3D Printing Converter & Calculator
Convert between mm, inches, microns, and thou — plus filament estimator, layer height reference, and print size checker.
3D Printing Calculators
Why Millimeters Are the Standard in 3D Printing
Every major 3D printer manufacturer and slicer software uses millimeters as the default unit. This convention traces back to the RepRap project — the open-source movement that spawned modern consumer 3D printing — which was based in the UK and continental Europe where metric is standard. Today, firmware (Marlin, Klipper, RepRapFirmware), G-code commands, and all slicer settings operate in millimeters.
The millimeter strikes the perfect balance for 3D printing precision. Layer heights range from 0.05mm to 0.3mm, nozzle diameters from 0.2mm to 1.0mm, and typical print tolerances fall around 0.1–0.2mm. All of these values are expressed as simple decimals in mm. Using inches would require awkward fractions or many decimal places (0.4mm = 0.01575 inches), while using microns for build plate dimensions would yield unwieldy numbers (220mm = 220,000µm).
Common Nozzle Sizes: mm vs Inches
| Nozzle (mm) | Nozzle (in) | Max Layer Height | Use Case |
|---|---|---|---|
| 0.2mm | 0.008" | 0.15mm (150µm) | Ultra-detail, miniatures |
| 0.4mm | 0.016" | 0.30mm (300µm) | Standard (most printers) |
| 0.6mm | 0.024" | 0.45mm (450µm) | Faster prints |
| 0.8mm | 0.031" | 0.60mm (600µm) | Speed & strength |
| 1.0mm | 0.039" | 0.75mm (750µm) | Large objects, vases |
Understanding Layer Heights and Microns
Layer height is the single most important quality setting in FDM 3D printing. It determines the thickness of each horizontal slice of your model, directly affecting surface finish, print time, and structural strength. Layer heights are commonly discussed in two units: millimeters (as displayed in slicer software) and microns (as used in community discussions and printer marketing).
The conversion is straightforward: 1mm = 1,000 microns. So when a forum post recommends "printing at 100 microns," that means setting your slicer to 0.1mm layer height. When your Bambu Lab printer advertises "50µm minimum layer height," that is 0.05mm — achievable but very slow.
The 75% rule is essential: your maximum layer height should not exceed 75% of your nozzle diameter. With a standard 0.4mm nozzle, that means 0.3mm (300µm) max. Going higher causes poor layer adhesion because the nozzle cannot properly press and spread the filament. Going lower than 25% of nozzle diameter (0.1mm / 100µm for a 0.4mm nozzle) is possible but pushes the limits of consistent extrusion.
STL Files and the Inch/mm Confusion
The STL file format — the most common format for 3D printable models — does not store unit information. It contains only raw numbers for vertex coordinates. This means a cube designed as "1 unit" could be 1mm, 1 inch, 1 centimeter, or 1 meter depending on the CAD software used to create it.
When you import an STL into your slicer, it assumes the numbers represent millimeters. If the original design was in inches, every dimension will be 25.4 times too small. A model that should be 100mm tall will appear as 3.94mm — barely visible on the build plate. The fix is to scale by 2540% (25.4 times). Conversely, if a model appears 25.4 times too large, it was designed in meters — scale to 0.1%.
- Model too small? Likely designed in inches. Scale by 2540%.
- Model too large? Likely designed in meters or centimeters. Scale to 0.1% or 10%.
- Model looks right? It was designed in millimeters. No scaling needed.