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Three ways to write the same place
One place can be written three ways, and every map, GPS device, and guidebook seems to pick a different one. Decimal degrees write a place as 51.5074, -0.1278. Degrees, minutes, and seconds write it as 51 degrees, 30 minutes, 26.64 seconds north. Degrees and decimal minutes write it as 51 degrees, 30.444 minutes north. All three point at the same spot, but typing one format into a device that expects another sends you somewhere else entirely.
This converter translates between all three formats, in both directions, with hemisphere handling. It is a small tool that prevents a large mistake, because a coordinate error of one degree is about 111 kilometres, and even a one-minute error is nearly 2 kilometres.
How to use the converter
To convert from decimal, enter the decimal latitude and longitude and press the button. The tool shows the DMS and DDM forms for both. Latitude uses north and south, longitude uses east and west, and negative numbers become south or west automatically.
To convert back, use the DMS section or the DDM section. Enter degrees, minutes, and seconds with the hemisphere, or degrees and decimal minutes with the hemisphere, and the tool gives the decimal value you can paste into a map or a GPS. Check the hemisphere twice: it is the most common source of errors, and it flips your position to the other side of the equator or the prime meridian.
How the maths works
The system is base sixty, inherited from ancient astronomy. One degree holds 60 minutes, and one minute holds 60 seconds. To go from DMS to decimal, divide minutes by 60 and seconds by 3600 and add them to the degrees. To go from decimal to DMS, take the whole degrees, multiply the remainder by 60 for minutes, and multiply the new remainder by 60 for seconds.
DDM sits in the middle: degrees plus decimal minutes, where the minutes are the remainder times 60, kept as a decimal. The hemisphere sets the sign: north and east are positive, south and west are negative. A longitude of 0 degrees 7 minutes 39 seconds west is negative 0.1275 degrees, not positive.
A worked example
London’s Big Ben sits near 51.5074 north, 0.1278 west. In DMS that is 51 degrees, 30 minutes, 26.64 seconds north, and 0 degrees, 7 minutes, 40.08 seconds west. In DDM it is 51 degrees, 30.444 minutes north, and 0 degrees, 7.668 minutes west. Paste any of the three into a modern map and you land on the same building, which is the test that the conversion worked.
Planning tips
When copying coordinates from a guidebook, copy the hemisphere letters as carefully as the numbers. A missing minus sign or a swapped N and S is invisible until you arrive in the wrong place. When sharing a location with a rescue service or a group, give the decimal form and the DMS form together, so a transcription error in one can be caught against the other.
Check precision against your need. Six decimal places pin a spot to about 10 centimetres, which is far more than a hiker needs. Four decimal places give about 11 metres, which is plenty for a trailhead. Seconds to two decimal places are the matching precision in DMS, and there is no benefit in copying more digits than your device can use.
Frequently asked questions
Why do old maps use DMS? Because DMS predates decimal notation and matches the way angles were measured with instruments divided into minutes and seconds. Modern devices prefer decimal because computers handle decimals more easily. Both describe the same angles.
What is the difference between DDM and DMS? DDM keeps minutes as a decimal number, while DMS splits minutes into whole minutes and seconds. DDM is common on marine GPS units, DMS on paper maps and older devices, and decimal on most phone maps.