Decimal degrees to degrees decimal minutes (DDM)

About this conversion

DDM (degrees decimal minutes) keeps whole degrees but writes minutes as a decimal (48°51.5'), avoiding seconds. It's the default on many chartplotters and marine/aviation GPS units.

To convert from decimal degrees: keep the whole degrees, and multiply the fractional part by 60 to get decimal minutes.

How to use it

1. Enter your latitude and longitude above (any of decimal degrees, DMS or DDM, auto-detected).
2. Read the highlighted result and click Copy. Every other format is shown too.
3. Nothing is uploaded, it all runs in your browser.

How decimal degrees to DDM works

Degrees decimal minutes keeps the whole degrees and writes everything smaller as a decimal number of minutes. The degrees are the whole-number part; multiply the fraction by 60 to get the decimal minutes, and stop there, no seconds. For 48.85833: degrees = 48, and 0.85833 × 60 = 51.5, giving 48°51.5'. It sits halfway between decimal degrees and full DMS.

As always the sign maps to a hemisphere letter (S for negative latitude, W for negative longitude). DDM is easy to confuse with DMS at a glance: in 48°51.5' the 51.5 is minutes-and-a-fraction, whereas in DMS you would see whole minutes and separate seconds.

Worked examples

A straightforward conversion

For 48.8584: keep 48°, then multiply the fraction 0.8584 by 60 to get 51.504 minutes. The result is 48°51.504'N. DDM stops here, it never splits the minutes into seconds.

A southern coordinate

For -33.8688 (a latitude): use the magnitude 33.8688. Degrees 33, minutes 0.8688 × 60 = 52.128. The negative sign means south, so the DDM is 33°52.128'S. As always, the sign becomes the hemisphere letter at the end.

Marine-style precision

Chartplotters usually show three decimals of minutes, e.g. 48°51.504'. Three decimal minutes is about two metres, plenty for navigation, so there is no need to carry more.

Precision and rounding

The decimal minutes carry the precision. Three decimal places of minutes is about 1.8 m, which is why GPS receivers that use DDM typically show three decimals (for example N48°51.500'). Two decimals is roughly 18 m. Keep three for navigation-grade accuracy.

A closer look at the numbers

DDM sits exactly halfway between decimal degrees and DMS: it keeps the whole degree and writes everything below it as a single decimal number of minutes. Because one minute is 1/60 of a degree, the decimal part of your degrees simply gets multiplied by 60. A decimal of 0.5° is 30 minutes, 0.25° is 15 minutes, 0.1° is 6 minutes. There is no seconds step at all, which is what makes DDM quick to read on a moving boat or aircraft.

Reading the result back

To read a DDM value back into decimal degrees, divide the minutes by 60 and add them to the degrees: 48°51.504' is 48 + 51.504/60 = 48.8584. Keep the hemisphere letter, it becomes the minus sign for south and west. A DDM waypoint from a chartplotter goes back into GIS this way, then straight onto a map with the spreadsheet to map converter.

Quick reference

A few well-known places, converted, so you can sanity-check your own numbers against them.

Accepted input formats

Paste a decimal-degrees pair such as 48.8584, 2.2945, or type latitude and longitude separately. The result is degrees decimal minutes, the format most marine chartplotters and aviation GPS units use, ready to key straight into the device.

You do not have to reformat anything first. The box detects the notation from what you paste, so every one of these is understood:

The hemisphere is taken from an N, S, E or W letter anywhere in the value, or from a leading minus sign. If both are missing, a positive number is assumed north or east.

Where you will use it

DDM is the working format of marine and aviation GPS: chartplotters, handheld marine units and flight computers almost all display position as degrees and decimal minutes. If you are transcribing a waypoint from a boat's GPS or an aeronautical chart, it is likely in DDM, and you convert it to decimal degrees to use it in GIS or a web map.

Why the format works this way

Decimal degrees minutes is a compromise that suits how navigators actually work. On a nautical chart, one minute of latitude is defined as one nautical mile, so reading position in minutes lets a sailor measure distance straight off the latitude scale with a pair of dividers, no conversion needed. Seconds add precision a moving vessel rarely needs and are fiddly to read at a glance, so marine and aviation electronics dropped them and kept a decimal on the minutes instead. That is why a chartplotter shows N48°51.500' rather than N48°51'30" or N48.8583°: it is the notation that lines up with the chart and the instruments in front of you.

What to do next

DDM is what you key into a chartplotter or aviation GPS. If your device wants full degrees minutes seconds instead, use decimal degrees to DMS; to bring a DDM waypoint back into GIS, convert it with DMS to decimal degrees (it reads DDM too) and then map it via the spreadsheet to map converter.

Common mistakes to avoid

• Treating DDM like DMS. 48°51.5' is decimal minutes, not 51 minutes 5 seconds. There are no seconds in DDM.
• Rounding minutes to whole numbers. Dropping the decimal part of the minutes loses up to a nautical mile; keep at least two decimals.
• Losing the hemisphere. Marine coordinates always carry N/S and E/W; keep the letter when you enter the device.

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