File totals, drives needed, unit equivalents and physical volume
Storage questions come in three kinds, and this page keeps them apart. How much space a set of files needs, stated in both decimal and binary units with room for overhead, and turned into a number of drives. What a capacity figure equals in every other unit, for when a label and a system disagree. And how much physical room a set of boxes takes, against the usable volume of a storage unit. Each mode computes from exact byte or volume counts and states its conventions on the results.
Mode 1: space for a set of files
Mode 2: unit equivalents
Mode 3: physical storage (boxes and units)
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. File totals are as exact as the average sizes entered; a set whose sizes vary widely is better weighed by its real total.
What this calculator does
It answers the three capacity questions that share a keyword with three separate computations, because mixing them is how storage gets mis-bought. The file mode sums quantity times size across up to two kinds of item, adds the overhead percentage you set, reports the total in decimal units (the language of drive labels) and binary units (the language of operating systems) side by side, and divides by a drive size to give a count of drives that covers it. The equivalents mode converts one figure into every unit in both families plus bits and bytes, which settles label-versus-system arguments with arithmetic. The physical mode works in volumes: box volume times quantity against a storage unit’s volume discounted by a packing factor, returning units required and how full one unit would be. Nothing is estimated silently; the conventions are printed on the results they produced.
How to use it
- Averages hide the one huge file. Quantity times average size is exact only when sizes cluster. A photo library averages honestly; a folder of videos does not, because three long recordings can outweigh three hundred clips. Where sizes vary wildly, enter the real total size as one item of “1 x total”, or split the set so the giants are their own item type, which is what the second row is for.
- Overhead is not pessimism. Filesystems take their cut, versioning and thumbnails take theirs, and a drive filled to 100% is a drive misbehaving. The default instinct should be at least 10%, and more for anything that grows: a backup target sized to today’s data with no margin is next year’s emergency purchase, already scheduled.
- Drives needed assumes the data must fit with the margin. The count divides the with-overhead total by the drive size and rounds up, because 1.2 drives is not a purchasable object. It does not build in redundancy: a mirrored or RAID set needs the RAID calculator’s figures on top, and a backup copy needs its own drive entirely, which the honest reading of “drives needed” for precious data is at least two.
- Read both unit columns before buying. A “2 TB” drive holds 2 trillion bytes, which a system reports as about 1.82 TiB. If your file total is stated in GiB by your system and you buy against the TB label without converting, the shortfall is about 7% at TB scale, every time. Mode 2 exists so the conversion happens before the purchase, not during the restore.
- Packing factor is where physical plans go wrong. A storage unit’s volume is a perfect box; your boxes are not the same shape as the unit, cannot be stacked to the ceiling everywhere, and need a path to the back. Uniform boxes at 85% is realistic; mixed furniture in the same arithmetic is fantasy, which is why the factor is yours to lower.
How it is calculated
- File mode: bytes = sum over item types of (count x size x unit multiplier), with decimal units as powers of 1,000 and binary as powers of 1,024 exactly. With-overhead total = bytes x (1 + overhead %). Drives needed = ceiling(with-overhead bytes / drive bytes). Totals are reported in GB and TB (decimal) and GiB and TiB (binary), all from the same byte count.
- Equivalents: the entered value is converted to bytes through its unit’s exact multiplier (bits divide by 8), then out into each unit. Physical mode: box volume = L x W x H cubic inches / 1,728 = cubic feet; total = box volume x quantity. Unit volume = L x W x H feet. Effective unit volume = unit volume x packing %. Units required = ceiling(total / effective). Fill of one unit = total / unit volume x 100.
Worked examples
Photos and videos. 5,000 photos averaging 4 MB are 20.00 GB. Add 200 videos averaging 0.7 GB (entered as size 0.7, unit GB) for 140.00 GB: the set totals 160.00 GB. With the 10% overhead the planning total is 176.00 GB, which is 163.91 GiB in the binary units a system will show. Against a 2 TB drive, drives needed is 1, with the honest rider from the usage notes: that is the fit figure, not a backup plan.
