In this Instructable, you will learn how to calibrate a steel, retractable tape measure. This may sound unnecessary or even silly. However, damage from dropping the tape measure or long use will make measurements inaccurate. Successful calibration will return the tape measure to accuracy.

If the tape measure is still accurate, only the initial tests (Step 2 and Step 3) will be needed. If the tape passes the test, you can skip to the end knowing your tape measure is accurate. Proceed with your fun projects with confidence in accurate measurements.

The hook on the end of the tape measure is made with a small amount of travel at the attaching rivets. As they say, this is not a problem, it is a feature. The travel is to compensate for the thickness of the hook when measurements are made with the hook pushed against inside surfaces or when hooked on the edge for outside measurements. When the hook travel is the same as the hook thickness, the inside and outside measurements will both be accurate.

Without proper calibration, a tape measure used for inside measurement and then used to measure and mark material for cutting will result in shorter or longer material than expected.

Step 1: Tools Needed:

The retractable steel tape measure you want to calibrate. This may be a new tape measure you want to check for accuracy or an old, well used tape measure you suspect is not accurate.
A straight measuring ruler. This should be a trusted ruler with fine, engraved lines for markings. A framing square or tri-square would be good. Increments of 1/16 inch or better is needed. A printed, promotional yard stick is not a good choice, since it will have wide marking lines.
Two pair of pliers. These can be basic slip joint pliers, needle nose pliers or locking type pliers. They do not need to be identical types.

Step 2: Supplies Needed:

A moderate amount of patience.

Step 3: Examine for Damage

Carefully examine the hook end of the tape measure. Do you need to clean dirt and gunk? Is the tape cracked at the rivets attaching the hook? If the tape is damaged, calibration may not improve accuracy and the useful life of the tape may be over.

Step 4: Check Inside Measurement Accuracy

Check the tape accuracy with inside measurement. Press the outside against a surface while the ruler is also pushed against the same surface. Observe the difference between the 1 inch mark for the tape measure and the ruler. If the 1 inch marks are aligned, the tape is accurate for inside measurements.

If the 1 inch marks are not perfectly aligned, write down the TAPE measurement at the 1 inch mark of the ruler. Mark this measurement with INSIDE.

Step 5: Check Outside Measurement Accuracy

Next place the hook of the tape measure over the end of the ruler. Carefully position the hook so the end of the tape hook is extended the maximum distance from the tape. Observe the difference between the 1 inch mark of the tape measure and the 1 inch mark of the ruler.

If the marks are perfectly aligned, the tape measure is accurate for outside measurements. If there is a difference, write down the TAPE measurement at the 1 inch mark of the ruler. Mark this measurement with OUTSIDE.

If both inside and outside accuracy checks are aligned with the ruler, then the tape measure hook does not need adjustment. Your tape measure is accurate and can be used with confidence! Skip down to Step 13 for Bonus Tips.

Step 6: Determine How to Adjust the Hook

Gather your notes from your initial checks for inside and outside accuracy. Remember these measurements were read on the TAPE at the 1 inch mark of the ruler.

INSIDE CHECK If the tape measurement is GREATER THAN the ruler on inside check, then the tape hook is too long and must be bent toward the tape. If the tape measurement is LESS THAN the ruler on inside check, the tape hook is too short and the hook must be bent out from the tape. The amount of the measurement difference from 1 inch is the amount the hook need to bend.

OUTSIDE CHECK If the tape measurement is GREATER THAN the ruler on outside check, the tape hook is too short and needs to be bent toward the tape. If the tape measurement is LESS THAN the ruler on the outside check, the tape hook is too short and must be bent out from the tape. The measurement difference form 1 inch on the ruler is the amount the hook tab needs to be bent.

If the inside check and outside checks indicate opposite tab adjustments, you will not be able to calibrate the tape measure. If you have any doubts about your initial inside and outside checks, repeat the process.

Step 7: Adjusting the Hook

Grip the tape hook at the rivet area with one of the pliers. The locking pliers worked well here if you have them. Grip the hook tab with the second pliers. Be careful to hold the rivet end stationary and only bend the hook tab. Do not damage the end of the flexible tape. Make very small adjustments and then retest for accuracy as noted in the steps below. If you fear you may damage the marks on the tape, wrap a couple turns of electrical tape around the tape measure to protect the marks.

Step 8: Repeat Inside Accuracy Check

Repeat Step 4 to confirm you have adjusted the hook tab as needed for inside measurements.

Step 9: Repeat Outside Accuracy Check

Repeat Step 5 to confirm you have adjusted the hook tab as needed for outside measurement accuracy.

Step 10: Adjust the Hook Again If Needed

If you have not adjusted the hook correctly to pass the latest accuracy check, repeat Step 7 again. Try to make very small adjustments to the hook tab.

Step 11: Check Inside Accuracy Again

Repeat Step 4 to confirm you have adjusted the hook tab as needed for accuracy.

Step 12: Recheck Outside Accuracy

Repeat Step 5 again to confirm you have adjusted the hook tab as needed.

If the inside and outside tests are both correct, you have completed the calibration process! You can now make measurements with assurance you will have accuracy on both inside and outside measurements.

If repeated adjustments cannot make the tape accurate for both inside and outside measurement, the tape may be beyond calibration. You may decide you want to keep it have projects with 'good enough' measurements. If you decide this, mark the tape with a permanent marker.

Consider buying a new tape for the high quality work you do.

Industry Note: If you worked in a factory with a Quality Assurance group. they would require this test and calibration periodically as part of the plant quality program. The idea is that quality work is only done with accurate tools.

Step 13: Bonus Tip 1 - Measure Inside Spaces

You can use a retractable tape measure to measure inside closed spaces like a closet shelf. Most tape measures have the case length marked ( may be 2 to 3 1/2 inches). Place the hook end at one side of the inside space to be measured and push the case against the other side of the opening. Read the distance on the tape at the opening of the case. Then ADD the case length to the first measurement. Be careful to add the fractions correctly.

Step 14: Bonus Tip 2 - Clean Your Tape

Clean the tape measure with water, mild soap and a cloth. Do not submerge the tape in water - it may rust on the inside. Wet the cloth with soapy water and rub the cloth on the tape for cleaning. A clean tape is easier to read, has less friction and will last longer.

Step 15: Bonus Tip 3 - Don't Let the Hook Slam Into the Case

Avoid letting the hook end retract into the case too quickly. the impact of the hook on the case can loosen the rivets or break off the hook. Better tapes have a cushion on the hook stop to reduce this damage.

Step 16: Bonus Tip 4 - Using the Stud Markings

Some tapes have special markings every 16 inches. These marks can be used when laying out carpentry framing for wall stud spacing. Standard wood frame wall studs are spaced every 16 inches.

Disclaimer: These are my suggestions only. I am not responsible for any injury, material or property damage or other undesirable results.

Introduction: Steel Measuring Tape Calibration