What flow rate does
Flow rate, also called extrusion multiplier, scales how much plastic the slicer asks for on every line. Filament diameter, material behavior and hotend temperature all nudge the real output away from the ideal, so each filament benefits from its own value. Too high and top surfaces get ridges, edges bulge and holes shrink. Too low and you see gaps between lines, weak layer bonding and rough top skins.
Flow rate is a slicer setting and is specific to each filament profile. It is different from e-steps, which is a firmware setting for the extruder hardware. Get e-steps right first, if your printer needs it, and use flow to fine tune each material.
How the calculation works
Print a hollow object with a single perimeter and measure how thick the wall came out compared with the line width you asked for:
new flow = current flow × target width / average measured width
If you asked for 0.45 mm and got 0.47 mm, the printer is putting out about 4% too much and flow comes down by the same ratio.
How to print and measure the test
- Make a 20 to 30 mm cube or cylinder. In the slicer turn on vase mode (spiralize outer contour) or set one wall, zero top layers and zero infill.
- Set a known line width for the outer wall, such as 0.45 mm for a 0.4 mm nozzle, and note your current flow.
- Print at your normal temperature and a moderate speed for that material.
- Once cool, measure the wall with digital calipers at four spots: the middle of each side, away from corners and the seam, and away from the first few layers, which are squished.
- Average the four readings.
Caliper pressure matters a lot here. A single wall of plastic is soft and easy to crush or to measure loosely. Close the jaws gently until they just touch, the same way each time. Some makers use the thumb roller lightly or a micrometer with a ratchet for consistent pressure. A tenth of a millimeter of difference from heavy hands is a 20% error.
Worked example
Current flow is 100%, outer wall line width is 0.45 mm. Your four readings are 0.47, 0.48, 0.46 and 0.47 mm.
- Average: (0.47 + 0.48 + 0.46 + 0.47) / 4 = 0.47 mm
- New flow: 100 × 0.45 / 0.47 = 95.7%
- Set the filament profile to about 96% and print a test with a top surface to confirm.
Typical values
| Material | Common flow range | Notes |
|---|---|---|
| PLA | 92–100% | Often slightly under 100% |
| PETG | 90–98% | Tends to over-extrude and blob |
| ABS / ASA | 92–100% | Check in an enclosure |
| TPU | 95–105% | Soft walls make measuring hard |
These are starting points, not answers. Slicers also define 100% differently, so values copied from another slicer may not transfer.
Built-in slicer calibration
OrcaSlicer, Bambu Studio and PrusaSlicer-based tools include flow calibration prints. These usually print a set of patches with different flow values and you pick the smoothest top surface. Some printers also run automatic flow dynamics calibration. That approach avoids caliper error entirely and is a good alternative or cross-check. Many people use the single-wall method for a quick number, then confirm with a top surface patch test.
Common mistakes
- Squeezing the calipers. The most common error. Light, repeatable pressure only.
- Measuring at the seam or corners. Both are thicker than the straight wall.
- Using a different line width in the formula than you printed. Check the outer wall line width in the slicer, not the nozzle size.
- Tuning flow to fix other faults. Wet filament, a partial clog or too low temperature all look like flow problems.
Practical tips
- Keep separate flow values per brand and color. Pigments change behavior.
- Change one thing at a time. Adjust temperature first, then flow.
- Make small steps. If the answer is more than 10% off, look for a hardware or setting problem.
After setting the new value, print something with a flat top surface and a few holes. A good flow gives a smooth top with lines just touching and holes close to their modeled size.