Units of Measurement

Why do we need standard units?

The seven base SI units

Prefixes make big and small quantities easier

Frequently asked questions

More lessons

• 👤 My Body Parts — Learn all the important parts of your body — head, arms, legs, and what each one does!
• 👁️ My Five Senses — See, hear, smell, taste and touch — your five senses connect you to the whole world!
• 🧼 Keeping Clean — Washing hands, brushing teeth, and bathing every day keeps germs away and keeps us healthy!
• 🐕 Pets and Farm Animals — Dogs, cats, cows, and hens — some animals live with us as pets or on farms to help us!
• 🦁 Wild Animals — Lions, elephants, tigers — wild animals live in forests and jungles far from our homes!
• 🦋 Birds and Insects — Parrots, sparrows, butterflies and bees — birds have wings and beaks, insects have six legs!
• 🌳 Types of Plants — Trees, shrubs, herbs, and climbers — plants come in many sizes and shapes all around us!
• 🌻 What Plants Need — Sunlight, water, soil, and air — plants need all four to grow big and strong!
• 🍎 Plants Give Us Food — Fruits, vegetables, grains, and pulses — so much of what we eat comes from plants!
• 🍽️ Meals of the Day — Breakfast, lunch, dinner and snacks — eating regular meals keeps us energetic all day!
• 🥦 Healthy Food Choices — Fruits, vegetables and milk keep us strong — choose healthy food over junk food!
• 🌾 Where Food Comes From — Food comes from plants, farmers, and animals — there is a long journey before food reaches our plate!
• ☀️ Day and Night Sky — The sun lights our day and the moon and stars shine at night — the sky is always changing!
• 🌤️ Types of Weather — Sunny, rainy, windy, cold — weather changes every day and tells us what to wear!
• 🌧️ Three Seasons — India has summer, monsoon, and winter — each season brings its own weather and fun!
• 👶 Growing Up — We grow from baby to child to adult — good food, sleep, and exercise help us grow strong!
• 💪 Our Bones and Muscles — 206 bones give us our shape and muscles move us — together they make every action possible!
• 🏃 Staying Healthy — Eat well, drink water, exercise, and sleep — four golden habits for a healthy body!
• 🐄 What Animals Eat — Herbivores eat plants, carnivores eat meat, and omnivores eat both!
• 🏠 Animal Houses — Birds build nests, rabbits dig burrows, lions live in dens — every animal has its own home!
• 🐣 Baby Animals — Some babies hatch from eggs, others are born alive — all are cared for by their mothers!
• 🌱 Seeds to Plants — A tiny seed holds a sleeping baby plant — water and warmth wake it up to grow!
• 🍃 Leaves and Roots — Leaves make food using sunlight and roots drink water from soil — together they keep the plant alive!
• 🌿 Plants Have Uses — Plants give us food, medicine, wood, cotton, and clean air to breathe — they are our most important resource!
• 🌊 Water Around Us — Water covers most of Earth — in oceans, rivers, lakes, rain, and underground wells!
• 💧 Uses of Water — We use water for drinking, cooking, cleaning, farming, and every living thing needs it to survive!
• 🚰 Saving Water — Only 3% of Earth's water is fresh — closing taps, short showers, and collecting rain all help save it!
• 💨 Air We Breathe — Air is all around us — we breathe in oxygen and breathe out carbon dioxide, and plants keep our air fresh!
• 🌬️ Wind and Storms — Wind is moving air — it can be a gentle breeze or a powerful storm, and it shapes our weather every day!
• ☁️ Clouds and Rain — Clouds form when water vapour rises and cools — and when they get heavy, they release rain that grows our food!
• 🌴 Plants in Habitats — Plants have amazing adaptations — cacti store water in deserts, lotus floats in ponds, and pine trees shed snow!
• 🌸 Flower Parts — Flowers have petals, sepals, stamens and a pistil — together they create seeds and new plants through pollination!
• 🍇 Seeds and Fruits — Fruits grow from flowers and carry seeds — seeds travel by wind, water, animals, and even explosion to grow new plants!
• 🕸️ Food Webs — Food chains show who eats whom — producers, consumers, and decomposers are all connected in a food web!
