Boot Time Calculator
Break down system boot time across firmware, kernel and systemd unit startup phases.
About Boot Time Calculator
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
Worked example
With the default values below, Boot Time Calculator returns Total Boot Time of 13.40. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Kernel Timeminimum 0 seconds
- 3.2 seconds
- Initrd Timeminimum 0 seconds
- 1.5 seconds
- Userspace Timeminimum 0 seconds
- 8.7 seconds
Results
- 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%
Step by step
- 1Values used
- 2Total boot time
- 3Total Boot Time= 13.40 seconds
- 4Slowest Phase= Userspace (systemd unit startup)
- 5Kernel Phase Share= 23.9
- 6Initrd Phase Share= 11.2
- 7Userspace Phase Share= 64.9
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.