How to Calibrate A Protractor

How to know that your protractor is accurate? What is the way to assess that your protractor’s accuracy is still within the permissible accuracy (tolerance)? What if your protractor actually needs a replacement? All of these questions can be answered with a protractor calibration.

This article will provide a basic idea of the techniques for calibrating protractors conducted by certified calibration laboratories as well as a simple guide and idea of protractor calibration beneficial for people who use their protractors for hobbies, interests, and small businesses and workshops.

There are different types of protractors (half-circle, single-arm, double-arm, vernier model, etc). Different types mean different shapes. That said, the way to calibrate for each type is also different.

In this article, we are covering only the bevel vernier protractor and the double arm protractor models, because these two models are frequently used for a considerable accuracy.

But still, it is advisable that an accredited/certified calibration lab shall conduct that protractor calibration, like any measuring device. International standard such as ISO 9001 and ISO 17025 which covers general calibration requirements define personnel or lab who can conduct calibration.

A. Bevel Protractor Calibration

Bevel protractor or vernier protractor or micro protractor or universal bevel protractor is the most precision model of protractor. It measures down to 5 minute resolution (the smallest reading). This model is available in analog and digital version. The analog version employs a vernier scale.

The process of calibrating protractors discussed below is based on the standard requirements of bevel protractors stated in the BS 1685:2008 standard, entitled “Bevel Protractors (Mechanical and Optical) – Requirements and Test Methods”. So, if you want to learn further, you can read the standard entirely.

Measuring tools and equipments to prepare for this bevel protractor calibration are:

  1. Sine Bar
  2. Angle gauge Block Set
  3. Surface Plates
  4. Gauge blocks
  5. Height gauge with protractor holder
  6. Magnetic Blocks / V-Blocks
  7. Dial Indicator
  8. Cleaning solvents and oil
  9. Lint-free cloth
  10. Arkansas Stone
  11. Soft-bristle brush

1. Preliminary Inspection of the Bevel Protractor

In this stage, conduct a visual inspection of the bevel protractor.

1.1 All parts of the protractor must be complete.

1.2 Scales and graduations must be visible, including the vernier scale and main scales. For micro protractors, the graduations on the thimble and sleeve must also be checked properly.

1.3 Check the knobs, and locking nuts are functioning well.

1.4 On the magnifying lens of the vernier scale, there shouldn’t be any noticeable scratches that might obscure the scale.

1.5 The blades, and working edges, must be free from rust. Burrs, deformation, and warpage are also not allowed.

1.6 For digital bevel protractors, the readings on the LCD must be clear and visible.

1.7 Perform preliminary flatness and parallelism checks on the working edges and blades of the protractor. Place the working edges and blade on a surface plate or any toolmaker’s flat. Hold both ends slowly. There must be no movement or rocking between the blade and the surface plate or toolmaker’s flat.

1.8 Examine the blade clamps and locks operation. Move the blade from side to side when unclamped. The sliding motion of the blades shall be smooth.

1.9 The protractor must not be contaminated with excessive oil, dirt, or dust.

1.10 Prepare a Calibration Data Sheet. Important content of the calibration datasheet basically includes the following:

a. Protractor Name / Type

b. Serial No., if there’s any

c. Actual Calibration Date indicating Start Date and Finish Date

d. Time of Inspection may also be included if necessary

e. Previous Calibration Date

f. Next Calibration Date

g. Temperature (Before, After and necessary Time intervals During calibration)

h. Table form of a list of Calibration Check Points and Measurement Points as well as the blank portion for the recording of actual results.

i. Calibration Evaluation Result/Findings.

1.11 Record the result on the prepared datasheet. Determine the category if the defects can be repaired. Major defects must be reported to authorized repair facilities.

2. Reconditioning of the Bevel Protractor and Other Materials for Calibration

2.1 Disassemble the protractor head and the blade.

2.2 Clean the protractor. Thoroughly wipe the blade clean using cleaning oil and solvents. Any dirt, dust, and excessive oil must be removed using a cleaning solvent and lint-free cloth. Any unnecessary foreign object must be removed.

2.3 Using Arkansas stone remove the rust and burrs on the blade.

2.4 Use a soft bristle brush and cleaning oil to remove rust and dust on the screw locks.

2.5 Apply only a thin layer of oil to the metal parts of the bevel protractor to prevent rust. Lubricate the bolt threads properly. Wipe any excess oils and lubricants that may affect the measurement performance of the instrument.

2.6 Demagnetize the instrument.

2.7 Clean all the reference tools and equipment that are needed for the measurements during the calibration process, such as the sine bar, angle gauge, height gauge, V-blocks, and gauge blocks.

2.8 Clean the surface plates to be used for calibration.

2.9 Store the protractor and all the tools and instruments in the calibration room at a temperature of 20°C or 63°F ± 2 and relative humidity of 30% to 40% at least 3 hours before the calibration.