• 🦔 Animal Defences — Animals use camouflage, armour, spines, poison, speed, and even ink to protect themselves from predators!
• 🦋 Animal Life Cycles — Every animal goes through a life cycle — the butterfly changes from egg to caterpillar to pupa and finally a butterfly!
• ⚡ Food for Energy — Food gives us energy, builds our body, and protects us — carbohydrates, proteins, fats, vitamins, and water all have roles!
• 🍊 Vitamins and Minerals — Vitamins A, B, C, D and minerals like calcium and iron are essential for healthy eyes, bones, blood, and immunity!
• 🧼 Food Safety — Washing hands, covering food, and avoiding stale food protects us from germs and food poisoning!
• 🏔️ Water Sources — Water comes from rivers, lakes, groundwater, rain, and dams — it travels a long journey to reach our taps!
• 🔄 Water Cycle — Water evaporates from oceans, rises as vapour, forms clouds through condensation, and falls as rain — endlessly!
• 🚰 Safe Water — Not all water is safe — germs, chemicals, and sewage contaminate water, but boiling, filtering, and chlorination make it safe!
• 🌍 Air, Water and Soil — Air, water, and soil are the three pillars of our environment — all living things depend on them working together!
• 🏭 Types of Pollution — Air, water, and soil pollution harm all living things — factories, vehicles, and waste are the main causes!
• ♻️ Reduce, Reuse, Recycle — Reduce what we use, reuse what we have, and recycle what we can — together the three Rs protect our planet!
• 🧪 The Iodine Test — Find the hidden starch in your food.
• 🛡️ Tiny Protectors: Vitamins — A, B, C, D — what each vitamin does.
• 🥗 What's a Balanced Diet? — Build the perfect plate, food by food.
• 🚜 Food Miles: Farm to Plate — How does a chapati reach your kitchen?
• 🌾 Millets: Tiny Super Grains — Jowar, bajra and ragi — old and powerful.
• 🧲 Magnetic or Not? — Find out which everyday things a magnet can pull.
• 🧭 Two Poles, Two Rules — Why magnets sometimes snap and sometimes push apart.
• 🌍 Earth is a Giant Magnet — Why a compass needle always points north.
• 🛠️ Make Your Own Compass — Build a working compass with a needle and water.
• 🔧 Magnets in Everyday Life — The hidden magnets in things you use every day.
• 🤲 Hand-picking — Pick the bad bits out with your fingers.
• 🍚 Sieving — Separate small from big using a sieve.
• 🌾 Winnowing — Use wind to blow the lighter bits away.
• 💧 Cleaning Muddy Water — Sedimentation, decantation and filtration.
• 🧂 Evaporation — Get salt from saltwater by drying it out.
• 🧲 Magnetic Separation — Use a magnet to pull iron out of a mix.
• 🧊 Three States of Water — Solid, liquid, gas — all the same water.
• 🔥 Melting & Freezing — How ice and water swap places.
• 💨 Evaporation & Condensation — How water sneaks between liquid and gas.
• 🍳 Boiling — Water's fastest change — bubbles all the way through.
• 🌧️ The Water Cycle — How rain, rivers and clouds are all the same water.
• 🌍 Day & Night — Why day turns to night and back again.
• 🪐 The Eight Planets — A quick tour of our solar system.
• 🌗 Phases of the Moon — Why the Moon changes shape every night.
• ⭐ Stars & Constellations — Connect the dots in the night sky.
• ☄️ Comets, Meteors & Asteroids — The small visitors of the solar system.
• 🧱 Grouping Materials — Hard, soft, shiny, rough — every material tells a story.
• 🧂 Soluble and Insoluble — Some materials vanish in water. Others never do.
• 🔦 Transparent, Translucent & Opaque — Can light pass through? That's the question.
• 🌡️ What is Temperature? — The measure of how hot or cold something is.
• 🩺 Reading a Thermometer — Clinical, lab — how to read them right.
• 🌱 Living and Non-Living Things — What makes something truly alive?
• 🌵 How Plants Adapt — Different homes, different shapes — plants are clever.
