Real Disk Capacity Calculator: See Your True Hard Drive Size

Ever wonder why your new 1 Terabyte (TB) hard drive only shows 931 Gigabytes (GB) of usable space? You’re not alone, and your drive isn’t broken or faulty. Our calculator instantly shows you the real-world capacity your computer will report, and our detailed guide below explains the simple marketing vs. math reason for the difference.

Find out the actual usable space of your hard drive or SSD.

Advertised Capacity

Actual Capacity (in OS)

931.32GiB

"Lost" Capacity

68.68GiB

Actual Difference

Why is there a difference?

It comes down to a difference in how numbers are counted:

10
Manufacturers advertise capacity using the decimal system (base 10), where 1 Gigabyte (GB) = 1,000,000,000 Bytes.
2
Operating Systems (like Windows, macOS) measure capacity using the binary system (base 2), where 1 Gibibyte (GiB) = 1,073,741,824 Bytes.

Because a decimal "gigabyte" is smaller than a binary "gibibyte," the operating system reports a lower number for the same amount of space. This calculator shows you the real capacity you'll see on your computer.

How to Use Our Real Disk Capacity Calculator

Find out your drive’s actual usable space in two simple steps.

Advertised Capacity: Enter the storage capacity written on the product’s box or in its description (e.g., for a 2 TB drive, enter “2”).
Unit: Select the advertised unit from the dropdown menu, either Gigabytes (GB) or Terabytes (TB).

The calculator will instantly show you the “Real Capacity”—the space your operating system (like Windows) will actually display.

Understanding Your Results: The ‘Lost’ Space Explained

The difference between the advertised size and the real capacity isn’t due to lost space or a faulty drive. It all comes down to a simple math problem: manufacturers and computers count using two different number systems.

Manufacturers use the Decimal (Base-10) system. This is the system we use every day. “Kilo” means 1,000, “Mega” means 1 million, “Giga” means 1 billion. It’s simple and great for marketing.
Computers use the Binary (Base-2) system. Computers work with powers of 2. For them, a “Kilo” is not 1,000, but 1,024 ( $2^{10}$ ).

This creates a discrepancy that gets larger as the storage size increases. To clear this up, official scientific terms were created: a “Gigabyte” (GB) is the decimal term, while a “Gibibyte” (GiB) is the binary term.

Term (What Marketers Use)	Decimal (Base-10) Value	Term (What Computers Use)	Binary (Base-2) Value	The Difference
1 Kilobyte (KB)	1,000 Bytes	1 Kibibyte (KiB)	1,024 Bytes	2.4%
1 Megabyte (MB)	1,000,000 Bytes	1 Mebibyte (MiB)	1,048,576 Bytes	4.8%
1 Gigabyte (GB)	1,000,000,000 Bytes	1 Gibibyte (GiB)	1,073,741,824 Bytes	7.4%
1 Terabyte (TB)	1,000,000,000,000 Bytes	1 Tebibyte (TiB)	1,099,511,627,776 Bytes	9.9%

The 1 Terabyte Example

When you buy a 1 TB drive, you are getting 1 trillion bytes. The manufacturer calculates this as: $1 TB = 1, 000 GB = 1, 000, 000, 000, 000 Bytes$ .

But your Windows PC divides by 1024 to calculate the space: $1, 000, 000, 000, 000 Bytes \div 1024 \div 1024 \div 1024 = 931.32 GiB$ .

Your computer often incorrectly labels this as “GB,” causing the confusion. You have the full 1 trillion bytes you paid for; they are just being measured differently.

Common Hard Drive Sizes: Advertised vs. Real Capacity

Here’s a quick reference chart for the usable space you can expect from common drive sizes.

Advertised Capacity (Decimal)	Real Capacity (Binary – What You See)
250 GB	~ 232 GiB
500 GB	~ 465 GiB
1 TB	~ 931 GiB
2 TB	~ 1.81 TiB
4 TB	~ 3.63 TiB
8 TB	~ 7.27 TiB
16 TB	~ 14.55 TiB

Frequently Asked Questions

So, am I being scammed by hard drive companies?

No, you are not being scammed, but the practice is certainly confusing. This decimal measurement has been a standard in the storage industry for decades. Both the advertised number (e.g., 1,000,000,000,000 bytes) and the number your computer reports (931 GiB) are correct representations of the same amount of physical storage. The problem is the inconsistent labeling (“GB”) used by operating systems.

