TORQUE STANDARD REFERENCE
ISO 6789 Torque Wrench Calibration:
Accuracy Classes, Tolerances and What to Verify
ISO 6789 sets the accuracy classes and calibration method for hand torque tools, so a setting of 100 N·m means a known, tested range — not a number stamped on a handle. The 2017 edition defines Class 1 (±4%) and Class 2 (±6%).
THE STANDARD
What Is ISO 6789?
ISO 6789 is the international standard titled Assembly tools for screws and nuts: Hand torque tools. It applies to hand-operated torque wrenches and torque screwdrivers — and not to powered or assembly-line tools, which follow separate standards.
HAND TORQUE TOOLS
One Standard, Split Into Parts
2017
Current Edition
The standard is deliberately split into parts, and the split matters because each part produces a different document. A buyer who asks for one and receives the other has not received what they asked for.
Hand tools only.
Powered and assembly-line torque tools are covered by separate standards and cannot cite ISO 6789 for conformity.
APPLIES TO
Hand tools
Torque wrenches and torque screwdrivers used for controlled tightening.
EXCLUDES
Powered tools
Assembly-line and power tools follow separate standards entirely.
FOUR PARTS
How the Standard Is Organised
The 2017 revision separated conformance testing from calibration to protect the word “calibration,” so a manufacturer’s declaration could no longer be mistaken for a traceable measurement.
01
ISO 6789-1:2017
Requirements and methods for design conformance testing and quality conformance testing, plus marking. It produces a declaration of conformance.
02
ISO 6789-2:2017
The calibration method and the determination of measurement uncertainty. It is the only part that produces a calibration certificate.
03
ISO 6789-3
Performance verification and interim checking for users, introduced after the 2017 revision.
04
ISO/TS 6789-4
Planned, covering angle measurement, which was separated out of the earlier scope.
ACCURACY CLASSES
ISO 6789 Accuracy Classes and Tolerances
The current (2017) edition uses a two-class system. The class is the maximum permissible deviation between the torque the tool is set to indicate and the torque it actually applies, expressed as a percentage of the indicated value.
CURRENT EDITION
ISO 6789:2017
CLASS 1
±4%
CLASS 2
±6%
VALID RANGE
20% – 100%
01
CURRENT SYSTEM
The 2017 Two-Class System
The class is a statement of how tightly the tool controls the torque it delivers — not a general measure of build quality.
| Class | Maximum Permissible Deviation | Typical Tool Type |
|---|---|---|
| Class 1 | ±4% of indicated value | Adjustable click wrenches; indicating wrenches (dial or electronic) |
| Class 2 | ±6% of indicated value | Preset (fixed-setting) wrenches |
02
WITHDRAWN SYSTEM
The Superseded 2003 Letter Classes
Before 2017, ISO 6789 classed tools first by type — Type I for indicating tools, Type II for setting tools — and then by a letter from A to G. Documentation that cites “Type II Class B” is using the withdrawn system.
| 2003 Class (Withdrawn) | What It Described | 2017 Equivalent |
|---|---|---|
| Type II Class A | Adjustable graduated wrench | Class 1 |
| Type II Class B | Fixed-setting wrench | Class 2 |
| Type II Class C | Adjustable non-graduated wrench | Class 2 |
| Type I Class A | Torsion / flexion-bar wrench | Class 1 (indicating) |
PRODUCT POSITIONING
The Class Is a Positioning Decision, Not Just a Compliance Box
A ±4% Class 1 wrench and a ±6% Class 2 wrench are two different products at two different price points, and the class drives what a distributor can claim.
ACCURACY POSITION
Class 1
A line positioned on accuracy needs Class 1 documentation to back the claim.
VALUE POSITION
Class 2
A line positioned on value can sell a Class 2 tool honestly without overstating it.
SEQUENCE
Class First
Choose the class before you write the marketing — that is the difference between a defensible claim and one that collapses at a retail listing.
VERIFY BEFORE YOU CLAIM
ISO 6789 conformance is product- and market-specific.
Confirm the exact standard, certificate, test report and SKU for your market, and check which edition the documentation cites before you rely on a class designation.
THE DOCUMENT QUESTION
ISO 6789-1 vs ISO 6789-2 The Document You Actually Need
The single most important distinction on this page is between the two documents the two parts produce. A buyer who asks for a calibration certificate and receives a declaration of conformance has not received a calibration.
DECLARATION ≠ CALIBRATION
A Declaration of Conformance Is Not a Calibration
A declaration states that the tool was produced to conform to the standard. A calibration certificate is a traceable measurement. Only the second one is a calibration.
ISO 6789-1
Declaration
Issued by the manufacturer
→
ISO 6789-2
Certificate
Issued by an accredited lab
The 2017 revision made this split on purpose.
Manufacturers’ earlier “calibration certificates” often lacked enough information to be traceable.
01
ISO 6789-1:2017
DECLARATIONCovers design conformance testing and quality conformance testing, plus marking. Issued by the manufacturer, with no measurement uncertainty and no validity period.
02
ISO 6789-2:2017
CERTIFICATECovers calibration and the determination of measurement uncertainty. Issued by a laboratory accredited to ISO/IEC 17025 — the only part that produces a calibration certificate.
03
What the Certificate Reports
CONTENTA calibration certificate reports the actual readings with their measurement uncertainty. It does not declare the tool “in tolerance” — the user makes that judgement against the class.
BUYER’S RULE
Ask for the certificate, not the declaration.
A certificate issued under ISO 6789-2 by an ISO/IEC 17025-accredited laboratory is the only document that can properly be called a calibration under ISO 6789.
