Error: cannot parse 'disk_size' as uint (Packer)

Error: cannot parse 'disk_size' as uint: strconv.ParseUint: parsing "5G": invalid syntax (Packer)

TL;DR

disk_size takes a bare number in MiB, not a human-readable size. Change "5G" to "5120".

The error

Build 'ubuntu-aarch64' prepare failure: 1 error(s) decoding:

* cannot parse 'disk_size' as uint: strconv.ParseUint: parsing "5G": invalid syntax

Fix it

1. Find the disk_size (or your build_disk_size variable) in the template.

   • Success check: you see the quoted "5G" or similar suffix.

2. Replace with mebibytes as a bare number: 5 GiB becomes 5120, 20 GiB becomes 20480.

   • Success check: the value is digits only (quotes are fine; the suffix is the problem).

3. Run packer validate.

   • Success check: the decoding error is gone.

4. Re-run packer build.

   • Success check: the build prepares and proceeds.

When to use this

You hit this at prepare/validate time with a disk_size carrying a unit suffix.

When NOT to use this

Do not use this for disk space errors during provisioning (guest ran out of room; increase the size) or for cloud block-device size limits.

Compatibility

Packer 1.x, qemu and other builders with numeric disk_size.

Variants

• cannot parse '[other-field]' as uint for any numeric field given a suffixed string
• "5GB", "5120M", "20g" all fail the same way

Root cause

The field decodes as an unsigned int in MiB. Go's ParseUint rejects anything but digits, so unit suffixes fail at config decode time, before the build starts.

Edge cases

• Some builders document the unit as MiB explicitly; others just say "size". When in doubt, assume MiB.
• Template variables pass the string through; fix the variable default or the -var value, not just the template.