How to Multiply Decimals With the Grid Method

Written by brenda scottsdale
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How to Multiply Decimals With the Grid Method
Number grids break down numbers greater than nine into their components. (Ryan McVay/Photodisc/Getty Images)

The grid method involves breaking down numbers greater than nine into components, multiplying each component and adding these together. This method allows a person to multiply even large numbers in their head as opposed to having to use a calculator. The grid method is an excellent teaching tool to help students visualise numbers and understand that multiplication is just a short form of adding. Whole numbers (integers), fractions and decimals can all be multiplied using the grid method.

Draw a grid with one box representing each component to be multiplied. For example, 3.4 multiplied by 2.8 has four components 3 and .4 and 2 and .8. Draw two sets of two vertically stacked boxes.

Colour in the boxes. Colour the northwest box yellow, the southwest box brown, the northeast box pink and the southeast box blue.

Write the number 3 to the left of the yellow box. Write the number .4. to the left of the brown box. Across the top, write the number 2 on top of the yellow box and the number .8 on top of the pink box.

Fill in your boxes by multiplying numbers on the left with numbers across the top. Write 6 in the yellow box because 6 is the product of 3 multiplied by 2. To multiply the decimals, calculate the multiplication of the numbers as if they did not have decimals and then insert the decimals afterward. To fill in the brown box, 4 multiplied by 2 is 8, and because there is one decimal in the question (.4) you would insert one decimal to the right of the 8, so .8 is the correct answer. For the pink box, multiply 3 by 8 to get 24 and insert one decimal to the right -- 2.4 goes in the pink box. For the blue box, 8 multiplied by 4 is 32, and two decimals are inserted because there are two altogether in the question (.4 and .8 each have one). So, you would write .32 in the blue box.

Add the rows. To add decimals, line up the decimals first. So, 6 can be written 6.0 plus 2.4, equalling 8.4. The decimal .8 can also be written .80. Therefore, .80 plus .32 is 1.12.

Add the row totals -- 8.4 plus 1.12 equal 9.52. Therefore, 3.4 multiplied by 2.8 equals 9.52.

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