Significant figures are the digits in a number that reliably indicate the true value.

For example, if using a scale reliable to the nearest gram, digits beyond the gram are insignificant. If using a ruler with ticks every 1 centimeter, then
the ruler is certain to 1 centimeter but reliable to one decimal despite some uncertainty.

Lastly, exact numbers ("3 people" or "1/2" in "1/2mv^2) don't have significant figures, though some may consider them to have infinite significant figures for calculation purposes.

How many significant figures are there in 100?

"100" has 1 significant figure because trailing zeros are insignificant by default. If written "100" (with an underline on digit 2), it
would have 2 significant figures. rule 5

How many significant figures are there in "100."

"100." has 3 significant figures because zeros up to the decimal are significant. rule 3

How many significant figures are there in "100.00"?

"100.00" has 5 significant figures because zeros after the decimal are significant. rule 4

Counting significant figures

We count significant figures using the following rules:

Non-zero digits are significant ("123" has 3 significant figures)

Leading zeros are insignificant ("005" and "0.001" have 1 significant figure)

Zeros between digits and up to the decimal are significant ("3.01" and "320." have 3 significant figures)

Trailing zeros after the decimal are significant ("4.10" has 3 significant figures)

Trailing zeros before the decimal are ambiguous, but default to insignificant ("4500" has 2 significant figures unless otherwise denoted)

We have a calculator on our homepage to help you with your significant figure calculations.