Length Conversion Calculator
Convert inches, feet and metric lengths without grabbing a chart
Length conversion FAQ
Why do some plans use inches while others are in millimetres?
It mostly comes down to what the industry and country are used to. In the US, construction, woodworking and retail packaging often use inches and feet, while engineering drawings, machining and most of the world rely on millimetres. This calculator helps you move cleanly between both systems without cluttered charts.
What is the exact relationship between inches and millimetres?
By international agreement, 1 inch is defined as exactly 25.4 millimetres. That means 1 mm is exactly 0.03937 in, 10 mm is about 0.394 in, and 25.4 mm is exactly 1 in. Because the definition is exact, you can use that factor confidently in any precise calculation or CAD model.
How many metres are in a mile?
The international mile used on road signs and maps is exactly 1,609.344 metres. That’s also 1.609344 kilometres. The calculator uses this relationship internally any time you convert miles to kilometres, metres or smaller metric units.
When should I work in millimetres instead of centimetres?
Many designers and machinists prefer millimetres because they avoid decimals: a board is 18 mm thick, not 1.8 cm. Millimetres make sense whenever tolerances matter, from cabinetry to 3D printing, PCB layout and metal fabrication. Centimetres are more common in everyday descriptions like someone’s height or the size of a box.
What are micrometres and nanometres for?
Micrometres (µm) and nanometres (nm) are tiny metric units used for things like film thickness, semiconductor features, optical wavelengths and precision surface finishes. One micrometre is one millionth of a metre, and one nanometre is one billionth of a metre. They’re overkill for a bookshelf, but essential in optics, electronics and materials work.
Does this tool round too much for precise work?
The calculator shows a tidy rounded result but keeps full precision in the background. For cutting wood or checking product specs, a couple of decimal places are usually enough. If you’re doing metrology or tight-tolerance machining, treat the result as a quick sanity check and keep as many significant figures as your process requires.
From tape measure to metric in one line
This length conversion calculator is meant to sit between your tape measure and your plans. You enter a value, choose the starting unit (for example inches, feet or miles) and the target unit (millimetres, centimetres, metres, kilometres, and so on). The result box then shows a clean line like “72 in = 1.8288 m” that you can read at a glance or drop into an email or project note.
1. Choose units that match your task
The dropdowns cover the most common everyday and technical length units:
- Miles (mi), yards (yd), feet (ft) and inches (in) for road distances, room dimensions and product sizes.
- Kilometres (km) and metres (m) for maps, sports distances and engineering drawings.
- Centimetres (cm) and millimetres (mm) for packaging, furniture, cutting lists and sewing patterns.
- Micrometres (µm) and nanometres (nm) for films, coatings, optics and electronics work.
US-style units appear at the top of the list so you can quickly convert tape-measure numbers into metric values your tools, spreadsheets or clients expect.
2. Base-unit method: everything through metres
Internally, every conversion runs through metres, the SI base unit of length. Each unit has a fixed factor that tells the calculator how many metres it equals:
- 1 in = 0.0254 m exactly.
- 1 ft = 0.3048 m (12 in).
- 1 yd = 0.9144 m (3 ft).
- 1 mi = 1,609.344 m (5,280 ft).
- 1 km = 1,000 m, 1 cm = 0.01 m, 1 mm = 0.001 m.
- 1 µm = 0.000001 m, 1 nm = 0.000000001 m.
The calculator first converts your input to metres, then divides by the target unit’s factor to get the new value. That two-step process keeps everything consistent with modern definitions of the inch, foot, mile and SI metric units.
3. Length conversion factors at a glance
If you prefer to eyeball numbers, this table lists some of the common length conversions that the calculator uses internally. Multiply the starting unit by the conversion factor to get the resulting unit.
| Starting Unit | Resulting Unit | Conversion Factor |
|---|---|---|
| US and metric length | ||
| miles (mi) | kilometres (km) | 1.609344 |
| miles (mi) | metres (m) | 1,609.344 |
| yards (yd) | metres (m) | 0.9144 |
| feet (ft) | metres (m) | 0.3048 |
| metres (m) | feet (ft) | 3.28084 |
| kilometres (km) | metres (m) | 1,000 |
| Small-scale conversions | ||
| inches (in) | centimetres (cm) | 2.54 |
| inches (in) | millimetres (mm) | 25.4 |
| centimetres (cm) | millimetres (mm) | 10 |
| millimetres (mm) | inches (in) | 0.03937 |
| micrometres (µm) | millimetres (mm) | 0.001 |
| nanometres (nm) | micrometres (µm) | 0.001 |
4. Reading and using the result
The result card is intentionally simple. If you enter 2.5 and convert from metres to feet, you’ll see a line like “2.5 m = 8.2021 ft”. For most real-world jobs, rounding to something like 8.2 ft or 8′ 2½″ is enough. For drawings, motion control or simulations, you can keep the full number and treat this tool as a quick cross-check between unit systems.
Because every value passes through metres and uses the modern exact definitions for the inch, foot, mile and SI prefixes, you get conversions that line up cleanly with standards used in engineering, physics, mapping and manufacturing.
References and further reading on length units
These sources describe how key length units are defined and relate to each other:
- Inch — exact definition and history — explains that the inch is defined as exactly 25.4 millimetres and how that standard was adopted.
- Mile — international mile and metric equivalents — notes that the international mile is exactly 1,609.344 metres and 1.609344 kilometres.
- Metre — SI base unit of length — covers the metre as the base SI unit and explains how prefixes like milli-, micro- and nano- generate mm, µm and nm for very small distances.
For legal documents, manufacturing standards or survey work, always check against your organisation’s official unit definitions and tolerances before finalising numbers.