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What is a fat finger error in trading?

In one line: A fat finger is a trading mistake caused by typing an order wrong, such as an extra zero, the wrong price, or buy instead of sell.

Trading glossary · Updated 2026-10-21

A famous case

In December 2005, a trader at the Japanese firm Mizuho meant to sell 1 share of a new stock for ¥610,000. Instead, the order went in as 610,000 shares at ¥1 each.

The mistake caused chaos in that stock and cost the firm a very large loss, reported at around ¥40 billion. It's still the classic example of a fat finger.

A simple everyday example

You mean to buy 10 shares of a $90 stock: $900. You type 100. Now it's $9,000.

Your Stop loss is $2 below entry. On 10 shares that risk is $20. On 100 shares it's $200. One extra zero made the trade ten times riskier.

In forex, typing 1.0 lots instead of 0.10 does the same thing: about $10 a pip instead of $1.

Why it matters

Fat-finger errors are one reason exchanges have price bands and Circuit breakers. Big errors can cause sudden spikes, then sharp snap-backs.

For you, the danger is simpler. One mistyped order can do more damage than a week of normal losses.

How to avoid it

1. Work out the size first with the position size calculator. Then type that number.
2. Read the order ticket out loud: buy or sell, size, price, stop.
3. Use a Limit order when you can. A Market order fills at whatever price is there.
4. Set a default size of the smallest amount on your platform, so a missed edit stays small.
5. Never place orders in a rush, half-watching, or on a tiny phone screen without a check.

A common mistake

Trusting one-click trading too soon. It's fast, which is the point and the danger. Turn it on only once your routine is automatic, and keep size defaults small.

Questions