2 minute read

Context

Most engineers know that there’s a difference between decimal and binary byte units.

Fewer engineers can confidently say:

  • which one a given system is using
  • when the distinction matters
  • when it’s safe to ignore

This post explains byte size units in the way that’s actually useful in practice—without turning it into a standards lecture.

The Two Systems You’ll Encounter

There are two byte size systems in common use:

Decimal (Base-10)

Used primarily for:

  • disk marketing
  • network throughput
  • vendor specifications
1 KB = 1,000 bytes
1 MB = 1,000,000 bytes
1 GB = 1,000,000,000 bytes
1 TB = 1,000,000,000,000 bytes

These scale cleanly by powers of 10.

Binary (Base-2)

Used primarily by:

  • operating systems
  • memory reporting
  • filesystems
  • low-level tooling
1 KiB = 1,024 bytes
1 MiB = 1,048,576 bytes
1 GiB = 1,073,741,824 bytes
1 TiB = 1,099,511,627,776 bytes

These scale by powers of 2.

Why the Names Look So Similar

The confusion comes from history.

For years, binary quantities were labeled using decimal names:

  • “KB” meant 1024 bytes
  • “MB” meant 1024² bytes

That shorthand stuck—long after it became misleading.

The IEC standard introduced:

  • KiB, MiB, GiB, TiB

Not to complicate things—but to be precise.

Where This Actually Matters

In practice, you’ll most often see:

  • Disks advertised in GB/TB (decimal)
  • Operating systems reporting GiB/TiB (binary)
  • Memory measured in GiB
  • Network speeds measured in Gb/s (decimal bits)

This is why a “1 TB disk” doesn’t show up as “1 TB” in your OS.

Nothing is missing. Nothing is broken.

The units changed.

A Practical Example

A disk advertised as 1 TB contains:

1,000,000,000,000 bytes

Your OS reports in GiB:

1,000,000,000,000 ÷ 1,073,741,824 ≈ 931 GiB

That ~7% difference is expected.

It’s not overhead. It’s arithmetic.

When You Should Care

You should pay attention to units when:

  • capacity planning
  • comparing vendor claims
  • sizing storage or memory limits
  • troubleshooting “missing” space
  • interpreting monitoring metrics

This is especially true in:

  • Kubernetes resource limits
  • cloud storage pricing
  • filesystem usage reports

When You Can Mostly Ignore It

You can often ignore the distinction when:

  • working at small scales
  • eyeballing approximate usage
  • doing relative comparisons within the same system

Just don’t mix unit systems mid-calculation.

Practical Guidance

A simple rule of thumb:

  • If it’s hardware, bandwidth, or marketing → decimal
  • If it’s an OS, memory, or filesystem → binary

When precision matters, check the unit label explicitly.

If the tool says GiB, believe it.

Why This Is Still Worth Knowing

This confusion persists because:

  • both systems are valid
  • both are widely used
  • tools are inconsistent about labeling

Understanding the distinction once prevents years of second-guessing.

It’s a small mental model with a long shelf life.

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