Break down system boot time across firmware, kernel and systemd unit startup phases.
systemd-analyze breaks a Linux boot into three sequential phases: kernel initialization (from bootloader handoff until the kernel is ready), initrd (mounting and preparing the real root filesystem from the initial ramdisk), and userspace (systemd bringing all configured units up to the default target). Total boot time is simply the sum of these three phases, and identifying which phase consumes the largest share tells you where to focus optimization — a slow userspace phase points to bloated or serially-dependent systemd units (diagnosed with systemd-analyze blame and systemd-analyze critical-chain), while slow kernel/initrd phases usually point to firmware, driver probing, or disk/initramfs issues.
Total boot time
total = kernel_time + initrd_time + userspace_time
Run `systemd-analyze` with no arguments — it prints a one-line summary breaking down firmware, loader, kernel, initrd and userspace time, which is the exact source for the kernel, initrd and userspace figures this calculator combines.
Common causes include services with long, serial startup dependency chains (visible via `systemd-analyze critical-chain`), units waiting on network availability before other services can proceed, or a single slow-starting service (identified with `systemd-analyze blame`) blocking everything that depends on it.
No — this covers only the OS-level phases systemd-analyze measures (kernel, initrd, userspace). Firmware/BIOS/UEFI POST time happens before the bootloader hands off to the kernel and is reported separately by systemd-analyze as 'firmware' time on UEFI systems with the right ACPI support.