Measurement allows scientists to describe observations using numbers and agreed units.
Instead of saying that an object is long, hot or heavy, a measurement can give much more specific information.
For example:
The pencil is 15 cm long.
That statement contains both a number and a unit.
This guide introduces common measurements, useful scientific units and simple habits that make measurements clearer and more reliable.
What is a measurement?
A measurement compares a quantity with an agreed unit.
The quantity might be:
- length
- mass
- time
- temperature
- volume
- another property that can be measured
A complete measurement normally includes a numerical value and a unit.
For example:
25 cm
The number is 25.
The unit is centimetres, represented by the symbol cm.
Why do scientists use agreed units?
Imagine that one learner says a table is six hand spans long while another learner says it is seven hand spans long.
The learners may have different sized hands.
The measurement is therefore difficult to compare.
Standard units solve this problem.
If both learners measure the table in centimetres, they can compare their results using the same reference.
Agreed units also allow scientists in different places to understand one another's measurements.
The International System of Units
The International System of Units is commonly called SI.
It provides an internationally agreed system for expressing measurements.
Seven SI base units form the foundation of the system.
At beginner level, some especially useful ones are:
- metre, symbol m, for length
- kilogram, symbol kg, for mass
- second, symbol s, for time
- kelvin, symbol K, for thermodynamic temperature
Scientists also use many units built from these base units and some other units that are accepted for use with SI.
Measuring length
Length describes distance from one point to another.
The SI base unit of length is the metre.
Common units include:
- millimetre, symbol mm
- centimetre, symbol cm
- metre, symbol m
- kilometre, symbol km
The useful relationships are:
10 mm = 1 cm
100 cm = 1 m
1000 m = 1 km
Choose a sensible length unit
Different units are convenient for different sizes.
You might measure:
- the thickness of a small object in millimetres
- the length of a pencil in centimetres
- the length of a classroom in metres
- the distance between towns in kilometres
Using kilometres for the length of a pencil would be possible mathematically, but it would be inconvenient.
A useful unit makes the number easy to understand.
Tools for measuring length
Common tools include:
- a ruler
- a metre rule
- a measuring tape
- more specialized instruments when greater precision is needed
When using a ruler, place the starting point of the object at the correct zero mark rather than simply at the physical edge of the ruler.
Read the scale carefully and keep your eye positioned so that the marking is easy to see correctly.
Measuring mass
Mass describes the amount of matter in an object.
The SI base unit of mass is the kilogram.
Common units include:
- milligram, symbol mg
- gram, symbol g
- kilogram, symbol kg
Useful relationships include:
1000 mg = 1 g
1000 g = 1 kg
A small laboratory sample might be measured in grams or milligrams.
A school bag might be measured in kilograms.
Mass and weight are not exactly the same
In everyday conversation, people often use the word weight when talking about kilograms.
In science, mass and weight describe different quantities.
Mass describes an object's amount of matter.
Weight is a force caused by gravity acting on that mass.
This beginner guide focuses mainly on measuring mass.
Tools for measuring mass
A balance is commonly used to measure mass.
Before measuring, check that the balance is ready for use and displays the expected starting value.
If a container is used to hold a material, the balance may allow the container's contribution to be removed before the material is measured.
Follow the instructions for the particular balance you are using.
Measuring time
The SI base unit of time is the second.
Common time units include:
- second, symbol s
- minute, symbol min
- hour, symbol h
Useful relationships are:
60 s = 1 min
60 min = 1 h
A stopwatch or timer can be used when measuring the duration of an event.
Choose a sensible time unit
You might measure:
- a short reaction in seconds
- a classroom activity in minutes
- a long journey in hours
The choice depends on the duration being measured and the precision needed.
Measuring temperature
Temperature describes how hot or cold something is according to a temperature scale.
The SI base unit for thermodynamic temperature is the kelvin, symbol K.
In everyday science activities, temperature is also commonly measured in degrees Celsius, symbol °C.
For example:
The water temperature is 22 °C.
A thermometer or temperature sensor can be used to make the measurement.
Kelvin and degrees Celsius
The kelvin and degree Celsius use the same size temperature interval.
Their zero points are different.
For many beginner classroom investigations, degrees Celsius are convenient because they are familiar and suitable for ordinary environmental and laboratory temperatures.
More advanced science often uses kelvin when working with thermodynamic temperature.
Measuring liquid volume
Volume describes how much three dimensional space something occupies.
The SI unit for volume can be expressed using cubic metres.
For many liquid measurements, litres and millilitres are convenient and widely used.
Common units include:
- millilitre, symbol mL
- litre, symbol L
The relationship is:
1000 mL = 1 L
A measuring cylinder can be used for many laboratory liquid volume measurements.
Read a liquid scale carefully
Place the measuring container on a stable level surface.
Bring your eye close to the level of the liquid scale rather than looking steeply from above or below.
Some liquids form a curved surface inside a narrow container.
Your teacher or laboratory instructions can show you which part of that curve should be read for the liquid and equipment being used.
Metric prefixes help describe larger and smaller amounts
Prefixes change the size represented by a unit.
Three common prefixes are:
- kilo, meaning one thousand times the unit
- centi, meaning one hundredth of the unit
- milli, meaning one thousandth of the unit
For example:
1 km = 1000 m
1 cm = 0.01 m
1 mm = 0.001 m
Because the metric system uses powers of ten, many conversions can be understood by thinking about place value.
Unit symbols matter
Scientific unit symbols are standardized.
