Holding a perpetual costs funding at every settlement your position is open across. The interval is the part almost every calculator gets wrong, and it is the part that changes the answer by a multiple rather than a rounding.
Eight hours is the common case and it is not the rule. Kraken Futures accrues funding hourly. Bybit drops to four, two or one hour when rates reach their cap. A backtest that assumes eight hours everywhere is not slightly wrong on those symbols; it counts a fraction of the settlements that actually happened.
Funding is signed: you pay it on one side of the market and receive it on the other, and over a long enough sample the mean is close to nothing while individual trades swing by more than the commission. And the rate is not constant. A report over your real trade list uses the rate that actually settled in each window and shows the distribution rather than an average — which is the difference between "ignore this" and "one trade in twenty pays more funding than fees".
Cost your trade list to get that, free.