Marine Mammal Observers mitigate for the potential impact of sound exposure on marine fauna during seismic surveys, UXO clearance or civil engineering projects. A fundamental function of the observer is to calculate the distance of an animal to the source of noise irrespective of their own position and recommend actions based on specified mitigation zones around the source of the sound. Depending on the survey type and in-water configuration, MMOs can be located 100's of metres from the centre of the source. This creates time-consuming complications when estimating where the animal is in relation to the mitigation zone.This app calculates the distance from the animal (Target) to the origin of the sound (Source) using a trigonometric cosine function. The user simply enters the distance and bearing to the Target and Source from their observation position and the app calculates the rest. The app also implements a reticule converter. This applies spherical trigonometry* per the formulae presented by Lerczack and Hobbs (1998). Specify the MILs per reticule from your binocular manufacturers guidelines, set your eye-heights then simply enter your reticule measurement in the Target Distance field and hit the reticule button.*earth as a perfect sphere with a mean radius of 6,371,000m.The MMO can also enter all data manually using either relative or true bearings (in degrees) and distances (in metres) which they have estimated using standard survey methodology. The MMO can also use their devices in-built compass to automatically fill out bearing data by simply pointing towards the Target or Source and clicking the relevant "Fix" buttons**.**Compass interference, particularly onboard vessels with lots of electronics and metals may be significant and differ in sensitivity between devices. Therefore the user should verify compass bearings against a calibrated compass onboard their vessel. Some android devices are particularly sensitive to pitch and roll orientation so best results will be achieved when the device is on a flat surface.When Location Services are enabled and the device has good GPS satellite connectivity the app displays the users current position, speed over ground (SOG) and course made good (or Course over Ground; COG).MMO Range Finder App should be used as a reference tool and is only as precise as the users ability to estimate range. This is not a replacement for experience and training in the field. Sea state and other factors may induce errors in calculations that may increase exponentially by distance to the animal. Any decision making is the responsibility of the user. If in use, the compass and location should be verified often. Raw values returned by the digital compass component of the orientation sensor are relative to magnetic north only.