Is a bigger hard drive always cheaper per terabyte?

No, and this is the myth worth killing. The received wisdom is that price per terabyte falls as capacity rises, so the biggest drive is always the best value. It is a smooth curve in a slide deck and a bumpy one in a shopping cart. On Amazon.com today the cheapest 16TB is $25.56/TB, the 18TB is $18.91/TB and the 20TB is $26.00/TB. The 18TB in the middle is 26% cheaper per terabyte than either neighbour. Capacity sets a rough floor, but the actual cheapest drive at each size is decided by which model is on a good price right now, not by how many terabytes it holds.

Why is the 18TB cheaper per terabyte than the 16TB and the 20TB?

Because the cheapest 18TB I can find today is an external. The SanDisk Professional G-Drive at $340.38 is a USB-C desktop drive with an Ultrastar enterprise HDD inside, and it happens to be priced below the cheapest bare 16TB and 20TB internals this week. That is the honest catch: it is a shuck candidate, not a bare internal you drop straight into a NAS bay. If you want a bare internal 18TB you will pay more than $18.91/TB. The mid-range of the price-per-TB curve is set by individual deals, not by capacity, which is exactly why it is worth checking each size rather than assuming.

Should I just buy small used drives to get the lowest price per terabyte?

Only if you have thought about the whole cost. The single lowest price per terabyte on the site today is a 4TB used Seagate Constellation ES.3 at $18.25/TB. To build 18TB out of 4TB drives you need five of them. That is five SATA ports, five drive bays, five drives worth of idle power, and five times the annual odds that one dies and you are doing a rebuild. A used enterprise pull can be a genuinely good buy, but per-terabyte price alone hides the bay, power and failure costs that a bigger drive folds into one unit. I wrote about that trade in more detail in the piece on fewer big drives or more small ones.

Will the 18TB still be the cheapest per terabyte next week?

Probably not, and that is the point of this page rather than a flaw in it. Every price here is Amazon.com in US dollars, pulled on 16 August 2026, and drive prices move hourly. The value knee sits at 18TB today because of one well-priced external. Next week it could be a 16TB on sale, or a 22TB that just dropped. The durable advice is not the number, it is the habit: before you buy, open the page for the capacity you want and the one on either side of it, look at the price per terabyte rather than the sticker price, and buy the knee. The capacity pages update as prices change, so the number is always current even when this article is not.

Which Hard Drive Capacity Is Cheapest Per Terabyte Right Now? Bigger Is Not the Answer (2026)

Q: Which hard drive capacity is cheapest per terabyte right now?

18TB, on 16 August 2026. The cheapest 18TB I can buy on Amazon.com today is a SanDisk Professional G-Drive at $340.38, which works out at $18.91/TB. That is cheaper per terabyte than the cheapest 8TB ($26.13/TB), 12TB ($24.92/TB), 16TB ($25.56/TB) and 20TB ($26.00/TB). The only thing that beats it is a 4TB used enterprise drive at $18.25/TB, and you would need four or five of those to match one 18TB, using four or five bays and four or five drives worth of power and failure risk. So among drives you would actually build around, 18TB is the value spot today. It will not stay the value spot forever, because the number depends on which specific drive happens to be discounted this week.

Ask most people which hard drive is the best value and they will tell you to buy the biggest one you can afford. Price per terabyte falls as capacity rises, so the 20TB beats the 8TB, and you are done. It is repeated so often that nobody checks it against a cart.

So I checked it. Every figure below is Amazon.com in US dollars, pulled on 16 August 2026, and it is the cheapest drive I can actually buy at each capacity, not a list price or a spec-sheet ideal. The product links are affiliate links, which is how this site pays for itself.

The short version: bigger is not cheaper per terabyte right now. The curve does not slope smoothly downward. It bends, and today it bends at 18TB.

The price-per-TB curve today

Here is the cheapest usable pick at each capacity, and what it works out to per terabyte.

Capacity	Cheapest pick today	Price	Per TB	Condition
4TB	Seagate Constellation ES.3	$73.00	$18.25/TB	used
8TB	WD Gold	$209.00	$26.13/TB	used
12TB	HPE Helium (Ultrastar inside)	$299.00	$24.92/TB	used
16TB	Toshiba MG08	$409.00	$25.56/TB	renewed
18TB	SanDisk Professional G-Drive	$340.38	$18.91/TB	new
20TB	Avolusion M5	$519.99	$26.00/TB	renewed

Read down the per-TB column and the smooth-curve story falls apart. From 8TB to 20TB the price per terabyte sits in a band around $25 to $26, give or take. Capacity barely moves it. The 16TB is $25.56/TB and the 20TB, two whole sizes larger, is $26.00/TB. Larger, and dearer per terabyte.

Then there is 18TB at $18.91/TB, sitting 26% below both of its neighbours. That is the knee. If you want the most storage for the least money per terabyte in a single drive today, that is where it is.

Why the curve bends

Two things are going on, and neither is about capacity.

At the small end, the cheapest price per terabyte is always used enterprise pulls. The 4TB Seagate Constellation ES.3 at $18.25/TB is the single lowest number on the whole site. There is a steady supply of decommissioned 2TB to 4TB enterprise drives coming off corporate fleets, and that supply keeps their per-terabyte price under almost everything else. It is real, but it comes with a real bill attached that the per-TB number hides, which is the next question below.

In the middle, the number is set by whichever drive is on a good price this week, not by how big it is. The 18TB is cheap today because of one specific listing: a SanDisk Professional G-Drive at $340.38. That is an external USB-C desktop drive with an Ultrastar enterprise HDD inside, and right now it is priced below the cheapest bare 16TB and 20TB internals. There is nothing structural about 18TB being cheaper than 20TB. It is a deal on one model, and a deal on one model is exactly what moves the knee.

Which is the honest catch on today’s winner. The cheapest 18TB is an external, so it is a shuck candidate rather than a bare internal you drop straight into a NAS bay. If you want a bare internal at that capacity, you will pay more than $18.91/TB. I went through the shucking maths in detail in is shucking still worth it, and the same caution applies here: a great price on an external is only a great price on a bare drive if you are willing to open the box and void the warranty.

What to actually do with this

The takeaway is not “buy 18TB”. By the time you read this the 18TB deal may be gone and the knee may have moved to 16TB, or to a 22TB that just dropped. The takeaway is the method.

Before you buy, do three things. Open the page for the capacity you were going to buy, and the pages for the size on either side of it. Look at the price per terabyte, not the sticker price, because the sticker price tells you almost nothing about value. Then buy the knee, the capacity where the per-TB number dips below its neighbours.

That is the whole trick, and it is why every page on this site sorts by price per terabyte and updates as prices change. The number in the table above was true on 16 August 2026. The number on the cheapest per TB page is true right now, whenever right now is for you.

One last thing, because it is the mistake I see most. Do not chase the absolute lowest number off the top of the list if it means five small drives instead of one big one. The 4TB at $18.25/TB looks unbeatable until you count the five bays, the five lots of idle power, and the five chances a year that one fails and you are rebuilding an array. A single 18TB at $18.91/TB is 66 cents per terabyte more expensive, about $12 across the whole drive, and a great deal less hassle. I put numbers on that trade in fewer big drives or more small ones if you want to see the arithmetic.

Buy the knee. Just check where it is first.