RAID 6 Calculator
The formula
Why dual parity became the default for big disks
RAID 5 tolerates exactly one failure, and rebuilding that failure takes hours on modern 16–22 TB nearline drives. During the rebuild every remaining disk is read end-to-end at high duty cycle — precisely when latent sector errors surface. The arithmetic is uncomfortable: with a 1% annual failure rate and a 30-hour rebuild, a twelve-disk RAID 5 array carries a few tenths of a percent chance of total loss on each rebuild, which compounds across an estate of hundreds of arrays. RAID 6 removes most of that risk by surviving a second failure, at the cost of one extra disk and a heavier write path. For arrays of eight or more large disks it is now the standard recommendation.
The write penalty, honestly stated
A single small write to a RAID 6 stripe requires reading the old data block, reading both parity blocks, recomputing parity and writing three blocks — six I/O operations for one logical write. On random-write-heavy workloads such as transactional databases and mail stores this is a serious tax, and it is why RAID 10 wins there despite worse capacity efficiency. On sequential or read-heavy workloads the penalty nearly disappears, because full-stripe writes need no read-modify-write. Match the RAID level to the access pattern rather than to the capacity spreadsheet.
Frequently asked questions
Is RAID a backup?
What is the minimum number of disks for RAID 6?
Is RAID 6 slower than RAID 5?
RAID 6 or RAID 10 for a database?
Does RAID 6 protect against ransomware?
Source & methodology
Patterson, Gibson & Katz (1988) RAID taxonomy; Plank (2013), "A Tutorial on Reed-Solomon Coding for Fault-Tolerance in RAID-like Systems"; rebuild-window risk per Schroeder & Gibson (2007) and Elerath & Pecht (2009).
RAID 6 explained: formula, worked example and honest limits
You are here because you need a specific number, and you want to see how it was produced before you rely on it. RAID 6 is the kind of number people argue about from memory. This page ends that argument with arithmetic you can watch happen: enter your figures, get Usable capacity in real time, and see every substitution printed below.
There is a habit that separates people who get useful answers from Networks & Storage numbers: they decide what question the figure answers before they compute it. RAID 6 answers “Usable capacity?” — and nothing else, which is precisely why the result is trustworthy.
In plain terms, RAID 6 answers the question "Usable capacity?". The measurement matters because it converts a situation that feels complicated — several inputs pulling in different directions — into a single number you can act on. Within Technology & Computing, Networks & Storage is the context where this figure is quoted most often, and professionals treat it as a first-pass screen rather than a verdict.
Nothing below is stock text pasted around a number: the worked example computes usable capacity = 48.00 TB from this tool’s own defaults, and the sensitivity section ranks the inputs that move the result — Number of disks first, at ≈13.3 % of the headline for a ten-percent error either way in that input — which is the thread the rest of this guide pulls on.
The RAID 6 formula, explained
The engine implements a single stated model: usable = (N − 2) × disk size; efficiency = (N − 2)/N; write penalty = 6 I/O per small write. There is no hidden coefficient and no login-gated precision — the formula printed is the formula executed, which is also what makes the result citable.
Number of disks (example value 8) is a plain measured quantity — the unit shown in the label is the unit the formula expects. Quantities scale everything downstream — a one-unit error here is multiplied across the whole result.
Capacity per disk (TB) (example value 8) is a numeric input whose unit is fixed by the label; a wrong unit is a wrong answer wearing a plausible disguise. If you are unsure what belongs in this field, look at the printed steps: the substitution shows exactly how the engine uses it.
Throughput per disk (MB/s) (example value 250) is a plain measured quantity — the unit shown in the label is the unit the formula expects. This input feeds the formula directly; a defensible figure here is worth more than false precision anywhere else.
Disk MTBF (hours) (example value 1,400,000) is a plain measured quantity — the unit shown in the label is the unit the formula expects. Be precise about the boundary (exact date, start of month); time inputs decide the exponent in most formulas.
Rebuild time for one disk (hours) (example value 30) is a measured number in the unit named by the label; the formula does not convert for you. Use the value as of today, not as of the last time you looked — time fields drift silently.
