Copy the whole measurement before calculating
A shelf, frame or storage box might be described as 30 × 12 × 18 in. That is a set of three lengths. Convert each with the same factor, keep them in order and check which label means width, depth and height. Changing units cannot determine the manufacturer’s dimension order for you.
The label in or double-prime symbol ″ means inches. The label ft or prime symbol ′ means feet. Plain text often substitutes quotation marks. If the notation is ambiguous, establish the unit first: a number alone is not a complete dimension. The examples below are fictional and use the international inch.
An exact factor can still produce a rounded display
One international inch is exactly 2.54 cm, or 25.4 mm. One foot contains 12 inches. Multiply inches by 2.54 to obtain centimeters; divide centimeters by 2.54 to obtain inches. NIST lists these unit relationships in its conversion tables.
cm = in × 2.54 in = cm ÷ 2.54 mm = in × 25.4
Unit relationships: NIST — Guide to the SI, Appendix B.8.
For an individual value, use inches to centimeters, centimeters to inches or millimeters to inches. Choose the starting unit from the original label, rather than the unit you want the final answer to use.
Why 5′ 8″ does not mean 5.8 feet
Measurement: 5 feet 8 inches
5 × 12 + 8 = 68 in 68 × 2.54 = 172.72 cm
A foot is divided into twelve inches, not ten. Rewriting 5′ 8″ as 5.8 ft changes the measurement: 5.8 ft represents 5 feet 9.6 inches, or 176.784 cm. The error is 4.064 cm.
A clear method is to combine the feet and remaining inches into one inch measurement first. Enter 68 in the inches-to-centimeters converter. If the original quantity really is decimal feet, use the length converter and select feet as the starting unit.
How to enter a half, an eighth or a mixed fraction
Divide the numerator by the denominator to turn a fraction into a decimal. For a mixed number, retain the whole part and add the fraction. The measurement 2½ in means 2 + 1 ÷ 2, giving 2.5 in. That equals 6.35 cm or 63.5 mm. It does not mean 2.05 in.
Premeri accepts a decimal value rather than notation such as 2 1/2 or ⅜. Enter 2.5 or 2,5 in the numeric field. For 3/8 in, calculate 3 ÷ 8 = 0.375 first, then enter that decimal value.
| Inches | Decimal inches | Millimeters |
|---|---|---|
| ¼ in | 0.25 in | 6.35 mm |
| ⅜ in | 0.375 in | 9.525 mm |
| ½ in | 0.5 in | 12.7 mm |
| ¾ in | 0.75 in | 19.05 mm |
| 2½ in | 2.5 in | 63.5 mm |
Worked example: will the shelf fit?
Shelf: 30 × 12 in; available space: 75 × 32 cm
Width: 30 × 2.54 = 76.2 cm Depth: 12 × 2.54 = 30.48 cm
The 30.48 cm depth is less than 32 cm, but the 76.2 cm width exceeds the available 75 cm by 1.2 cm. The shelf therefore does not fit in this fictional orientation. Checking only its depth would miss the problem. Check height and any other obstructions separately when they matter to the actual arrangement.
Do not combine the dimensions into one sum and treat that total as the width. Do not multiply width by depth when you need an ordinary length, either: their product is an area. To check a fit, create matching pairs: item width against available width, depth against depth, and height against height.
Round at the end, especially when clearance is small
Suppose a flat piece measures 10 in across and an opening measures 25.3 cm. The converted width is 25.4 cm, making the difference 1 mm. If you round the item to 25 cm too early, the calculation suggests clearance that is not there. Extra digits do not make the original measurement more accurate, but dropping useful digits can conceal a difference.
Keep the unrounded converted values, put both measurements in the same unit and subtract before choosing a display precision. For example, 20 cm is approximately 7.874 in. Converting the coarser rounded value 7.9 in back gives 20.066 cm. The factor has not changed; the discrepancy comes from rounding the intermediate value.
A quick check before using the answer
- Does the original label use inches, feet or centimeters?
- Have feet and remaining inches been combined using 1 ft = 12 in?
- Have fractions been converted to decimals before entering them?
- Have all necessary dimensions been converted and matched by their meaning?
- Have you retained enough digits to see a small difference between sizes?
Sources and how this guide was prepared
The explanations and worked examples on this page were prepared for Premeri. Examples are fictional calculations, rather than reports about real plots or products. Unit definitions were checked against the references cited; the arithmetic follows those relationships.
If you spot an error in a factor, calculation or explanation, send the page address and a specific example through our contact page.