On Termux, /data is a real mount, so df from coreutils works. For machine-readable output in bytes:

df -B1 --output=avail /data | tail -n 1

This prints just the available space in bytes (e.g. 48373227520), with no header or units.

Alternatives:

# Available KiB, single number
df -k /data | tail -n 1 | awk '{print $4}'

# stat-based: free blocks × block size, computed to bytes
stat -f -c '%a * %S' /data | bc

Notes:

• --output=avail avoids locale/parsing issues since you select exactly one column; tail -n 1 drops the header line.
• Prefer avail (available to a regular user) over free (--output=itotal-style free column), since the system reserves some blocks.
• If you want the total/used/avail triple: df -B1 --output=size,used,avail /data | tail -n 1.