--- title: "Perimeters of polygons" description: "Solve problems involving perimeters of polygons: find perimeter from side lengths, find an unknown side length, and explore rectangles with same perimeter but different areas (or vice versa)" canonical: https://lightmysky.com/learn/mathematics/perimeters-of-polygons-mt_WtcFrxGOgw source: https://lightmysky.com/learn/mathematics/perimeters-of-polygons-mt_WtcFrxGOgw.md retrieved: 2026-10-19 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Perimeters of polygons Solve problems involving perimeters of polygons: find perimeter from side lengths, find an unknown side length, and explore rectangles with same perimeter but different areas (or vice versa) Subject: Mathematics · Area: Measurement · Ages 8 to 9 Page: https://lightmysky.com/learn/mathematics/perimeters-of-polygons-mt_WtcFrxGOgw ## Ready when they can - Calculate the perimeter of a rectangle with sides 8 cm and 5 cm - Find the missing side of a pentagon with perimeter 30 cm and four known sides - Draw two rectangles both with perimeter 24 cm but different areas ## Lesson: Perimeters of polygons You already know how to measure a perimeter by tracing around a shape. Now you can find it just by adding up side lengths you're given, with no ruler needed. Add the length of every side of a polygon, and that total is the perimeter. *(drawing: This rectangle has sides of 8 cm and 5 cm, twice each. Perimeter = 8 + 5 + 8 + 5 = 26 cm.)* **Example.** A pentagon's perimeter is 30 cm. Four of its sides measure 4 cm, 5 cm, 7 cm, and 8 cm, and the fifth is hidden. Add the known sides: 4 + 5 + 7 + 8 = 24 cm. Subtract that from the total: 30 - 24 = 6 cm. The missing side is 6 cm. *(drawing: Both rectangles have a perimeter of 24 cm, but they don't cover the same amount of space inside.)* **Tip.** When you're hunting for a missing side, add up the sides you know first, then subtract from the whole perimeter. And don't assume two shapes are the same just because their perimeters match. **Recap.** Add every side for the perimeter, subtract known sides from the total to find one that's missing, and remember that equal perimeters can hold different amounts of space. ## Practice 16 questions on this page, each with its working shown. ## Needs first - [Rectangle area by multiplying](https://lightmysky.com/learn/mathematics/rectangle-area-by-multiplying-mt_GzcJEVkNRn) - [Measuring Perimeters](https://lightmysky.com/learn/mathematics/measuring-perimeters-mt_wE7-Gs9ENL) ## Opens up - [Area and perimeter of rectangles](https://lightmysky.com/learn/mathematics/area-and-perimeter-of-rectangles-mt_eiB3-6pu6a) - [Perimeter of Compound Shapes](https://lightmysky.com/learn/mathematics/perimeter-of-compound-shapes-mt_n0AlyLQwC9) - [Multi-Step Problem Solving (age 8+)](https://lightmysky.com/learn/mathematics/multi-step-problem-solving-age-8-mt_SmghasIvbT)