Length Conversion Guide - ConcretesMath

Convert between millimetres, centimetres, metres, kilometres, inches, feet, yards, and miles — instantly and accurately.

Length Unit Conversion guide

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Length Conversion Reference Tables

Quick-reference tables for the most common length conversions — millimetres to inches, metres to feet, and feet to metres. Switch between tables using the tabs below.

Millimetres (mm): 1 mm; Inches (in): 0.03937 in; Inches (fraction): ≈ 1/25 in; Notes: Thickness of a credit card
Millimetres (mm): 2 mm; Inches (in): 0.07874 in; Inches (fraction): ≈ 3/32 in; Notes: Pencil lead diameter
Millimetres (mm): 3 mm; Inches (in): 0.11811 in; Inches (fraction): ≈ 1/8 in; Notes: Common grout joint width
Millimetres (mm): 5 mm; Inches (in): 0.19685 in; Inches (fraction): ≈ 3/16 in; Notes: Small bolt diameter
Millimetres (mm): 10 mm; Inches (in): 0.39370 in; Inches (fraction): ≈ 13/32 in; Notes: 10 mm rebar nominal dia.
Millimetres (mm): 25 mm; Inches (in): 0.98425 in; Inches (fraction): ≈ 1 in; Notes: ≈ 1 inch (1 in = 25.4 mm)
Millimetres (mm): 25.4 mm; Inches (in): 1.00000 in; Inches (fraction): = 1 in exactly; Notes: By definition
Millimetres (mm): 50 mm; Inches (in): 1.96850 in; Inches (fraction): ≈ 2 in; Notes: Common concrete cover
Millimetres (mm): 100 mm; Inches (in): 3.93701 in; Inches (fraction): ≈ 4 in; Notes: Minimum slab thickness
Millimetres (mm): 150 mm; Inches (in): 5.90551 in; Inches (fraction): ≈ 6 in; Notes: Standard slab thickness
Millimetres (mm): 200 mm; Inches (in): 7.87402 in; Inches (fraction): ≈ 8 in; Notes: Thick slab / wall
Millimetres (mm): 300 mm; Inches (in): 11.81102 in; Inches (fraction): ≈ 12 in; Notes: Column / wall width
Millimetres (mm): 450 mm; Inches (in): 17.71654 in; Inches (fraction): ≈ 18 in; Notes: Beam depth
Millimetres (mm): 600 mm; Inches (in): 23.62205 in; Inches (fraction): ≈ 24 in; Notes: Large column / pile dia.
Millimetres (mm): 1000 mm; Inches (in): 39.37008 in; Inches (fraction): ≈ 39.4 in; Notes: 1 metre
Inches (in): 1/8 in; Millimetres (mm): 3.175 mm; Centimetres (cm): 0.318 cm; Notes: Small gap / mortar joint
Inches (in): 1/4 in; Millimetres (mm): 6.350 mm; Centimetres (cm): 0.635 cm; Notes: Tile spacer size
Inches (in): 1/2 in; Millimetres (mm): 12.700 mm; Centimetres (cm): 1.270 cm; Notes: Rebar dia. (US #4 = 12.7 mm)
Inches (in): 3/4 in; Millimetres (mm): 19.050 mm; Centimetres (cm): 1.905 cm; Notes: Copper pipe nominal
Inches (in): 1 in; Millimetres (mm): 25.400 mm; Centimetres (cm): 2.540 cm; Notes: 1 inch = 25.4 mm exactly
Inches (in): 2 in; Millimetres (mm): 50.800 mm; Centimetres (cm): 5.080 cm; Notes: Cover to rebar (typical)
Inches (in): 3 in; Millimetres (mm): 76.200 mm; Centimetres (cm): 7.620 cm; Notes: Pipe diameter
Inches (in): 4 in; Millimetres (mm): 101.600 mm; Centimetres (cm): 10.160 cm; Notes: Nominal brick height (US)
