A PHP library for representing and converting dimensional units of measure.

conversion, unit, measurement, measurements, units


Measurements & Units

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This is a PHP library for representing and converting dimensional units of measure. This is inspired by the Measurement API in the Apple Foundation Framework.


The package can be installed via Composer:

$ composer require nmarfurt/measurements

Library Classes


Unit is the abstract superclass of units. Each instance of an Unit subclass consists of a symbol, which can be used to create string representations of Measurement objects.


Dimension is an abstract subclass of Unit that represents unit families or a dimensional unit of measure, which can be converted into different units of the same type. Each instance of an Dimension subclass has a converter, which is used to represent the unit in terms of the dimension's base unit provided by the baseUnit() method.

The library provides concrete subclasses for many of the most common types of physical units (listed below). If you need a custom unit type to represent a custom or derived unit, you can subclass Dimension. If you need to represent dimensionless units, subclass Unit directly.


A UnitConverter describes how to convert a unit to and from the base unit of its dimension. UnitConverterLinear is a UnitConverter subclass for converting between units using a linear equation. You can define your own converters if needed.


A Measurement object represents a measured quantity, using a unit of measure and a value. The Measurement class provides a programmatic interface to converting measurements into different units, as well as calculating the sum or difference between two measurements.

Measurement objects are initialized with a Unit object and double value. They are immutable and cannot be changed after being created.

The library provides specific subclasses (so called Quantities) for measurements that correspond to the provided units (see the list below).

Provided Units & Quantities

The library provides concrete subclasses for many of the most common types of physical units:

Dimension Subclass Description Base Unit Measurement Subclass
UnitAcceleration Unit of measure for acceleration meters per second squared m/s² Acceleration
UnitAngle Unit of measure for planar angle and rotation degrees ° Angle
UnitArea Unit of measure for area square meters Area
UnitConcentrationMass Unit of measure for concentration of mass milligrams per deciliter mg/dL ConcentrationMass
UnitDispersion Unit of measure for dispersion parts per million ppm Dispersion
UnitDuration Unit of measure for duration seconds sec Duration
UnitElectricCharge Unit of measure for electric charge coulombs C ElectricCharge
UnitElectricCurrent Unit of measure for electric current amperes A ElectricCurrent
UnitElectricPotentialDifference Unit of measure for electric potential difference volts V ElectricPotentialDifference
UnitElectricResistance Unit of measure for electric resistance ohms Ω ElectricResistance
UnitEnergy Unit of measure for energy joules J Energy
UnitFrequency Unit of measure for frequency hertz Hz Frequency
UnitFuelEfficiency Unit of measure for fuel consumption liters per 100 kilometers L/100km FuelEfficiency
UnitIlluminance Unit of measure for illuminance lux lx Illuminance
UnitLength Unit of measure for length meters m Length
UnitMass Unit of measure for mass kilograms kg Mass
UnitPower Unit of measure for power watts W Power
UnitPressure Unit of measure for pressure newtons per square meter N/m² Pressure
UnitRadioactivity Unit of measure for radioactivity becquerel Bq Radioactivity
UnitSpeed Unit of measure for speed meters per second m/s Speed
UnitTemperature Unit of measure for temperature kelvin K Temperature
UnitVolume Unit of measure for volume liters L Volume


Using Measurements

You can define measurements as follows:

use Measurements\Measurement;
use Measurements\Units\UnitLength;
use Measurements\Units\UnitDuration;

$length = new Measurement(4.48, UnitLength::meters());
echo $length; // = 4.48 m

$duration = new Measurement(1.5, UnitDuration::hours());
echo $duration; // = 1.5 hr

You may want to enforce the unit type of a measurement by using the provided measurement subclasses (aka Quantities):

use Measurements\Units\UnitLength;
use Measurements\Units\UnitDuration;
use Measurements\Quantities\Length;
use Measurements\Quantities\Duration;

$length = new Length(4.48, UnitLength::meters());
echo $length; // = 4.48 m

$duration = new Duration(1.5, UnitDuration::hours());
echo $duration; // = 1.5 hr

$invalid = new Length(4.48, UnitDuration::hours()); // Will throw a UnitException exception

The Quantities objects also provide the benefit of an expressive syntax to create measurements. They make use of the __callStatic() magic-method to create a new instance by resolving the derived dimension.

use Measurements\Quantities\Length;
use Measurements\Quantities\Duration;

$length = Length::meters(4.48);
echo $length; // = 4.48 m

$duration = Duration::hours(1.5);
echo $duration; // = 1.5 hr

$invalid = Length::hours(4.48); // Will throw a BadMethodCallException exception

Some of the Measurement subclasses expose convenience methods to easily create instances from other measurements.

