Match the exposure before comparing prices
Rolling a long closes the expiring exposure by selling and opens the later exposure by buying. The relevant executable quotes are therefore the old contract's bid and the new contract's ask. Use the same underlying quantity, quote currency and linear payoff convention for both legs. A contract count alone is insufficient when multipliers differ. Inverse contracts require a different settlement calculation. The additional tenor must be positive, and both quotes should refer to comparable market conditions rather than prices collected hours apart.
Separate replacement spread from cash settlement
For underlying quantity q, expiring bid B and next ask A, the quoted replacement spread is q × (A − B). Add the closing fee on q × B and the opening fee on q × A. A positive total represents a modeled cost; a negative total represents a modeled credit under these conventions. Futures do not require paying the entire purchase notional like spot assets, so this spread measure should not be presented as the literal movement of cash through a margined futures account.
Interpret simple annualization
To compare different extension lengths, divide modeled roll cost by the old contract notional q × B, then multiply by 365 divided by the days between expiries. This is a simple annualized comparison, not a promised yield or financing rate you can lock repeatedly. It excludes prior position profit, collateral returns, margin changes and future rolls. A wider spread can reflect changing market conditions. The page only evaluates a long roll; short rolls need the opposite bid and ask legs and their own signed calculation.