What is the difference between ISO 10816 and ISO 20816?

ISO 20816 is the current standard and progressively supersedes the ISO 10816 series, merging it with the former ISO 7919 shaft-vibration standards into one framework. ISO 10816-1 was superseded by ISO 20816-1:2016, and ISO 10816-3 was carried forward as ISO 20816-3. The zone concept and the numeric limits were kept, so the two are continuous rather than conflicting.

What do vibration zones A, B, C, and D mean?

Zone A is the vibration of a newly commissioned machine, Zone B is acceptable for unrestricted long-term operation, Zone C is unsatisfactory for long-term running and calls for planned corrective action, and Zone D is severe enough that damage can occur and the machine should not keep running. In practice: A and B leave alone and trend, C open a work order, D stop and investigate.

What is an acceptable vibration level in mm/s?

It depends on the machine's size and mounting. Using ISO 10816-3 / 20816-3, a rigidly mounted medium machine (15 to 300 kW) crosses from acceptable into the action zone at about 2.8 mm/s RMS and into the danger zone at about 4.5 mm/s, while a large machine on a flexible support is allowed considerably higher levels before crossing the same lines.

Is 4.5 mm/s vibration bad?

It depends entirely on the machine. On a medium machine (15 to 300 kW) on a rigid base, 4.5 mm/s RMS sits at the C/D boundary and is a danger-level reading. On a large machine on a flexible support, the same 4.5 mm/s is only the B/C boundary, worth acting on but not an emergency. You cannot judge vibration without classifying the machine first.

Where do you measure vibration on a machine?

On the non-rotating parts, normally each bearing housing, measured in the horizontal, vertical, and axial directions at the same marked points every time. ISO 10816/20816 uses broadband RMS velocity, typically over 10 to 1000 Hz. Consistent sensor location and mounting are essential, because a poor mount can add or hide several mm/s on its own.

Why is the vibration trend more important than a single reading?

Because the standard itself notes that a change in level can be more significant than the absolute value. A machine that doubles from 1.0 to 2.0 mm/s is developing a fault even though it is still in Zone B. Alarming on the rate of rise, not only the zone boundary, gives weeks of warning to plan a repair before the reading ever reaches the danger zone.

What causes a machine to move into Zone C?

The faults vibration is best at catching: imbalance, misalignment, mechanical looseness, and bearing wear. A rise at running speed after a coupling job often points to alignment, while a climb in the bearing-defect frequencies points to the bearing or its lubrication. Diagnose the cause with spectral analysis before disassembling anything.

ISO 10816 / 20816 Vibration Severity Limits

Is 4.5 mm/s bad? It depends on the machine. Here is how the four severity zones and the RMS-velocity limits work, and why ISO 20816 now replaces ISO 10816.

ISO 10816 (now largely superseded by ISO 20816) sets vibration severity limits for rotating machines by sorting measured RMS vibration velocity into four zones: A for newly commissioned machines, B for unrestricted long-term operation, C for restricted operation needing action, and D for dangerous levels where damage can occur. The limits scale with machine size and how it is mounted.

Every plant that owns a vibration meter eventually asks the same question: is 4.5 mm/s bad? The honest answer is "it depends on the machine," and ISO 10816/20816 exists precisely to turn that judgment into a table anyone can apply. This guide covers what the standard measures, what the four zones mean, the actual velocity limits by machine group, why ISO 20816 replaced ISO 10816, and how to apply the numbers without misreading a healthy machine as a sick one.

What does ISO 10816 / 20816 actually measure?

It measures broadband vibration on the non-rotating parts of a machine, typically the bearing housings, reported as RMS velocity in millimetres per second over a standard frequency band, usually 10 to 1000 Hz. Velocity is used because, across that band, it correlates well with the destructive energy in most rotating-machinery faults, which makes it a fair single yardstick for overall machine health.

Two words in that sentence matter. Broadband means it is one overall number, not a spectrum, it tells you the machine is rough, not why. Diagnosing the cause still needs a spectrum and the training behind it, the kind covered by condition-based maintenance analysis. Non-rotating parts means the sensor goes on the bearing housing, not the shaft; shaft-relative vibration is covered by the companion measurements that ISO 20816 now folds in. Getting the sensor location and mounting right is its own discipline, see accelerometer mounting methods because a bad mount can add or hide several mm/s all by itself.

The four evaluation zones are universal; only the mm/s values that separate them change with machine group and mounting. A and B are healthy, C means take action, D means stop.

What do the four zones A, B, C, and D mean?

The zones translate a velocity reading into a decision. They are the same across every part of the standard, even though the numeric boundaries differ by machine:

Zone A the vibration of a newly commissioned or reconditioned machine. This is what "good" looks like off the shop floor.
Zone B acceptable for unrestricted long-term operation. A machine living in A or B is fine; keep trending it.
Zone C unsatisfactory for long-term continuous operation. The machine can usually run for a limited period while you plan and schedule corrective action, but it is on the clock.
Zone D severe enough to cause damage. This is a stop-and-investigate level, not a "watch it" level.

The practical reading: A and B are "leave it alone and trend it," C is "get a work order open," and D is "do not keep running this if you can avoid it." That maps cleanly onto the priorities in your maintenance KPIs and work-order backlog.

