Arduino millis()/micros() Rollover Calculator

About this tool

Both millis() and micros() return an unsigned long that overflows back to 0 after it fills up — this tool shows exactly when that happens, and lets you check whether a specific "current value + interval" calculation crosses that rollover point.

The good news: the standard non-blocking timing pattern — if (currentMillis - previousMillis >= interval) — keeps working correctly across a rollover, because unsigned integer subtraction wraps around too, giving the right answer automatically. You almost never need to handle rollover specially if you use subtraction (not > comparisons) against a target value.

Frequently asked questions

Do I need to write special rollover-handling code?

Usually not — as long as you compute elapsed time as currentMillis - previousMillis (both unsigned) and compare that difference to your interval, unsigned wraparound arithmetic makes the subtraction correct even when millis() has rolled over in between.

Why does micros() roll over so much sooner than millis()?

Both are 32-bit counters with the same maximum value (about 4.29 billion), but micros() counts 1,000× faster (microseconds vs milliseconds), so it fills up 1,000× sooner — about 71.6 minutes instead of about 49.7 days.

Is this different on ESP32 or other non-AVR boards?

The core Arduino API still returns a 32-bit unsigned long from millis()/micros() on essentially all boards for API compatibility, so the same 32-bit rollover math shown here applies even on 32-bit or 64-bit-capable chips like ESP32.