PLC IO Count Calculator

Work out plc io count instantly with clear inputs, formula shown and shareable results.

About PLC IO Count Calculator

IO counts are sized with spare capacity because adding a module later means rack space, wiring and often a rebuild of the address map. Twenty percent spare on each type is the usual allowance, and module counts round up since partial modules do not exist.

Formula

IO sizing

points required = actual x (1 + spare); modules = ceil(points / channels per module)

Worked example

With the default values below, PLC IO Count Calculator returns Total IO points of 149. Change any input above and every figure here is recalculated in your browser.

Inputs used

Digital inputs: 64 points
Digital outputs: 48 points
Analogue inputs: 12 channels
Spare capacity: 20 %

Results

Total IO points: 149 points
16-channel DI modules: 5 modules
16-channel DO modules: 4 modules
8-channel AI modules: 2 modules

Step by step

1Values used
2IO sizing
3Total IO points= 149 points
416-channel DI modules= 5 modules
516-channel DO modules= 4 modules
68-channel AI modules= 2 modules

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, fees or charges or compounding frequency into account.
Figures are rounded for display to whole numbers, 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 much spare should I allow?

20% is standard, more on a first-of-type installation. It is far cheaper to fit a module now than to extend a rack in a live plant.

Do analogue points need more allowance?

Usually yes. Analogue additions are more common during commissioning, and each channel needs its own scaling and alarm configuration.