Nikon D800 - Manual (Page 57)

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Press the I (Q) and O (Q) buttons again.
Press the I (Q) and O (Q) buttons together a second time
while C is blinking to format the memory card. Do not
remove the memory card or remove or disconnect the power
source during formatting.
When formatting is complete, the control panel and
viewfinder will show the number of photographs that can be
recorded at current settings.