Rounding to decimal places and significant figures
Round numbers and measures to an appropriate degree of accuracy including to a specified number of decimal places or significant figures
What a learner can do afterwards
- Round numbers to a given number of decimal places
- Round numbers to a given number of significant figures
- Choose an appropriate degree of accuracy for a given context
1 · Read
Every digit in a decimal has a place: tenths, hundredths, thousandths, and so on. Rounding means shortening a decimal to a set number of those places. To round to a certain number of decimal places, look at the digit right after your cut-off point. If it is 5 or more, the last digit you keep rounds up by one. If it is less than 5, that digit stays the same. Then everything after it is dropped.
Round 12.3746 km to 2 decimal places. Keep the tenths and hundredths digits: 3 and 7. The next digit, the thousandths digit, is 4. Since 4 is less than 5, the 7 stays a 7. The answer is 12.37 km.
Significant figures are a different way to control accuracy. Start counting from the first digit that isn't zero, then count every digit after it, including digits after the decimal point. Rounding to 3 significant figures means keeping the first three of those digits. You still look at the next digit to decide whether to round up.
Round 12.3746 km to 3 significant figures. The first three significant figures are 1, 2, and 3. The next digit is 7. Since 7 is 5 or more, the 3 rounds up to 4. The answer is 12.4 km.
Match your rounding to what the measurement is for. A recipe might only need whole grams, while a science reading might need 2 or 3 decimal places to be useful. More decimal places isn't automatically better.
Look at the digit right after your cut-off point: 5 or more rounds the last digit up, below 5 leaves it the same.
2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.