🔢 Decimal numbers CM1: understanding the decimal point, with exercises

“2.65 is bigger than 2.7, there are more digits!” If you’ve heard this at home, don’t worry: it’s the most common mistake with decimal numbers in CM1. The decimal point changes the rules your child has known since CP, and it takes a little time to adjust.

Here’s how you can help your child understand what the decimal point means, then read, compare, order and place decimal numbers. At the end, you’ll find six corrected exercises to do in ten minutes.

Decimal numbers in CM1: what your child needs to know

In CM1, children mostly work with numbers that have one or two digits after the decimal point: tenths and hundredths. The progression guidelines published by the French Ministry of Education (Eduscol) expect that by the end of the year, your child should be able to:

• name the place values: units, tenths, hundredths, and the links between them (1 unit = 10 tenths, 1 tenth = 10 hundredths);
• switch between a decimal fraction and the decimal notation, and vice versa (3.12 = 3 + 12/100);
• place a decimal number on a number line;
• compare and order decimal numbers;
• find which two whole numbers a decimal falls between.

The starting point is decimal fractions (tenths and hundredths written as fractions). If your child is still struggling with this, start with our article on decimal fractions and decimal numbers: the decimal point is just another way to write these fractions.

Reading a decimal number correctly

The trap is reading it as “three point forty-five”. That’s correct, but it doesn’t explain the value of each digit. Get your child used to reading the number in place values too: 3.45 means 3 units and 45 hundredths. Or 3 units, 4 tenths and 5 hundredths.

Each digit has its place, just like in large numbers, and each place is worth ten times less than the one to its left.

Comparing decimal numbers: the rule that works

To compare two decimal numbers, move from left to right, place by place:

1. First, compare the whole number parts. 4.9 is less than 5.1, because 4 is less than 5. What comes after the decimal point doesn’t matter here.
2. If the whole numbers are equal, compare the tenths. 2.7 and 2.65: 7 tenths versus 6 tenths, so 2.7 is bigger.
3. If the tenths are equal, compare the hundredths. 1.34 and 1.38: 4 hundredths versus 8, so 1.38 is bigger.

A classic mistake comes from whole numbers: “the more digits, the bigger the number”. With decimals, that’s not true. 2.7 only has one digit after the decimal, but 7 tenths is 70 hundredths, which is more than 65.

Ordering and placing decimal numbers

Ordering means comparing several numbers. Take 4.3, 4.03, 4.33 and 3.9. Start with the whole number part: 3.9 is the only one with 3 units, so it’s the smallest. For the other three, add zeros to match: 4.30, 4.03, 4.33. Ordering becomes obvious: 3.9 < 4.03 < 4.3 < 4.33.

Placing means finding which numbers a decimal falls between. In CM1, this usually means between two consecutive whole numbers: 7.48 is between 7 and 8, so 7 < 7.48 < 8. To go further, you can place it to the nearest tenth: 7.4 < 7.48 < 7.5.

A number line is very helpful here. Draw a line from 7 to 8, divide it into 10 equal parts: each mark is a tenth. Your child can see that 7.48 is almost in the middle, just before 7.5.

Common mistakes with decimals

Some mistakes crop up with almost every child. Spotting them helps you correct them quickly, without making a fuss:

• Reading the decimal part as a separate whole number. The child thinks 1.12 is bigger than 1.2 “because 12 is bigger than 2”. Solution: add a zero to match (1.12 and 1.20).
• Forgetting the zero in the tenths place. “3 units and 5 hundredths” is written 3.05, not 3.5. Solution: use a place value chart, one box for each place.
• Misaligning numbers in a written addition. For 3.5 + 1.25, line up the decimal points, not the last digits. Result: 4.75.
• Thinking the decimal point ‘splits’ the number in two. 3.45 is still a single number, between 3 and 4.

6 worked exercises on decimal numbers for CM1

Do these orally or on paper, one or two per day. Answers are just below—cover them with your hand.

1. Break down 6.82.
   Answer: 6 units, 8 tenths and 2 hundredths. 6.82 = 682/100.
2. Write in digits: 3 units and 5 hundredths.
   Answer: 3.05 (zero tenths, five hundredths).
3. Compare 5.4 and 5.38.
   Answer: 5.40 > 5.38, so 5.4 > 5.38.
4. Order from smallest to largest: 0.9; 0.09; 0.19; 1.01.
   Answer: 0.09 < 0.19 < 0.9 < 1.01.
5. Find two whole numbers that 12.6 falls between.
   Answer: 12 < 12.6 < 13.
6. Work out 4.7 + 2.15 by lining up the decimal points.
   Answer: 4.70 + 2.15 = 6.85.

If your child gets all six right, they’ve got the basics. If they’re stuck on one, go back to using money or a number line—practical examples almost always help. To build speed, mental maths for CM1 is a great addition, and fractions in CM1 set the stage for what’s next.

A short routine to make progress

No need for long sessions. Ten minutes, three or four times a week, is enough: read a decimal number in units, compare two numbers, sort a small list. Use different materials: receipts, supermarket prices, height or weight measurements. And praise your child’s reasoning as much as the correct answer.

To keep a written record at home, download our free printable workbook: a few practice pages, screen-free, that your child can do independently.