What’s the difference between a Gigabyte (GB) and a Gibibyte (GiB)?

A Gigabyte (GB) is a decimal unit equal to exactly 1 billion bytes. This is the unit used in marketing. A Gibibyte (GiB) is a binary unit equal to $102 4^{3}$ or 1,073,741,824 bytes. This is the unit your computer uses to actually measure the space. The difference is about 7.4%.

Does this measurement difference apply to all types of storage?

Yes. This discrepancy applies to virtually all modern storage media you can buy, including:

Traditional Hard Disk Drives (HDDs)
Solid-State Drives (SSDs)
USB Flash Drives (“thumb drives”)
Memory Cards (SD, microSD, etc.)
External Hard Drives

Is there other ‘lost’ space besides the measurement difference?

Yes. Even after accounting for the decimal vs. binary conversion, you will lose a little more space to a few things:

Formatting: When you format a drive, the file system (like NTFS or APFS) creates a “table of contents” to keep track of your files. This formatting overhead can consume several hundred megabytes.
Recovery Partitions: Most computers with pre-installed operating systems have a hidden recovery partition. This contains a copy of the OS for system restore purposes and can take up 10-20 GB or more.
The Operating System Itself: Your OS (like Windows 11 or macOS Sonoma) takes up a significant amount of space, often 20-30 GB or more, which is subtracted from your usable free space.

How can I calculate the real capacity myself without a calculator?

You can use a simple formula. Let ‘A’ be the advertised capacity.

From GB to GiB: Real Capacity = $(A \times 100 0^{3}) \div 102 4^{3}$
From TB to TiB: Real Capacity = $(A \times 100 0^{4}) \div 102 4^{4}$

For a quick estimate, you can use the following rule of thumb:

For GB, multiply by 0.9313. (e.g., $500 GB \times 0.9313 \approx 465 GiB$ )
For TB, multiply by 0.9095. (e.g., $4 TB \times 0.9095 \approx 3.63 TiB$ )

Why don’t operating systems just use the decimal system to avoid confusion?

This is due to historical reasons. The fundamental architecture of computers is built on binary (base-2) logic, so measuring storage in powers of 2 (1024) was the natural method for early engineers. While it would be less confusing to switch, changing decades of convention in software is difficult. Interestingly, some operating systems, like Apple’s macOS, have started displaying storage capacity in decimal units (true GB and TB) to match the marketing, which helps reduce confusion for their users. Windows, however, continues to use binary measurements (GiB, TiB) while often labeling them as GB and TB.

How much storage do I actually need for a new computer?

Light Use (256-512 GB): Ideal for students and users who primarily browse the web, use cloud applications (Google Docs, Office 365), and do some light photo storage.
Average Use (1 TB): The perfect choice for most users. It provides plenty of room for the OS, dozens of applications, and a large library of photos, music, and documents.
Heavy Use/Professional (2 TB+): Essential for gamers (games can be over 150 GB each), video editors, photographers, and other professionals who work with massive files and need fast local access to their data.

What is a partition and does it affect my total capacity?

A partition is a logical division of a physical drive. For example, you can partition a 1 TB drive into two 500 GB “drives” (C: and D:). Partitioning organizes your drive but does not change the total physical capacity. The only exception is hidden partitions, like a recovery partition, which are created by the manufacturer and do subtract from the space available to you.

Does this affect RAM (memory) too?

No, this is one area of computing where measurements are consistent. RAM (Random Access Memory) has always been manufactured and measured using the binary (powers of 1024) system. So when you buy 16 GB of RAM, you are getting 16 gibibytes (GiB). There is no discrepancy between the advertised and actual amount.

Other Tools You Might Find Useful

Understanding your storage is the first step. Next, see how that storage interacts with your daily use.

Convert between any data unit, from kilobytes to terabytes, with our Data Storage Converter.
Curious how long it will take to download a game to fill your new drive? Use our Download Time Calculator.

Creator

Huy Hoang

A seasoned data scientist and mathematician with more than two decades in advanced mathematics and leadership, plus six years of applied machine learning research and teaching. His expertise bridges theoretical insight with practical machine‑learning solutions to drive data‑driven decision‑making.

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