• 🌍 Classifying Living Things — Fish, amphibians, birds — sorting life into kingdoms and groups.
• 🐫 How Animals Adapt — Humps, gills, and flippers — built for a reason.
• 🔭 What is Science? — Curiosity, observation, and the scientific way of thinking.
• 🧪 The Scientific Method — Observe, hypothesise, experiment, conclude — the scientist's recipe.
• 🔭 Science Tools & Measurement — Rulers, balances, cylinders — scientists don't guess, they measure.
• 📐 Measuring Length — From millimetres to kilometres — length has a unit for every scale.
• 🏃 Types of Motion — Straight, circular, back-and-forth, or random — motion comes in types.
• 🔬 The Cell — Basic Unit of Life — Zoom in on any living thing and find the same tiny unit.
• 🌿 How Plants Make Food — Sunlight, water, air — plants cook their own meals.
• 🫀 Organ Systems of the Body — Cells build tissues, tissues build organs, organs build systems.
• 🌳 What is a Forest? — More than trees — a whole living community of plants, animals and soil.
• 🌿 Why Forests Matter — Oxygen, water, climate — forests run the life-support system of Earth.
• 🌱 Save Our Forests — Deforestation, its consequences, and how we can protect what remains.
• 🐕 Super Senses — Dogs smell 10,000× better than us. Eagles see 8× sharper. Nature's senses are extraordinary.
• 🦇 Echolocation — Bats and dolphins navigate the dark using sound — a sonar system evolution built from scratch.
• 🐦 Animal Migration — The Arctic Tern flies 90,000 km every year — from pole to pole and back.
• 👁️ Our Five Senses — Your eyes, ears, nose, tongue and skin work together to build your picture of the world.
• 🍎 Digestion — Nine metres of tubing turns food into fuel — a complete guide to the digestive system.
• 🫙 Food Preservation — Why does food go bad — and how do we stop it? Freezing, drying, pickling and more.
• 🌱 Seed Germination — Inside every seed is a sleeping baby plant — here's how water wakes it up.
• 🌬️ Seed Dispersal — Dandelion fluff, coconuts and hooked burrs — how seeds travel far from their parent plant.
• 🌸 Parts of a Flower — Petals, stamens, pistils — every flower part has a job in making the next generation.
• 💧 Every Drop Counts — Only 1% of Earth's water is drinkable — water conservation matters more than ever.
• 🫧 Water Properties — Universal solvent, surface tension, capillarity — water's physics explain so much.
• 🚰 Clean Water — 5-step water treatment plant, boiling, SODIS, filtration — how to make water safe.
• 🌍 Earthquakes — Tectonic plates collide, slide and separate — the science of shaking ground.
• 🌡️ Blow Hot, Blow Cold — Heat always moves from hot to cold — through conduction, convection, and radiation.
• ⛽ What if it Finishes? — Coal, oil, and gas took 300 million years to form — we're burning them in decades.
• 🍎 Gravity — The invisible force that holds you to Earth, keeps the Moon orbiting, and holds the solar system together.
• 🧑‍🚀 Life in Space — In space, everything floats — sleep, food, water and all. Here's why and how astronauts cope.
• ⭐ Stars and the Sun — The Sun is a star — a giant ball of nuclear fire 109 Earths wide, 8 light-minutes away.
• 🌿 How Plants Grow — A seed becomes a tree using sunlight, water, and air — photosynthesis is nature's solar panel.
• 🐝 Bees and Flowers — Bees need flowers for food. Flowers need bees to make seeds. The perfect team.
• 🌳 Trees Are Homes — One tree can be home to hundreds of animals — birds, insects, squirrels and more.
• 🦔 Animal Homes — Birds build nests, bees build hives, rabbits dig burrows — every animal has its own home.
• 🦁 Who Eats Whom? — Grass feeds the rabbit, rabbit feeds the fox — nature's eating chain connects every living thing.
• 🐜 Amazing Ants — An ant can carry 50× its own weight. A colony of millions works like one giant brain.
• 🌧️ Rain and Rivers — Rain fills streams, streams fill rivers — rivers carry freshwater across the whole land.
• ♻️ The Water Cycle — Water evaporates, forms clouds, falls as rain, and flows back to the sea — endlessly.
