What Is a Length Converter?
A length converter translates a distance measurement from one unit to another — feet to meters, inches to centimeters, miles to kilometers. Length is arguably the most commonly converted quantity online, since the US measures nearly everything in feet, inches, and miles while the rest of the world runs on meters and kilometers.
Every unit on this page reduces to a single base unit — the meter — before converting back out to whatever unit you asked for. That's also exactly how the calculator above works internally, per NIST's published SI length standards.
How to Use This Converter
- Enter a value in the From field — the result updates as you type, no button needed
- Pick units from either dropdown — twelve length units are available, from micrometers to light-years
- Swap directions anytime with the ⇄ button between the two fields
- Check the panel below to see your value converted into all twelve units at once
- Type into the To field if you want to convert backward — it works in both directions
The Conversion Formula
Length conversion is simpler than most other unit types — no offsets, just multiplication, because every length unit is a fixed multiple of a meter:
Because 0.0254, 0.3048, and 0.9144 are all exact by international definition, any two US-to-metric length conversions you run will always agree — there's no measurement uncertainty baked in, only the rounding you choose to display.
Height Conversions (Feet-Inches to Centimeters)
One of the most common length lookups is human height. Every row here comes from the same math: total inches × 2.54.
| Feet-Inches | Total Inches | Centimeters |
|---|---|---|
| 5'0" | 60 in | 152.4 cm |
| 5'2" | 62 in | 157.5 cm |
| 5'4" | 64 in | 162.6 cm |
| 5'6" | 66 in | 167.6 cm |
| 5'8" | 68 in | 172.7 cm |
| 5'10" | 70 in | 177.8 cm |
| 6'0" | 72 in | 182.9 cm |
| 6'2" | 74 in | 188.0 cm |
| 6'4" | 76 in | 193.0 cm |
Going from 5'0" to 6'4" — a 16-inch span — covers 40.6 cm (193.0 minus 152.4), which is why a 2-inch height difference (5.1 cm) reads as noticeable on a form but usually isn't visually dramatic in person.
Running and Road Distances
Race distances are where mi-to-km conversion gets used constantly, and the numbers aren't round in either direction — worth having on hand rather than recalculating every time:
| Distance | Kilometers | Miles |
|---|---|---|
| 100 m sprint | 0.1 km | 0.0621 mi |
| 5K | 5 km | 3.1069 mi |
| 10K | 10 km | 6.2137 mi |
| 1 mile run | 1.6093 km | 1 mi |
| Half marathon | 21.0975 km | 13.1094 mi |
| Marathon | 42.1951 km | 26.2188 mi |
Notice the marathon isn't a clean 42 km or 26 miles in either system — it's 42.195 km, a distance fixed by the International Olympic Committee based on the 1908 London Olympics course, not a round number in either unit. A half marathon, being exactly half, comes out to 21.0975 km.
Small-Scale: Millimeters and Inches
Below the human scale, mm-to-inch conversions show up in tools, screens, and hardware specs — a wrench sized in mm won't fit a bolt sized in inches, and the mismatch isn't always obvious at a glance:
| Millimeters | Inches |
|---|---|
| 1 mm | 0.0394 in |
| 5 mm | 0.1969 in |
| 10 mm | 0.3937 in |
| 25 mm | 0.9843 in |
| 50 mm | 1.9685 in |
| 100 mm | 3.9370 in |
25 mm lands just short of a full inch (0.9843 in, not 1.0) — close enough that a "25mm ≈ 1 inch" approximation works for rough carpentry, but not for anything that needs a precise fit, like threading a bolt or fitting a phone case.
Nautical Miles and Light-Years
Two length units live outside everyday use but show up often enough to be worth explaining. A nautical mile — used in aviation and maritime navigation — is defined as 1,852 m exactly, which works out to 1.15078 statute (regular) miles. It isn't arbitrary: one nautical mile equals one minute of latitude along a meridian, tying the unit directly to the Earth's geometry.
A light-year measures distance, not time, despite the name — it's how far light travels in one Julian year: 9,460,700,000,000 km (about 9.4607 × 10¹⁵ meters). Proxima Centauri, the nearest star to the Sun, sits roughly 4.2465 light-years away, which converts to about 40.17 trillion km — a distance that makes even a marathon's 42.195 km look microscopic by comparison.
6 Practical Tips for Length Conversion
- The three "exact" US units make chained conversions safe. Since 1 in = 0.0254 m, 1 ft = 0.3048 m, and 1 yd = 0.9144 m are all defined exactly, converting inches → feet → meters → back to inches will always return your original number with no rounding drift, unlike some other unit categories.
- A mile is 1.609344 km — not 1.6 flat. For short distances the rounding barely matters, but over a marathon's 26.2188 miles, using 1.6 instead of 1.609344 introduces an error of about 0.24 km — noticeable if you're pacing a race by kilometer markers.
- 25mm is not quite an inch. It's 0.9843 in, about 1.6% short. For woodworking or 3D-printing tolerances under a millimeter, that gap matters; for a rough visual estimate, it doesn't.
- Nautical miles and statute miles differ by about 15%. 1 nmi = 1.15078 mi, so a boat's speed in knots (nautical mph) reads faster than the same speed would in regular mph — a 20-knot boat is moving at about 23 mph, not 20.
- Height differences compress visually more than they read numerically. The 40.6 cm gap between 5'0" and 6'4" (152.4 cm to 193.0 cm) spans the entire adult human height range, yet most of that range clusters within about 20 cm of average.
- For screens and diagonal measurements, remember size ≠ width. A 55-inch TV is measured corner-to-corner, not side-to-side — the same inch-to-cm math applies (55 in × 2.54 = 139.7 cm), but it's the diagonal, not the horizontal width, that the number describes.