How to read a tape measure
Every tape measure has the same simple plan. The big numbers are inches. Between each pair of big numbers are shorter lines, and each size of line is half of the size before it. The longest lines in between are halves, the next are quarters, then eighths, and on a standard tape the shortest are sixteenths. To read a measurement you do three things: find the last whole inch number before the mark, count the small lines after it, and say what size they are. If the mark is 5 lines past the 3 and the shortest lines are sixteenths, you have 3 and 5/16 inches. Tap anywhere on the tape above and the page does exactly that, step by step, and Count it for me lights up each line as it counts.
What the lines mean
An inch is cut in half, then each half in half again, and so on. That makes 2 halves, 4 quarters, 8 eighths and 16 sixteenths in an inch. You can tell the size of a line by its length: the longer the line, the bigger the piece. A line at the middle of an inch is the half. The lines halfway between that and the ends are quarters. The lines halfway between those are eighths, and the shortest lines fill in the sixteenths. One tape maker's guide notes that on some tapes the quarter and eighth lines are drawn the same length, so when in doubt, count from the nearest line you are sure of, and the lines on the tape are always evenly spaced.
Reading fractions and simplifying
You will often count a number of sixteenths that can be made simpler. If you count 4 lines of 1/16 past the 2, that is 4/16, and the same spot is the quarter line, so it reads 2 1/4. To simplify, divide the top and the bottom by the same number: 4/16 divided by 4 on both is 1/4. A shortcut is to read the longest line you pass on the way. If you pass the half line and then count 3 more sixteenths, the reading is 1/2 plus 3/16, which is 11/16. A reading is in its simplest form when you cannot divide the top and the bottom by the same number any more.
Half of a measurement
People search for half of 5/8 and half of 3/4 on a tape because halving a fraction is hard in your head. To halve a fraction, double the bottom. Half of 5/8 is 5/16. Half of 3/4 is 3/8. For a mixed number, halve the whole length: half of 1 3/8 inches is 11/16 inch, because 1 3/8 is 11/8, and half of 11/8 is 11/16. To split a length into three equal parts, multiply the bottom by three: 7 1/4 inches divided by 3 is 2 5/12 inches, which is not on a tape, and the nearest 1/16 line is 2 7/16. The Find tab does these exactly and shows the line on the tape.
The metric side
The metric side is simpler because everything counts in tens. The numbered lines are centimeters. Each centimeter is split into 10 millimeters, with a longer line at 5 millimeters in the middle. If the mark is 4 small lines past the 8, it reads 8 centimeters and 4 millimeters, which is 8.4 centimeters or 84 millimeters. An inch is exactly 25.4 millimeters, so the two sides line up exactly only every 5 inches, because 5 inches is exactly 127 millimeters. Everywhere else they are a little off: 3 5/16 inches is 84.1375 millimeters, about 84 on the metric side. The Both view puts them one above the other.
Tips for accurate measuring
- Do not use the very end if it is bent or worn. Start from the 1 inch line instead and subtract 1 inch from the reading, a habit people call burning an inch.
- Look straight down at the line. Looking from the side shifts the mark against the cut.
- Mark with a V, with the point on the line, instead of a single dash. The point shows the exact spot.
- Measure twice and write the number down as a fraction, not as a decimal, so you do not mix up 5/16 and 0.5.
- Many tapes add layout marks: one every 16 inches for studs (often red) and one every 19.2 inches (often a small black diamond) for joists or trusses. Five spaces of 19.2 inches make 96 inches, so five fit evenly on an 8-foot sheet.
Rounding a measurement to a line
Most real lengths fall between two lines. Choose the nearest line, and decide how close you need to be. A rough cut for framing may only need the nearest 1/8 inch. A cabinet or a trim cut may need the nearest 1/16. The Find tab shows the nearest line for every size and tells you how far off it is, so you can decide. When a length is exactly halfway between two lines, this page rounds up to the larger line. That is this page's rule, not a standard, so cut on the side that keeps the piece from being too short.
Mistakes when reading a tape
- Counting the line you start on. Count the first line after the whole inch as number one.
- Forgetting the whole number. 5/16 and 3 5/16 are three inches apart.
- Reading a quarter as a fourth sixteenth. The quarter line is the 4th sixteenth line, but it reads 1/4.
- Mixing the two sides. Check which side you are reading before you write the number.
- Treating 0.5 as 5/16. A half inch is 0.5 inch and also 8/16.
Where the numbers come from
- One inch is exactly 25.4 millimeters (NIST SP 811, Appendix B).
- The halving pattern of the lines (two sixteenths make an eighth, two eighths a quarter, two quarters a half) is from a tape maker's reading guide. Common builders' tapes mark sixteenths along the blade; some also mark thirty-seconds in the first foot, as store listings for Stanley tapes show.
- The decimal values such as 1/16 = 0.0625 and 1/64 = 0.015625 are exact division and match published decimal-inch charts.
- Stud marks every 16 inches and every 19.2 inches: Stanley lists both on its tapes ("16-in and 19.2-in stud center markings"). The colors vary by maker.
- The tips are common workshop practice, not a standard.
Questions
How do you read a tape measure in inches?
Find the last whole inch number before the mark, count the small lines after it, and say the size of those lines. 5 lines past the 3 on a tape with sixteenths is 3 5/16 inches.
What do the lines on a tape measure mean?
The longest lines are inches, then half inches, quarters, eighths and sixteenths, each half the size of the one before. The shorter the line, the smaller the piece.
What is half of 5/8 on a tape measure?
5/16. To halve a fraction, double the bottom number.
What is half of 3/4 on a tape measure?
3/8. Double the bottom: 3/4 becomes 3/8.
How do you read the metric side?
The numbers are centimeters and each small line is a millimeter. A mark 4 lines past the 8 reads 8.4 centimeters, or 84 millimeters.
How many sixteenths are in an inch?
16. An inch also holds 8 eighths, 4 quarters and 2 halves.
What does the red arrow in the practice mean?
It points at the line you should read. Count the lines from the last whole inch number, then type the reading as a fraction, like 3 5/16.
Can I share a mark?
Yes. Share saves only the mark and the type of tape for 90 days so the link can open the same puzzle, and makes a picture for Facebook that hides the answer.