Result in Micrometres
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Equivalent length
Accurate length conversion between millimetres (mm) and micrometres (µm / microns) — and back
Convert millimetres to micrometres instantly using the exact factor of 1,000 µm per mm. Full multi-unit breakdown into µm, nm, cm, m, inches, and mils — all in one free tool for 2026.
Professional length conversion for engineering tolerances, manufacturing, optics, biology, materials science, and metrology applications
The millimetre and the micrometre are both SI metric units of length, connected by an exact factor of 1,000 within the International System of Units. By definition, 1 mm = 1,000 µm and 1 µm = 0.001 mm — perfectly exact, no rounding involved. This follows from the SI prefix hierarchy: milli- = 10⁻³ and micro- = 10⁻⁶, so 1 mm = 10⁻³ m and 1 µm = 10⁻⁶ m, giving a ratio of 10⁻³ / 10⁻⁶ = 10³ = 1,000. The micrometre (µm) is also commonly called the micron — both terms are fully accepted in scientific and engineering literature. This is one of the most important conversions in precision engineering, semiconductor fabrication, and microscopy.
Switch instantly between Millimetres → Micrometres and Micrometres → Millimetres conversion modes. The results panel simultaneously displays the equivalent length in six units — µm, mm, nanometres (nm), centimetres (cm), metres (m), inches (in), and mils (thou) — giving you complete cross-unit context from a single input. This is particularly useful for precision engineers and scientists working across drawing dimensions in mm (CAD/engineering drawings) and tolerance specifications in µm (surface finish, machining tolerance, coating thickness), allowing immediate visibility of all relevant scales at once.
The mm-to-µm conversion is critical across many high-precision fields: Precision machining — CNC tolerances as tight as ±1–5 µm (±0.001–0.005 mm); Surface finish — Ra (roughness average) measured in µm, drawings in mm; Semiconductor fab — chip feature sizes in µm and nm (process nodes); Optics & lenses — wavelengths and coatings in µm, lens dimensions in mm; Biology & medicine — cell sizes (1–100 µm), tissue sections, and microcapillary diameters; PCB manufacturing — trace widths and via diameters; Metrology — CMM measurement accuracy in µm; 3D printing — layer heights in µm, part dimensions in mm.
Select conversion direction, enter your length value, and get instant multi-unit results
The millimetre (mm) is 1/1,000 of a metre — the smallest graduation on most everyday rulers and a fundamental dimension unit in engineering drawing, construction, and manufacturing. The micrometre (µm), also called the micron, is 1/1,000,000 of a metre — exactly one-thousandth of a millimetre. The micrometre is the workhorse unit of precision engineering, microscopy, and nanotechnology: too small to see with the naked eye (the unaided human eye can resolve to approximately 100–200 µm), yet large enough to be highly significant in machining, biological processes, and semiconductor manufacturing.
The conversion is perfectly exact within the SI system: 1 mm = 1,000 µm and 1 µm = 0.001 mm = 10⁻³ mm. In the broader SI length scale: 1 mm = 1,000 µm = 1,000,000 nm = 0.1 cm = 0.001 m. The micrometre sits between the millimetre and nanometre scales — bridging the world of human-scale mechanical engineering and the nanoscale world of semiconductor devices and molecular biology. For reference: a human hair is approximately 50–100 µm (0.05–0.1 mm) in diameter; a red blood cell is about 6–8 µm; a bacterium is typically 1–10 µm; visible light wavelengths span approximately 0.38–0.75 µm (380–750 nm).