2.10 Record the actual temperature on the check sheet.

3. Selection of Calibration Measurement Points for Bevel Protractor

3.1 Define the measurement points during calibration. Input on the datasheet.

3.2 Choose at least five measurement points based on the principle of 5-point calibration. Based on the availability of your reference masters such as the angle gauge, you may choose at least 3 or 5 measurement points of a specific angle. You may also wring the angle gauge up to the maximum range of the device. There are no rules on how many measurement points you need but you shall always consider the measurement range of your protractor.

However, if you intend to use other equipment, such as a universal measuring scope, profile projector, or CMM, you can also select custom calibration points, but always include the minimum angle of 0 degrees or 90 degrees, 180 degrees, and the protractor’s maximum angle range. There are no existing rules or standards that define the measurement points, but having a reference from the previous calibration is recommended.

3.3 The accuracy of measurement or the maximum permissible error in accordance with the content of BS 1685:2008 shall be within the tolerance of 5 arc minutes for Type A and B mechanical bevel protractors. The protractor’s accuracy is also indicated in the product’s technical specifications as defined by its manufacturer.

3.4 Measurement uncertainty must also be considered when evaluating the calibration result, so data needed for measurement uncertainty value must also be recorded. Common sources of measurement uncertainty for protractor calibration are uncertainty of the instruments, reference masters or the standards used for data gathering and measurement comparison, temperature, wringing error in case of wringing angle gauges, gauge blocks, or any other precision blocks, coefficient of thermal expansion (CTE), the parallelism of blade and ruler, instruments’ resolution, and repeatability. You may use several available software tools or calculators to compute the uncertainty value for interpreting calibration results. You may also make your own by putting formulas on an excel worksheet.

4. General Procedure for the Calibration of Bevel Protractor

4.1 Start by recording the calibration room temperature and humidity using the hygrometer. Calibration checksheets must be filled out properly.

4.2 Make sure that the protractor and other instruments to be used have already undergone preliminary inspection and reconditioning before starting the measurement process.

4.3 Clean the surface plate properly.

4.4 Measure the flatness and straightness of the blades and the working edges of the stock and acute angle attachment.

Below are the measurement requirement of flatness and straightness according to BS 1685:2008.

A. Both side of blades


For blade length of 150mm (6″), the flatness is 0.125 mm (.005″)
For blade length of 300 mm (12″), the flatness is 0.25 mm (.010″)


For blade length of 150 mm (6″), the straightness is 0.010 mm (.0004″)
For blade length of 300 mm (12″), the straightness is 0.020 mm (.0008″)


For blade length of 150 mm (6″), the parallelism is 0.0125 mm (.0005″)
For blade length of 150 mm (6″), the parallelism is 0.0250 mm (.001″)

B. Stock (Working edge)

The stock’s straightness is 0.010 mm (.0004″)

C. Acute Angle Attachment (Working edge)

The working edge’s straightness is 0.005 mm (.0002″)

Parallelism to the working edge of the stock is 0.0125 mm (.0005″)

There are several methods to check the straightness, flatness, and parallelism of the blades, as well as the working edges of a bevel protractor. The easiest method is by using a dial indicator. The accuracy of the tool or equipment must be higher than the instrument under test.

4.5 Assemble the gage block and the sine bar to measure the certain angle or the measurement points of calibration. Properly wring the gauge blocks where the sine bar will be mounted. If using a sine bar as the tool for calibration, make sure to indicate also on the checksheet the needed gauge block along with its corresponding angle to prevent the possible mistake in wringing gauge blocks. An angle gauge may also be used as a reference as long as it is 3 times more accurate than the protractor under test. When using an angle gauge, simply mount the angle gauge on the calibration grade surface plate and get the measurement result using the protractor under calibration.

4.5 Double-check the angle set on the sine bar using an angle gauge. Position the angle gauge on the slope of the sine bar. Using a dial indicator measure the surface of the angle gauge. Measurement results shall be accurate within 0.

4.6 Mount the protractor if possible on a height gauge using the appropriate holder. You may also use a V-block as mounting support for the protractor or the calibration grade surface plate.

4.7 Record the result on the data sheet or check sheet.

4.8 Repeat the process to all measurement points.

4.9 Interpret the result. The result of the actual maximum error shall be compliance with the maximum permissible error specified by BS 1685:2008.

B. Two Arm Protractor Calibration (Mechanical and Digital Model)

For the two arm protractor, the examples are General Tools 822 Digital Angle Finder and typical miter protractor.

Both these tools have the two arms rule whose straight edges serve as the measurement faces for the angles.

The calibration process for these two types of protractors must include some of the same procedure for the calibration of the steel rules which is one of the main components of these angular devices, however, detailed procedure will not be discussed here. But, if the ruler contains no graduation, then the graduation accuracy should not be subject to inspect.

Gauges and equipments for two arm protractor are:

  1. Angle Gauge Block Set
  2. Surface Plates
  3. Precision L-Square
  4. Cleaning solvents and oil
  5. Lint-free cloth
  6. Arkansas Stone

1. Preliminary Inspection of the Two-Arm Protractor

1.1 All parts of the Angle finder and Miter protractor must be complete.

1.2 Scales and graduations must be visible.

1.3 The locking knobs must be functioning well and the rules shall be moved smoothly.

1.4 For the digital angle finder, the readings on the LCD must be clear and readable.

1.5 There shall not be any deformation, burrs, or chip-off portion on the rules. Any visible wear or obvious deformation on the rules shall be subject to necessary repair if possible.