• 💧 Save Every Drop — Only 3% of Earth's water is fresh — and most of that is frozen. Every drop we save matters.
• 🌾 Farm to Table — Before food reaches your plate, it travels through fields, factories, trucks, and shops.
• 🌈 Eat a Rainbow — Red tomatoes, orange carrots, green spinach — each colour gives your body something different.
• 🌱 Soil and Seeds — A handful of soil contains more living things than people on Earth — and seeds can wait decades to grow.
• 🦷 Teeth and Food — Your teeth are the first step in digestion — different teeth are shaped for different jobs.
• 💪 Bones and Muscles — 206 bones give your body its shape. Over 600 muscles pull those bones to create every movement.
• ❤️ Heart and Lungs — Your heart pumps blood to every cell, and your lungs fill that blood with the oxygen cells need.
• 🌸 Four Seasons — Spring, summer, autumn, winter — Earth's tilt as it orbits the Sun creates the four seasons.
• 🪨 Rocks and Soil — Rocks are not just stones — they are the raw material that becomes the soil all our food grows in.
• 🌙 Sun, Earth and Moon — Earth spins to give us day and night, orbits the Sun to give us a year, and the Moon follows along.
• 🌿 Photosynthesis — Plants convert sunlight, water and CO₂ into glucose — the foundation of almost all food on Earth.
• 🪴 Other Modes of Nutrition — Not all plants photosynthesise — parasites steal food, insectivores trap prey, and saprophytes eat the dead.
• 🌱 Replenishing Soil Nutrients — Crops deplete the soil — crop rotation, compost, and nitrogen-fixing bacteria keep it fertile.
• 🦁 Nutrition in Animals — Every animal must find, eat, and digest food — the strategies are as varied as life itself.
• 🦷 Animal Feeding Strategies — Sharp canines, flat molars, hooked beaks — every animal's teeth and jaws are shaped by what it eats.
• 🧫 Digestive Juices and Absorption — Your gut produces powerful enzymes that break any food into tiny molecules small enough to enter your bloodstream.
• 🌿 Natural Fibres — Cotton, silk, wool, and jute — fibres harvested from plants and animals that humans have woven for millennia.
• 🧪 Synthetic Fibres — Nylon, polyester, acrylic — fibres made in factories from petrochemicals that revolutionised clothing.
• 🪡 Fibre to Fabric — From raw fibre to finished cloth: spinning twists fibres into yarn, weaving interlocks them into fabric.
• 🌡️ Heat and Temperature — A teaspoon of boiling water and a bathtub of warm water — same temperature, very different amounts of heat.
• 🔥 Heat Transfer: Conduction — Metal feels cold even at room temperature — because it conducts heat away from your hand so fast.
• 🌊 Convection and Radiation — Sea breezes, boiling water, and the warmth of the Sun — heat travels in three very different ways.
• 🍋 Acids in Daily Life — Lemon juice, vinegar, fizzy drinks — acids are sour, corrosive, and all around us.
• 🧼 Bases and Alkalis — Soap, bleach, baking soda — bases are the opposites of acids and feel slippery to touch.
• ⚗️ Neutralisation and Salts — Mix an acid and a base and they cancel each other out — producing salt and water.
• 🔄 Physical Changes — Melting, dissolving, bending — physical changes alter shape or state but never create a new substance.
• ⚗️ Chemical Changes — Burning, rusting, cooking — chemical changes produce entirely new substances that cannot easily be reversed.
• 🟤 Rusting and Corrosion — Iron plus water plus oxygen equals rust — a slow chemical reaction destroying metal worldwide.
• ⛅ Weather and Climate — Weather changes daily; climate is what you expect — understanding both explains how life adapts.
• 📊 Reading Weather Data — Barometer, thermometer, rain gauge — meteorologists read these instruments to forecast tomorrow's weather.
• 🌍 Climate Adaptations — Camels store fat, polar bears have tiny ears, cacti spike their leaves — life finds ways to survive anywhere.
• 💨 How Wind Forms — Hot air rises, cool air rushes in — this simple principle drives everything from sea breezes to the monsoon.
