A rate is the quotient of two quantities with different, but related, units. A unit rate is a rate with a unit quantity, usually the denominator. A vector rate is a rate with a vector quantity, usually the numerator. Rates with the same units may be added, subtracted, and averaged.

Addition

Rates with the same units in their denominator are added using ordinary addition, which will be called *arithmetic addition* since addition takes place in the numerator. For example, if *x*_{1} and *x*_{2} are lengths, and *t* is a given time interval, then the time speed rates *v*_{1} and *v*_{2} are added by arithmetic addition:

If **x**_{1} and **x**_{2} are displacements, and *t* is a given time interval, then the time velocity rates **v**_{1} and **v**_{2} are added by arithmetic addition:

where vector addition means addition of dimensions, i.e., parallelogram addition.

If *t*_{1} and *t*_{2} are time intervals, and *x* is an independent length, then the space pace rates *w*_{1} and *w*_{2} are added by arithmetic addition:

If **t**_{1} and **t**_{2} are dischronments, and *x* is an independent length, then the space lenticity rates **w**_{1} and **w**_{2} are added by arithmetic addition:

Rates with the same units in their numerator are added using *harmonic addition*, also known as parallel addition, in which addition takes place in the denominator. For this reason it could be called *denominator addition*. If *t*_{1} and *t*_{2} are time intervals, and *x* is an independent length, then the space velocity rates *u*_{1} and *u*_{2} are added by harmonic addition (symbolized here by a *circled plus*):

If **t**_{1} and **t**_{2} are time intervals, and *x* is an independent length, then the space velocity rates **u**_{1} and **u**_{2} are added by harmonic addition:

Averages

Rates with the same units in their denominator are averaged using the arithmetic mean. Rates with the same units in their numerator are averaged using the harmonic mean. For example, if *x*_{1} and *x*_{2} are lengths, and *t* is a given time interval, then the time speed rates *v*_{1} and *v*_{2} are averaged by the arithmetic mean:

If **x**_{1} and **x**_{2} are displacements, and *t* is a given time interval, then the time velocity rates **v**_{1} and **v**_{2} are averaged by the arithmetic mean:

If *t*_{1} and *t*_{2} are time intervals, and *x* is a given length, then the space pace rates *w*_{1} and *w*_{2} are averaged by the arithmetic mean:

If **t**_{1} and **t**_{2} are dischronments, and *x* is a given length, then the space lenticity rates **w**_{1} and **w**_{2} are averaged by the arithmetic mean:

If *t*_{1} and *t*_{2} are time intervals, and *x* is a given length, then the space speed rates *u*_{1} and *u*_{2} are averaged by the harmonic mean:

If **t**_{1} and **t**_{2} are dischronments, and *x* is a given length, then the space velocity rates **u**_{1} and **u**_{2} are averaged by the harmonic mean:

Another example would be electrical resistance, which equals voltage divided by current, and is connected in series with arithmetic addition or in parallel with harmonic addition. In a series circuit, the current across each resistor is the same, so resistance is added with arithmetic addition. On the other hand, resistors connected in parallel have the same voltage, so resistance is added with harmonic addition.

Conductance, the inverse of resistance, is connected in series with harmonic addition, or connected in parallel with arithmetic addition. Serial circuits are temporally sequential; parallel circuits are temporally parallel.