Has ISO 20816 replaced ISO 10816?

Yes. ISO 20816 is the current standard and progressively supersedes the ISO 10816 series, merging it with the old ISO 7919 shaft-vibration standards into one unified framework. ISO 10816-1 was superseded by ISO 20816-1:2016, and the widely used industrial-machine part, ISO 10816-3, has been carried forward as ISO 20816-3. The good news for anyone with existing procedures: the zone concept, the evaluation method, and the numeric boundaries were kept, so the transition is mostly a change of cover page.

In the field you will still hear people say "ISO 10816" for years, because that is the number stamped on a decade of analyzer presets and inspection sheets. It is not wrong to know the values by that name, just cite ISO 20816 as the current document when you write a new procedure, and treat the two as continuous rather than conflicting.

What are the actual vibration limits by machine size?

The limits come from ISO 10816-3 / 20816-3, which sorts most industrial machines into groups by power and rates them differently depending on whether they sit on a rigid or a flexible support. Larger machines and more flexible mounts are allowed higher velocities before they cross into the next zone. The table below shows the standard RMS-velocity boundaries most rotating-equipment programs use:

Machine group	Support	A/B (mm/s)	B/C (mm/s)	C/D (mm/s)
Group 2: medium, 15–300 kW	Rigid	1.4	2.8	4.5
Group 2: medium, 15–300 kW	Flexible	2.3	4.5	7.1
Group 1: large, 300 kW–50 MW	Rigid	2.3	4.5	7.1
Group 1: large, 300 kW–50 MW	Flexible	3.5	7.1	11.0

So the answer to "is 4.5 mm/s bad?" is now concrete. On a 45 kW pump (Group 2) on a rigid base, 4.5 mm/s sits right at the C/D line, dangerous. On a large machine on a flexible frame, the same 4.5 mm/s is only the B/C boundary, worth acting on, but not an emergency. Same number, very different verdict, which is exactly why you cannot judge vibration without knowing the machine.

Standard measurement points: each bearing housing, measured horizontally, vertically, and axially. Consistent locations are what make the trend meaningful across months.

How do you apply the standard without misreading a machine?

You apply it by measuring consistently, classifying the machine correctly, and paying as much attention to the trend as to the zone. The standard itself notes that a change in vibration level can be more significant than the absolute value, a machine that doubles from 1.0 to 2.0 mm/s is telling you something even though it is still deep in Zone B. Work through it in order:

Classify the machine. Find its power and decide Group 1 or Group 2, then determine whether the support is rigid or flexible. This picks the correct row of limits; guessing here throws off every judgment that follows.
Fix the measurement points. Mark each bearing housing and measure the same spots, in the same directions, every time. Repeatability is what turns readings into a trend.
Measure under normal operating conditions. Take readings at normal speed, load, and temperature. Vibration changes with all three, so a reading at idle is not comparable to one under load.
Compare to the zone boundaries, then to history. Place the reading in A, B, C, or D, but also compare it to this machine's own baseline. A sharp rise inside Zone B warrants a look before the absolute number ever reaches C.
Set alarms off the boundaries and act on them. A common scheme sets an alert near the B/C line and a danger alarm near C/D, then routes each exceedance to a work order rather than a note. Diagnose the cause with spectral analysis before you tear anything down.

When a machine does cross into C, the usual suspects are the faults that vibration is best at catching: imbalance, misalignment, looseness, and bearing wear. Rising velocity at running speed after a coupling job often points back to alignment, the reason a laser shaft alignment check pays for itself, while a climb in the bearing frequencies points at the bearing or its lubrication, tying straight into lubrication failure modes.

A machine trending upward inside Zone B is already talking. Alarming on the rate of change, not only the zone boundary, buys weeks of planning time.

The numbers worth knowing

The values in this guide come straight from the standards and the maintenance economics behind them:

ISO 20816-1:2016 defines the four-zone evaluation framework (A/B/C/D) and supersedes ISO 10816-1, unifying the earlier bearing-housing and shaft-vibration standards.
ISO 20816-3 (carrying forward ISO 10816-3) sets the industrial-machine boundaries used above, for example, roughly 2.8 mm/s at B/C and 4.5 mm/s at C/D for a rigidly mounted medium machine.
The U.S. Department of Energy's Federal Energy Management Program O&M Best Practices guidance reports that vibration-based predictive maintenance saves roughly 8–12% over preventive maintenance alone, which is the economic case for measuring against these zones in the first place.

Where do the zones fit in a reliability program?

The ISO zones are the scorecard, not the whole game. They give you an objective, defensible line for when a machine needs attention, but the value only compounds when the readings are trended over time and tied to what you do about them. A single Zone C reading is a data point; twelve months of readings on the same points, next to the work orders they triggered, is a reliability program.

That is where most vibration efforts stall, the numbers live in an analyzer or a spreadsheet and never meet the maintenance history. Plants that push their vibration data, machine monitoring feeds, and work orders into one searchable record can finally see which machines keep drifting toward Zone C and why, the same way the team in our CLS case study turned scattered paper logs into searchable plant knowledge. Tie the zones to that record and vibration stops being a periodic chore and becomes the backbone of your predictive maintenance and overall equipment reliability effort (see how Harmony connects floor data to decisions).