use Measurements\Quantities\Length;
use Measurements\Quantities\Duration;

$distance = Length::kilometers(18);
$time = Duration::hours(1);

$speed = Speed::fromLengthAndDuration($distance, $time);
echo $speed->toString(); // 5 m/s

Converting Measurements

Measurement objects of the same dimension can be converted from one unit of measure to another.

use Measurements\Measurement;
use Measurements\Units\UnitLength;

$meters = new Measurement(4.48, UnitLength::meters());

$centimeters = $meters->convertTo(UnitLength::centimeters());
echo $centimeters; // = 448 cm

Measurements created as Quantities objects also provide a short syntax to convert them. They make use of the __call() magic-method to resolve the derived dimension.

use Measurements\Quantities\Length;

$meters = Length::meters(4.48);

$centimeters = $meters->toCentimeters();
echo $centimeters; // = 448 cm

Making Arithmetic Operations

Measurement objects support different operations, including add (+), subtract (-), multiply (*) and divide (/). Since Measurement objects are immutable, new instances are returned.

use Measurements\Measurement;
use Measurements\Units\UnitLength;

$first = new Measurement(4.48, UnitLength::meters());
$second = new Measurement(2.02, UnitLength::meters());

echo $first->add($second); // = 6.5 m

echo $first->subtract($second); // = 2.46 m

echo $first->multiplyBy($second); // = 9.0496 m

echo $first->divideBy($second); // = 2.2178217822 m

Conversions are automatically applied while making operations on measurements with different units. The returned measurement is defined in the base unit.

use Measurements\Measurement;
use Measurements\Units\UnitLength;

$decimeters = new Measurement(44.8, UnitLength::decimeters());
$centimeters = new Measurement(202, UnitLength::centimeters());

echo $decimeters->add($centimeters); // = 6.5 m

echo $decimeters->subtract($centimeters); // = 2.46 m

echo $decimeters->multiplyBy($centimeters); // = 9.0496 m

echo $decimeters->divideBy($centimeters); // = 2.2178217822 m

It is also possible to make arithmetic operations on a measurement using values.

use Measurements\Measurement;
use Measurements\Units\UnitLength;

$centimeters = new Measurement(42, UnitLength::centimeters());

echo $centimeters->addValue(8); // = 50 cm

echo $centimeters->subtractValue(12); // = 30 cm

echo $centimeters->multiplyByValue(2); // = 84 cm

echo $centimeters->divideByValue(2); // = 21 cm

Working with Custom Units

In addition to provided units, you can define custom units. Custom units can be initialized from a symbol and converter of an existing type or implemented as a class method of an existing type for additional convenience. You can also define your own Dimension subclass to represent an entirely new unit dimension.

Initializing a Custom Unit with a Specified Symbol and Definition

The simplest way to define a custom unit is to create a new instance of an existing Dimension subclass.

For example, let define a jump as a custom, nonstandard unit of length (1 jump = 1.82 m). You can create a new instance of UnitLength as follows:

$jump = new UnitLength("jump", new UnitConverterLinear(1.82));

Extending Existing Dimension Subclasses

Alternatively, you can extend an existing Dimension subclass to define a new unit.

For example, let define our new jump unit as custom subclass:

class UnitJump extends UnitLength {

	public static function jumps()
		return new static("jump", new UnitConverterLinear(1.82));


Creating a Custom Dimension Subclass

You can create a new subclass of Dimension to describe a new unit dimension.

For example, let define units for digital data. In computing and telecommunications, a unit of information is the capacity of some standard data storage system or communication channel, used to measure the capacities of other systems and channels. Bytes, or multiples thereof, are common units used to specify the sizes of computer files and the capacity of storage units.

You can implement a UnitDigitalData class that defines units of digital information as follows:

class UnitDigitalData extends Dimension {

	public static function baseUnit()
		return static::bytes();
	public static function bytes()
		return new UnitDigitalData("B", new UnitConverterLinear(1.0));

	public static function kilobytes()
		return new UnitDigitalData("kB", new UnitConverterLinear(1000));
	public static function megabytes()
		return new UnitDigitalData("MB", new UnitConverterLinear(1000000));

	public static function kibibytes()
		return new UnitDigitalData("KiB", new UnitConverterLinear(1024));

	public static function mebibytes()
		return new UnitDigitalData("MiB", new UnitConverterLinear(1048576));
	// ...


Generating API Documentation

$ phpdoc -d ./src/ -t ./doc/generated --template="xml"
$ phpdocmd ./doc/generated/structure.xml doc/


This library is open-sourced software licensed under the MIT license.