Example: 0.025 mm × 1,000 = 25 µm | 500 µm ÷ 1,000 = 0.5 mm
1 mm = 1,000 µm | 1 µm = 1,000 nm | 1 mm = 10⁶ nm | 1 m = 10³ mm = 10⁶ µm = 10⁹ nm | 1 inch = 25,400 µm = 25.4 mm
To convert millimetres to micrometres, multiply by 1,000 (move the decimal point three places to the right). To convert micrometres back to millimetres, divide by 1,000 (move the decimal three places to the left). Here are three practical worked examples:
Input: 0.025 mm (typical CNC tolerance)
Formula: 0.025 × 1,000
= 25 µm
= ±25 µm finish machining tolerance
Input: 0.07 mm (average hair diameter)
Formula: 0.07 × 1,000
= 70 µm
= typical human hair width
Input: 250 µm (standard plastic film)
Formula: 250 ÷ 1,000
= 0.25 mm
= common packaging film gauge
mm → µm: Multiply by 1,000 — move decimal 3 places right. Example: 0.005 mm → 5 µm; 1.2 mm → 1,200 µm; 25 mm → 25,000 µm. µm → mm: Divide by 1,000 — move decimal 3 places left. Example: 500 µm → 0.5 mm; 8 µm → 0.008 mm; 25,400 µm → 25.4 mm. Key size references: Human hair ≈ 50–100 µm; red blood cell ≈ 7 µm; bacterium ≈ 1–5 µm; visible light ≈ 0.4–0.75 µm; 1 thou/mil = 25.4 µm = 0.0254 mm. Precision machining: H7 bore tolerance in 25 mm hole = ±13 µm (±0.013 mm); ground surface finish Ra ≈ 0.4–0.8 µm; milled surface Ra ≈ 1.6–3.2 µm. 3D printing layer heights: FDM 50–300 µm (0.05–0.3 mm); SLA/resin 25–100 µm (0.025–0.1 mm). Semiconductor nodes: 5 nm = 0.005 µm = 0.000005 mm; 7 nm = 0.007 µm; 28 nm = 0.028 µm.
Complete reference table covering the full range of practical mm/µm values — from sub-micron precision through engineering dimensions — with nm, cm, m, inch, and mil equivalents plus real-world context. Desktop shows the full table; mobile shows grouped cards.
| Millimetres (mm) | Micrometres (µm) | Nanometres (nm) | Inches (in) | Mils (thou) | Real-World Reference |
|---|---|---|---|---|---|
| 0.0001 mm | 0.1 µm | 100 nm | 0.0000039 | 0.0039 | Optical coating layer |
| 0.001 mm | 1 µm | 1,000 nm | 0.0000394 | 0.0394 | Bacterium / fine surface finish Ra |
| 0.005 mm | 5 µm | 5,000 nm | 0.000197 | 0.197 | Red blood cell / fine tolerance |
| 0.010 mm | 10 µm | 10,000 nm | 0.000394 | 0.394 | Precision fit tolerance (H6/h6) |
| 0.025 mm | 25 µm | 25,000 nm | 0.000984 | 0.984 | 1 mil = 25.4 µm; fine CNC tolerance |
| 0.0254 mm | 25.4 µm | 25,400 nm | 0.001000 | 1.000 | Exactly 1 mil (thou) |
| 0.050 mm | 50 µm | 50,000 nm | 0.001969 | 1.969 | Fine hair / SLA 3D print layer |
| 0.100 mm | 100 µm | 100,000 nm | 0.003937 | 3.937 | Human hair / FDM 3D print layer |
| 0.200 mm | 200 µm | 200,000 nm | 0.007874 | 7.874 | Thick hair / PCB thin trace |
| 0.250 mm | 250 µm | 250,000 nm | 0.009843 | 9.843 | Standard plastic film / thin foil |
| 0.500 mm | 500 µm | 500,000 nm | 0.019685 | 19.685 | PCB standard trace / thin card |
| 1.000 mm | 1,000 µm | 1,000,000 nm | 0.039370 | 39.370 | 1 mm — credit card thickness ≈ 0.76 mm |
| 2.000 mm | 2,000 µm | 2,000,000 nm | 0.078740 | 78.740 | Pencil lead diameter |
| 2.540 mm | 2,540 µm | 2,540,000 nm | 0.100000 | 100.000 | Exactly 0.1 inch |
| 5.000 mm | 5,000 µm | 5,000,000 nm | 0.196850 | 196.850 | Bolt head / engineering dimension |
| 10.000 mm | 10,000 µm | 10,000,000 nm | 0.393701 | 393.701 | 10 mm — coin diameter range |