Inches (in): 6 in; Millimetres (mm): 152.400 mm; Centimetres (cm): 15.240 cm; Notes: Standard slab thickness
Inches (in): 9 in; Millimetres (mm): 228.600 mm; Centimetres (cm): 22.860 cm; Notes: One-brick wall (UK)
Inches (in): 12 in (1 ft); Millimetres (mm): 304.800 mm; Centimetres (cm): 30.480 cm; Notes: 1 foot exactly
Inches (in): 18 in; Millimetres (mm): 457.200 mm; Centimetres (cm): 45.720 cm; Notes: Beam depth (approx)
Inches (in): 24 in; Millimetres (mm): 609.600 mm; Centimetres (cm): 60.960 cm; Notes: Large column section
Inches (in): 36 in (1 yd); Millimetres (mm): 914.400 mm; Centimetres (cm): 91.440 cm; Notes: 1 yard exactly
Metres (m): 0.10 m; Feet (ft): 0.328 ft; Feet & Inches: 3 15/16 in; Notes: 100 mm slab
Metres (m): 0.125 m; Feet (ft): 0.410 ft; Feet & Inches: 4 15/16 in; Notes: 125 mm slab (standard)
Metres (m): 0.15 m; Feet (ft): 0.492 ft; Feet & Inches: 5 29/32 in; Notes: 150 mm slab
Metres (m): 0.30 m; Feet (ft): 0.984 ft; Feet & Inches: 11 13/16 in; Notes: 300 mm column
Metres (m): 0.45 m; Feet (ft): 1.476 ft; Feet & Inches: 1 ft 5 23/32 in; Notes: 450 mm beam depth
Metres (m): 1.00 m; Feet (ft): 3.281 ft; Feet & Inches: 3 ft 3 3/8 in; Notes: 1 metre
Metres (m): 2.00 m; Feet (ft): 6.562 ft; Feet & Inches: 6 ft 6 3/4 in; Notes: Doorway height
Metres (m): 2.80 m; Feet (ft): 9.186 ft; Feet & Inches: 9 ft 2 1/4 in; Notes: Min residential storey
Metres (m): 3.00 m; Feet (ft): 9.843 ft; Feet & Inches: 9 ft 10 1/8 in; Notes: Standard storey height
Metres (m): 3.50 m; Feet (ft): 11.483 ft; Feet & Inches: 11 ft 5 13/16 in; Notes: Commercial storey height
Metres (m): 4.00 m; Feet (ft): 13.123 ft; Feet & Inches: 13 ft 1 1/2 in; Notes: Industrial building height
Metres (m): 5.00 m; Feet (ft): 16.404 ft; Feet & Inches: 16 ft 5 in; Notes: Beam span (medium)
Metres (m): 10.00 m; Feet (ft): 32.808 ft; Feet & Inches: 32 ft 9 3/4 in; Notes: Long-span beam
Metres (m): 100.00 m; Feet (ft): 328.084 ft; Feet & Inches: 328 ft 1 in; Notes: Football pitch width
Feet (ft): 1 ft; Metres (m): 0.3048 m; Millimetres (mm): 304.8 mm; Notes: By definition exactly
Feet (ft): 2 ft; Metres (m): 0.6096 m; Millimetres (mm): 609.6 mm; Notes: Step riser × 3.5
Feet (ft): 3 ft (1 yd); Metres (m): 0.9144 m; Millimetres (mm): 914.4 mm; Notes: 1 yard
Feet (ft): 4 ft; Metres (m): 1.2192 m; Millimetres (mm): 1219.2 mm; Notes: Sheet material width (US)
Feet (ft): 5 ft; Metres (m): 1.5240 m; Millimetres (mm): 1524.0 mm; Notes: Average person height (est.)
Feet (ft): 6 ft; Metres (m): 1.8288 m; Millimetres (mm): 1828.8 mm; Notes: Tall person / fence height
Feet (ft): 8 ft; Metres (m): 2.4384 m; Millimetres (mm): 2438.4 mm; Notes: Standard ceiling height (US)
Feet (ft): 9 ft; Metres (m): 2.7432 m; Millimetres (mm): 2743.2 mm; Notes: Min residential storey height
Feet (ft): 10 ft; Metres (m): 3.0480 m; Millimetres (mm): 3048.0 mm; Notes: Standard storey height (approx)
