Decimal place value (age 11+)
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
The lesson
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.
Watch it
Where it sits
Learn first
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
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.