In DayZ there are several ways to estimate the range between the player and an object within their sight, most of which work just as in real life.
The simplest one is the Rangefinder which directly tells the distance in meters between the player and the object in the center of the reticule.
Another method of estimating range requires a Map or GPS, and for the player to know their exact location on the map and the location of a landmark which can be any immoveable object with a distinct appearance. Open the map and zoom in until the smaller square grids are visible. Distance can be gauged by calculating the number of grid square between two points. Each of the smaller grid squares is 100 × 100 meters.
If way points are enabled on the server, simply set a way point on the map to match the location of the target, close the map, and the way point and distance will be visible on the player's heads-up display (HUD).
The GPS helps the player know their location to a higher accuracy than the map as it displays their grid coordinates among other information. Using default controls "ctrl+m" opens the GPS on their position on the HUD similar to a minimap. A Map is not required, a GPS has its own minimap built-in.
If the player has an SVD Camo or a scoped rifle with a similar reticle, they can use the built-in rangerfinger to gauge distance to a standing or couching player, or object of similar height, from 200 to 1000 meters (A player is 1.8 meters tall standing.) The rangerfinder is used by placing the target's lowest section (usually the feet) onto the baseline of the rangerfinger (horizontal line) and the targets highest section (the head) onto the curved line. Read the distance.
- (distance to object in distance-unit) = [(height in distance-unit) ÷ (height in mildots)] × 1000
- (distance to object in distance-unit) = [1.8m ÷ 2 mildots] × 1000
- (distance to object in distance-unit) = 900 meters
This holds true for distance-unit being anything, meters or yards, metric or imperial.
A quick way to do this in-game for standing players is to use 2 meters as height (instead of the "real" 1.8 meters) and then taking off 10% (0.1) away from the distance estimated through that, as it is somewhat easier this way. Note that the first level of zoom on the DMR does not work this way because the dots on the scope are in the second focal plane of the scope and do not get magnified when going into the second zoom level.
Similar methods as the one in the last paragraph exist for basically anything scoped, even for Binoculars but the heights of indicators in the scope, or even just field of view, have to be tested out, relative to mildots. An example would be the DMR in first zoomlevel, which would use the upper formula times (magnification in 2nd zoomlevel)/(magnification in 1st zoomlevel).