Feet (ft): 12 ft; Metres (m): 3.6576 m; Millimetres (mm): 3657.6 mm; Notes: Commercial / industrial storey
Feet (ft): 15 ft; Metres (m): 4.5720 m; Millimetres (mm): 4572.0 mm; Notes: Beam span (short)
Feet (ft): 20 ft; Metres (m): 6.0960 m; Millimetres (mm): 6096.0 mm; Notes: Shipping container length (short)
Feet (ft): 40 ft; Metres (m): 12.1920 m; Millimetres (mm): 12192.0 mm; Notes: Shipping container length (long)
Feet (ft): 100 ft; Metres (m): 30.4800 m; Millimetres (mm): 30480.0 mm; Notes: Building dimension reference
Centimetres (cm): 1 cm; Inches (in): 0.3937 in; Millimetres (mm): 10 mm; Notes: Width of a thumbnail
Centimetres (cm): 2 cm; Inches (in): 0.7874 in; Millimetres (mm): 20 mm; Notes: Tile spacer (large)
Centimetres (cm): 2.54 cm; Inches (in): 1.0000 in; Millimetres (mm): 25.4 mm; Notes: = 1 inch exactly
Centimetres (cm): 5 cm; Inches (in): 1.9685 in; Millimetres (mm): 50 mm; Notes: Standard concrete cover
Centimetres (cm): 10 cm; Inches (in): 3.9370 in; Millimetres (mm): 100 mm; Notes: 10 cm = 100 mm
Centimetres (cm): 15 cm; Inches (in): 5.9055 in; Millimetres (mm): 150 mm; Notes: Standard slab thickness
Centimetres (cm): 20 cm; Inches (in): 7.8740 in; Millimetres (mm): 200 mm; Notes: Thick slab
Centimetres (cm): 25 cm; Inches (in): 9.8425 in; Millimetres (mm): 250 mm; Notes: Narrow column
Centimetres (cm): 30 cm; Inches (in): 11.8110 in; Millimetres (mm): 300 mm; Notes: Standard column width
Centimetres (cm): 45 cm; Inches (in): 17.7165 in; Millimetres (mm): 450 mm; Notes: Beam depth
Centimetres (cm): 60 cm; Inches (in): 23.6220 in; Millimetres (mm): 600 mm; Notes: Large column / pile
Centimetres (cm): 100 cm (1 m); Inches (in): 39.3701 in; Millimetres (mm): 1000 mm; Notes: 1 metre
Kilometres (km): 0.1 km; Miles (mi): 0.0621 mi; Metres (m): 100 m; Notes: 100 m sprint distance
Kilometres (km): 0.5 km; Miles (mi): 0.3107 mi; Metres (m): 500 m; Notes: 5-minute walk
Kilometres (km): 1 km; Miles (mi): 0.6214 mi; Metres (m): 1,000 m; Notes: 1 kilometre
Kilometres (km): 1.609 km; Miles (mi): 1.0000 mi; Metres (m): 1,609 m; Notes: = 1 mile exactly
Kilometres (km): 2 km; Miles (mi): 1.2427 mi; Metres (m): 2,000 m; Notes: 20-minute walk
Kilometres (km): 5 km; Miles (mi): 3.1069 mi; Metres (m): 5,000 m; Notes: 5K race distance
Kilometres (km): 10 km; Miles (mi): 6.2137 mi; Metres (m): 10,000 m; Notes: 10K race distance
Kilometres (km): 21.097 km; Miles (mi): 13.109 mi; Metres (m): 21,097 m; Notes: Half marathon
Kilometres (km): 42.195 km; Miles (mi): 26.219 mi; Metres (m): 42,195 m; Notes: Full marathon
Kilometres (km): 50 km; Miles (mi): 31.069 mi; Metres (m): 50,000 m; Notes: City-to-city distance
Kilometres (km): 100 km; Miles (mi): 62.137 mi; Metres (m): 100,000 m; Notes: Approximate city distance
Kilometres (km): 1000 km; Miles (mi): 621.37 mi; Metres (m): 1,000,000 m; Notes: Inter-city / country distance