One terabyte, every way. Mode 2 on 1 TB returns 1,000.00 GB and 1,000,000.00 MB in decimal, 931.32 GiB in binary, and 8,000,000,000,000 bits. The GiB line is the one that explains a thousand support threads: nothing is missing from the drive; the two unit families simply count in different-sized steps, and the equivalents table shows both accountings of the same bytes at once.
Boxes into a unit. 100 boxes of 24 x 18 x 12 inches are 3.00 cubic feet each, 300.00 cubic feet in all. A 10 x 10 x 8 foot unit holds 800.00 cubic feet raw, 680.00 at an 85% packing factor, so the boxes need 1 unit and fill 37.50% of its raw volume. The gap between 300 and 680 is the plan’s shock absorber: late additions, a piece of furniture, and the aisle you swore you would not need.
Reading your results
In file mode, plan purchases on the with-overhead figure and sanity-check them in the binary column, because that is the column your system will argue in afterwards. A drives-needed result of 1 with a total near the drive’s size is a warning dressed as an answer: growth will evict you within the year. In physical mode, read units required together with the fill percentage: one unit at 95% fill is a tighter life than two at 50%, even when the second costs money, because storage you cannot reach the back of is storage rented for the front row.
Limitations
- Average-based totals inherit the average’s honesty. The mode is exact arithmetic on entered sizes; a wrong average produces a precisely wrong total. Where the real total is known from the system, enter it directly.
- No redundancy or backup is included. Drives needed is fit, once. Mirroring doubles it, RAID reshapes it (see the RAID calculator), and a backup copy is a separate device in a separate place; none of that is in this page’s division, by design, so it cannot be mistaken as included.
- Physical mode assumes stackable uniform boxes. It is volume arithmetic with a packing discount, not a 3D packing solver: shapes that interlock badly, weight limits on stacking, and unit doors narrower than the boxes all live outside the model, in the packing factor you choose.
- Formatted capacity varies slightly by filesystem. The overhead percentage is the allowance for it; no universal figure exists because filesystems, cluster sizes and reserved blocks differ by system and settings.
Frequently asked questions
How much storage do I need for my files?
Add up what the files actually total, then buy for the total plus growth, not the total. Mode 1 does the addition from counts and average sizes, applies your overhead percentage, and converts to a drive count. For most home sets the arithmetic surprises in one direction: photos are smaller than feared (5,000 at 4 MB is only 20 GB) and videos are larger (200 modest ones outweighed them seven to one in the worked example). Measure the real set where you can; averages are for the sets you cannot.
Why does my 2 TB drive show as 1.82 TB in my system?
It shows 1.82 in the binary unit, TiB, while labelled 2 in the decimal unit, TB: 2 trillion bytes divided by 2^40 is 1.82. Both describe the same drive. The equivalents mode prints the whole family at once (a terabyte is 931.32 GiB, for instance), and the practical rule is to compare like with like: a file total your system states in GiB must be set against the drive’s GiB figure, not its TB label, or you will plan about 7 to 10% more capacity than you bought.
How many drives do I need?
To fit the data once, the file mode’s division: total with overhead, divided by the drive size, rounded up. To keep the data, more: a second copy on a second device for anything irreplaceable, and the RAID calculator’s numbers if the fit must also survive a drive failure. The single number on this page is the floor of the answer, stated as such, because a page that quietly included backup would double the price for the users who only asked about fit.
How much can a 10 x 10 storage unit hold?
800 cubic feet of perfect geometry, and realistically about 680 of it in uniform boxes at an 85% packing factor, less for awkward furniture. In the worked example, 100 standard archive boxes take 300 cubic feet, a little over a third of the unit. Volume is the right first check and a poor final one: floor area and door access decide what actually goes in, which is why the mode reports the fill percentage as well as the unit count, so a plan that technically fits can still be recognised as miserable.
Related calculators
Method: exact byte arithmetic under the stated decimal and binary unit definitions, ceiling division for drives and units, and rectangular volume arithmetic with a user-set packing factor, exactly as defined on this page. Last reviewed: 13 October 2026.