There are a few ways to calculate a bid-ask spread, but a relatively straightforward way to find the spread percentage is to subtract the bid price from the ask price and then divide the difference by the ask price ([Ask – Bid] ÷ Ask).
Consider a stock with a bid of $9.95 and an ask of $10. The bid-ask spread is 5 cents. The spread as a percentage is $0.05 / $10 or 0.50%.