3D Printing MM to Inches Conversion
Complete unit conversion guide for 3D printing, STL files, Fusion 360, Blender, and digital modeling. Fix model scaling issues, calculate tolerances, and optimize your prints.
🔢 3D Model Scale Calculator
Calculate the perfect scale factor for resizing your 3D models. Essential for fixing STL file size issues.
🔩 3D Printing Screw Hole Sizes
Optimized screw hole dimensions for FDM printing. Add +0.2mm for clearance holes to compensate for printing tolerance.
M2 Screw Hole
Small models, drones, RC cars
M3 Screw Hole
Most common, electronics enclosures
M4 Screw Hole
Medium structural parts
M5 Screw Hole
Heavy structural connections
💡 Tip: For PLA, add +0.2mm to clearance holes. For ABS, add +0.3mm due to higher shrinkage. Always print a test piece with multiple hole sizes first.
🎨 Software Unit Setup Guide
Configure units correctly in popular 3D modeling software to avoid the dreaded 25.4x scaling error.
Fusion 360
Change Document Units:
- File → Preferences → Default Units
- Default Design Units: Millimeters → Inches
- Decimal Rounding: 3 places → 4 places
- Click Apply → OK
Scale Existing Model:
- Modify → Scale
- Select all bodies
- Scale Factor: 0.0393701 (mm→in) or 25.4 (in→mm)
Blender
Scene Units Setup:
- Scene Properties (right panel)
- Units → Unit System: Metric → Imperial
- Length: Millimeters → Inches
- Unit Scale: Keep at 1.0
Quick Scale Conversion:
- Select all objects (A key)
- Press S (scale)
- Type 0.03937 (mm→in) or 25.4 (in→mm)
- Press Enter, then Ctrl+A → Apply Scale
Tinkercad (Easiest)
✅ Automatic Conversion:
- Click gear icon ⚙️ (top right)
- Units: Metric (mm) ⟷ Imperial (in)
- All existing objects convert automatically!
🖨️ Printer Bed Size Reference
Common 3D printer build volumes to check if your model will fit.
| Printer Model | Size (mm) | Size (inches) |
|---|---|---|
| Creality Ender 3 | 220×220×250 mm | 8.66×8.66×9.84" |
| Prusa i3 MK3S+ | 250×210×210 mm | 9.84×8.27×8.27" |
| Bambu Lab X1 Carbon | 256×256×256 mm | 10.08×10.08×10.08" |
| Creality CR-10 | 300×300×400 mm | 11.81×11.81×15.75" |
| Creality CR-10 S5 | 500×500×500 mm | 19.69×19.69×19.69" |
💡 Tip: Always leave 10-20mm margin around edges for easier part removal and avoid edge warping.
🎯 RC Model & Miniature Scales
Common scale ratios for RC vehicles, military models, and miniatures.
| Scale | Real (mm) | Model (mm) | Model (in) | Common Use |
|---|---|---|---|---|
| 1:6 | 1800 | 300 | 11.81" | Large motorcycles/figures |
| 1:8 | 4000 | 500 | 19.69" | RC off-road cars |
| 1:10 | 4500 | 450 | 17.72" | Standard RC racing |
| 1:12 | 1800 | 150 | 5.91" | Motorcycle models |
| 1:18 | 4500 | 250 | 9.84" | Small RC cars |
| 1:24 | 4800 | 200 | 7.87" | Diecast car models |
| 1:35 | 10000 | 286 | 11.26" | Military models |
| 1:64 | 4800 | 75 | 2.95" | Hot Wheels/Matchbox |
| 1:87 | 1435 | 16.5 | 0.65" | HO model trains |
📐 Common 3D Printing Component Sizes
Standard dimensions for frequently used components in 3D printed designs.
Nozzle Sizes
0.2mm = 0.008" (fine detail)
0.4mm = 0.016" (standard)
0.6mm = 0.024" (fast prints)
0.8mm = 0.031" (large models)
Filament Diameter
1.75mm = 0.069" (most common)
2.85mm = 0.112" (also called "3mm")
🔬 FDM Printing Tolerances
Understand printing accuracy and design appropriate clearances for your parts.
FDM Accuracy
SLA/Resin Accuracy
Design Clearances (FDM)
Material Shrinkage
FAQ - 3D Printing Unit Conversion
Why is my STL file 25.4 times too large or too small?
This is a unit mismatch error. Your model was designed in inches but imported as millimeters (or vice versa). Since 1 inch = 25.4mm, the file appears 25.4× wrong. Solution: In your slicer (Cura/PrusaSlicer), scale the model by 3.937% (÷25.4) or 2540% (×25.4). Check screw holes: M3 should be 3mm, not 76mm or 0.12mm.
How do I fix Fusion 360 or Blender units before exporting STL?
Fusion 360: File → Preferences → Default Units, then choose mm or inches. For existing models, use Modify → Scale with factor 0.0393701 (mm→in) or 25.4 (in→mm).
Blender: Scene Properties → Units → set to Metric/Millimeters or Imperial/Inches. Keep Unit Scale at 1.0. When exporting STL, check "Scene Unit" option.
Why do I need to add 0.2mm to screw holes for 3D printing?
FDM printing has inherent tolerances. Plastic expands slightly when hot and contracts when cooling, plus layer adhesion causes slight inward bulging. For clearance holes, add +0.2mm (PLA) or +0.3mm (ABS). Example: M3 screw (3.0mm nominal) → design hole as 3.2mm for easy fit. Always print a test piece with 2.8mm, 3.0mm, 3.2mm holes to find your printer's sweet spot.
How accurate are FDM vs SLA/resin printers?
FDM (filament): ±0.1mm (±0.004") typical accuracy. Good for functional parts, tolerances need compensation.
SLA/Resin: ±0.025mm (±0.001") accuracy, 4× more precise. Excellent for miniatures, jewelry, dental models. Minimum feature size: 0.05mm vs 0.4mm for FDM.
Can I use this calculator for Thingiverse models?
Yes! Use the Scale Calculator above. If a Thingiverse model is 100mm but you need 3.5 inches (88.9mm), enter Original: 100mm, Target: 3.5in, then apply the scale factor (88.9%) in your slicer. Many Thingiverse models have unit errors—check comments for phrases like "scale to 25.4×" or "too small/large."
What's the minimum wall thickness for FDM printing?
For 0.4mm nozzle: minimum 0.8mm (2 perimeters). Recommended: 1.2mm (3 perimeters) for structural strength, 2.0mm for durable parts. Wall thickness should be a multiple of nozzle diameter. For 0.6mm nozzle: 1.2mm, 1.8mm, 2.4mm. SLA/resin can go down to 0.4mm for decorative parts.