Fathomline

About Fathomline

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.