The bid ask spread is the gap between the highest price buyers are willing to pay (the bid) and the lowest price sellers are willing to accept (the ask, or offer). It’s the price of instant execution and the most basic cost of trading. On the ES (E-mini S&P 500) it’s usually one tick; on a thin contract it can be several. Everything you read in order flow sits on top of this two-price structure, so it’s worth getting exactly right.
Bid, ask, and the gap between them
At any instant, a market shows two prices, not one:
- The bid is the best price a buyer will pay right now. It’s where resting limit buy orders sit.
- The ask (or offer) is the best price a seller will accept right now. It’s where resting limit sell orders sit.
- The spread is ask minus bid.
Say the ES is 5,285.00 bid and 5,285.25 offered. The spread is 0.25, one tick, the minimum set by the E-mini S&P 500 contract specs. If you send a market buy, you get filled at 5,285.25, the ask. If you send a market sell, you get filled at 5,285.00, the bid. The moment you enter, you’re down one tick, because you’d have to sell at the bid what you just bought at the ask. That one-tick round-trip cost is the spread, and it’s the toll every aggressive order pays.
This is where the market/limit distinction becomes real money. A market order crosses the spread and pays it. A limit order rests and, if filled, earns it. The full mechanics live in order types in trading, but the takeaway is simple: aggression pays the spread, patience collects it.
What the spread tells you about liquidity
The spread is a live gauge of liquidity in trading. A tight spread means lots of resting orders competing to be the best bid and offer, so the market is deep and cheap to trade. A wide spread means few participants, thin resting size, and expensive execution.
Three things drive the spread:
- Volume and participation. More active traders means tighter competition and a tighter spread. The ES trades a razor-thin one-tick spread during regular hours because thousands of participants are quoting it.
- Volatility. When price is whipping around, liquidity providers widen the spread to protect themselves from getting run over. Spreads blow out around news releases for exactly this reason.
- Time of session. The overnight and pre-market sessions have thinner books and wider spreads than the cash-hours session. The same contract can be a joy to trade at 10am New York time and a minefield at 3am.
When you watch the spread widen suddenly, the book is telling you liquidity just evaporated. That’s often your cue that a move is about to get fast, or that you should size down.
The spread in the order book
The bid and ask you see quoted are just the top of a deeper ladder. Below the best bid sit more limit buyers at lower prices; above the best offer sit more limit sellers at higher prices. That whole ladder is the depth of market DOM, and the spread is simply the distance between its two innermost rungs.
Depth and spread work together. A one-tick spread with 800 contracts resting on each side is a fortress of liquidity. A one-tick spread with 12 contracts on each side looks tight but is fragile; one decent market order clears the level and the spread jumps. Never read the spread without glancing at the size behind it.
How the spread powers order flow reads
Here’s why this page underpins everything else you do. Order flow classifies every trade by where it printed relative to the spread:
- A trade that prints at the ask was an aggressive buyer lifting the offer.
- A trade that prints at the bid was an aggressive seller hitting the bid.
That single rule is the foundation of the entire discipline. It’s how a footprint chart splits volume into a bid column and an ask column, and it’s how cumulative delta is calculated: delta is ask volume minus bid volume. Without a bid and an ask, there’s no way to separate aggressive buying from aggressive selling, and order flow wouldn’t exist. The spread isn’t just a cost; it’s the measuring stick.
The spread as a real trading cost
Don’t let a small number fool you. For a scalper, the spread is often the single biggest recurring expense, bigger than commissions. If you’re taking 20 round trips a day on the ES and paying one tick of spread on each, that’s 20 ticks of friction to overcome before you’re even. On the ES a tick is $12.50, so that’s $250 a day of pure spread cost per contract, on top of fees.
Two practical consequences. First, entering with limit orders instead of market orders can flip the spread from a cost into an edge on the fills you’re patient enough to wait for. Second, avoid trading contracts and sessions where the spread is wide relative to your target; a strategy that works on the ES in cash hours can bleed to death on a thin contract overnight purely from spread. If you’re deciding what to trade, the spread is a first-class input, and it’s worth weighing alongside the rest of what makes a market suitable for order flow trading.
Frequently Asked Questions
Why is the bid always lower than the ask?
Because buyers want to pay as little as possible and sellers want to receive as much as possible. The bid is the highest a buyer will pay; the ask is the lowest a seller will accept. If the bid ever reached the ask, a trade executes instantly and both are consumed, so at rest the bid always sits below the ask by at least the minimum tick.
What is a good bid ask spread?
The tightest possible spread is one tick, and liquid futures like the ES trade there most of the session. “Good” is relative to the contract: one tick on a deep market is excellent, while the same one tick on a thin market with little size behind it can still be risky. Always judge the spread together with the depth resting behind it.
Does the spread count as a trading cost?
Yes, and for active traders it’s often the largest one. Every time you enter and exit with market orders you pay the spread on both sides. On the ES that’s $12.50 per tick per contract each way. Using limit orders where possible reduces or reverses this cost, which is why patient entries matter so much to scalpers.
How does the spread affect the footprint and delta?
Directly, because both are built on it. Trades printing at the ask are counted as aggressive buying; trades printing at the bid are counted as aggressive selling. The footprint splits volume into those two columns, and delta is ask volume minus bid volume. Without a bid and an ask to classify trades against, neither tool could separate buyers from sellers.