Numford
Rounding

Round 0.9999 to 3 significant figures

The answer is 1.00. The carry runs through every 9, so the answer gains a digit before the point.

Significant figures

Non-zero digit. Every non-zero digit is significant.Zero between digits. A zero between two non-zero digits is significant.Leading zero. Leading zeros only fix the decimal point, so they are not significant.Trailing zero after the point. Trailing zeros in a number with a decimal point are significant.Trailing zero, no point. Trailing zeros in a number without a decimal point are not counted. Write 1.00 × 10² to make them count.Digit of a in a × 10ⁿ. In a × 10ⁿ every digit of a is significant, including its zeros. Counted and defined numbers, such as 3 coins or 100 cm in a metre, are exact and have unlimited significant figures.

Tap a digit to see why it counts.

4significant figures
Scientific notation9.999 × 10−1

Round to

3
significant figures

0.99991.00

The dropped part 9 is more than half, so the last kept digit 9 goes up by one.

Add, subtract, multiply, divide

Add or subtract. The answer keeps as many decimal places as the number that has the fewest. Multiply or divide. The answer keeps as many significant figures as the number that has the fewest. The rounding is done once, on the exact result.
  • 4.2374 significant figures
  • 2.513 significant figures

Exact result1.68804780876

Multiply or divide: keep the fewest significant figures.

Reported1.693 significant figures
Working

Step by step

  1. 0.9999 has 4 significant figures. Keep the first 3: 9 9 9.
  2. The dropped part 9 is more than half, so the last kept digit 9 goes up by one.
  3. 0.9999 rounded to 3 significant figures is 1.00.
Related

More roundings

Counting instead? See the significant figures in 0.0025.

Questions

0.9999 to 3: quick answers

What is 0.9999 to 3 significant figures?

0.9999 to 3 significant figures is 1.00. The dropped part 9 is more than half, so the last kept digit 9 goes up by one.

What if the dropped digit is exactly 5?

In the Class 11 rule, if the digit before it is even the 5 is dropped and nothing changes. If the digit before it is odd, that digit goes up by one. So 2.745 and 2.735 both become 2.74 to 3 significant figures.

What is special about this example?

The carry runs through every 9, so the answer gains a digit before the point.

Round your own number