For example:
- metre uses m
- centimetre uses cm
- kilometre uses km
- gram uses g
- kilogram uses kg
- second uses s
- millilitre uses mL
- litre uses L
- kelvin uses K
Capital and lowercase letters can have different meanings, so copy unit symbols carefully.
Leave a space between the number and most unit symbols
Standard SI writing normally places a space between the numerical value and the unit symbol.
Preferred
25 cm
4 kg
18 °C
The unit symbol does not normally take a full stop simply because it is a symbol.
Write the unit with every measurement
A number without a unit may be unclear.
Suppose someone records:
Length = 12
Is that 12 millimetres, centimetres or metres?
A clearer record is:
Length = 12 cm
Always record the unit with the value unless the table or graph clearly states the unit for the entire set of measurements.
Use the same unit when comparing measurements
Suppose one object is 80 cm long and another is 1 m long.
The values are easier to compare if you first express them using the same unit.
Since:
1 m = 100 cm
the comparison becomes:
80 cm and 100 cm
The second object is longer.
Measurement tools have limits
No measuring tool provides unlimited detail.
A ruler marked only in centimetres cannot reliably provide the same detail as an instrument designed to measure much smaller distances.
Record a measurement with a level of detail that the instrument can reasonably support.
Do not invent extra digits simply because a calculator or computer can display them.
Repeated measurements can be useful
Sometimes a measurement varies slightly when it is repeated.
Repeating a measurement can help you see whether the result is consistent.
For example, if you time the same event several times, your recorded times may differ slightly because of natural variation or the way the measurement was made.
Those differences are information worth noticing.
Accuracy and precision are useful ideas
These words have specific meanings in measurement science.
For a beginner, one useful distinction is:
- accuracy concerns how closely a measurement agrees with an appropriate reference value
- precision concerns how closely repeated measurements agree with one another
A set of measurements can be closely grouped without necessarily being close to the correct reference value.
You will study these ideas in more depth in later science work.
Choose the right tool for the question
The measuring instrument should suit the quantity and the amount of detail needed.
For example:
- use a ruler or measuring tape for length
- use a balance for mass
- use a stopwatch or timer for time
- use a thermometer or sensor for temperature
- use suitable graduated equipment for liquid volume
A tool designed for the wrong quantity will not produce a useful scientific measurement.
Practice 1: Choose the sensible unit
Choose a suitable unit for each measurement.
- The length of a pencil
- The distance between two towns
- The mass of a school bag
- The duration of a short race
- The amount of water in a drinking bottle
Suggested answers
- centimetres
- kilometres
- kilograms
- seconds
- millilitres or litres, depending on the bottle
Practice 2: Match the tool
Match each quantity with an appropriate measuring tool.
- Length of a desk
- Mass of an object
- Water temperature
- Duration of an event
- Liquid volume
Suggested answers
- ruler, metre rule or measuring tape
- balance
- thermometer or temperature sensor
- stopwatch or timer
- appropriate graduated measuring equipment
Practice 3: Complete the conversions
- 100 cm = ___ m
- 1000 m = ___ km
- 1000 g = ___ kg
- 1000 mL = ___ L
- 60 s = ___ min
Answers
- 1 m
- 1 km
- 1 kg
- 1 L
- 1 min
Practice 4: Find the incomplete measurement
Which record is incomplete?
- Temperature = 21 °C
- Mass = 450 g
- Length = 32
- Time = 18 s
Answer
Length = 32 is incomplete because the unit is missing.
Practice 5: Compare using the same unit
Which is longer?
Object A = 95 cm
Object B = 1 m
Convert 1 m to 100 cm.
Now compare 95 cm with 100 cm.
Object B is longer.
Practice 6: Correct the unit symbol
Which version uses the standard symbol correctly?
- 5 KG
- 5 kg
- 5 Kg
Answer
5 kg
The standard kilogram symbol uses lowercase letters.
Activity: Measure three classroom objects
If your teacher or responsible adult approves the activity, choose three safe ordinary objects.
For each object:
- decide which quantity you will measure
- choose an appropriate tool
- choose a sensible unit
- make the measurement
- record the number and unit together
- repeat the measurement once and compare the results
Do not measure anything dangerous, hot, electrical or otherwise unsuitable for handling.
A simple measurement table
You could record your results using columns such as:
- Object
- Quantity measured
- Tool
- Measurement 1
- Measurement 2
- Unit
A clear table helps keep the numerical values connected to the correct quantities and units.
Common measurement mistakes
- forgetting to write the unit
- using an inconvenient unit for the size being measured
- mixing units before making a comparison
- reading a scale from an awkward angle
- starting from the wrong point on a ruler
- recording more digits than the instrument can support
- confusing mass with weight
- using incorrect capitalization in unit symbols
- changing units without recording the conversion
A quick measurement check
Before accepting a measurement, ask:
- What quantity am I measuring?
- Is this the right tool?
- Is the unit suitable?
- Did I read the scale correctly?
- Did I write the numerical value and unit together?
- Does the amount of detail match the measuring tool?
- Would repeating the measurement be useful?
The main idea
Scientific measurement combines numerical values with agreed units.
The SI gives scientists a common international measurement system.
Common beginner measurements include length, mass, time, temperature and volume.
Choose a unit and measuring tool that suit the quantity, write the value and unit clearly, and record only the level of detail your equipment can reasonably support.
Careful measurement makes observations easier to compare, analyze and communicate.
Continue learning
Review The Scientific Method Explained Step by Step to see how measurements become evidence in an investigation.
Review Observation and Inference: What Is the Difference? to see how measured observations can support scientific inferences.
You will use these measurement skills again when you practise recording observations in a data table.