Bid-Ask Spread Explained

At a glance

Definition: The gap between the best bid and the best ask
What it pays for: Immediacy, and the liquidity provider’s inventory risk
Quoted vs effective: Effective spread is what you actually paid
Widens when: Volatility rises, liquidity thins, or information arrives

Key takeaways

The spread compensates liquidity providers for inventory risk and for the possibility of trading against someone better informed.
Crossing the spread costs roughly half of it on each side, so a round trip costs approximately the full spread before commissions.
The quoted spread is what you see; the effective spread is what you actually paid, and for larger orders it is wider.
Spreads widen predictably at the open, at the close in some markets, around scheduled news, and whenever volatility rises.
For short-horizon strategies, spread cost relative to the target move is usually the single factor that decides viability.

What the spread actually is

The bid is the highest price someone is willing to buy at. The ask is the lowest price someone is willing to sell at. The difference is the spread, and it exists because the participants quoting those prices are providing a service: they are willing to trade at any moment, and they need compensation for that willingness.

Quote: 49.98 bid / 50.02 ask     spread = 0.04
Midpoint = 50.00

Buy at the ask:     50.02   ->  paid 0.02 above midpoint
Sell at the bid:    49.98   ->  received 0.02 below midpoint
Round trip cost:    0.04    =  0.08% of a 50 price

For a strategy targeting a 0.5% move:
   spread cost = 0.08% / 0.5% = 16% of the target
   Sixteen percent of every winning trade consumed
   before commissions, slippage, or a single loss.

The cost of crossing, in practical terms.

What determines spread width

Factor	Effect on the spread	Why
Trading volume	Higher volume, tighter spread	More competition among liquidity providers
Volatility	Higher volatility, wider spread	Greater inventory risk for the provider
Price level	Lower priced instruments, wider relative spread	Minimum tick size becomes a larger percentage
Information asymmetry	More asymmetry, wider spread	Higher risk of trading against the informed
Time of day	Widest at open and in thin sessions	Fewer participants; uncertain valuation
Scheduled events	Dramatically wider around releases	Providers withdraw ahead of a repricing
Market structure	Fragmented or dealer markets vary	Competition and transparency differ by venue
Instrument type	Options and small caps much wider	Lower volume; harder to hedge inventory

Quoted versus effective spread

The quoted spread applies to the displayed size at the best prices. If your order exceeds that size, you consume deeper levels and your average price is worse. The effective spread measures what you actually paid relative to the midpoint at order time.

Effective spread = 2 x |fill price - midpoint at order time|

Example:
   Midpoint at submission  50.00
   Your average fill       50.035
   Effective spread        0.07

   The quoted spread was 0.04.
   You paid the equivalent of a 0.07 spread because
   your order was larger than the displayed size.

Do this for 30+ trades and average the result.
That number, not the quoted spread, is your real cost
and the figure that belongs in your backtest.

Measuring your true cost.

Typical spreads by instrument

Instrument	Typical spread	As a percentage	Viable for
Major index futures	1 tick	0.002 to 0.01%	Any strategy including intraday
Large-cap equities	1 to 5 cents	0.01 to 0.05%	Most strategies
Liquid index ETFs	1 to 2 cents	0.01 to 0.03%	Most strategies
Major FX pairs	0.5 to 1.5 pips	0.005 to 0.015%	Most strategies
Mid-cap equities	5 to 20 cents	0.1 to 0.5%	Swing and longer
Small-cap equities	0.5 to 3%	0.5 to 3%	Position trading only
Liquid options	1 to 5% of premium	Large	Swing and longer
Illiquid options	10 to 30% of premium	Very large	Generally avoid
Major crypto	0.02 to 0.1%	Moderate	Most strategies

Approximate round-trip spread costs during normal conditions.

The pattern determines strategy design more than most traders realise. An intraday strategy is viable in index futures and unviable in small caps purely because of this table, regardless of how good the signal is.

Managing spread cost

Trade liquid instruments. The difference between a large-cap and a small-cap round trip can be a factor of fifty.
Use limit orders where the strategy permits. You collect the spread rather than paying it, at the cost of missed fills.
Avoid the widest moments. The first and last minutes of the session, and the period around scheduled releases.
Hold longer. Spread is a fixed cost per round trip, so a longer holding period amortises it across a larger move.
Check the spread before entering. In options and thin instruments, the spread on the day can make an otherwise good setup unprofitable.
Measure your effective spread, not the quoted one, and put that number in your backtest.
Size below the displayed depth where possible, so you are not consuming deeper price levels.

Frequently asked questions

Why do spreads widen during volatile periods?

Because liquidity providers face greater inventory risk and a higher probability that whoever trades with them knows something. Both increase the compensation they require, which appears as a wider spread. The withdrawal is rational rather than opportunistic, which is why it happens in every market during stress.

Does a zero-commission broker mean zero cost?

No. The spread is charged regardless of commission, and zero-commission brokers earn revenue through order flow arrangements, margin interest, and securities lending. The effective spread you pay is the real cost, and measuring it against the midpoint at order time is the only way to know what it is.

What is the difference between the quoted and effective spread?

The quoted spread is the displayed difference between bid and ask at the best prices. The effective spread is twice the difference between your actual fill and the midpoint when you submitted, which captures the effect of order size and routing. For larger orders the effective spread is wider; for retail orders receiving price improvement it can be narrower.

How does the spread affect my strategy?

It is charged on every round trip, so it scales with trading frequency while your edge per trade generally does not. Express it as a percentage of your average winning trade: above roughly 15 percent, the strategy is unlikely to survive. This single calculation eliminates most short-horizon strategies in expensive instruments.

Can I avoid paying the spread?

Partly, by using limit orders and letting others cross to you, which means you collect the spread instead of paying it. The cost is that some orders never fill, and the missed trades may have been the profitable ones. Whether this is favourable depends on the strategy and must be measured rather than assumed.

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Referenced by

Educational use only. This guide explains how a strategy works. It is not investment advice, not a recommendation, and no result described here is a forecast. Test any approach on historical and out-of-sample data, size positions conservatively, and never risk money you cannot afford to lose.