Round number
Integer ending in zeroes
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Through Integer, Interval (mathematics) and Rounding
In decimal notation, a number ending in the digit "5" is also considered more round than one ending in another non-zero digit (but less round than any which ends with "0"). For example, the number 25 tends to be seen as more round than 24. Thus someone might say, upon turning 45, that their age is more round than when they turn 44 or 46. These notions of roundness are also often applied to non-integer numbers; so, in any given base, 2.3 is rounder than 2.297, because 2.3 can be written as 2.300. Thus, a number with fewer digits which are not trailing "0"s is considered to be rounder than others of the same or greater precision.
Numbers can also be considered "round" in numbering systems other than decimal (base 10). For example, the number 1024 would not be considered round in decimal, but the same number ends with a zero in several other numbering systems including binary (base 2: 10000000000), octal (base 8: 2000), and hexadecimal (base 16: 400). The previous discussion about the digit "5" generalizes to the digit representing b/2 for base-b notation, if b is even.
In b-adic number, two numbers are nearer if their difference is rounder (in base b).
Psychology and sociology
Psychologically, round numbers form waypoints in pricing and negotiation. So, starting salaries are usually round numbers. Prices are often pitched just below round numbers to avoid breaking the psychological barrier of paying the price of the round number.
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Culture
Round-number anniversaries are often especially celebrated. For example, a fiftieth birthday, the centenary of an event, or the millionth visitor or customer to a location or business.
On 1 January 2000, the round-number year 2000 was widely celebrated. Technically, according to some sources, the 3rd millennium did not begin until 1 January 2001, a year later, as there is no year zero in the Gregorian calendar. However, the astronomical year zero is −1 (1 BC) according to the latter.
Round number bias
Round number bias is the psychological tendency to prefer round numbers over others, which is passed onto a person through socialization. Round numbers are also easier for a person to remember, process, and perform mathematical operations on.
Round number bias has been observed mainly in retail and grocery, where prices are often just slightly less than a rounded number (ex. $9.99 or $9.95), in investments, including crowdfunding, in the real estate market through mortgages, and number milestones.
Round numbers are often used when estimating the time taken to complete a task.
Mathematics
A round number is mathematically defined as an integer which is the product of a considerable number of comparatively small factors as compared to its neighboring numbers, such as 24 = 2 × 2 × 2 × 3 (4 factors, as opposed to 3 factors for 27; 2 factors for 21, 22, 25, and 26; and 1 factor for 23).