Manhattan street addresses follow an algorithm that allows anyone with rudimentary math skills and the following charts to find the nearest cross street for a house number on an avenue.

Take the number of the street address, drop the last digit, divide by two (essentially dividing the address by 20 but accounting for odd numbers), and add or subtract the number in the chart. For example, 945 Madison Avenue (Whitney Museum of American Art, by the way) would yield the following equation:

945 / 2 = 47 + 27 (from the chart below) = 74th Street.
*The entrance is actually closer to 75th Street, but the museum takes up the whole block so it is just as close to 74th Street.

Avenues A, B, C, D: add 3
5th Avenue

• 63 - 108: add 11
• 109 - 200: add 13
• 201 - 400: add 16
• 401 - 600: add 18
• 601 - 775: add 20
• 776 - 1286: divide by 10 (instead of 20), subtract 18
• 1501 - 2000: add 24
Avenue of the Americas (6th Avenue): subtract 12
7th Avenue
• 1 - 1800: add 12
• 1801 and up: add 20
• 1-754: unnumbered streets, south of 8th Street
• 756 - 846: subtract 29
• 847 - 953: subtract 25
• above 953: subtract 31
Central Park West: divide by 10 (not 20), add 60