spread tighter.
• High leverage, simple margin. Perps are built for 5×, 10×, even 100× leverage with a clean margin model, which is exactly what a speculative retail market wants.
• 24/7, on-chain native. Crypto never closes, and a perp’s continuous cash settlement fits a market with no trading hours and no clearing house.

The result: perpetual-swap volume routinely runs several times the spot volume of the same assets. The perp is where crypto price discovery and leverage actually live.

The two prices: index vs. mark

Here’s the subtlety that trips up every beginner. A perp doesn’t have one price — it has two, and keeping them straight is the key to everything that follows.

Analogy. Think of a championship boxing match. The index price is the official scorecard kept by the judges — a sober, averaged, hard-to-rig measure of what’s really happening. The mark price is the roar of the betting crowd — where the perp is actually trading right now, which can run hot or cold relative to the scorecard. Funding is the mechanism that, over time, forces the crowd’s bet back toward the judges’ card.

Definition — index (or spot) price. The index price is the perp’s anchor: a reference price for the underlying asset, almost always a volume-weighted average across several spot exchanges. It’s deliberately robust and slow, so no single venue or wick can yank it around. This is “what the asset is really worth.”

Definition — mark price. The mark price is the price the perpetual contract itself is trading at on its own venue — set by supply and demand for the contract, i.e. by how aggressively traders are going long or short the perp. This is the price your PnL and your liquidation are measured against.

The gap between them is the whole ballgame:

$\text{premium} = \text{mark} - \text{index}.$

• Mark > index (positive premium): traders are paying up to be long the perp — the crowd is greedier than the scorecard.
• Mark < index (negative premium / discount): traders are paying up to be short — the crowd is more fearful than the scorecard.

The convergence puzzle — and a first look at its solution

So we have a contract with no expiry, trading at a mark price that can drift above or below the index. Nothing in the calendar pulls them together. If the perp could float free forever, it would be useless — it would stop tracking the asset it’s supposed to represent.

The fix, which the next lesson dissects in full, is the funding rate: a periodic payment exchanged directly between longs and shorts (the exchange just moves it; it doesn’t pay or receive). The rule is simple and self-correcting:

• When the perp trades above index (positive premium), longs pay shorts. Being long now has a holding cost, which bleeds out longs and tempts arbitrageurs to short the rich perp — pushing mark back down toward index.
• When the perp trades below index (negative premium), shorts pay longs, pushing mark back up.

It’s a thermostat. Whenever the mark wanders from the index, funding makes the wandering side pay, and that cost drags it home. The animation below shows the effect in the abstract: set a starting premium and watch the perp price decay back to the index as funding does its work.

Cash-settled, continuously: how your PnL actually accrues

One more mechanical point, because it shapes everything about risk. A perp is cash-settled and marked continuously. You never receive the underlying asset. Instead, your position’s value is recomputed against the mark price tick by tick, and your unrealized PnL rises and falls in your margin balance in real time.

Worked example. You go long 1 BTC of perp at a mark of $30,000, posting $3,000 of margin (so 10× leverage — more on that soon).

• The mark ticks up to $30,600 (+2%). Your notional is $30,000, so a 2% move is +$600 of unrealized PnL. Your margin balance now reads $3,600. That’s a +20% return on your $3,000 margin from a 2% price move — leverage cuts both ways.
• The mark ticks down to $29,400 (−2% from entry). Now you’re at −$600, a margin balance of $2,400 — a −20% hit on margin.

No expiry ever arrives to “true this up.” The position simply lives, marking to market continuously, accruing or paying funding each period, until you close it or the engine liquidates you. That’s the instrument in one breath: a never-expiring, leveraged, continuously cash-settled bet, kept honest by funding instead of a calendar.

You now know what a perp is: a leveraged, never-expiring, continuously cash-settled bet with two prices — a robust index and a live mark — whose gap is the premium. The one piece of machinery doing all the work is the funding rate, which we’ve only sketched. Next, we open it up completely: the exact formula, who pays whom and how much, the worked arithmetic of a funding payment, and why an unusually high or negative funding rate is one of the loudest signals in all of crypto.