The default RAID 6 calculation, written out in full
Abstract formulas are hard to trust, so here is the complete substitution using the default inputs, written the way an examiner would want it. Starting figures: Number of disks, Capacity per disk (TB), Throughput per disk (MB/s), Disk MTBF (hours). The endpoint of that chain is 48.00 TB of usable capacity, sitting in the “64.00 TB raw · 75.0% efficiency” band.
- Usable = (N − 2) × disk size = (8 − 2) × 8.00 TB = 48.00 TB.
- Efficiency = (N − 2)/N = 6/8 = 75.0%; 16.00 TB is consumed by the two parity sets.
- Read throughput ≈ (N − 2) × per-disk = 1,500 MB/s. A small random write costs six I/O operations: read data, read both parities, write data, write both parities.
- Per-disk failure probability across a 30-hour rebuild ≈ 0.00215%.
- Probability another of the 6 surviving disks fails in that window ≈ 0.0129% — this is the figure that makes rebuild speed and background scrubbing matter on large-capacity drives.
Raw capacity — 64.00 TB. This is the number experienced users quote when the headline alone would mislead.
Capacity efficiency — 75.0%. Compare it against the main result of 48.00 TB before drawing conclusions.
Fault tolerance — Survives any two simultaneous disk failures. This is the number experienced users quote when the headline alone would mislead.
Sequential throughput (approx.) — 1,500 MB/s. This is the number experienced users quote when the headline alone would mislead.
Annual failure rate per disk — 0.63%. Read next to the headline, this is the figure that shows the shape of the trade-off.
Re-running with your own figures is instant; re-running by hand once is the best way to know the tool is telling you the truth.
Scenario table: RAID 6 at ±10 % and ±20 % of the key input
Read this table as the practical error bar on the headline figure. It answers the question people actually have after seeing a single number: how wrong could this be if the main input is off by ten or twenty percent?
| Number of disks setting | Usable capacity |
|---|---|
| 80 % of the example value | 35.20 TB |
| 90 % of the example value | 41.60 TB |
| example inputs | 48.00 TB |
| 110 % of the example value | 54.40 TB |
| 120 % of the example value | 60.80 TB |
Across 5 settings of Number of disks, the result spans 35.20 TB to 60.80 TB. A decision that only works at one row of this table is a decision waiting for an input to move. Reproduce it in the form above by editing Number of disks alone; the substitution steps will follow your figures exactly.
Where RAID 6 is fragile — and where it is not
Generic advice says “it depends on your inputs”; this section says exactly how much. Each field was perturbed by ±10 % in isolation and the usable capacity recomputed, so the ranking below is measured rather than asserted. It is the fastest honest answer to “what matters most in RAID 6?” — and it is specific to these defaults, because sensitivity moves with the starting point.
- Number of disks +10 % → RAID 6 = 54.40 TB · −10 % → 41.60 TB (the usable capacity shifts ≈13.3 %)
- Capacity per disk: ten percent up gives 52.80 TB, ten percent down gives 43.20 TB — ≈10.0 % of the usable capacity either way
Number of disks is the dominant lever here: ten percent either way moves the result by about 13.3 %, from 41.60 TB to 54.40 TB. Spend your verification effort there first, and treat Capacity per disk as the second priority (≈10.0 % per ten percent). The remaining fields can sit at safe defaults without materially changing the answer — knowing which is which is what separates a careful calculation from an anxious one.
Is your RAID 6 good — reading the result bands
Nobody acts on a bare figure — they act on a figure with a comparison attached. The verdict attached to the default calculation — “64.00 TB raw · 75.0% efficiency” — is one band on a ladder your own figure may sit somewhere else on. The sensitivity list shows how far a modest input error moves you along that ladder: for Number of disks, ±10 % covers 41.60 TB to 54.40 TB. The practical reading rules: read the band as a starting hypothesis, then test it: change the two dominant inputs by ten percent and see whether the verdict survives. Verdicts that survive stress are the ones worth acting on.
Where RAID 6 sits next to similar measures: this tool answers “how much is the RAID 6”, while the related calculators answer “what do I do about it”. Reading the result alone tends to over-weight a single snapshot; pairing it with the companion tools above turns the number into a decision.
