Inch Fractions
FIELD GUIDE / INCH FRACTIONS
The small lines divide one inch into equal spaces. Once you know the size of one space, every reading is a whole number plus a fraction.
Start with one inch
The distance from the 0 mark to the 1 mark is one inch. Divide that distance into two equal spaces and each is 1/2 inch. Divide it into four and each is 1/4 inch. Eight equal spaces give eighths; sixteen give sixteenths.
The denominator tells you how many equal spaces make one inch. The numerator tells you how many of those spaces you have counted. 3/8 inch means three of the eight equal spaces in an inch.
| Smallest space | Spaces per inch | Exact decimal |
|---|---|---|
| 1/2 in | 2 | 0.5 in |
| 1/4 in | 4 | 0.25 in |
| 1/8 in | 8 | 0.125 in |
| 1/16 in | 16 | 0.0625 in |
Read a sixteenth-inch ruler
- Find the last whole-inch mark. If the pointer is between 1 and 2 inches, start with 1.
- Count spaces after that mark. If the pointer is three small spaces past 1, the extra part is 3/16.
- Combine them. The reading is 1 3/16 inches.
Do not count the starting whole-inch line as the first space. The first small line after it marks the end of the first space.
Recognise the longer marks
On the practice diagram, whole-inch lines are longest, halves are next, then quarters, eighths and sixteenths. Many real measuring tools use a similar pattern. Real designs vary, so confirm the subdivision spacing and printed numbers instead of relying only on line length.
On a sixteenth-inch scale, the eighth small space is halfway across: 8/16 = 1/2. The fourth is 4/16 = 1/4. The twelfth is 12/16 = 3/4.
Simplify when you can
Reduce a fraction by dividing its top and bottom by the same whole number. For example, 6/16 becomes 3/8 after dividing both by 2. The position on the ruler stays the same.
| Counted in sixteenths | Simplified | Exact decimal |
|---|---|---|
| 2/16 | 1/8 | 0.125 |
| 4/16 | 1/4 | 0.25 |
| 6/16 | 3/8 | 0.375 |
| 8/16 | 1/2 | 0.5 |
| 10/16 | 5/8 | 0.625 |
| 12/16 | 3/4 | 0.75 |
| 14/16 | 7/8 | 0.875 |
Three examples to try
One whole inch plus five eighths: 1 5/8 in, or 1.625 in.
Two whole inches plus four sixteenths: 2 4/16 = 2 1/4 in, or 2.25 in.
Eleven sixteenths from zero: 11/16 in, or 0.6875 in. There is no whole-inch part yet.
Inches and metric units
One inch is exactly 25.4 millimetres. Conversion does not change a length; it expresses the same length in another unit. For this exact relationship, see NIST's length-unit reference. The practice questions ask for the unit shown, so check it before answering.