| 25.000 mm | 25,000 µm | 25,000,000 nm | 0.984252 | 984.252 | ≈ 1 inch (25.4 mm = 1 in exactly) |
| 25.400 mm | 25,400 µm | 25,400,000 nm | 1.000000 | 1,000.000 | Exactly 1 inch |
| 50.000 mm | 50,000 µm | 50,000,000 nm | 1.96850 | 1,968.50 | Standard ruler width |
| 100.000 mm | 100,000 µm | 100,000,000 nm | 3.93701 | 3,937.01 | 100 mm — standard short length |
🟢 Green = microscale / precision | 🟧 Orange = sub-mm engineering | 🔵 Blue = mm-scale dimensions
Engineering drawings specify part dimensions in millimetres, but machining tolerances — the allowable deviation from nominal — are expressed in micrometres for precision work. A typical IT6 tolerance on a 25 mm shaft is 13 µm (±0.013 mm); IT7 is 21 µm; IT5 is 9 µm. Surface finish (roughness) is specified as Ra (arithmetic mean roughness) in µm: ground surfaces Ra 0.4–0.8 µm; turned surfaces Ra 1.6–3.2 µm; milled surfaces Ra 1.6–6.3 µm. A ground surface of Ra = 0.8 µm = 0.0008 mm. CMM (coordinate measuring machine) accuracy is typically ±0.5–2 µm. Understanding the mm-to-µm relationship is essential for reading engineering drawings alongside tolerance standards (ISO 286, ASME Y14.5).
3D printing layer heights are specified in both mm and µm depending on the technology. FDM (fused deposition modelling) layer heights range from 50 µm to 400 µm (0.05–0.4 mm) — a standard 0.2 mm layer = 200 µm. SLA/DLP resin printers achieve 25–100 µm (0.025–0.1 mm) layer resolution. Multi-Jet Fusion and SLS sintering: 80–120 µm per layer. Part accuracy on consumer FDM printers is typically ±100–200 µm (±0.1–0.2 mm) in XY and ±50–100 µm in Z. Industrial metal SLM/DMLS printers achieve ±50 µm (±0.05 mm) feature accuracy. Print bed levelling is usually adjusted in increments of 0.05–0.1 mm (50–100 µm). Nozzle diameter (0.4 mm = 400 µm) defines minimum feature width.
In optics, the micrometre is the natural unit for many critical dimensions. Optical fibre core diameters: single-mode fibre ≈ 8–10 µm (0.008–0.010 mm); multi-mode ≈ 50–62.5 µm (0.050–0.0625 mm). Anti-reflection optical coatings: quarter-wavelength thickness of visible light ≈ 0.1–0.2 µm (100–200 nm). Diffraction grating line spacing: typically 0.5–3 µm (lines per mm = 333–2,000). MEMS (micro-electromechanical systems) feature sizes: 1–500 µm. Laser spot diameters in cutting and engraving: 25–200 µm. Camera sensor pixels in modern smartphones: 0.7–1.2 µm pixel pitch. The mm-to-µm conversion is constant in optical engineering where lens dimensions are in mm and wavelengths/features in µm or nm.
The micrometre is the primary scale unit for cellular biology. Reference sizes: red blood cells 6–8 µm diameter; white blood cells 7–15 µm; platelets 2–3 µm; bacteria 0.5–10 µm (E. coli ≈ 2 µm × 0.5 µm); typical animal cell nucleus 5–10 µm; Paramecium ≈ 50–330 µm. Histology tissue sections cut on microtomes are typically 5–10 µm thick (0.005–0.010 mm). Capillary diameters: 5–10 µm. Hair follicle width: 70–120 µm (0.07–0.12 mm). Medical device catheter wall thickness: 50–500 µm (0.05–0.5 mm). Wound healing: fibroblast cell migration ≈ 30 µm/hour. Microscope slide coverslips: 0.13–0.17 mm thick (130–170 µm). Pathologists, cell biologists, and medical device engineers routinely convert between mm-scale device dimensions and µm-scale biological features.