The errors that quietly break an RAID 6 figure
Copying a single headline number without checking the companion outputs. The tool reports Raw capacity, Capacity efficiency and Fault tolerance alongside the main result precisely because RAID 6 in isolation misleads — the secondary columns are where outliers and trade-offs show up.
Copying a single headline number without checking the companion outputs. The tool reports Raw capacity, Capacity efficiency and Fault tolerance alongside the main result precisely because RAID 6 in isolation misleads — the secondary columns are where outliers and trade-offs show up.
One mistake is specific to this engine: assuming every input matters equally. The computation above shows Number of disks dominating — a careful figure there beats false precision everywhere else.
Another is comparing across contexts: an RAID 6 computed with one set of conventions should never be lined up against a figure computed with another. Check the inputs behind both before ranking them.
Before you trust your RAID 6: a short checklist
- State, in one sentence, which convention each rate, unit or period in the form is using.
- Check that every field the formula needs actually arrived from a document, not from recollection.
- Re-verify Number of disks against a source document — ten percent of error there moves the headline by ≈13.3 %.
- Validate inputs against a live measurement, not datasheet peak figures.
- Compare the result with one independent method or published benchmark.
- Run the ±10 % stress on the top two levers and check that your conclusion survives both.
Five steps from raw inputs to a figure you can defend
Using a calculator well sounds trivial, but the difference between a decision-grade figure and a throwaway one lives entirely in the routine. The routine for RAID 6 takes a minute to learn and applies to every input above.
Step 1 — start from evidence. The inputs behind RAID 6 are number of disks, capacity per disk (tb), throughput per disk (mb/s), disk mtbf (hours), rebuild time for one disk (hours). Write them down with their source next to each one; if you cannot name where a figure came from, that is the input most likely to be wrong. The inputs are the part of the work that nobody else can do for you.
Step 2 — start from the case you can defend. Enter pessimistic-but-honest figures first; the optimistic run is a ceiling, not a plan. A conclusion that holds on conservative inputs holds everywhere. A result worth acting on survives having its two shakiest inputs halved.
Step 3 — isolate your variables. Change one field, observe, revert. The live sensitivity list does the ranking for the default inputs; verify it against your own, because dominance shifts when the starting point shifts. If a result feels obviously right, that is the moment to check the inputs hardest.
Step 4 — compute a corridor, not a point. Three runs — worst, likely, best — turn RAID 6 from a snapshot into a range. Plan against the pessimistic end and treat the optimistic end as the upside story. Rounding early is the quietest way to introduce a permanent error.
Step 5 — schedule the next run. In Technology & Computing the inputs drift quietly. Put a reminder a quarter out, re-enter the same fields, and read the change rather than the level. Most surprises in numbers are surprises about inputs, not about formulas.
From calculation to decision: where RAID 6 earns its keep
Most Networks & Storage disagreements are arithmetic disagreements in disguise. When Usable capacity of 48.00 TB sits on the table with its working shown, the conversation stops being about who sounds more confident and starts being about which input is wrong. The inputs are the part of the work that nobody else can do for you.
The audience for this page is people who need one trustworthy number before their next decision. In professional Networks & Storage practice the pattern is always the same — screen with RAID 6, then invest slow analysis only in whatever survives the screen. Numbers are cheap to produce and expensive to retract; the extra minute is the cheapest insurance available.
Re-running this page costs nothing and catches the slow drifts that never announce themselves. In Technology & Computing the expensive mistakes are rarely sudden; they are small changes in Number of disks that nobody recalculated for a year. If the answer changes your behaviour, it deserves a second run with different assumptions.
RAID 6 — frequently asked questions
What affects RAID 6 the most?
The short answer: Number of disks. The longer answer is the sensitivity table on this page, where every numeric input was probed by ten percent and the usable capacity recalculated — Number of disks moved it furthest, by roughly 13.3 % (41.60 TB to 54.40 TB).
Which matters more: capacity per disk or number of disks?
Number of disks, by a factor of about 1.3. Concretely: ten percent on Number of disks is worth ≈13.3 % of RAID 6, the same nudge on Capacity per disk only ≈10.0 %. That ranking is re-computed for each page from that page’s own defaults.
What is a typical RAID 6?
There is no honest universal average for RAID 6, but there is a reference point: the defaults on this page produce 48.00 TB (64.00 TB raw · 75.0% efficiency). Replace the inputs with your own and compare — that comparison is the useful meaning of “typical”.
