Determine the place value you need to use when rounding for your particular math problem. For instance, if you are adding the prices of food, you may decide to round to the nearest ones or the nearest tens. When working with very large numbers, as when determining the distance between planets or stars, you may choose to round to the nearest million.
Round up if the digit next to the place value you are using is 5 or higher. Round down if the digit is 4 or lower. If you are rounding to the nearest 10 and your number is 46, you would round up to 50. If the number was 44, you would round down to 40. All of the numbers to the right of the place value you are rounding to become zeros.
Complete the arithmetic for the problem. Add, subtract, multiply or divide as needed.
Locate the highest place value of the numbers. If the numbers all have the same highest place value, for instance, the highest digit in each number is in the hundreds place, you can use front-end estimation. If the numbers have large differences in value, front-end estimation will not work.
Add the first digit of each number. Do not round the numbers. As an example, if you are adding 4,392 + 6,834 + 9,603 you would add 4 + 6 + 9 = 19. Adjust the answer to the place value of your front-end digits. Your answer so far would be 19,000. For larger numbers, you may want to use the first two digits for greater accuracy.
Round the remaining digits. When adding 4,392 + 6,834 + 9,603, you would round the remaining digits to the nearest 1,000 using mental math: 392 rounds to zero, 834 rounds to 1,000 and 603 rounds to 1,000. So using mental math, you would add 0 + 1,000 + 1,000 = 2,000.
Add the sum of the front-end digits to the estimation of the remaining digits: 19,000 + 2,000 = 21,000.