What the tool is for. Model spot-perp funding carry with fees, funding flips, exchange risk and capital lockup. The number that matters is what survives after both legs are funded and all four fills are paid, not the rate on the ticker.
Carry is a spread trade with two exit bills.
Funding carry is not free yield. It is a bundle of basis risk, fee drag, venue risk, borrow pressure, funding flips and capital lockup. The simulator below exists to make those line items add up in one place, because in isolation each of them looks small enough to ignore.
The first thing that quietly ruins the arithmetic is the denominator. Funding is quoted against the perpetual's notional, but your return has to be measured against every dollar tied up on both legs — the spot inventory plus the margin backing the short. Splitting capital across two venues can roughly halve a headline annualized figure before anything goes wrong. The second is that a carry position is opened twice and closed twice, so the round-trip fee is four fills, not two. On a rate of a few basis points per settlement, those fills can eat weeks of accrual, which is why a carry that only makes sense if you hold it for months is not really a carry at the size you can actually exit.
The third is that the exit is not symmetric. Unwinding in a calm market is two limit orders; unwinding while the perpetual is being force-deleveraged means you may lose the short leg at the exchange's chosen moment and be left holding naked spot. That is not a tail scenario to model later — it is the scenario that decides how large the position should be today.
Set the funding input to zero and see what the simulator returns. Whatever is left is your pure cost of carrying the trade — fees, spread and locked capital. If that number is large relative to the yield you were hoping for, the position needs funding to stay positive every single settlement just to break even, and no perpetual market has ever promised that.
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Read the gap between gross and net first
Four figures come back, and the informative one is the distance between the first two. Gross is the funding you would collect if trading were free; net is what survives the fills and the spread. When a short holding period leaves those two far apart, the position is mostly paying for its own execution. Treat the annualised line with the most suspicion of the four — it extends a rate that is reset every settlement out to a full year, so it describes today rather than the year.
Cross-check the yield against the exit
The only yield worth trusting is the one that shows up on your own account statement after both legs have been funded, all four fills have been paid and the capital has sat idle for the full holding period; the rate ticker describes none of that. Before sizing, price the exit rather than the entry — ask what the short leg would cost to replace if the venue deleveraged it mid-squeeze, and whether the spot leg can be sold at the same moment on the same day. A carry that only clears its costs if funding stays positive at every settlement is not a spread trade; it is a directional bet on funding wearing a spread trade's clothes.
Risk note. Crypto assets are volatile and not suitable for every investor. This page is editorial analysis, not financial advice.