ASK FOR
ISO 6789-2
CALIBRATION METHOD
How Torque Wrench Calibration Works Under ISO 6789
Calibration compares the tool against a reference device and records the difference. The procedure is defined tightly enough that the result can be reproduced by a different laboratory.
TEST PARAMETERS
Where the Tool Is Checked
3
points
Calibration is verified at three points across full scale, with the reading repeated at each point so the result is not a single sample.
20
%
60
%
100
%
5
readings
Readings below 20% of capacity
They are not used to judge conformance, which is why a torque wrench should be selected so the working torque falls inside its upper four-fifths.
ACCEPTANCE CRITERIA
How the Error Is Calculated
The error at each calibration point is expressed as a percentage of the indicated value. A tool passes when its error at every point is within the class tolerance.
(measured − nominal) ÷ nominal × 100
THE ONE-QUARTER RULE
The device that checks the wrench must be at least four times more accurate than the tolerance it is checking, or its own error contaminates the result. A certificate whose reference device does not meet that rule is not a valid certificate under ISO 6789-2.
CALIBRATION INTERVAL
How Often Should a Torque Wrench Be Calibrated?
The common industry interval is every 12 months or every 5,000 load cycles, whichever comes first. This is a practice, not a fixed clause — different quality systems impose their own. What matters for a buyer is confirming the supplier states an interval and knows who owns it.
12 months
Scheduled
5,000 cycles
Scheduled
After a drop
Event
Out of tolerance
Interval halved
RECALIBRATION TRIGGERS
What Shortens the Interval
A stated interval is the baseline. Three events move the next calibration forward regardless of where the tool is in its cycle.
01
SCHEDULED
12 Months or 5,000 Cycles
The routine interval. Confirm the supplier states one and that someone owns it after the goods ship.
Standard Practice
02
EVENT TRIGGERED
Drop, Overload or Repair
Any event that can change the calibration state of the tool should be followed by an unscheduled check.
Event Driven
03
CORRECTIVE
Found Out of Tolerance
If a tool fails a scheduled check, the interval is typically halved going forward until it demonstrates consistency.
Interval Halved
RANGE SELECTION
Keep the Working Torque Inside the Upper Four-Fifths of the Tool
The tolerance applies only from 20% to 100% of capacity. A wrench chosen with its working torque near the bottom of its range is the wrong size, whatever the class.
USED FOR CONFORMANCE
20
%
–
100
%
NOT USED
<20
%
PRACTICAL RULE
Size Up, Not Down
Choose a tool whose range brackets your working torque
BUYER CHECKLIST
What to Verify Before You Buy
This section is written for the buyer sourcing torque wrenches as a category, not for the technician holding one. Five things to confirm before the purchase order, not after the container lands.
5-STEP CHECK
Buyer Verification Checklist
Each item below is a question the supplier should be able to answer with a document, not a verbal assurance.
01
Which Class, and Which Edition?
Confirm whether the tool is Class 1 (±4%) or Class 2 (±6%) under the 2017 edition. If the documentation cites letter classes, it is the withdrawn 2003 system.
02
Calibration Certificate or Declaration?
A calibration certificate is issued under ISO 6789-2 by an ISO/IEC 17025-accredited laboratory. A declaration of conformance is not a calibration.
03
Does the Calibration Cover Your Working Range?
The tolerance applies from 20% to 100% of capacity. If your working torque sits below 20% of the tool’s range, the tool is the wrong size.
04
Does the Class Match What You Intend to Claim?
If you plan to market the tool as a precision instrument, the documentation has to be Class 1. If the documentation is Class 2, your claim should be too.
05
Who Owns the Recalibration Interval?
Confirm the supplier states an interval and can provide recalibration documentation, rather than leaving it as your problem after the container lands.
ISO 6789 conformance is product- and market-specific.
Confirm the exact standard, certificate, test report and SKU for your market before making a compliance claim. This is a verification step, not an assumption.
Frequently Asked Questions (FAQs)
Short answers to the questions buyers ask most often when comparing ISO 6789 documentation from different suppliers.
What is the difference between a declaration of conformance and a calibration certificate?
A declaration of conformance, issued under ISO 6789-1, states that a manufacturer produced the tool to conform to the standard. A calibration certificate, issued under ISO 6789-2 by an ISO/IEC 17025-accredited laboratory, is a traceable measurement that reports the actual readings and their uncertainty. Only the second is a calibration.
What is ISO 6789?
ISO 6789 is the international standard for hand torque tools. It sets accuracy classes and calibration methods for torque wrenches and torque screwdrivers. The 2017 edition defines Class 1 at ±4% and Class 2 at ±6% of indicated value.
What is the difference between ISO 6789-1 and ISO 6789-2?
ISO 6789-1 covers design and quality conformance testing and produces a declaration of conformance. ISO 6789-2 covers calibration and the determination of measurement uncertainty, and is the only part that produces a calibration certificate.
How often should a torque wrench be calibrated?
Every 12 months or 5,000 load cycles, whichever comes first, is the common industry interval. Recalibrate sooner after a drop, an overload or a repair. If a tool is found out of tolerance, the interval is usually halved until it proves consistent.
What does a Class 1 torque wrench mean?
A Class 1 torque wrench is allowed a maximum deviation of ±4% of the indicated value under ISO 6789-1:2017. Set to 100 N·m, it may apply between 96 N·m and 104 N·m and still conform.
What is the tolerance of a torque wrench?
Under the current ISO 6789 edition, the tolerance is ±4% for Class 1 tools and ±6% for Class 2 tools. The tolerance applies from 20% to 100% of the tool’s capacity; below 20%, readings are not used to judge conformance.