Printed circuit board (PCB) manufacturing specifications use both mm and µm. Standard PCB trace widths: 100–250 µm (0.1–0.25 mm) for general purpose; fine-pitch traces as narrow as 50 µm (0.05 mm). Via hole drill diameters: 0.2–0.3 mm (200–300 µm) for HDI boards. Copper foil thickness: 17.5 µm (0.5 oz) to 70 µm (2 oz) per layer. Solder mask thickness: 20–30 µm. Semiconductor process nodes: 28 nm = 0.028 µm = 0.000028 mm; 5 nm = 0.005 µm. Die sizes for modern SoCs: typically 5–20 mm² (dimensions in mm). Bond wire diameter: 17–25 µm. Wafer thickness: 775 µm (0.775 mm) for 300 mm wafers. IPC-2221 and IPC-7351 standards specify PCB dimensions in mm and µm, requiring frequent conversion between the two scales.
Industrial metrology — the science of measurement — operates almost entirely at the µm scale for precision inspection. Vernier callipers resolve to 20–50 µm (0.02–0.05 mm); digital micrometers resolve to 1 µm (0.001 mm); CMMs (coordinate measuring machines) achieve 0.1–2 µm accuracy. Gauge block accuracy grades: Grade K = ±0.05 µm; Grade 00 = ±0.1 µm; Grade 0 = ±0.15 µm. ISO 1302 surface finish codes specify Ra values in µm on drawings dimensioned in mm. Roundness and cylindricity tolerances under GD&T (ASME Y14.5 / ISO 1101) are specified in µm on mm-dimensioned drawings. Go/No-Go gauges for m6/h6 shaft fits operate at tolerances of 8–15 µm on shafts dimensioned in mm — making mm-to-µm conversion a daily requirement for quality engineers.
1 mm = 1,000 µm (exact). 1 µm = 0.001 mm. 1 mm = 1,000,000 nm. 1 µm = 1,000 nm. 1 inch = 25.4 mm = 25,400 µm. 1 mil (thou) = 0.0254 mm = 25.4 µm. Common size references: bacteria 1–10 µm; red blood cell ≈ 7 µm; human hair ≈ 50–100 µm; credit card ≈ 760 µm (0.76 mm); SIM card 0.76 mm thick. Engineering tolerances: IT6 at 25 mm = 13 µm (0.013 mm); Ra surface finish range 0.1–12.5 µm. 3D printing layers: 25–400 µm = 0.025–0.4 mm. Quick rule: mm × 1,000 = µm; µm ÷ 1,000 = mm. Perfect exact powers of 10 — no approximation ever needed.
In engineering, especially when working with both metric and imperial specifications, the abbreviation "mil" can cause confusion. A mil (also called a thou) is 1/1,000 of an inch = 25.4 µm = 0.0254 mm — it is NOT the same as a millimetre or a micrometre. For example: a 3 mil PCB coating = 3 × 25.4 µm = 76.2 µm = 0.0762 mm — NOT 3 mm or 3 µm. In contrast, the metric abbreviation "mm" is always millimetres (0.001 m) and "µm" (or "um" in text) is always micrometres (0.000001 m). Some US-sourced engineering documents use "mil" for thousandths of an inch (thou) — always verify which unit system is in use before applying conversion factors. This calculator outputs both inches and mils for full cross-unit clarity.
The International Bureau of Weights and Measures (BIPM) defines all SI units and their prefixes. The metre is the SI base unit of length; milli- (10⁻³) and micro- (10⁻⁶) are SI prefixes that define the millimetre and micrometre exactly. The 9th edition of the SI Brochure (2019) confirms all prefix multipliers and the exact relationships between metric length units. The micrometre is a coherent derived unit fully accepted within the SI, and the millimetre is the most commonly used sub-unit of the metre in engineering, manufacturing, and everyday measurement worldwide.
Visit BIPM →ISO 286 is the international standard governing limits and fits for cylindrical parts — specifying tolerances in micrometres on drawings dimensioned in millimetres. It defines 20 IT (International Tolerance) grades from IT01 through IT18, covering precision instrument work through heavy engineering. Understanding µm tolerances on mm dimensions is fundamental to reading ISO 286 tables and applying fits (clearance, transition, interference) to shaft and bore combinations. ISO 1302 covers surface finish notation (Ra values in µm) on engineering drawings, and ISO 1101 covers geometric dimensioning and tolerancing (GD&T) in µm.
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