Perimeter = Sum of All Side Lengths

P = side₁ + side₂ + side₃ + ...

1. Perimeter is a LENGTH

Measured in linear units: cm, m, ft, in, etc.

2. Add ALL sides

Include every side of the shape

3. Works for ANY polygon

Triangles, squares, rectangles, pentagons, etc.

4. Think of it as walking around

If you walk around the outside, perimeter is the total distance traveled

Step 1: Identify all the side lengths

Step 2: Add all the side lengths together

Step 3: Write the answer with the correct unit

Problem: Find the perimeter of a square with side length 7 cm.

Problem: Find the perimeter of a rectangle with length 12 m and width 8 m.

Problem: Find the perimeter of a triangle with sides 5 in, 6 in, and 7 in.

Problem: Find the perimeter of a pentagon with sides: 3 ft, 5 ft, 4 ft, 6 ft, 5 ft.

Tip: With whole numbers, simply add them up! No need for complicated calculations.

Step 1: Line up the decimal points vertically

Step 2: Add zeros if needed to make same number of decimal places

Step 3: Add like regular numbers

Step 4: Keep the decimal point in the answer

Problem: Find the perimeter of a square with side length 4.5 cm.

Problem: Find the perimeter of a rectangle with length 16.2 m and width 9.4 m.

Problem: Find the perimeter of a triangle with sides 3.5 cm, 4.2 cm, and 5.8 cm.

Important Rule for Decimals:

Always line up the decimal points!

Step 1: Find a common denominator (if needed)

Step 2: Convert fractions to equivalent fractions with same denominator

Step 3: Add the numerators, keep the denominator

Step 4: Simplify if possible

Step 5: Convert improper fractions to mixed numbers

Problem: Find the perimeter of a square with side length 3/4 inch.

Problem: Find the perimeter of a rectangle with length 2 1/2 ft and width 1 3/4 ft.

Problem: Find the perimeter of a triangle with sides 1/2 m, 3/4 m, and 5/8 m.

Same Denominator:

1/4 + 2/4 = 3/4

(Add numerators, keep denominator)

Different Denominators:

1/2 + 1/4 = 2/4 + 1/4 = 3/4

(Find common denominator first)

Step 1: Check the scale of the grid

Example: Each square = 1 cm, 1 m, 1 unit, etc.

Step 2: Count the unit lengths around the OUTSIDE of the shape

Count the edges (lines), NOT the squares!

Step 3: Start at one corner and go around clockwise or counterclockwise

Mark each edge as you count to avoid counting twice

Step 4: Write your answer with the correct unit

Problem: Find the perimeter of a rectangle on a 1 cm grid.

The rectangle is 4 squares long and 3 squares wide.

Problem: Find the perimeter of an L-shaped figure on a 1 meter grid.

Problem: Find the perimeter of this shape on a grid (each square = 1 unit).

✓ Count LINES (edges), not squares

✓ Mark edges as you count (to avoid double counting)

✓ Always check the grid scale (1 cm, 1 m, etc.)

✓ Only count the OUTSIDE edges

✓ Start at a corner and go all the way around

✓ Perimeter = Distance around the outside

✓ Always use same units for all measurements

✓ For whole numbers: Just add them up!

✓ For decimals: Line up decimal points before adding

✓ For fractions: Find common denominator first

✓ On grids: Count the edges (lines), not squares

✓ Perimeter is measured in linear units (cm, m, ft, in)

✓ Square: All 4 sides equal, so multiply by 4

✓ Rectangle: Opposite sides equal, use 2(l+w)

✓ Always include the unit in your answer!

Perimeter:

"Peri-meter = Peri-meter around the shape"

Think of the periphery (outside edge)

Square Formula:

"A square has 4 equal sides, so multiply by 4"

Rectangle Formula:

"2 lengths + 2 widths = 2(l + w)"

Decimals:

"Line up the dots (decimal points)!"

Fractions:

"Common denominator = Common ground for adding"

Grid Counting:

"Count the lines, not the squares!"

"Mark as you count, so you don't lose count!"