Boot Time Calculator
Break down total system boot time across kernel, initrd and userspace/systemd startup phases from systemd-analyze figures.
Inputs
Time from bootloader handoff to kernel finishing its own initialization
Time spent in the initial ramdisk before switching to the real root filesystem
Time for systemd to reach the default target, starting all units
Total Boot Time
13.40seconds
Slowest Phase
Userspace (systemd unit startup)
Kernel Phase Share
23.9%
Initrd Phase Share
11.2%
Userspace Phase Share
64.9%
Step by step
Values used
Kernel Time = 3.20 seconds; Initrd Time = 1.50 seconds; Userspace Time = 8.70 seconds
Total boot time
total = kernel_time + initrd_time + userspace_time
Total Boot Time
= 13.40 seconds
Slowest Phase
= Userspace (systemd unit startup)
Kernel Phase Share
= 23.9
Initrd Phase Share
= 11.2
Userspace Phase Share
= 64.9
How it works
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.
Formula
Total boot time
total = kernel_time + initrd_time + userspace_time
- K
- kernel initialization time
- I
- initrd time
- U
- userspace/systemd startup time
Frequently Asked Questions
How do I get these timing figures on my own system?
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.
What usually causes a slow userspace boot phase?
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.
Does this calculator include firmware/BIOS boot time?
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.