Reading a Tape Measure
Read fractions on a tape quickly and accurately, and take and mark measurements without errors.
Table of contents
A tape measure is the first tool most people use on a job, and it is also where many expensive mistakes start. Cut a board 1/8 inch short and it goes in the scrap bin. Lay out a wall 1 inch off and every trade after you has to work around it. Reading a tape fast and correctly takes only some practice, and once you have it, the skill stays with you.
What You Will Be Able to Do
- Name the parts of a tape measure and explain why the hook is loose.
- Read any mark to the nearest 1/16 inch.
- Take inside and outside measurements correctly.
- Mark a measurement so it stays accurate.
- Add and subtract tape readings.
Parts of a Tape
- Blade: the marked steel ribbon.
- Hook: the metal tab at the end of the blade.
- Case: the housing. Most print the case length on the back or bottom.
- Lock: holds the blade out.
- Standout: how far the blade reaches before it folds over.
Why the Hook Is Loose
The hook slides about 1/16 inch on purpose, and it is not broken. The slide equals the thickness of the hook itself. When you pull the hook over an edge, it slides out, so the measurement starts at the inside of the hook. When you push the hook against a surface, it slides in, so the measurement starts at the outside of the hook. Either way you get a true reading. If the rivets get bent or the hook is damaged, the tape can no longer be trusted, so check it against another tape or replace it.
How the Marks Work
Most US tapes divide an inch into 16 parts. The length of each line tells you what it is.
| Line length | Fraction | Count from the inch |
|---|---|---|
| Longest | 1 inch | Numbered |
| Next longest | 1/2 | Halfway |
| Next | 1/4 | 1/4, 3/4 |
| Short | 1/8 | 1/8, 3/8, 5/8, 7/8 |
| Shortest | 1/16 | 1/16, 3/16, 5/16 and so on |
Some tapes go down to 1/32 inch for the first few inches, and some print the fractions on the blade to help you learn.
Reading a Measurement
- Read the last whole inch before the mark.
- Count the 1/16 lines past that inch.
- Reduce the fraction.
Worked example: The end of the board lands 6 small lines past 14 inches.
- Whole inch: 14
- Lines past it: 6, so 6/16
- Reduce: divide top and bottom by 2 to get 3/8
- Reading: 14-3/8 inches
Reducing is a shortcut. If the top and bottom numbers are both even, cut them in half and repeat. So 12/16 becomes 6/8, then 3/4.
Instead of counting from the inch every time, learn to find the nearest easy landmark. If the mark is one small line past 1/2, you know it is 9/16. One line short of 3/4 is 11/16.
Feet and Inches
Most tapes also show feet, often with a small "1F" or a marker each 12 inches, so a reading of 40 inches may be shown near "3F 4". Plans use feet and inches, such as 3'-4". Keep the two straight.
Worked example: Convert 87 inches to feet and inches. 87 / 12 = 7 feet with 3 inches left over, so 7'-3".
Other Marks on the Blade
- Red numbers or boxes each 16 inches show standard stud spacing, called 16 inches on center. That means center to center.
- Small black diamonds each 19.2 inches show a spacing for joists and trusses, which fits five spaces into 8 feet.
- Metric marks: some tapes show millimeters along one edge. Each 10 mm is 1 cm. Do not mix the two scales on one job. 1 inch equals 25.4 mm.
Taking a Measurement
Outside Measurements
- Hook the end over the edge of the material.
- Pull the blade tight and straight, without letting it sag or bow.
- Keep it square to the edge. A slanted tape reads long.
- Read the mark at eye level, straight on, so you do not get a false reading from the side.
Inside Measurements
For the inside of a window opening or between two walls:
- Push the hook against one side.
- Run the case to the other side.
- Read the blade at the case, then add the case length printed on the housing. Many cases are 3 inches long.
Or make two readings: mark a spot, measure from each side to it, and add them together.
Burning an Inch
If the hook is damaged or the end is hard to hold, start at the 1-inch line and subtract 1 inch from your reading. This is called burning an inch. It is accurate, but it is also the classic way to end up an inch short. If you do it, say it out loud or write it down.
Worked example: You hold the 1-inch mark on the edge and the tape reads 37-1/4 inches. The real length is 36-1/4 inches.
Marking a Measurement
A fat pencil line can be 1/16 inch wide by itself. To keep your mark accurate:
- Use a sharp pencil or a marking knife.
- Make a crow's foot, a small V with its point exactly on the mark. A single line can leave you guessing which edge counts.
- Square the mark across the piece.
- Mark an X on the waste side of the line.
- Cut on the waste side, so the blade's width comes out of the scrap.
Allow for the Kerf
The kerf is the slot a saw blade cuts. The blade turns that strip of wood into sawdust, so it comes off your board. A circular saw kerf is about 1/8 inch. Always cut on the waste side of your mark, so that the kerf comes out of the scrap.
Worked example: You want a 30-inch piece and a 24-inch piece from a 54-inch board. The numbers add up, but the cut removes 1/8 inch, so the second piece comes out at 23-7/8 inches.
Adding and Subtracting Readings
To add fractions, use the same bottom number.
Worked example: 3-1/2 inches plus 2-3/4 inches.
- Change 1/2 to 2/4.
- Add fractions: 2/4 + 3/4 = 5/4, which is 1-1/4.
- Add whole numbers: 3 + 2 = 5.
- Total: 6-1/4 inches.
To find the center of a board, divide the length by 2. Half of 30-3/4 is 15-3/8, since half of 30 is 15 and half of 3/4 is 3/8.
Common Mistakes
| Mistake | What it costs | How to avoid it |
|---|---|---|
| Misreading 6 as 9 or a 1/8 for a 1/16 | Wrong cut | Read twice, and check the whole inch |
| Forgetting a burned inch | A piece 1 inch short | Say it out loud, or do not burn it |
| Ignoring the kerf | A piece about 1/8 inch short | Cut on the waste side of the mark |
| Letting the tape sag | The reading comes out long | Pull it tight and support long spans |
| Reading at an angle | The reading is off by 1/16 or more | Look straight down at the mark |
| Using two different tapes on one job | Tapes can differ slightly | Use one tape for the whole layout |
| Forgetting the blade thickness of the cut | Pieces come up short | Cut on the waste side |
The old saying is true: measure twice, cut once. It takes seconds, while a new board costs money and time.
Check Your Understanding
1. The edge of a part lands 3 short lines past 8 inches on a tape divided into sixteenths. What is the reading?
8-3/16 inches. Three sixteenths cannot be reduced.
2. You measure inside a cabinet. The blade reads 21-1/2 inches at the case, and the case is 3 inches long. How wide is the opening?
24-1/2 inches.
3. A coworker notices the hook on your tape slides and offers to tighten the rivets. Should you let them?
No. The hook is supposed to slide, by the thickness of the hook, so that inside and outside readings are both true.
4. You need to find the middle of a 48-5/8 inch board. Where is it?
24-5/16 inches. Half of 48 is 24 and half of 5/8 is 5/16.
Key Takeaways
- The longer the line, the bigger the fraction. Count sixteenths, then reduce.
- The loose hook is by design, and it makes inside and outside measurements both correct.
- For an inside measurement, add the case length.
- Mark with a sharp pencil and a crow's foot, and cut on the waste side.
- Keep the tape tight, straight and square, and read it straight on.
- Measure twice before you cut.