--- title: "Calculating with measurements" description: "Measure, compare, add, and subtract lengths (m/cm/mm), mass (kg/g), and volume/capacity (l/ml) using standard units" canonical: https://lightmysky.com/learn/mathematics/calculating-with-measurements-mt_6oxQPNLHNv source: https://lightmysky.com/learn/mathematics/calculating-with-measurements-mt_6oxQPNLHNv.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`. # Calculating with measurements Measure, compare, add, and subtract lengths (m/cm/mm), mass (kg/g), and volume/capacity (l/ml) using standard units Subject: Mathematics · Area: Measurement · Ages 7 to 8 Page: https://lightmysky.com/learn/mathematics/calculating-with-measurements-mt_6oxQPNLHNv ## Ready when they can - Measure a length in centimetres and millimetres - Weigh an object using grams and kilograms - Check attributes and units before combining measurements; add or subtract lengths, masses and liquid volumes after changing them to matching units ## Lesson: Calculating with measurements Line up one end with zero on the ruler and read the other end. Check cm or mm. If a label is missing, use the equal steps to find it. Each cm is 10 mm: 4 cm is 40 mm. For 4 cm 2 mm, change the centimetres first: 40 mm + 2 mm = 42 mm. *(drawing: Start with the empty scale at zero. Check its unit. When the reading stops changing, record the number with that unit.)* **Example.** Match units and measurement kinds. Add hundreds, tens and ones: 900 g + 600 g = 1500 g (9 + 6 hundreds). Subtract removed amounts in parts: 900 ml - 200 ml = 700 ml. Find missing amounts by counting up to the target and adding jumps. Check a proposed amount: add it to the start and compare with the target. One litre is 1000 ml. One kilogram (kg) is 1000 grams (g). Use 1000 g per kg; group each 1000 g into 1 kg: 1400 g is 1 kg 400 g. One litre is 1000 ml; change litres to ml before calculating. For 1000 ml - 625 ml, count up: 625 to 630 is 5, to 700 is 70, to 1000 is 300. Add the jumps: 375 ml more. When units differ, check what they measure. You can change centimetres to millimetres because both measure length. For 5 cm + 20 mm, change 5 cm to 50 mm, then add: 50 mm + 20 mm = 70 mm. You cannot add grams to millilitres to find a mass or volume. They measure different things. **Recap.** Read the number and unit. Add or subtract measurements of the same kind in matching units. Change units first when needed. ## Practice 17 questions on this page, each with its working shown. ## Needs first - [Comparing and ordering measurements](https://lightmysky.com/learn/mathematics/comparing-and-ordering-measurements-mt_BFJ-ch_8QU) - [Choosing measurement units](https://lightmysky.com/learn/mathematics/choosing-measurement-units-mt_DOe893F6gN) ## Opens up - [Measuring liquids and masses](https://lightmysky.com/learn/mathematics/measuring-liquids-and-masses-mt_-af65bxfdp) - [Converting measurement units](https://lightmysky.com/learn/mathematics/converting-measurement-units-mt_d8al9JcajP) - [Precision in mathematical language and notation](https://lightmysky.com/learn/mathematics/precision-in-mathematical-language-and-notation-mt_HY8Yycu_rz) - [Building the Pyramids](https://lightmysky.com/learn/history/building-the-pyramids-mt_kgTN6yk4oE) - [Estimating and comparing measures](https://lightmysky.com/learn/mathematics/estimating-and-comparing-measures-mt_mFJ-2ZF6Tk) - [Measuring Perimeters](https://lightmysky.com/learn/mathematics/measuring-perimeters-mt_wE7-Gs9ENL)