How do I calculate RAID 6 by hand?
usable = (N − 2) × disk size; efficiency = (N − 2)/N; write penalty = 6 I/O per small write is the entire method. Work the inputs in that order — Usable = (N − 2) × disk size = (8 − 2) × 8.00 TB = 48.00 TB. — and you will reach 48.00 TB on the default figures, the same number this tool prints.
What happens to RAID 6 if number of disks changes by 10 %?
On these inputs a ±10 % change in number of disks takes RAID 6 to 54.40 TB against the 48.00 TB example — around 13.3 % of the headline. The same test on every other field is in the sensitivity section.
Is the RAID 6 result of 48.00 TB good or bad?
Good and bad are comparisons, so the honest test is three questions: where does your own RAID 6 sit against the 48.00 TB reference above, how far would a plausible input error move it (≈13.3 % per ten percent on Number of disks), and what does the alternative option compute to?
What affects RAID 6 the most?
Sensitivity depends on where you start, but the inputs Number of disks and Capacity per disk (TB) typically move RAID 6 further than the rest because they enter the formula multiplicatively. If you can only measure two things well, measure those and hold the others at conservative defaults.
How often should I check RAID 6?
Whenever an input changes by more than a few percent, and otherwise on the rhythm of your decisions — before a purchase, a review, a renewal. RAID 6 is a snapshot, not a subscription; stale snapshots are the most common silent error in Technology & Computing planning.
Is RAID a backup?
No. RAID protects against disk failure, not against deletion, ransomware, controller faults, fire, or a bad firmware flash propagating to every disk at once. The "R" is redundancy, not recovery — you still need independent, off-site copies.
What is the minimum number of disks for RAID 6?
Four — two data disks plus two parity disks. Efficiency only becomes attractive from six or eight disks upward.
Is my data private on this page?
Yes — computation happens locally in your browser, with no account, no tracking of your inputs and nothing sent to a server. The printed formula is the whole story of what happens to your figures.
The vocabulary of RAID 6
- Raw capacity
- 64.00 TB for the RAID 6 worked case; it sits next to the main result because it carries context the headline leaves out.
- Capacity efficiency
- 75.0% in the default RAID 6 calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- Fault tolerance
- Survives any two simultaneous disk failures on the example inputs — a companion figure printed beside the headline of RAID 6 so that no result is read in isolation.
- Usable capacity
- The primary output of this tool: 48.00 TB for the default inputs, computed exactly from usable = (N − 2) × disk size; efficiency = (N − 2)/N; write penalty = 6 I/O per small write.
- Worked example
- Every substitution in order, from the example inputs to 48.00 TB — the audit trail for the headline figure.
- Verdict band
- The plain-language label — here “64.00 TB raw · 75.0% efficiency” — mapping the computed figure onto experience-based ranges for Technology & Computing.
- Sensitivity (±10 %)
- How far the RAID 6 moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
Method, sources and what this tool cannot tell you
Everything above runs on one stated formula, applied without hidden coefficients: usable = (N − 2) × disk size; efficiency = (N − 2)/N; write penalty = 6 I/O per small write. Source of record: Patterson, Gibson & Katz (1988) RAID taxonomy; Plank (2013), "A Tutorial on Reed-Solomon Coding for Fault-Tolerance in RAID-like Systems"; rebuild-window risk per Schroeder & Gibson (2007) and Elerath & Pecht (2009). Where a professional version of this calculation differs, it differs in inputs and jurisdiction-specific coefficients, not in the algebra — which is why re-checking your inputs beats hunting for a second calculator.
This page is an estimation aid, not professional advice. The design rule is simple: two people entering identical figures must get identical answers. Nothing is smoothed, weighted or adjusted behind the scenes; where judgement enters, this page says so and names the input responsible.
Turning this RAID 6 into a decision
Save the number together with its inputs and today’s date; an RAID 6 figure without context is unusable the moment an input changes.
Whoever you are (people who need one trustworthy number before their next decision), the next step is the same: replace the example inputs with your own, note the date, and re-run whenever a figure moves. In Technology & Computing, a dated figure is a trend, and an undated one is a rumour.