RAID 5 Calculator
The formula
The large-disk problem
RAID 5’s Achilles heel is rebuild time. With 8–20 TB drives, a rebuild reads the entire array and can take 24–72+ hours, during which every remaining disk runs hot and one unrecoverable read error (URE, rated ~1 per 10¹⁴ bits for consumer drives) means total data loss — statistically meaningful at multi-terabyte scale. For arrays above ~3 disks of large capacity, prefer RAID 6/RAID-Z2 (two-disk tolerance) and keep real backups: RAID is uptime protection, not backup.
Frequently asked questions
RAID 5 vs RAID 6?
Can I mix disk sizes?
Source & methodology
Standard RAID parity math; URE rates from consumer/enterprise drive datasheets (e.g., 1×10¹⁴ bits desktop, 1×10¹⁵ enterprise); rebuild-rate estimates per industry experience.
RAID 5: the number, the method and the caveats
This page was written for one job: turning your own inputs into a defensible figure, with the working shown. RAID 5 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.
Most confusion about RAID 5 comes from mixing up the input you control with the input you merely observe. The formula treats them identically, but your decisions should not: one deserves measurement, the other deserves a defensible estimate.
In plain terms, RAID 5 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.
What makes a calculation page trustworthy is that its copy is derived from its own arithmetic: here usable capacity = 24.0 TB, recomputed every time a field changes, 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. Work down the page and by the end you could re-derive the result in a spreadsheet from memory.
Every input in RAID 5, and what it does to the answer
The engine implements a single stated model: Usable = (N − 1) × disk capacity; efficiency = (N − 1)/N. 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 4) is a plain measured quantity — the unit shown in the label is the unit the formula expects. Count from the record, not the estimate; whole-number inputs are cheap to verify and expensive to get wrong.
Capacity per disk (TB) (example value 8) is a quantity rather than a category — precision here is cheap and worth having. The example value is there to demonstrate the mechanics, not to describe your situation — overwrite it.
Hot spare included? (example value No) is a branching choice rather than a figure you measure; the per-option effects are computed below. If you are unsure what belongs in this field, look at the printed steps: the substitution shows exactly how the engine uses it.
What each “Hot spare included?” choice actually does: No → 24.0 TB · Yes (1 hot spare) → 16.0 TB These figures are computed from this engine, not estimated — switch the dropdown above and you will land on exactly these numbers.
RAID 5 on the example inputs: the complete substitution
The tool prints its own working; this is the same arithmetic on the example values, reproduced so you can follow it by hand. Starting figures: Number of disks, Capacity per disk (TB), Hot spare included?. The computed usable capacity comes out at 24.0 TB, which the interpretation band labels “(4 − 1) × 8 TB”.
- Usable = (N − 1) × disk size = (4 − 1) × 8 = 24.0 TB
- Efficiency = (N−1)/N = 75.0% — improves as you add disks
- Rebuild ≈ usable TB × 10⁶ MB ÷ 150 MB/s ÷ 3600 = 44.4 h
Raw capacity — 32.0 TB. Read next to the headline, this is the figure that shows the shape of the trade-off.
Parity overhead — 8.0 TB. Compare it against the main result of 24.0 TB before drawing conclusions.
Storage efficiency — 75.0%. This is the number experienced users quote when the headline alone would mislead.
Fault tolerance — 1 disk. Read next to the headline, this is the figure that shows the shape of the trade-off.
Approx. rebuild time — 44.4 hours. This is the number experienced users quote when the headline alone would mislead.
Overwrite any input above and every line of this example updates with it — the substitution is live, not a screenshot of somebody else’s numbers.
The RAID 5 corridor: your result across a realistic band
The honest version of any estimate is a corridor, and this one is cheap to produce: RAID 5 was recomputed at 5 points along the dominant input, with every other field left untouched.
| Number of disks setting | Usable capacity |
|---|---|
| 80 % of the example value | 17.6 TB |
| 90 % of the example value | 20.8 TB |
| example inputs | 24.0 TB |
| 110 % of the example value | 27.2 TB |
| 120 % of the example value | 30.4 TB |
Across 5 settings of Number of disks, the result spans 17.6 TB to 30.4 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.
