A coordinate pair such as 25.4687, -80.4776 pins down one spot on Earth. The first number is latitude, the second is longitude. Once you know how to read them, you can check a delivery location, fill in a form that asks for coordinates, or confirm that a map pin is where you think it is.
Latitude: north or south
Latitude measures how far north or south of the equator a point is, from 0 at the equator to 90 at each pole. Positive numbers are in the Northern Hemisphere, negative numbers in the Southern Hemisphere. Everywhere in the United States has a positive latitude. The lines of equal latitude run east to west and are evenly spaced: one degree of latitude is about 69 miles, and that distance stays nearly constant from the equator to the poles [1].
Longitude: east or west
Longitude measures how far east or west of the prime meridian, the line through Greenwich, England, a point is, from 0 to 180. Positive numbers are east, negative numbers are west. The whole mainland United States is west of Greenwich, so its longitudes are negative. If a form shows a U.S. longitude without a minus sign, something has gone wrong and the point is in Asia.
Unlike latitude lines, longitude lines meet at the poles, so a degree of longitude shrinks as you move away from the equator. At 38 degrees north, roughly the latitude of Stockton, California or Charlottesville, Virginia, one degree of longitude covers about 54.6 miles, compared with about 69 miles for a degree of latitude [1]. In South Florida it is a little wider; in Alaska, much narrower.
Decimal degrees versus degrees, minutes and seconds
Modern tools use decimal degrees: 25.4687. Older maps, deeds and GPS devices may use degrees, minutes and seconds: 25° 28' 7" N. They describe the same point. A minute is one sixtieth of a degree and a second is one sixtieth of a minute, so to convert, divide minutes by 60 and seconds by 3,600 and add them to the degrees. For 25° 28' 7": 25 + 28/60 + 7/3600 = 25.4686. Going the other way, the whole number is the degrees, the decimal part times 60 gives the minutes, and the remainder times 60 gives the seconds.
How many decimal places matter
Because a degree of latitude is about 69 miles, or about 111 kilometers, each decimal place narrows the position by a factor of ten [1]. One decimal place (25.5) places you within about 11 kilometers, a town. Three places (25.469) is about 110 meters, a block. Four places (25.4687) is about 11 meters, a building. Five places is about a meter. Anything beyond six places is finer than ordinary GPS can measure and usually just reflects how a program stored the number, not real precision.
Why an address lookup is approximate
Turning a street address into coordinates is called geocoding. The U.S. Census Bureau runs a free public geocoder that does this for addresses in the United States, Puerto Rico and the U.S. Island Areas [2]. It works from address ranges: the Census Bureau knows that, say, numbers 100 to 198 on the even side of a block run between two intersections, and it places your house number proportionally along that stretch. The result is usually on the right block and often within a few houses, but it is not a rooftop position, and it can be off where house numbers are irregular or a street has been renumbered.
That is good enough for most uses: checking which side of a boundary a property falls on, filling in a coordinate field, or getting a map centered on the right block. It is not good enough for emergencies, surveying or anything where a few dozen feet matter. For those, use a GPS reading taken on site or a surveyor's measurement.
Finding coordinates for a U.S. address
- Open the Everyday Tools address to coordinates tool and leave Look up set to Address → coordinates.
- Type the full street address, including city and state, in U.S. street address. A ZIP code helps when a street name exists in more than one town.
- Press Find coordinates. The tool sends the address from its own server to the Census geocoder, so the geocoder never sees your IP address, and it shows the matched address alongside the latitude and longitude.
- Check that the matched address is the one you meant. If the geocoder matched a different street or town, add more detail and search again.
The tool can also run in reverse: give it a latitude and longitude and it returns the nearest mapped address or place from OpenStreetMap data, which covers locations worldwide where the map is complete. As with forward lookups, treat the answer as a nearby reference point, not an exact address.