Length Conversion Formulas – All Units

Every length unit conversion is based on a single exact anchor:1 inch = 25.4 mmby international definition (since 1959). All other conversions are derived from this one exact relationship.

The Anchor — 1 Inch = 25.4 mm

1 in = 25.4 mm (exact)

This is the international definition adopted in 1959, making it the exact foundation from which all metric-imperial conversions are derived. All other conversions — feet to metres, yards to metres, miles to kilometres — are estimated from this single exact equality.

1 ft = 12 in = 12 × 25.4 = 304.8 mm (exact). 1 yd = 3 ft = 914.4 mm (exact).

mm ↔ Inches

inches = mm ÷ 25.4

mm = inches × 25.4

The most common construction conversion — drawing dimensions in mm to site measurements in inches. Quick mental estimate: divide mm by 25 for a rough inch equivalent (5% overstatement).

Example: 300 mm ÷ 25.4 = 11.811 inches ≈ 11 13/16 in

Metres ↔ Feet

feet = metres × 3.28084

metres = feet × 0.30480

The most essential civil engineering conversion. Exact value: 1 ft = 0.3048 m. Inverse: 1 m = 3.280839895… ft. Quick mental estimate: metres × 3.3 ≈ feet (1% overstatement).

Example: 3.0 m × 3.28084 = 9.843 ft ≈ 9 ft 10 1/8 in

cm ↔ Inches

inches = cm ÷ 2.54

cm = inches × 2.54

Since 1 in = 2.54 cm (exact). Used frequently in everyday measurements, clothing sizes, screen sizes, and piping specifications. Quick estimate: multiply cm by 0.4 for approximate inches.

Example: 15 cm ÷ 2.54 = 5.906 in ≈ 5 29/32 in

km ↔ Miles

miles = km × 0.621371

km = miles × 1.609344

1 mile = 1609.344 m (exact, derived from 1 ft = 0.3048 m and 1 mile = 5280 ft). Quick mental estimate: km × 0.6 ≈ miles; miles × 1.6 ≈ km.

Example: 10 km × 0.621371 = 6.214 miles

Feet ↔ Inches

inches = feet × 12

feet = inches ÷ 12

1 foot = 12 inches (exact definition). When converting mixed feet-and-inches to a single unit: first convert inches to feet (divide by 12), then add the feet. Example: 9 ft 6 in = 9 + 6/12 = 9.5 ft = 2.8956 m.

Example: 9 ft 6 in = 114 inches = 2895.6 mm = 2.8956 m

Yards ↔ Metres

metres = yards × 0.9144

yards = metres ÷ 0.9144

1 yard = 3 feet = 36 inches = 914.4 mm = 0.9144 m (exact). Yards are used in US construction (concrete ordering in cubic yards), fabric measurements, and sports field dimensions.

Example: 1 cubic yard = 0.9144³ = 0.7646 m³ (concrete ordering conversion)

mm ↔ cm ↔ m ↔ km (Metric)

1 km = 1000 m = 100,000 cm = 1,000,000 mm

The metric system uses powers of 10. Moving left (smaller to larger unit): divide by 10, 100, or 1000. Moving right (larger to smaller): multiply. No conversion factors to remember — just move the decimal point.

10 mm = 1 cm · 100 cm = 1 m · 1000 m = 1 km

Length Units – What Each One Means

A quick guide to all eight length units supported by this conversion guide — their origins, how they are used today, and their exact metric equivalent.

Millimetre

One thousandth of a metre. The standard unit for structural drawings and engineering dimensions across South Asia, Europe, and most of the world. Concrete elements are specified in mm on all IS and BS drawings.

Centimetre

One hundredth of a metre. Widely used in everyday measurements — clothing, furniture, human height, and medical applications. Less common in structural engineering than mm or m. 1 cm = 10 mm.

Metre

The SI base unit of length. Used for room dimensions, building heights, span lengths, and all structural quantity estimates. The primary unit for concrete volume estimates (m³). Defined as the distance light travels in 1/299,792,458 seconds in vacuum.

Kilometre

One thousand metres. Used for road distances, infrastructure project lengths, civil engineering alignment chainages, and geographic distances. Road and highway design uses kilometres as the primary chainage unit worldwide except the USA and UK.

Inch

The fundamental imperial unit. Defined as exactly 25.4 mm since 1959. Widely used in the USA, UK (for some applications), and wherever imperial construction practice persists. Structural drawings in the US use feet and inches. Screen sizes, pipe diameters, and screw gauges use inches globally.

Foot

12 inches. The primary construction dimension unit in the USA. Building heights, room dimensions, slab spans, and concrete volumes (in cubic feet) are all specified in feet in US practice. 1 foot = 304.8 mm exactly — a commonly memorised constant in international engineering.

Yard

3 feet = 36 inches = 0.9144 m. Concrete in the USA is ordered by the cubic yard (cy). Sports field dimensions — American football (100 yards), cricket (22 yards per pitch) — use yards. Also used in fabric measurement in many countries.

Mile

5280 feet = 1760 yards = 1609.344 m. Used for road distances in the USA, UK, and some other countries. Speed limits and odometers in the USA and UK use miles per hour. Nautical miles (1852 m) are a separate unit used in aviation and navigation.

Construction Length Reference Guide – Common Dimensions 2026

Frequently needed dimension conversions for construction practice in Pakistan and South Asia — structural elements, room sizes, storey heights, and material dimensions in both metric and imperial.

