Storage: What You Buy vs What You Get

Storage is sold in one unit and delivered in another. Every tool here measures a different gap between the two — redundancy, tiers and egress, or one file type crowding out another.

24 TB of drives is 18.2 TB on screen

The RAID Usable Capacity Calculator shows the shrinkage happening twice.

Six 4 TB drives in RAID 5:

Raw capacity you paid for	24 TB
Usable after redundancy	20 TB
What the OS will report	18.2 TB
Share of what you bought	83.3%
Drives that can fail	1
Cost per usable TB	$30

Two separate losses. The first is real: 4 TB goes to parity so one drive can die without taking your data. The second is not a loss at all — drives are sold in decimal terabytes and operating systems report binary ones, so 20 TB becomes 18.2 TiB on screen with no bytes missing.

Cost per usable terabyte is the number to compare with, not the price per drive. It is the only figure that lets you weigh more small drives against fewer large ones honestly.

⚠️ RAID is not a backup. It survives a drive failing. It does not survive deletion, ransomware, a failed controller, fire or theft — all of which write to every drive at once.

The cloud bill has a line nobody quotes

The Cloud Storage Cost Calculator separates the price you compare on from the price you pay.

5 TB stored, 0.5 TB downloaded a month:

Standard tier, all in	$160/mo
Archive tier, all in	$75/mo
Saving	$85/mo — $1,020 a year
Egress alone	$45/mo
Egress share of the standard bill	28%

More than a quarter of the standard bill is the cost of getting your own data back out. Storage is advertised per terabyte stored; egress is what turns a cheap quote into a real bill, and it scales with how you use the data rather than how much you have.

The tool gives the crossover directly: the tiers cost the same at 4.75 TB of monthly download. Below that, archive wins; above it, retrieval fees overtake the storage saving.

And the framing that makes the trade-off concrete: moving 1 TB out costs the same as storing 3.9 TB for a month. If you are pulling data regularly, you are not really buying storage — you are buying transfer.

One video costs 852 photographs

The Memory Card Capacity Calculator handles the everyday version: two file types competing for one card.

A 128 GB card, 24 MP raw files, and 30 minutes of 100 Mbps video:

One frame	26.4 MB
Photos if the card held nothing else	4,848
With 30 min of video	3,996
What the video cost, in frames	852
Video's appetite	45 GB an hour

Video is not slightly hungrier than stills — it is a different order of magnitude. Half an hour of it eats the space of 852 photographs, and an hour would take 45 GB on its own.

Which is why "how many photos fit" is the wrong question the moment a camera also shoots video. The useful figure is 2.64% of the card per 100 frames — that scales to any card, and it makes the trade-off visible before you are standing somewhere with a full card.

The shape they share

	Quoted	Delivered
RAID	24 TB of drives	18.2 TB on screen
Cloud	a per-TB storage price	+28% egress
Memory card	4,848 photos	3,996, with some video

In each case the advertised number is true and incomplete, and the gap is structural rather than sneaky: parity has to live somewhere, transfer has to be paid for, and a card does not know what you plan to shoot.

The three questions worth asking

1. What is the cost per usable unit? Not per drive, not per advertised terabyte.
2. What does using it cost, as opposed to keeping it? Egress and retrieval are usage charges wearing a storage bill's clothing.
3. What else is competing for the same space? A card, an array and a bucket all get shared eventually.

⚠️ And whatever the arithmetic says, keep a copy somewhere else. Every tool here measures capacity or cost. None of them measures whether you would still have your data after the one event that takes out the whole device — and that is the only question that matters on the day it happens.