Sensitivity: which inputs change RAID 5 the most
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 5?” — and it is specific to these defaults, because sensitivity moves with the starting point.
- Nudge Number of disks by ±10 % and RAID 5 lands between 20.8 TB and 27.2 TB (sensitivity ≈13.3 %)
- Nudge Capacity per disk by ±10 % and RAID 5 lands between 21.6 TB and 26.4 TB (sensitivity ≈10.0 %)
Number of disks is the dominant lever here: ten percent either way moves the result by about 13.3 %, from 20.8 TB to 27.2 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.
Interpreting the RAID 5 verdict
Nobody acts on a bare figure — they act on a figure with a comparison attached. The default case lands in the “(4 − 1) × 8 TB” band, and bands are deliberately wide because real situations overlap. Before treating the band as fact, note its width: ten percent either way on Number of disks already spans 20.8 TB to 27.2 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 5 sits next to similar measures: this tool answers “how much is the RAID 5”, 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.
Why RAID 5 results go wrong — and how to prevent it
Reusing an RAID 5 from last season. In Technology & Computing the inputs are the thing that drifts; the formula does not.
Reusing an RAID 5 from last season. In Technology & Computing the inputs are the thing that drifts; the formula does not.
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 5 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.
RAID 5 verification checklist
- State, in one sentence, which convention each rate, unit or period in the form is using.
- Compare the result with one independent method or published benchmark.
- Check Number of disks before anything else: it is the measured dominant input, ≈13.3 % of RAID 5 per ten percent.
- Record the date and the inputs alongside the number so future-you knows what was assumed.
- Re-run the calculation with a ±10 % stress on the two most sensitive inputs.
- 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
The tool does the arithmetic; the routine below is what makes the arithmetic trustworthy. Five steps, a minute total — and afterwards you will know not just what the number is, but which assumption produced it.
Step 1 — one document, one pass. Open the statement, label or record that holds number of disks, capacity per disk (tb), hot spare included? and transcribe straight into the form. Transcription from a single source removes the mismatch errors that appear when figures are assembled from three different places. Comparisons are cheap: run the identical calculation for the alternative and look at the gap.
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. Write the figure down before moving on; memory flatters whichever number you saw last.
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. Say the answer out loud before you save it; if it sounds wrong, the model did not change, your understanding did.
Step 4 — run the scenario trio. Expected, pessimistic, optimistic. The gap between pessimistic and expected is the number to plan against; the optimistic result is useful only as a ceiling. Write the figure down before moving on; memory flatters whichever number you saw last.
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. A figure with a date beside it becomes evidence; without one it stays an anecdote.
The real-world conversations RAID 5 settles
In Networks & Storage the figure computed here is usually the first one somebody asks to see. Having it — with the formula behind it — changes your position in the conversation from participant to reference. If the answer changes your behaviour, it deserves a second run with different assumptions.
Professionals in Networks & Storage use this style of calculation as a screening filter before spending real money or effort: a quick RAID 5 check eliminates weak options in minutes, and the survivors earn the deeper, slower analysis. If the answer changes your behaviour, it deserves a second run with different assumptions.
There is a personal-administration use people overlook: periodic self-checking. RAID 5 quietly drifts as life changes — rates move, bodies change, costs compound — and re-running it a few times a year catches drift while correction is still cheap. The input you reach for first is usually the one you measured longest ago.
The questions that come up most about RAID 5
What affects RAID 5 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 % (20.8 TB to 27.2 TB).
Which matters more: capacity per disk or number of disks?
By the numbers, Number of disks: a ten-percent change in it moves the result ≈13.3 %, versus ≈10.0 % for Capacity per disk. Spend your measuring effort in that order — precision on the weaker lever buys almost nothing.
How much does the “Hot spare included?” choice change the answer?
The spread is not cosmetic: this choice alone takes RAID 5 from 16.0 TB to 24.0 TB with every other input frozen, which is more influence than most of the numeric fields on this page can claim.