Concrete Element Dimensions

Min slab thickness100 mm = 3.94 in
Standard slab (residential)125 mm = 4.92 in
Standard slab (commercial)150 mm = 5.91 in
Min column size (IS 456)200 mm = 7.87 in
Standard column (residential)300 mm = 11.81 in
Standard beam width230 mm = 9.06 in
Standard beam depth450 mm = 17.72 in
Concrete cover (moderate)40 mm = 1.57 in

Building Heights & Storey Dimensions

Min storey height (residential)2.8 m = 9.19 ft
Standard storey height3.0 m = 9.84 ft
Commercial storey height3.5 m = 11.48 ft
US standard ceiling (8 ft)2.438 m = 2438 mm
Standard door height2.1 m = 6.89 ft
Standard door width0.9 m = 35.43 in
Window sill height0.90 m = 2.95 ft
Window head height2.10 m = 6.89 ft

Brick & Masonry Dimensions

Standard brick (South Asia)230×115×75 mm
Brick length230 mm = 9.06 in
Brick width115 mm = 4.53 in
Brick height75 mm = 2.95 in
Mortar joint (standard)10 mm = 3/8 in
Half-brick wall115 mm = 4.53 in
One-brick wall230 mm = 9.06 in
US standard brick (ASTM)194×92×57 mm

Rebar / Steel Reinforcement Sizes

T8 bar dia.8 mm = 0.315 in
T10 bar dia.10 mm = 0.394 in
T12 bar dia.12 mm = 0.472 in
T16 bar dia.16 mm = 0.630 in
T20 bar dia.20 mm = 0.787 in
T25 bar dia.25 mm = 0.984 in
T32 bar dia.32 mm = 1.260 in
US #4 bar dia.12.7 mm = 1/2 in

Road & Civil Engineering

Standard lane width3.5 m = 11.48 ft
Pavement thickness (light)150 mm = 5.91 in
Pavement thickness (heavy)300 mm = 11.81 in
Kerb height150 mm = 5.91 in
Standard pipeline depth1.0 m = 3.28 ft
Chain (surveying)20.12 m = 66 ft
Link (surveying)0.201 m = 7.92 in
Chainage interval (roads)100 m = 328.08 ft

Common Imperial to Metric Quick-Ref

1/2 inch12.7 mm
1 inch25.4 mm
6 inches152.4 mm
1 foot304.8 mm = 0.3048 m
10 feet3048 mm = 3.048 m
1 yard914.4 mm = 0.9144 m
100 yards91.44 m
1 mile1609.344 m = 1.609 km

Pro Tips for Length Unit Conversion

The One Number to Remember: 25.4

Every metric-imperial length conversion derives from a single exact number: 1 inch = 25.4 mm. Memorise this and you can derive any other conversion on the fly — feet = inches × 12 = mm × 12 ÷ 25.4 = mm ÷ 0.3048 ÷ 3.28084. One number unlocks everything.

Quick Mental Estimates for Site Use

Fast approximate conversions you can do in your head on site: metres × 3.3 ≈ feet (accurate to 0.5%). mm ÷ 25 ≈ inches (5% high). km × 0.6 ≈ miles (3% low). inches × 2.5 ≈ cm (1.6% low). These are good enough for most quick checks — use the guide for procurement and drawings.

Structural Drawings: Always Check the Title Block

Before converting any dimension from a structural drawing, check the title block — it states the drawing unit system. IS drawings use mm; US drawings use ft and inches; some international projects use metres. Never assume the unit — a 300 that means 300 mm (30 cm) is very different from 300 that means 300 inches (7.62 m).

Feet & Inches — Convert Mixed Units First

When converting mixed feet-and-inches measurements: convert everything to inches first (feet × 12 + remaining inches), then divide by 25.4 for mm or by 39.3701 for metres. Example: 9 ft 6 in = (9 × 12 + 6) = 114 inches = 114 × 25.4 = 2895.6 mm = 2.8956 m. Never convert feet and inches separately and add afterwards without care.

Use mm Throughout — Never Mix Units on Site

In construction, the best practice is to work entirely in millimetres throughout a project — all dimensions, all drawings, all site measurements. Mixing mm and m on the same document causes errors. On a mm-based drawing, a 1000 means one metre; on a metres-based BOQ, 1000 means one kilometre. Consistency prevents disastrous mistakes.

