Real Disk Space Calculator

See how much usable space a hard drive or SSD actually gives you versus the capacity printed on the box.

Space reserved by formatting (NTFS, ext4, APFS typically reserve a few percent). Leave at 0% to compare raw advertised vs OS-reported capacity.

Rate this tool

How useful was this tool?

Why your new drive shows less space than advertised

Buy a "1 TB" drive and your computer reports about 931 GB. Nothing is broken and nothing is missing - the number printed on the box and the number your operating system shows are simply measured with two different definitions of a "gigabyte." This Real Disk Space Calculator converts between the two so you can see the true, usable capacity of any hard drive, SSD, USB flash drive or memory card before and after you buy it.

Decimal vs binary: the real reason capacity seems to "disappear"

Storage manufacturers measure capacity in decimal (SI) units, where 1 gigabyte is exactly 1,000,000,000 bytes and 1 terabyte is 1,000,000,000,000 bytes. Operating systems such as Windows measure the very same bytes in binary (IEC) units - 1 gibibyte (GiB) equals 1,073,741,824 bytes - yet they still print the label "GB". Because a binary gigabyte is about 7.4% larger than a decimal one, the reported figure is always lower than the box:

  • A 1 TB drive = 1,000,000,000,000 bytes = about 931 GiB, shown as "931 GB"
  • A 2 TB drive = about 1.82 TiB, shown as "1.82 TB"
  • A 500 GB drive = about 465 GiB, shown as "465 GB"
  • A 256 GB SSD = about 238 GiB, shown as "238 GB"

The gap is a fixed ~6.87% for every drive, regardless of size or brand. Interestingly, macOS and most Linux distributions now report capacity in decimal units, so the same drive can look "bigger" on a Mac than on a Windows PC even though the physical capacity is identical.

Formatting and system overhead

Beyond the unit difference, a freshly formatted drive loses a little more space to the filesystem itself. NTFS, exFAT, ext4 and APFS all reserve room for file tables, journals and metadata - typically a fraction of a percent up to a few percent. Manufacturer recovery partitions, the Windows page file and hibernation file, and hidden "system reserved" areas reduce what you finally see in File Explorer as well. Use the filesystem overhead slider to estimate the space left after formatting.

How to use this calculator

  1. Enter the capacity printed on the drive and choose its unit (MB, GB or TB).
  2. Set how many identical drives you are combining.
  3. Optionally add an estimated formatting overhead percentage.

You will instantly see the advertised capacity, what your operating system will actually report, the apparent shortfall, and the usable space after formatting. Every calculation runs locally in your browser, so nothing you type is ever uploaded.

A quick note on SSDs

Many solid-state drives also set aside hidden capacity for over-provisioning (wear levelling and spare blocks). That reserve is separate from the decimal-versus-binary difference and is not exposed to the operating system at all, which is why some SSDs read slightly lower than this estimate. If your usable space is far below what this calculator predicts, check for large recovery partitions or a restore image rather than assuming a fault.

Note: All processing is done client-side. We do not store or see your data.

Related Tools

English to GenZ Converter

Transform your standard English text into Gen Z slang and internet culture speak.

Fake Identity Generator

Instantly generate comprehensive random personas including names, addresses, and bios for testing or creative use.

Leet Speak Converter

Transform your text into \\\1337\\\ speak by replacing standard characters with numbers and symbols.

Zodiac Sign Finder

Discover your zodiac sign, elemental nature, and ruling planet based on your date of birth.

Glitch Text Generator

Zalgo / glitch text generator. Free creepy text maker.

Bubble Text Generator

Bubble text generator. Free bubble letters (circles).

Upside Down Text Generator

Flip text upside down. Free upside down text converter.

Currency Converter

Convert currencies. Free currency converter with live rates.

Dice Roller

Roll dice online. Free dice roller (d6, d20, custom).

Virtual Coin Flip

Flip a coin online. Free heads or tails coin flip.

Aim Trainer

Aim trainer game. Free browser aim practice.

Age Calculator

Calculate age from birth date. Free age calculator.

Between Dates Calculator

Days between two dates. Free date difference calculator.

BMI Calculator

Calculate BMI from height & weight. Free BMI calculator.

Profit Calculator

Compound interest & profit calculator. Free compound growth calculator.

Interest Calculator

Calculate interest on loan or amount. Free interest calculator.

GPA Calculator

Calculate your GPA online by entering your courses, credit hours, and grades.

Aspect Ratio Calculator

Calculate the aspect ratio of any image or video, or scale dimensions while maintaining a fixed ratio.

Binary Operations Calculator

Perform arithmetic operations on binary numbers (Add, Subtract, Multiply, Divide) with instant conversion to decimal and hex.

Calorie (BMR) Calculator

Estimate your daily calorie needs based on your Basal Metabolic Rate (BMR) and activity level using the Mifflin-St Jeor equation.

Decimal to Fraction Calculator

Convert any decimal number into its simplest fraction form, including mixed numbers and improper fractions.

Electricity Cost Calculator

Calculate the estimated cost of running any household appliance based on its wattage and your local energy rates.

Loan (EMI) Calculator

Calculate Equated Monthly Installments (EMI) for any loan with detailed interest and total payment breakdowns.

Macronutrient Calculator

Break down your daily calorie goal into Protein, Carbohydrates, and Fats based on your fitness goals.

Ohm's Law Calculator

Calculate Voltage, Current, Resistance, and Power instantly by entering any two known values.

Password Entropy Calculator

Measure the mathematical strength of your passwords using bit entropy to determine how resistant they are to brute-force attacks.

Percentage Calculator

Calculate percentages, percentage increases, and percentage decreases instantly with this versatile mathematical tool.

Pet Real Age Calculator

Calculate your pet's equivalent human age using scientifically-backed aging curves for dogs and cats.

RAID Calculator

Calculate usable capacity, fault tolerance, and efficiency for RAID 0, 1, 5, 6, 10, 50, and 60 arrays.

Readability Score Calculator

Analyze text readability and complexity using industry-standard formulas like Flesch-Kincaid and Gunning Fog.

Sales Tax / GST Calculator

Quickly calculate the total price including tax or extract the tax amount from a gross total.

Standard Deviation Calculator

Calculate the sample and population standard deviation, variance, and mean for a set of numbers.

Time Duration Calculator

Calculate the exact time difference between two dates and times with detailed breakdowns.

Tip Calculator

Quickly calculate tips and split bills among friends with real-time per-person breakdowns.

Count Down Timer

Countdown timer. Free online countdown (hours, minutes, seconds).

Stop Watch

Online stopwatch. Free stopwatch & lap timer.

Scientific Calculator

Scientific calculator online. Free with trig, log, etc.

World Clock

World clock & time zones. Free time zone converter.

What is My Browser

What browser am I using? Free browser detection.

Credit Card Validator

Validate credit card number (Luhn). Free card validator.

Date Picker Calendar

Date picker & calendar. Pick date and year.

YouTube Thumbnail Downloader

Download YouTube video thumbnails. Free thumbnail downloader.

Contact

Missing something?

Feel free to request missing tools or give some feedback using our contact form.

Contact Us
This website uses Cookies to ensure optimal user experience.