CalcTabs
Our own calculator app: every tab is a separate calculator with its own history — and a tab can hold a reusable formula instead of a running sum.
The brief
Phone calculators are built for one sum at a time. Start a second calculation halfway through the first and the first is gone; anything you work out regularly — a percentage, a markup, a split — gets typed in from scratch every time, because the calculator remembers nothing about what you were doing.
CalcTabs is our own small product, built on an idea the web browser already proved: tabs. Each tab is its own calculator with its own name, colour and history, and a tab can hold a formula instead of a running sum. The brief was to keep it a calculator — quick, offline, nothing to sign up for — and let the tabs carry the extra work.
The details
The calculator shows the answer while you type, which means evaluating an expression that is unfinished most of the time. Every keypress re-parses the whole line; if it doesn’t parse yet — an open bracket, a trailing operator — the last good answer simply stays put, and only = writes to the tab’s history or reports an error. The keypad speaks in the symbols people know (√, π, sin⁻¹, %, mod) and the expression library speaks its own grammar, so a small translation layer sits between them: π and e become bound constants, log becomes log base 10, % becomes ×0.01, and a multiplication sign is slipped in after a bracket, π, e, ! or %, because the parser won’t infer one. Trigonometry works in radians only for now — the degrees switch was left out rather than shipped half-built.
Custom formula tabs are what the tabs are really for. A formula is written once with up to four variables, V1 to V4, then reused by typing new values into four boxes, while the result recalculates on every digit. The variables are bound into the evaluation rather than pasted into the text, so the formula is stored exactly as it was written, and a box that is empty or half-typed counts as zero instead of breaking the result. A first launch opens with one tab of each kind already set up — a percentage formula and a plain calculator — so the idea explains itself.
Everything lives on the phone. Each tab is a single row in an on-device ObjectBox database — name, colour, type, formula, variables and history — written on every change, so closing the app halfway through a sum loses nothing, and tabs reopen in order of last use. There is no account and no server of ours. The app is free with one banner ad, shown only after Google’s consent form where consent is required, and a single one-time Premium purchase removes it for good — no subscription.
The outcome
CalcTabs is free on Google Play and on the App Store for iPhone and iPad. The version on Google Play dates from 27 September 2023, eleven days after the first commit, and it works entirely offline — the only network traffic is the ad banner and the purchase check.