Practice mode — tap any bead to slide it. Heaven beads (above the bar) are worth 5; earth beads (below the bar) are worth 1. A bead "counts" only when pushed against the bar. Each rod is a decimal place — leftmost is trillions, rightmost is ones. The dot under a rod marks the unit rod. Tap a digit on the pad to push it in from the right; tap ← to shift back; tap C to clear the abacus.
Calculator mode — type an expression (e.g. 137 × 89) using the pad, then tap =. The beads animate to the result. Integer arithmetic; division truncates. Switch back to practice anytime to manipulate the result by hand.
Roman numerals use the standard subtractive notation (IV, IX, XL, etc.) up to 3,999. Above that, single-overlined groups represent thousands (V̄ = 5,000), double-overlined represent millions, triple-overlined represent billions — the same vinculum convention Romans actually used.
The soroban (Japanese 4+1 abacus, refined from the Chinese suanpan) is a 2,000-year-old mechanical computer. A trained operator can add long columns of numbers faster than someone with a calculator, because the answer is already half-formed by the bead positions during entry. This is a faithful digital rendering — the mechanics are exactly what a physical soroban does.
The irony: this calculator app contains an abacus simulating a calculator running on a calculator (your phone) that contains roughly a billion abacuses' worth of computational power. Use it to add 2 + 2.