• ⛈️ Thunderstorms and Lightning — A bolt of lightning carries a billion volts and reaches thirty thousand degrees — hotter than the Sun's surface.
• 🌀 Cyclones and Safety — At the eye of a cyclone it is eerily calm — but a wall of catastrophic winds surrounds it.
• 🌱 Soil Profile — Cut through the ground and you find distinct layers — each horizon tells a story of rock, time, and life.
• 🌊 Soil Erosion and Conservation — Rain, wind, and human activity strip topsoil that took a thousand years to form — but we can stop it.
• 🪱 Soil as a Habitat — A teaspoon of healthy soil contains more organisms than there are people on Earth.
• ⚡ What Is Respiration? — Every cell in your body burns glucose for energy — this chemical process is respiration, not breathing.
• 💪 Aerobic and Anaerobic Respiration — Sprinting flat out, your muscles run out of oxygen and switch to a less efficient backup — and that is what causes cramps.
• 🫁 Breathing in Animals — Fish breathe through gills, insects through spiracles, and your lungs have 700 million air sacs to maximise surface area.
• ❤️ The Circulatory System — Your heart beats 100,000 times a day, pumping blood through 100,000 km of vessels to every cell.
• 🌿 Plant Transport System — A tall tree moves hundreds of litres of water upward every day — with no pump, just physics.
• 💛 Lymph and Excretion — Beyond blood: the lymphatic system filters fluids and your kidneys clean 150 litres of blood every day.
• 🌿 Vegetative Reproduction — Strawberries send runners, potatoes grow from tubers, willows root from cuttings — no flowers needed.
• 🌸 Pollination and Fertilisation — A bee visits a flower and inadvertently delivers pollen that will become a seed — every fruit begins this way.
• 🌱 Seed Dispersal Methods — Dandelion seeds float on air, coconuts float on oceans, burdock hooks onto fur — plants evolved remarkable travel strategies.
• ⚡ Speed and Distance — Speed is simply distance divided by time — but understanding it unlocks all of mechanics.
• ⏱️ Measuring Time — From sundials to atomic clocks — measuring time precisely has driven science, navigation, and technology.
• 📈 Distance-Time Graphs — A line on a graph can tell the entire story of a journey — speed, stops, and direction changes at a glance.
• ⚡ Electric Circuits — Electricity needs a complete loop to flow — break it anywhere and the current stops instantly.
• 🔥 Heating Effect of Current — A light bulb glows because electricity makes its thin wire so hot it becomes white — resistance creates heat.
• 🧲 Magnetic Effect of Current — A live wire deflects a compass — every electric current creates a magnetic field around it.
• 💡 How Light Travels — Light races through space at 300,000 km per second — and always in perfectly straight lines.
• 🪞 Reflection of Light — When light bounces off a surface, the angle it arrives at always equals the angle it leaves — perfectly symmetrical.
• 🔍 Refraction and Lenses — Light bends when it moves from one material to another — and lenses use this bending to focus, magnify, or correct vision.
• 🌊 Water Distribution on Earth — Though 71% of Earth is covered in water, less than 1% is freshwater we can actually use.
• 🚿 Water Scarcity — Two billion people lack safe drinking water — understanding why helps us protect this finite resource.
• 💧 Groundwater and Conservation — Billions depend on underground water — but we're using it faster than rain can replenish it.
• 🌳 What Forests Give Us — Forests clean our air, regulate rainfall, prevent floods, and harbour most of life on land — yet we lose them rapidly.
• 🌿 Forest Biodiversity — A single hectare of tropical forest can hold more species than all of Europe — understanding this web of life reveals why forests matter.
• 🪓 Deforestation and Its Consequences — The world loses forest the size of a football field every second — and the consequences reach far beyond the trees themselves.
• 🚿 What Is Sewage? — Every tap we turn and toilet we flush creates wastewater — understanding what it contains explains why treatment is essential.
• 🔬 Wastewater Treatment — A sewage treatment plant cleans dirty water through a remarkable sequence of physical, biological, and chemical steps.
• 🚽 Sanitation for All — Access to a toilet and handwashing with soap are among the most powerful public health tools ever discovered.