What it means
Markets quote two prices at the same time. The bid is the highest price a buyer will pay and the ask is the lowest price a seller will accept.
The difference between them is the bid-ask spread, and the mid price sits exactly halfway. Because nobody can usually trade at the mid price, it is a benchmark rather than a deal price.
A buyer pays the ask and a seller receives the bid, so the gap between the mid price and the actual trade price shows the cost of trading. Analysts often measure this cost as the half-spread.
The mid price is widely used for valuation. Funds and banks that need to mark positions to market (value them at current prices) often use the mid price because it avoids favouring buyers or sellers.
It is also used to assess how well a broker executed an order, by comparing the fill price with the mid price when the order arrived. In less liquid markets, the spread can be wide and the mid price becomes less reliable.
A bond with a bid of $96 and an ask of $100 has a mid price of $98, but few trades may happen near that level. Valuation policies often specify which price to use and when to override it with other evidence.
Mid price should not be confused with the last traded price, which is the price at which the most recent trade occurred. The last price can be stale or may reflect a trade on one side of the spread, while the mid price reflects the current quotes on both sides.
Many traders watch both together.
In practice
Real-world examples
Example 1
A fund accountant needs to value a portfolio of listed shares at month end. The valuation policy says to use the mid price at the close of trading. She pulls the bid and ask for each holding and averages them.
Example 2
An investor buys 5,000 shares at the ask price of $20.10 when the mid price is $20.05. A trading cost report shows that he paid $0.05 per share, or $250 in total, above the mid price. He compares this with the broker's fees to understand the total cost.
Example 3
A bond trader values an illiquid corporate bond. The bid is $94 and the ask is $98, so the mid price is $96. Because trades are rare, he tests this against the prices of similar bonds before using it for reporting.
Think of it
Think of selling a used car. A prospective buyer offers 4,000 pounds, while you are holding out for 6,000 pounds. The mid price is the 5,000 pounds fair market value sitting right in the middle of your negotiation gap.
Formula
Calculation
Mid price = (Bid + Ask) / 2
Half-spread = (Ask - Bid) / 2
Suppose a share is quoted with a bid of $49.80 and an ask of $50.20. Mid price = (49.80 + 50.20) / 2 = 100.00 / 2 = $50.00. The spread = 50.20 - 49.80 = $0.40, so the half-spread = $0.20. A buyer of 10,000 shares at the ask pays 10,000 x 0.20 = $2,000 more than at the mid price, which is the implied cost of trading immediately.Case study
Seen in the real world
Lakeshore Asset Management is an illustrative, fictional investment firm that reviewed its trading costs. The head of trading compared the price paid on each order with the mid price at the time the order was sent. Across 1,000 orders with a total value of $80,000,000, the shortfall averaged 0.12% of value.
That equated to 80,000,000 x 0.0012 = $96,000 of cost for the year. The review showed that most of the cost came from a handful of small, illiquid shares where the spread was wide. The desk began splitting orders and using limit orders that rested at or near the mid price.
After six months, the average shortfall dropped to 0.08%, a saving of about $32,000 on the same value traded. The illustrative lesson is that measuring execution against the mid price turns trading costs into a number that can be managed.
Watch out
Common mistakes
- Assuming that trades can be done at the mid price, when buyers pay the ask and sellers receive the bid.
- Using the mid price as a reliable value in illiquid markets, where the spread is wide and few trades occur.
- Confusing the mid price with the last traded price, which may be stale or on one side of the spread.
Questions
People also ask
Why do funds value assets at the mid price?
Because it is neutral between buyers and sellers, and avoids overstating or understating the value by choosing one side of the market.
Can the mid price be negative?
For some interest rate products and commodity spreads, prices can be negative, and the mid price follows the same arithmetic.
How is mid price used to judge trade quality?
By comparing the executed price with the mid price when the order was placed, the difference shows how much of the spread the investor gave up.