Hammers have a small real body and a long lower shadow.
Hammers occur after a price decline.
The hammer candlestick shows sellers came into the market during the period but by the close the selling had been absorbed and buyers had pushed the price back to near the open.
The close can be above or below the open, although the close should be near the open in order for the real body to remain small.
The lower shadow should be at least two times the height of the real body.
Hammer candlesticks indicate a potential price reversal to the upside. The price must start moving up following the hammer; this is called confirmation.
Confirmation -
Hammers are most effective when they are preceded by at least three or more declining candles. A declining candle is one which closes lower than the close of the candle before it.
A hammer should look similar to a "T". This indicates the potential for a hammer candle. A hammer candlestick does not indicate a price reversal to the upside until it is confirmed.
Confirmation occurs if the candle following the hammer closes above the closing price of the hammer. Ideally, this confirmation candle shows strong buying. Candlestick traders will typically look to enter long positions or exit short positions during or after the confirmation candle. For those taking new long positions, a stop loss can be placed below the low of the hammer's shadow.
Hammers aren't usually used in isolation, even with confirmation. Traders typically use price or trend analysis, or technical indicators to further confirm candlestick patterns.