Bitcoin futures basis is commonly calculated as futures minus spot, in currency or percentage terms. A positive basis is associated with contango and a negative basis with backwardation; annualizing it requires the exact time remaining to expiry.
Basis is a relationship between two prices at one moment, not a guaranteed return. The result changes with the selected exchange, spot index, contract month, timestamp and whether fees are included.
A simple percentage basis is (futures ÷ spot − 1). Annualized basis scales that value by time to expiry, but simple and compounded conventions differ and become unstable very near settlement.
Futures and spot converge through settlement mechanics and arbitrage incentives, not by a law that removes every mismatch. Funding costs, collateral, balance-sheet capacity and counterparty limits affect the observed spread.
A cash-and-carry trade can lose through fees, slippage, margin calls, liquidation, custody failure or imperfect hedging. Perpetual-swap funding is a different mechanism and must not be silently mixed with dated futures basis.
Always state venue, contract, spot benchmark, expiry, timestamp and annualization method. Comparing a clean series over time is more informative than quoting a large percentage without its assumptions.
For the clearest picture, read this entry together with Contango, Backwardation, Bitcoin basis trade, Bitcoin futures. The reverse links also lead from ProShares Bitcoin Strategy ETF (BITO), Bitcoin basis trade, Cash-and-carry arbitrage, Backwardation.