Definition: liquidity and spreads
Liquidity in forex is about how easily market participants can buy and sell at prices that match closely. In practical terms, it reflects the presence of nearby buy and sell orders (depth) and how quickly those orders can be filled.
A spread is the difference between the bid (sell price) and the ask (buy price) shown by a provider. If the bid is lower than the ask, the spread represents an immediate transaction cost for a new position because you typically buy at the ask and sell at the bid.
Key distinction: mechanics like “spread equals ask minus bid” are stable, while the actual bid/ask values and how much depth exists are variable and depend on market conditions and the provider’s quoting/execution setup.
Worked example with explicit assumptions
Below is a numerical scenario that isolates costs from assumptions about future price movement.
Assumptions
- You want to open one trade of EUR/USD exposure.
- You are quoted these prices at the moment you enter:
- Bid = 1.10000
- Ask = 1.10002
- The spread is therefore 0.00002 (2 “pips” for a typical 5-decimal quote).
- Contract size effects and pip-value conversions are not calculated from first principles here; instead, we show the cost in price terms (ask minus bid).
- You later close at a quoted price with bid/ask again.
Entry cost from the spread
If you open by buying, you use the ask. In this scenario, the entry price is 1.10002.
If you immediately tried to close, you would sell at the bid 1.10000.
Instant difference (spread cost) = 1.10002 − 1.10000 = 0.00002.
This cost occurs even before any “price move” because the spread is paid at entry/exit in the form of worse effective prices.
How liquidity changes what can happen next
Now add liquidity assumptions to show a limitation.
Additional assumptions about liquidity
- Imagine there is thin depth near 1.10000/1.10002.
- After your entry, suppose incoming orders quickly move available quotes outward by widening the spread.
For example, after entry, the provider quotes:
- New bid = 1.09990
- New ask = 1.10005
Now the spread is 1.10005 − 1.09990 = 0.00015.
Closing cost illustration
If you later close by selling, you use the bid. In this example, your effective sell price is 1.09990.
Compare that to your entry ask 1.10002: price-term difference = 1.09990 − 1.10002 = −0.00012.
Important limitation: this scenario does not prove a future outcome. It only demonstrates that when liquidity is weaker, quotes can change quickly, and the spread and effective exit price can both worsen relative to entry.
Limitations and risks (what a reader should verify)
- Spreads are variable: the bid/ask values used in the example are assumed only to illustrate arithmetic. Real quotes can differ at different times.
- Liquidity is not guaranteed: thin depth can cause fast quote changes or partial fills depending on execution model.
- Costs include more than spread: fees, commissions, financing charges, and execution slippage can also affect realized results.
- “Worked example” is not predictive: historical behavior or intuitive expectations about liquidity do not establish future relationships.
- Provider differences matter: different execution and pricing approaches can change how bid/ask and fills translate into your realized cost.
Verification and next question to ask
To independently verify the relevant facts, you can check how a provider defines and displays bid/ask and how spreads are shown (for example, during different market conditions). You can also compare how spreads and available quotes behave when liquidity is typically lower (without assuming a fixed pattern).
A useful follow-up question is: what specific execution model applies (market vs limit behavior, and whether you can experience slippage), because that determines how “spread widening” translates into actual filled prices.