Torque wrench: types, ISO 6789 tolerances and calibration intervals

Torque wrench calibration compares the torque a wrench delivers or indicates with a calibrated reference, usually a torque transducer, at several points of its range, and records the deviation at each point with its uncertainty (ISO 6789-2; VIM 2.39). Under ISO 6789-2:2017 that means the lowest value of the range (usually 20 % of maximum), 60 % and 100 %, five readings at each, compared with the permissible deviation for the wrench’s type and class, typically ±4 % for a click wrench with a maximum above 10 N·m (ISO 6789-1, clause 5.1.5; Torque Lab). Turning the adjustment screw until the wrench clicks at one setting is an adjustment, not a calibration (VIM 3.11).

This page is the overview. For the home check with a weight and the full lab procedure, see how to calibrate a torque wrench.

Torque wrench types and permissible deviations

ISO 6789-1 sorts hand torque tools into Type I, indicating tools, which show the torque, and Type II, setting tools, which release at a set value, each with classes (clause 4). The class and the maximum torque of the tool set the permissible deviation (clause 5.1.5, Tables 3 and 4):

Common wrench	ISO 6789-1 type and class	Max. ≤ 10 N·m	Max. > 10 N·m
Beam wrench	Type I, Class A	±6 %	±6 %
Dial or digital wrench (rigid housing)	Type I, Classes B and C	±6 %	±4 %
Adjustable click wrench	Type II, Class A	±6 %	±4 %
Preset click wrench	Type II, Classes B and C	±6 %	±4 %
Click torque screwdriver	Type II, Classes D, E, F	±6 %	±6 %

The columns refer to the maximum torque of the tool, not the value set, and the limits apply in each direction the wrench is made to work. A tighter limit declared by the manufacturer takes precedence (clause 5.1.5). The full table, including torque screwdrivers and flexion bar wrenches, is in the procedure.

Standards and guides that apply

Document	Scope	Access
ISO 6789-1:2017	Design and manufacture: types, classes, permissible deviations, the manufacturer’s declaration of conformance	Paid
ISO 6789-2:2017	Calibration: method, calibration system, measurement uncertainty, certificate content	Paid
ASME B107.300-2021	US standard for hand torque tools and torque testers, with an appendix that correlates it with ISO 6789	Paid
DKD-L 10-1 (2020)	How to determine the uncertainty components of ISO 6789-2	Free
DKD-R 10-8 (2020)	Static calibration of the devices used to calibrate torque wrenches	Free

Both parts of ISO 6789 are being revised; the 2017 editions are current (ISO 6789-1, preview). Under the 2017 edition, a new wrench comes with a declaration of conformance, not a calibration certificate (ISO 6789-1, foreword). In the US, many manufacturers rate their wrenches to ASME B107.300 instead, and often declare ±4 % clockwise and ±6 % counterclockwise (Harbor Freight, ICON product page). Use the limit on your wrench’s own declaration and record which standard it comes from.

Calibration intervals

If you have no data of your own, the default commonly cited is 12 months or 5,000 cycles, whichever comes first, and sooner after an overload of 25 % or more, a drop or a repair (Snap-on; Norbar USA). The reference needs its own calibration too: DKD-R 10-8 asks for recalibration of torque wrench calibration devices after 26 months at the latest (§6.2). Then let your as-found results move the interval: see calibration intervals.

In-house or a calibration lab?

An in-house calibration needs a torque calibration system with a valid certificate whose uncertainty is no more than a quarter of the wrench tolerance, about ±1 % for a ±4 % wrench (ISO 6789-2, 4.3, as described by Norbar and Torque Lab; DKD-L 10-1). Check your own ratio with the TUR calculator. If you send wrenches out, ask for an ISO 6789-2 calibration with as-found results and uncertainty from a lab accredited for torque (what accreditation means), and review the certificate with our certificate guide.

FAQ

Which type of torque wrench is the most accurate?

Under ISO 6789-1, it depends on the type, the class and the maximum torque. Dial and digital wrenches with a rigid housing and adjustable click wrenches are allowed ±4 % when their maximum torque is above 10 N·m and ±6 % at or below it; beam wrenches are allowed ±6 % at any size. A manufacturer can declare a tighter limit for a model, and then that limit applies. The calibration certificate tells you how a particular wrench actually performed.

Does a torque wrench need to be calibrated in both directions?

Only if it is made to work in both. ISO 6789-1 applies the permissible deviation in each direction the wrench is designed for, and a calibration tests each of those directions separately, with the same points and readings. Many US manufacturers declare different limits for the two directions, such as ±4 % clockwise and ±6 % counterclockwise. If you only ever use a wrench clockwise, say so when you order the calibration.

Who calibrates the torque tester?

The tester or calibration system needs its own calibration, with a certificate stating its uncertainty; a lab accredited for torque is the easiest way to show it is traceable. The German calibration service guideline for these devices, DKD-R 10-8, asks for recalibration after 26 months at the latest, and sooner after an overload, repair or mishandling. For calibrating wrenches, the system's uncertainty should be no more than a quarter of the wrench tolerance.