What is degrees decimal minutes (DDM)?

DDM expresses a coordinate as whole degrees plus minutes carried to decimals, for example 51 30.500 N. It is the standard format on GPS units, marine charts, and aviation, sitting between full decimal degrees and the older degrees-minutes-seconds notation.

How do I convert DD to DDM?

Take the absolute value of the decimal degree. The whole-number part is the degrees. Multiply the fractional part by 60 to get the decimal minutes. The sign or hemisphere comes from whether the original value was positive or negative.

How do I convert DDM back to decimal degrees?

Divide the decimal minutes by 60 and add the result to the whole degrees. Then apply the hemisphere: south latitudes and west longitudes are negative in decimal degrees.

What sign conventions does this use?

North latitude and east longitude are positive. South and west are negative. The tool shows the hemisphere letter in DDM output and uses the sign in decimal output.

How precise should decimal minutes be?

Three decimal places of a minute is about 1.8 metres of latitude, which is fine for most navigation. The tool keeps enough precision that a round trip back to decimal degrees reproduces your input closely.

What is the Decimal Degrees ↔ Degrees Decimal Minutes?

Convert latitude and longitude between decimal degrees (DD) and degrees decimal minutes (DDM), the GPS and marine navigation format. Handles hemisphere signs and rounding both ways. Runs entirely in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Decimal Degrees ↔ Degrees Decimal Minutes

Name: Decimal Degrees ↔ Degrees Decimal Minutes
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Decimal degrees and degrees-decimal-minutes (DDM) are the two coordinate formats you meet most often in modern navigation. GPS receivers, Garmin units, marine chartplotters, and VHF radio reporting use DDM. Mapping APIs, spreadsheets, and web services such as Google Maps use plain decimal degrees. Converting between them is routine when you move data between devices or systems.

The three coordinate formats compared

It helps to understand where DDM sits among the three common notations:

Format	Example (London, roughly)	Common use
Decimal degrees (DD)	51.5074° N, 0.1278° W	APIs, databases, spreadsheets
Degrees decimal minutes (DDM)	51° 30.444’ N, 0° 7.668’ W	GPS units, marine charts, aviation
Degrees minutes seconds (DMS)	51° 30’ 26.6” N, 0° 7’ 40.1” W	Survey maps, ordnance data, older charts

DDM is a middle ground: it keeps whole degrees as integers (convenient for compass work) but expresses the sub-degree portion as decimal minutes rather than minutes-and-seconds. Most recreational GPS devices default to DDM because it is easier to read at a glance than DMS while being more precise than whole degrees.

How the conversion works

The arithmetic is exact — there is no approximation involved, only display rounding.

Decimal degrees → DDM:

degrees    = floor(abs(DD))
minutes    = (abs(DD) - degrees) × 60
hemisphere = DD < 0 ? (S or W) : (N or E)

DDM → decimal degrees:

DD = degrees + (minutes / 60)
DD = (hemisphere is S or W) ? -DD : +DD

Latitude uses N (positive) and S (negative). Longitude uses E (positive) and W (negative).

Precision: how many decimal places on the minutes?

Decimal places on minutes	Approximate ground resolution
0 (whole minutes)	~1.85 km
1 (`30.5'`)	~185 m
2 (`30.50'`)	~18.5 m
3 (`30.500'`)	~1.85 m
4 (`30.5000'`)	~0.18 m

Three decimal places gives roughly two-metre precision on latitude, which is more than adequate for marine navigation, hiking GPS, and general surveying. When entering coordinates into a chartplotter or GPS device, match the precision shown on the chart or waypoint list you are working from.

Worked example

Decimal degree coordinate: 51.50833 N

degrees = 51
minutes = (51.50833 - 51) × 60 = 0.50833 × 60 = 30.4998 ≈ 30.500
result  = 51° 30.500' N

Converting back: 51 + (30.500 / 60) = 51 + 0.50833 = 51.50833 — the original value is restored.

Nothing is uploaded; the conversion runs entirely in your browser.