by a hair

By an extremely short or slim margin (of distance, time, or another measure). They're just about to close the gates! It looks like we made the flight by a hair. The race was neck and neck till the very end, but Sally won it by a hair. Hundreds of people avoided death by a hair when their apartment building caught on fire in the middle of the night.

by a whisker

By an extremely short or slim margin (of distance, time, or another measure). They're just about to close the gates! It looks like we made the flight by a whisker. The race was neck and neck till the very end, but Sally won it by a whisker. Hundreds of people avoided death by a mere whisker when their apartment building caught on fire in the middle of the night.

by a hair/whisker

Very narrowly, by a very small distance or amount. For example, “He missed sideswiping that car by a hair,” or “That ball was in, but just by a whisker.” Both versions of this cliché allude to the fineness of a single hair. The first, also put as by a hairbreadth, dates from the fourteenth century, whereas the second originated in early twentieth-century America.