1.6 Using the surface plate, check for flatness. Place all of the measuring edges against the surface plate. Check each side individually.

1.7 Prepare a Calibration Datasheet. Basic contents must be included.

1.8 Fill out the datasheet. If the defects are repairable, classify them. Significant defects must be confirmed to authorized repair facilities.

2. Reconditioning of the Double Arm Protractor

2.1 Clean the surface to remove any dirt, dust, and foreign substances and materials on the instrument.

2.2 Burrs on the steel rules can be corrected using Arkansas Stone without damaging the scales and graduations.

2.3 Demagnetize the protractor.

2.4 Clean the precision square, angle gauge block set, and the surface plate.

2.5 Store the protractor and all tools and instruments in the calibration room for at least 3 hours before the calibration at a temperature of 20°C or 63F ±2 and relative humidity of 30% to 40%.

3. Selection of Calibration Measurement Points of the Two-Arm Protractor

3.1 Select the measurement points.

3.2 Input the measurement points and tolerance on the prepared datasheet.

4. General Procedure for the Calibration of Double-Arm Protractor

4.1 Conduct inside measurement by placing the angle gauge between the measuring face or the inside edge of the protractor. For the digital angle finder, zero set first the LCD by properly closing the two rule arms together.

4.2 To conduct outside measurement, set first the protractor to 90° or 0° by placing one outside edge of the protractor against the surface plate and the other side to one side of the precision L-square to verify the 90° setting of the protractor. When using a digital angle finder for measuring outside acute angles, set the display of the protractor to zero.

4.3 Same with the 90° and 0° setting, conduct outside measurement by placing one outside edge against the surface place and the other edge to the measuring face of the angle gauge.

4.4 Repeat the measurement process to all measurement points.

4.5 Interpret the result.

Why Do Protractors Need Calibration?

Protractors require calibration just like any other measuring tools such as rulers, calipers, micrometers, square tools, and different gauges. Protractors will undergo calibration since a variety of circumstances and factors could compromise the measurement accuracy to measure angles.

Many businesses, especially those in compliance with ISO standards such as ISO 9001 and ISO 13485, require calibration certificates for all the measuring devices they employ. Additionally, this is done to guarantee that ISO-certified businesses provide their clients with high-quality products and services.

Small businesses and amateurs alike seek the development of quality products, and services, just like these ISO-certified companies do. Although calibration is not a requirement, quality instruments like protractors significantly impact the potential and success of businesses.

Are All Types of Protractors Need Calibration?

In general, all protractors, regardless of the material they are built of as long as they are used for precise measurement and inspection applications, require calibration.

However, the price of calibration services is expensive. In addition to the protractor’s intended use, the cost of the protractor compared to the cost of calibration must be taken into account.

For instance, you perhaps suspect that your half-circle protractor no longer provides accurate measurement while you use it for simple projects and mechanical drawings. Before opting to send your protractor to a calibration laboratory, you can conduct a simple comparison of its measurement to those of other protractors you have. It’s also a good idea to replace it with a new protractor because this kind may not be able to be adjusted and corrected if the measurement became too off. It is advised to do verification rather than calibration for this kind of semi-precision measuring instrument.

A simple method of verification and calibration to check the accuracy of this type of protractor will be briefly discussed in this article.

In the case of other types of precision protractors, such as bevel protractors and digital protractors, it is suggested to send such protractors to a traceable calibration laboratory.

How Often Do Protractors Need Calibration?

The calibration cycle or calibration interval for protractors can be decided by the owner of the measuring instrument.

Only the owners are aware of how and where their measuring tools and instruments are being utilized. Since owners are accountable for their protractors, they shouldn’t be persuaded to extend the calibration intervals in order to cut costs for calibration service since calibration will prevent unintended damage that cannot be repaired and will necessitate an additional expense for owners in the form of buying a replacement. In fact, some businesses shorten the calibration cycle and calibration interval of their measuring tools not only protractors to avoid future issues, lessen non-conforming products, and most importantly attract new customers. Profit will increase and expenses will decrease as a result.

Protractors containing gears, such as bevel protractors and digital protractors, as well as those whose materials are temperature sensitive, should ideally be calibrated annually from the viewpoint of someone who uses a protractor on a daily basis. Aside from these, frequently used protractors must also undergo annual calibration.

Protractors’ proper maintenance and care may also be taken into account while setting the calibration cycle. If you are the owner and you are absolutely sure you operate your protractors correctly and that you schedule preventative maintenance, you may extend the calibration period to at least two to three years.

Aside from the owners, their customers may also choose the protractors’ calibration cycle and interval.

There are also guides to deciding the calibration interval as suggested by The International Organization of Legal Metrology (OIML) documented in ILAC G24:2007 Guidelines for the determination of calibration intervals of measuring instruments.