In science, a measurement is a collection of quantitative or numerical data that describes a property of an object or event. A measurement is made by comparing a quantity with a standard unit. Since this comparison cannot be perfect, measurements inherently include error, which is how much a measured value deviates from the true value. The study of measurement is called metrology.

There are many measurement systems that have been used throughout history and across the world, but progress has been made since the 18th century in setting an international standard. The modern International System of Units (SI) bases all types of physical measurements on seven base units.

### Methods of Measurement

- The length of a piece of string can be measured by comparing the string against a meter stick.
- The volume of a drop of water may be measured using a graduated cylinder.
- The mass of a sample may be measured using a scale or balance.
- The temperature of a fire may be measured using a thermocouple.

### Comparing Measurements

Measuring the volume of a cup of water with an Erlenmeyer flask will give you a better measurement than trying to gauge its volume by putting it into a bucket, even if both measurements are reported using the same unit (e.g., milliliters). Accuracy matters, so there are criteria that scientists use to compare measurements: type, magnitude, unit, and uncertainty.

The level or type is the methodology used for taking the measurement. Magnitude is the actual numerical value of a measurement (e.g., 45 or 0.237). Unit is the ratio of the number against the standard for the quantity (e.g., gram, candela, micrometer). Uncertainty reflects the systematic and random errors in the measurement. Uncertainty is a description of confidence in the accuracy and precision of a measurement that is typically expressed as an error.

### Measurement Systems

Measurements are calibrated, which is to say they are compared against a set of standards in a system so that the measuring device can deliver a value that matches what another person would obtain if the measurement were repeated. There are a few common standard systems you may encounter:

**International System of Units (SI)**:*Système International d'Unités.*It is the most commonly used metric system.**Metric System**: SI is a specific metric system, which is a decimal system of measurement. Examples of two common forms of the metric system are the MKS system (meter, kilogram, second as base units) and CGS system (centimeter, gram, and second as base units). There are many units in SI and other forms of the metric system that are built upon combinations of base units. These are called derived units.**English System**: The British or Imperial system of measurements was common before SI units were adopted internationally. Although Britain has largely adopted the SI system, the United States and some Caribbean countries still use the English system for non-scientific purposes. This system is based on the foot-pound-second units, for units of length, mass, and time.