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In mathematics, a **rate** is the ratio between two related quantities in different units.^{[1]} If the denominator of the ratio is expressed as a single unit of one of these quantities, and if it is assumed that this quantity can be changed systematically (i.e., is an independent variable), then the numerator of the ratio expresses the corresponding *rate of change* in the other (dependent) variable.

The most common type of rate is "per unit of time", such as speed, heart rate and flux. Ratios that have a non-time denominator include exchange rates, literacy rates and electric field (in volts/meter).

In describing the units of a rate, the word "per" is used to separate the units of the two measurements used to calculate the rate (for example a heart rate is expressed "beats per minute"). A rate defined using two numbers of the same units (such as tax rates) or counts (such as literacy rate) will result in a dimensionless quantity, which can be expressed as a percentage (for example, the global literacy rate in 1998 was 80%) or fraction or as a multiple.

Often *rate* is a synonym of rhythm or frequency, a count per second (i.e., Hertz); e.g., radio frequencies or heart rate or sample rate.

Rates and ratios often vary with time, location, particular element (or subset) of a set of objects, etc. Thus they are often mathematical functions. For example, velocity v (distance tracity on segment i (v is a function of index i). Here each segment i, of the trip is a subset of the trip route.

A rate (or ratio) may often be thought of as an output-input ratio, benefit-cost ratio, all considered in the broad sense. For example, miles per hour in transportation is the output (or benefit) in terms of miles of travel, which one gets from spending an hour (a cost in time) of traveling (at this velocity).

A set of sequential indices i may be used to enumerate elements (or subsets) of a set of ratios under study. For example, in finance, one could define i by assigning consecutive integers to companies, to political subdivisions (such as states), to different investments, etc. The reason for using indices i, is so a set of ratios (i=0,N) can be used in an equation so as to calculate a function of the rates such as an average of a set of ratios. For example, the average velocity found from the set of v_{i}'s mentioned above. Finding averages may involve using weighted averages and possibly using the Harmonic mean.

A ratio r=a/b has both a numerator a and a denominator b. a and/or b may be a real number or integer. The inverse of a ratio r is 1/r = b/a.

Rates occur in many areas of real life. For example:
*How fast are you driving?* Miles per hour is a rate. *What interest does your savings account pay you?* Interest paid / year is a rate.

Consider the case where the numerator of a rate is a function where happens to be the denominator of the rate . A rate of change of with respect to (where is incremented by ) can be formally defined in two ways:^{[2]}

where *f(x)* is the function with respect to *x* over the interval from *a* to *a*+*h*. An instantaneous rate of change is equivalent to a derivative.

For example, the average speed of a car can be calculated using the total distance travelled between two points, divided by the travel time, while the instantaneous speed can be determined by viewing a speedometer.

In chemistry and physics:

- Speed, the rate of change of position, or the change of position per unit of time
- Acceleration, the rate of change in speed, or the change in speed per unit of time
- Power, the rate of doing work, or the amount of energy transferred per unit time
- Frequency, the number of occurrences of a repeating event per unit of time
- Reaction rate, the speed at which chemical reactions occur
- Volumetric flow rate, the volume of fluid which passes through a given surface per unit of time; e.g., cubic meters per second

- Radioactive decay, the amount of radioactive material in which one nucleus decays per second, measured in Becquerels

In computing:

- Bit rate, the number of bits that are conveyed or processed by a computer per unit of time
- Symbol rate, the number of symbol changes (signalling events) made to the transmission medium per second
- Sampling rate, the number of samples (signal measurements) per second

Miscellaneous definitions:

- Rate of reinforcement, number of reinforcements per unit of time, usually per minute
- Heart rate, usually measured in beats per minute

- Exchange rate, how much one currency is worth in terms of the other
- Inflation rate, ratio of the change in the general price level during a year to the starting price level
- Interest rate, the price a borrower pays for the use of money they do not own (ratio of payment to amount borrowed)
- Price–earnings ratio, market price per share of stock divided by annual earnings per share
- Rate of return, the ratio of money gained or lost on an investment relative to the amount of money invested
- Tax rate, the tax amount divided by the taxable income
- Unemployment rate, the ratio of the number of people who are unemployed to the number in the labor force
- Wage Rate, the amount paid for working a given amount of time (or doing a standard amount of accomplished work) (ratio of payment to time)

- Birth rate, and mortality rate, the number of births or deaths scaled to the size of that population, per unit of time
- Literacy rate, the proportion of the population over age fifteen that can read and write
- Sex ratio or Gender ratio, the ratio of males to females in a population

**^**See Webster's new international dictionary of the English language, second edition, unabridged. Merriam Webster Co. 2016. p.2065 definition 3. while this definition doesn't say "related" and while the ratio of two non-related quantities is technically a ratio, such a ratio has little (if any meaning). For example, what would be the utility of finding the ratio of such unrelated numbers as ratio of the weight of ones residence to an integer selected at random between -10^{−9}and +10^{9}?**^**Adams, Robert A. (1995).*Calculus: A Complete Course*(3rd ed.). Addison-Wesley Publishers Ltd. p. 129. ISBN 0-201-82823-5.