Fathomline

About this app

Made for a clear purpose.

Fathomline is a beautiful, private dive logbook and study companion that works completely offline. No account, no subscription, no ads - buy it once and your dives stay on your phone, forever.

Whether you're a new Open Water diver building your first pages or a divemaster with hundreds of dives, Fathomline keeps your logbook fast, gorgeous and yours.

LOG EVERY DIVE • Site, location, dive type, water and conditions • Max and average depth, bottom time, water and air temperature, visibility and current • Gas mix (air or nitrox), cylinder, start/end pressure and weighting • Buddy, dive centre, a 5-star rating, tags and free-form notes • Carry over a paper-log total so your grand total is always right

KNOW YOUR NUMBERS • Automatic SAC and RMV (your true breathing rate) for every dive with two pressures • A clean Stats screen: totals, deepest, longest, distinct sites and countries • Depth distribution, dives by year, dive-type breakdown and your top sites • An RMV trend chart so you can watch your gas efficiency improve

OFFLINE STUDY TOOLS • Nitrox calculator: MOD, EAD, best mix and ppO₂ at any depth, plus a full EAN21–EAN40 reference table • Gas-planning aids: rule of thirds, half-plus-reserve turn pressure and a runtime estimate • Unit converter for depth, pressure, temperature and gas volume • Switch the whole app between metric and imperial any time

A POCKET DIVE LIBRARY • 50+ marine-life ID cards - what it is, how to identify it, and how to behave around it • 140+ term dive glossary, from "bottom time" to "thermocline" • 30+ underwater hand signals grouped by purpose • Skills & drills refreshers, dive-type guides and equipment-care schedules • Plain-English safety and best-practice summaries

YOUR DATA, YOUR DEVICE • 100% offline - works in airplane mode, on the boat, anywhere • No account, no login, no cloud, no tracking, nothing leaves your phone • Export your whole logbook as a text report or CSV spreadsheet whenever you like • One-time purchase, every feature included

Fathomline is a personal logbook and educational reference - not a dive planner, dive computer, or decompression tool. Always dive within your training and certification, use a dive computer and/or tables, plan conservatively, and follow your certifying agency's guidelines.

Dive safe. Log beautifully. Keep your data.