Length Converter
Convert between metric, imperial and nautical units of length with exact, treaty-defined factors and full double-precision accuracy across sixteen units.
Length Converter
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Results are a model, not a quotation — an institution's own figures govern.
- Every input is an assumption; change one and the answer changes with it.
- Rounding is applied only at the display layer, so totals may differ by a cent from a statement that rounds each line.
In short: Length conversion works by expressing the input in metres — the SI base unit — then dividing by the target unit's metre value. Because the inch is defined as exactly 0.0254 m and the mile as exactly 1609.344 m, imperial conversions are exact rather than approximate.
Formula
Each factor is the number of metres in one of that unit. Converting through the SI base unit means only two multiplications are ever required, regardless of the pair chosen.
Worked Example
- Identify the source factor. One metre is 1 m by definition.
- Convert to the base unit. 1 × 1 = 1 m.
- Identify the target factor. One inch is exactly 0.0254 m.
- Divide. 1 ÷ 0.0254 = 39.370079 in.
- Check the inverse. 0.0254 ÷ 1 = 0.0254, so one inch is 2.54 cm exactly.
Metrology note. Since the 1959 International Yard and Pound Agreement the inch has been defined as exactly 0.0254 metres. That makes every imperial-to-metric length conversion exact by definition, not an approximation — a distinction that matters in engineering tolerances.
Strengths & Limits Of This Model
Where this engine is strong
- Runs entirely in your browser — no figure you type is transmitted or stored.
- Shows the full working, so every number can be traced and challenged.
- Free, unmetered and free of affiliate incentives.
Where it stops
- Generalised assumptions cannot capture every individual circumstance.
- Jurisdiction-specific rules and mid-year changes may not be reflected.
- A model output is not a substitute for a professional review of your position.
Practical Use Cases
Engineering drawings and machining
Mixed-unit drawings are a common source of costly error. Converting at full precision and rounding only at the final dimension avoids tolerance stack-up, particularly on parts specified to thousandths of an inch.
Construction and material take-offs
Timber, steel and fixings are frequently specified in one system and sold in another. Convert once, then work in a single system throughout — see the Area Calculator for area take-offs.
Navigation and aviation
The nautical mile is defined as exactly 1852 m, one minute of latitude at the equator. It remains standard in marine and air navigation precisely because it maps directly onto angular position.
Methodology & Editorial Standards
Every factor in this engine is the exact number of metres in one unit of the source. Metric prefixes are exact powers of ten; the inch, foot, yard and mile derive from the 1959 International Yard and Pound Agreement and are exact; the nautical mile is exactly 1852 m by international agreement; the light-year is exactly 9,460,730,472,580,800 m, following from the defined speed of light and the Julian year. Results display to eight significant figures, switching to exponent notation outside the range 10⁻⁴ to 10¹². All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Length Converter — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many centimetres are in an inch?
Exactly 2.54 cm. This is a definition rather than a measurement, established by the 1959 International Yard and Pound Agreement, so the conversion carries no rounding error at any precision.
How do I convert metres to feet?
Divide by 0.3048, since one foot is exactly 0.3048 m. One metre is therefore 3.2808399 feet. To go the other way, multiply feet by 0.3048.
How many kilometres are in a mile?
Exactly 1.609344 km, because the international mile is defined as exactly 1609.344 m. A 5 km run is therefore 3.1068560 miles.
What is a nautical mile?
Exactly 1852 metres by international agreement. It was originally defined as one minute of arc of latitude, which is why it remains the standard unit in marine and air navigation — distance maps directly onto angular position on a chart.
Why is the metre defined by the speed of light?
Since 1983 the metre has been defined as the distance light travels in vacuum in 1/299,792,458 of a second. Defining it against a fundamental constant makes it reproducible in any laboratory, unlike the physical prototype bar it replaced.
What is the difference between a US survey foot and an international foot?
The international foot is exactly 0.3048 m; the US survey foot was 1200/3937 m, longer by about two parts per million. The survey foot was officially deprecated at the end of 2022. This engine uses the international foot.
How precise are these conversions?
Calculations run at full IEEE-754 double precision, roughly 15 to 17 significant decimal digits, and display eight significant figures. Because conversion routes through the SI base unit, only two operations occur regardless of the pair chosen.
How far is a light-year?
Exactly 9,460,730,472,580,800 metres, about 9.46 trillion kilometres. It follows arithmetically from the defined speed of light multiplied by the Julian year of 365.25 days.
What are rods, chains and furlongs?
Traditional surveying units still encountered in land records and legal descriptions. A rod is 5.0292 m, a chain is 4 rods or 20.1168 m, and a furlong is 10 chains or 201.168 m. Eight furlongs make a mile.
Why does my result show scientific notation?
Because the value falls outside the range where fixed decimal notation is readable — below 10⁻⁴ or at or above 10¹². Displaying 0.000000001 m as 1×10⁻⁹ is both shorter and less error-prone to read.
Should I round before or after converting?
Always convert first and round last. Rounding an intermediate value and then converting propagates the error through every subsequent step, which is how tolerance stack-up occurs in multi-stage calculations.
Is the yard used outside the imperial system?
Rarely for general measurement, but it persists in sport — American football, cricket pitch length and golf are all measured in yards. It is defined as exactly 0.9144 m.
Is this length converter free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.
Which currency does it use?
Amounts display in US$ accounting format, but the underlying mathematics is currency-agnostic. The result is identical in any currency, so simply read the figures in your own.
Why does a result show an em-dash?
An em-dash indicates the calculation is not defined for the inputs given — typically a division by zero or a value outside the valid domain. We show a dash rather than a misleading number.
How do I report an error?
Email apexconverter.praxiscalc@gmail.com with the tool URL, your exact inputs, the output received and the output you expected. Verified mathematical errors are patched within 72 hours.