Smartwatch Battery Life Calculator

This calculator builds a milliwatt-level power budget for your smartwatch rather than pulling a single "battery life" number from a spec sheet. It converts your battery's mAh rating into watt-hours using a 3.85V nominal Li-Po voltage, then adds up the average continuous draw from every subsystem: a processor idle baseline (~8mW), the display (weighted by type — AMOLED, LCD, or MIP/transflective — brightness, and how much of the day it's actively on versus showing an always-on face), GPS (75mW while active, averaged across your daily usage minutes), heart rate monitoring (off, periodic, or continuous), a constant Bluetooth radio floor plus a small per-notification bump for the ~2 seconds of active transfer each one triggers, an optional LTE cellular allowance, and the vibration motor's brief per-notification pulse. Dividing total battery watt-hours by this blended average power draw gives battery life in hours and days, and the same model runs a second time under workout conditions (continuous GPS and HR, less screen dimming) and a GPS-only scenario to bound your outdoor tracking time.

The biggest lever by far is always-on display and screen-on time — AMOLED's efficiency advantage nearly disappears once AOD or bright, frequent wake-ups are factored in, while MIP/transflective displays (as used in many Garmin watches) sip power by comparison. GPS is the next biggest swing: even modest daily GPS minutes materially shorten the days-of-battery figure because its 75mW draw dwarfs everything else running in the background. Keep in mind this is a physics-based average, not a manufacturer-tested rating — real devices vary with radio strength, temperature, battery age and chemistry degradation, and firmware power management, so use these numbers to compare settings and habits against each other rather than as a guaranteed runtime.