calculate disk usage percentage to match df Use% output

Question

I am trying to calculate and display in percentage the current disk usage with this:

use nix::sys::statvfs::statvfs;

macro_rules! cast {
    ($x:expr) => {
        u64::from($x)
    };
}

fn main() {
    let stat = statvfs("/var/db".as_bytes()).unwrap();
    // f_frsize
    // let total_space = cast!(stat.fragment_size()) * cast!(stat.blocks());
    // let avail_space = cast!(stat.fragment_size()) * cast!(stat.blocks_available());
    // f_bsize
    let total_space = cast!(stat.block_size()) * cast!(stat.blocks());
    let avail_space = cast!(stat.block_size()) * cast!(stat.blocks_available());
    let used = total_space - avail_space;
    let usage = used * 100 / total_space;
    println!("total: {}", total_space);
    println!("avail: {}", avail_space);
    println!("used:  {}", used);
    println!("use%:  {}%", usage);
}

The output on my current test system is:

total: 6779424768
avail: 3697811456
used:  3081613312
use%:  45%

Output of df -B 1 /var/db

# df -B 1 /var/db
Filesystem      1B-blocks       Used  Available Use% Mounted on
/dev/loop3     6779424768 2713636864 3697811456  43% /var/db

Used field not matching 2713636864 I am calculating it by doing 6779424768-3697811456 = 3081613312

Output of df -h /var/db:

# df -h /var/db
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop3      6.4G  2.6G  3.5G  43% /var/db

I think I am missing something probably converting the units, I tried already using f_frsize instead of f_bsize and result still the same, the code above returns 45 and df 43.

Any ideas?