Ratios are unitless when they relate quantities of the same dimension. When the two quantities being compared are of different types, the units are the first quantity "per" unit of the second — for example, a speed or velocity can be expressed in "miles per hour". If the second unit is a measure of time, we call this type of ratio a rate.
A ratio can be written as two numbers(the terms) separated by a colon (:) which is read as the word "to". For example, a ratio of 2:3 ("two to three") means that the whole is made up of 2 parts of one thing and 3 parts of another — thus, the whole contains five parts in all. To be specific, if a basket contains 2 apples and 3 oranges, then the ratio of apples to oranges is 2:3. If another 2 apples and 3 oranges are added to the basket, then it will contain 4 apples and 6 oranges, resulting in a ratio of 4:6, which is equivalent to a ratio of 2:3 (thus ratios reduce like regular fractions). In this case, 2/5 or 40% of the fruit are apples and 3/5 or 60% are oranges in the basket.
Note that in the previous example the proportion of apples in the basket is 2/5 ("two of five" fruits, "two out of five" fruits, "two fifths" of the fruits, or 40% of the fruits). Thus a proportion compares part to whole instead of part to part.
Throughout the physical sciences, ratios of physical quantities are treated as real numbers. For example, the ratio of metres to 1 metre (say, the ratio of the circumference of a certain circle to its radius) is the real number . That is, m/1m = . Accordingly, the classical definition of measurement is the estimation of a ratio between a quantity and a unit of the same kind of quantity. (See also the article on commensurability in mathematics.)
- The ratio of heights of the Eiffel Tower (300 m) and the Great Pyramid of Giza (139 m) is 300:139, so one structure is more than twice the height of the other (more precisely, 2.16 times).
- The ratio of the mass of Jupiter to the mass of the Earth is approximately 318:1, meaning that Jupiter's mass is 318 times that of the earth.
- If two axles are connected by gear wheels, the number of times one axle turns for each turn of the other is known as the gear ratio, one familiar example of which is the number of turns of the pedals of a bicycle compared with number of turns of the rear wheel.
- The ratio of hydrogen atoms to oxygen in water (H2O) is 2:1, which means for every one oxygen atom, there would be two hydrogen atoms as well.
- Most movie theater screens have an aspect ratio of 16:9, which means that the screen is 16/9 as wide as it is high.
- In statistics, the ratio of the probability of something happening to the probability of it not happening is called the odds of the event happening.
- In music, the interval of a perfect fifth is formed by two pitches, or frequencies, at a ratio of 3:2, with the higher note being 1.5 times the frequency of the lower.