Significant Figures — Match to Measurement Accuracy

Don’t over-specify precision when converting. A site steel tape measure is accurate to ±1 mm at best. Converting “3 ft” to “914.4 mm” implies ±0.1 mm accuracy — false precision. Match the number of significant figures in your output to the actual measurement accuracy: if you measured to the nearest foot, report to the nearest 10 mm or 30 mm — not to 0.1 mm.

Why Use Our Length Conversion guide?

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All conversions use IEEE 754 double-precision arithmetic — 15+ significant digits internally, displayed to a practical number of decimal places for each unit.

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Frequently Asked Questions – Length Conversion

How many millimetres in an inch?

There are exactly25.4 millimetres in 1 inch. This is an exact definition adopted internationally in 1959. To convert inches to mm: multiply by 25.4. To convert mm to inches: divide by 25.4.1 in = 25.4 mm. Examples: 12 inches (1 foot) = 304.8 mm. 6 inches = 152.4 mm. 1/2 inch = 12.7 mm. This single conversion factor is the basis for all metric-imperial length conversions.

How many feet in a metre?

There are3.28084 feet in 1 metre(to 5 decimal places). Equivalently, 1 foot = 0.3048 metres exactly.1 m = 3.28084 ft. The exact value derives from 1 in = 25.4 mm: 1 ft = 12 in = 12 × 25.4 mm = 304.8 mm = 0.3048 m. So 1 m = 1/0.3048 = 3.28084 ft. Quick mental estimate: multiply metres by 3.3 for approximate feet (slightly overstated by 0.6%).

How do I convert metres to feet and inches?

To convert metres to combined feet and inches: (1) Multiply metres by 3.28084 to get decimal feet. (2) The whole number part is the feet. (3) Multiply the decimal part by 12 to get the inches. Example — convert 2.85 m: Step 1: 2.85 × 3.28084 = 9.3504 ft. Step 2: 9 feet. Step 3: 0.3504 × 12 = 4.20 inches. Result:2.85 m = 9 ft 4.2 in ≈ 9 ft 4 3/16 in. To go the other way: feet + (inches/12) × 0.3048 = metres.

How many km in a mile?

There are1.609344 kilometres in 1 mileexactly. This derives from 1 mile = 5280 feet and 1 foot = 0.3048 m: 5280 × 0.3048 = 1609.344 m = 1.609344 km.1 mile = 1.609344 km. Conversely: 1 km = 0.621371 miles. Quick mental estimate: miles × 1.6 ≈ km (0.4% low). Or: km × 0.6 ≈ miles (3.4% low). For road speed: 60 mph ≈ 96.6 km/h. 100 km/h ≈ 62.1 mph.

How do I convert feet and inches to millimetres?

To convert a mixed feet-and-inches measurement to mm: (1) Convert everything to inches: total inches = (feet × 12) + remaining inches. (2) Multiply total inches by 25.4 to get mm. Example — convert 9 ft 6 in: Step 1: (9 × 12) + 6 = 108 + 6 = 114 inches. Step 2: 114 × 25.4 =2895.6 mm. Never convert feet and inches to mm separately and add them — convert to total inches first to avoid errors.

What is the difference between a mile and a nautical mile?

Astatute mile(the everyday “mile”) = 5280 feet = 1609.344 metres. Anautical mile= 1852 metres exactly (by international definition). The nautical mile was originally defined as one minute of arc of latitude along a meridian — making it directly tied to the Earth’s geometry. 1 nautical mile ≈ 1.151 statute miles. Nautical miles are used in aviation, maritime navigation, and meteorology. Knots (nautical miles per hour) is the speed unit for ships and aircraft.

How do I convert cm to inches quickly in my head?

The exact conversion is: inches = cm ÷ 2.54. For a quick mental estimate, use:multiply cm by 0.4(accurate to within 2%). Example: 15 cm × 0.4 = 6 inches (actual: 5.91 in — error 1.5%). Another useful mental shortcut: subtract 60% from cm and you have rough mm, but that is less intuitive. The most practical on-site shortcut: 2.5 cm ≈ 1 inch (accurate within 1.6%). So count how many 2.5 cm chunks fit in your measurement.

How accurate is this length conversion guide?

All conversions use IEEE 754 double-precision floating-point arithmetic internally — providing 15–17 significant digits of precision. The base conversion factor (1 in = 25.4 mm) is exact by international definition, so all derived metric-imperial conversions are exact to the precision of the floating-point representation. Results are rounded for display to a practical number of decimal places (e.g. 4 for metres, 2 for km and miles, 3 for feet and inches). The rounding happens only at the display stage — never during intermediate estimates.

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