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.

One 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 per 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%), fraction, or multiple.

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

- 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:

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
- 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

## Economics/finance rates/ratios

- 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 unem
Miscellaneous definitions:

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

## Economics/finance rates/ratios