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Redundancy Calculator (N+1, N+2)

Size an N+1 or N+2 cluster: units required, spare capacity, per-unit load after a failure and the cost of redundancy.

About Redundancy Calculator (N+1, N+2)

N is the number of units the load actually demands, and the redundancy level adds units on top so that failures do not reduce served capacity. The post-failure figure is the check that matters: if it exceeds 100%, the cluster is redundant on paper but overloads the survivors. N+1 that pushes the remaining units to 115% of rated capacity is not redundancy, it is a delayed cascading failure.

Formula

Redundancy Calculator (N+1, N+2)

N = ceil(load ÷ unit capacity); deployed = N + spares (or 2N for full duplication); post-failure load per unit = load ÷ (deployed − 1).

N
Units needed to carry the load with nothing spare
spares
Units that may fail without breaching capacity

Worked example

With the default values below, Redundancy Calculator (N+1, N+2) returns Units to Deploy of 5. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total load to carryaccepts 1–100,000,000 units of load
3,600 units of load
Capacity per unitaccepts 1–100,000,000 units of load
1,000 units of load
Redundancy levelone of N (no redundancy), N+1, N+2, N+3, 2N (full duplication)
N+1
Cost per unitaccepts 0–100,000,000 currency
15,000 currency

Results

Units to Deploy
5
Units Needed to Carry the Load
4
Per-Unit Load After One Failure
90.0%
Cost of the Redundant Units
$15,000
Simultaneous Failures Survivable
1

Step by step

  1. 1Values used
  2. 2Redundancy Calculator (N+1, N+2)
  3. 3Units to Deploy= 5
  4. 4Units Needed to Carry the Load= 4
  5. 5Per-Unit Load After One Failure= 90.0
  6. 6Cost of the Redundant Units= 15,000
  7. 7Simultaneous Failures Survivable= 1

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Redundancy Calculator (N+1, N+2) calculated?

N = ceil(load ÷ unit capacity); deployed = N + spares (or 2N for full duplication); post-failure load per unit = load ÷ (deployed − 1). N is the number of units the load actually demands, and the redundancy level adds units on top so that failures do not reduce served capacity. The post-failure figure is the check that matters: if it exceeds 100%, the cluster is redundant on paper but overloads the survivors.

Why does Redundancy Calculator (N+1, N+2) matter?

N+1 that pushes the remaining units to 115% of rated capacity is not redundancy, it is a delayed cascading failure.

What values do I need to enter?

This calculator takes 4 inputs: Total load to carry, Capacity per unit, Redundancy level, Cost per unit. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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Redundancy Calculator (N+1, N+2) – Calculate Units to Deploy