What are vibration zones A, B, C, and D?

ISO 20816-1 defines four evaluation zones: Zone A — newly commissioned machines in excellent condition; Zone B — acceptable for long-term unrestricted operation; Zone C — not suitable for long-term operation, remedial action needed; Zone D — dangerous vibration levels, damage may occur, immediate shutdown recommended.

How do I select the correct machine group?

Group 1: Large machines >300 kW on rigid foundations. Group 2: Medium machines 15–300 kW on rigid foundations. Group 3: Pumps (multistage, vertical, etc.). Group 4: Large machines >300 kW on flexible foundations. If unsure, consult the machine manufacturer or use Group 2 as a conservative default.

What is the difference between velocity and displacement?

Vibration velocity (mm/s RMS) is the standard measurement for evaluating machine condition per ISO 20816-1 and is preferred for most rotating machinery in the 10–1000 Hz range. Displacement (μm peak-to-peak) is useful for low-speed machines (<600 RPM) where velocity readings may be very small.

What action should I take for Zone C?

Zone C indicates the machine is not suitable for long-term continuous operation. Plan maintenance action promptly — investigate root cause (misalignment, unbalance, bearing wear, looseness), schedule corrective action, and increase monitoring frequency. The machine may operate for a limited period while preparing for repair.

What is the difference between ISO 20816 and ISO 10816?

ISO 20816 is the modern replacement for ISO 10816. The zone boundaries and methodology are largely the same, but ISO 20816 consolidates and updates the older standard series. ISO 20816-1 replaces ISO 10816-1, with minor editorial improvements and clarifications.

Free Engineering Tool #019

Vibration Evaluation Zones A/B/C/D

Evaluate machine vibration severity per ISO 20816-1. Select machine group, enter measured vibration, and get the zone classification with recommended actions.

ISO 20816-1 Zones A/B/C/D General Guidelines

Machine Group

Measurement Type

Measured Value (mm/s RMS)

Quick presets

Results

Vibration Zone

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Measured Value

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Machine Group

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Zone	Boundary	Description

Zone Evaluation Concept

ISO 20816-1 provides general guidelines for evaluating machine vibration by comparing measured vibration values against zone boundaries. The standard defines four zones:

Zone A — Vibration of newly commissioned machines typically falls within this zone
Zone B — Machines with vibration in this zone are normally considered acceptable for unrestricted long-term operation
Zone C — Machines with vibration in this zone are normally considered unsatisfactory for long-term continuous operation; remedial action should be taken at the next convenient opportunity
Zone D — Vibration values within this zone are normally considered severe enough to cause damage to the machine

Vibration Measurement

The primary measurement parameter is vibration velocity in mm/s RMS (root mean square), measured on the bearing housing in the frequency range 10–1000 Hz. This parameter has been shown to correlate well with machine condition for most rotating machinery.

Zone Boundaries by Machine Group

Zone Boundary	Group 1 Large rigid	Group 2 Medium rigid	Group 3 Pumps	Group 4 Large flexible
A/B	2.3 mm/s	1.4 mm/s	2.3 mm/s	3.5 mm/s
B/C	4.5 mm/s	2.8 mm/s	4.5 mm/s	7.1 mm/s
C/D	7.1 mm/s	4.5 mm/s	7.1 mm/s	11.0 mm/s

ℹ️ Note: ISO 20816-1 provides general guidelines. For specific machine types, refer to the dedicated parts: ISO 20816-2 (steam turbines >40 MW), ISO 20816-3 (industrial machines 15 kW–50 MW), ISO 20816-4 (gas turbines).

⚠️ Important: Zone boundaries are guideline values. Machine-specific acceptance criteria may be defined by the manufacturer. Always consider trending — a significant increase in vibration may warrant investigation even if the absolute level is within Zone B.

ISO 20816-1 Vibration Zones A/B/C/D Evaluation Tool

Published by Nikolai Shelkovenko on February 15, 2026 March 5, 2026

Results

Zone Evaluation Concept

Vibration Measurement

Zone Boundaries by Machine Group