What is a typical RAID 5?
There is no honest universal average for RAID 5, but there is a reference point: the defaults on this page produce 24.0 TB ((4 − 1) × 8 TB). Replace the inputs with your own and compare — that comparison is the useful meaning of “typical”.
How do I calculate RAID 5 by hand?
Three steps: write the formula (Usable = (N − 1) × disk capacity; efficiency = (N − 1)/N), insert your values, and simplify. The worked example section shows exactly that chain for the default inputs; replicating it once on paper teaches you more about RAID 5 than any number of calculator runs.
What happens to RAID 5 if number of disks changes by 10 %?
The engine was re-run with that field moved by ten percent while everything else stayed fixed: the usable capacity goes across 20.8 TB to 27.2 TB, a swing of roughly 13.3 % around the 24.0 TB baseline. That ratio is the practical definition of how much this input matters.
Is the RAID 5 result of 24.0 TB good or bad?
Good and bad are comparisons, so the honest test is three questions: where does your own RAID 5 sit against the 24.0 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?
Is RAID 5 accurate?
The arithmetic is exact: Usable = (N − 1) × disk capacity; efficiency = (N − 1)/N is evaluated with full floating-point precision and every substitution is printed in the worked example. What limits accuracy is input quality and the model’s assumptions, so treat the 24.0 TB result as the correct answer to the question you typed — and check the assumptions section before acting on it.
How do I calculate RAID 5 manually?
Write down the inputs, then follow the printed steps in order — Usable = (N − 1) × disk capacity; efficiency = (N − 1)/N. A spreadsheet replicates it with two or three cells. The manual route is worth doing once to build intuition; after that the calculator saves time and eliminates transcription slips.
RAID 5 vs RAID 6?
RAID 6 tolerates two simultaneous disk failures at the cost of one more disk of parity and slightly slower writes. With large modern disks, RAID 6 is the safer default for 4+ drives.
Can I mix disk sizes?
Technically the array sizes to the smallest disk, wasting the rest. Use matched disks — same model and capacity ideally.
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 5
- Raw capacity
- 32.0 TB on the example inputs — a companion figure printed beside the headline of RAID 5 so that no result is read in isolation.
- Parity overhead
- 8.0 TB for the RAID 5 worked case; it sits next to the main result because it carries context the headline leaves out.
- Storage efficiency
- 75.0% on the example inputs — a companion figure printed beside the headline of RAID 5 so that no result is read in isolation.
- Usable capacity
- The primary output of this tool: 24.0 TB for the default inputs, computed exactly from Usable = (N − 1) × disk capacity; efficiency = (N − 1)/N.
- Worked example
- The step-by-step substitution printed above, so the arithmetic can be re-derived by hand or in a spreadsheet.
- Verdict band
- A named zone on the range of plausible results; the example inputs land in “(4 − 1) × 8 TB”, and your inputs may not.
- Sensitivity (±10 %)
- How far the RAID 5 moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
The provenance of this RAID 5 calculation
Everything above runs on one stated formula, applied without hidden coefficients: Usable = (N − 1) × disk capacity; efficiency = (N − 1)/N. Source of record: Standard RAID parity math; URE rates from consumer/enterprise drive datasheets (e.g., 1×10¹⁴ bits desktop, 1×10¹⁵ enterprise); rebuild-rate estimates per industry experience. Any other implementation of this model should return the same figure for the same inputs. If one does not, the difference is in its assumptions — which this page prints and that one probably does not.
This page is an estimation aid, not professional advice. The design rule is simple: two people entering identical figures must get identical answers. The promise to the reader is narrow and testable: same inputs, same output, no silent rounding, and no figure presented without the formula that produced it.
Next steps after the calculation
Save the number together with its inputs and today’s date; an RAID 5 figure without context is unusable the moment an input changes.
The audience here is people who need one trustworthy number before their next decision. Everything you need to defend the result — formula, substitution, sensitivity, source — is printed above, so the only remaining variable is the honesty of your inputs.