Blobby Walls, Rough Tops and Oversized Parts (Over-Extrusion)

Likely causes, most common first

1. Flow ratio or extrusion multiplier set too highMost common
   OrcaSlicer's wiki says too high a flow ratio gives excess material, rough surfaces and dimensional inaccuracy. Prusa notes the right value varies by material, color and even spool, so a value tuned for one filament can be high for the next.
2. Printer flow changed on the printer itselfCommon
   Many printers let you change flow during a print from the screen. Prusa notes this firmware value multiplies with the slicer's, so both stack.
3. Filament diameter in the profile does not match the filamentOccasional
   The slicer converts the plastic each line needs into a length of filament using the diameter in the profile. Filament thicker than the profile says pushes more plastic per millimeter. Prusa gives the industry diameter tolerance as plus or minus 0.05 mm Rule of thumb.
4. Profile for a different nozzle or materialOccasional
   Prusa recommends starting from the stock profile for your printer, filament type and nozzle size. A profile for another setup asks for the wrong amount of plastic.
5. Nozzle too close on the first layer (looks like over-extrusion)Occasional
   If only the first layer looks squashed and the layers above are fine, the problem is nozzle height, not flow. See Z offset.

Fix it, one change at a time

1. Reset the printer's flow to its default (100%) from the printer's screen if it has been changed.
2. Check the filament profile. Use the stock profile for your printer, nozzle and filament, and make sure the diameter matches the filament you have (1.75 mm or 2.85 mm). Measure the filament with calipers in several places along the spool.
3. Rule out nozzle height if the problem is only on the first layer: raise the nozzle slightly with Z offset or live adjust and print a first-layer test.
4. Calibrate in the right order. The OrcaSlicer guide recommends temperature first, then max volumetric speed, then pressure advance, then flow ratio, then retraction, and repeating the filament tests for each new filament. Flow comes after pressure advance; if flow changes a lot, OrcaSlicer suggests a quick pressure advance retest.
5. Run a flow test. In OrcaSlicer, the recommended flow ratio test prints blocks with modifiers from -0.05 to +0.05 in 0.01 steps Starting point around your current value; pick the block with the smoothest top, no gaps and the least material piled where the lines meet. With Prusa's visual cube, change the multiplier 1 to 2% Starting point at a time: too much material near the perimeters means lower it, visible gaps between lines mean raise it.
6. Or measure a wall. Prusa's precise method prints a single-wall cube in vase mode and divides the expected wall width by the measured one:
   FormulaExtrusion multiplier from a measured wall

   new multiplier = 0.45 ÷ average measured wall thickness (mm)

   0.45 mm is the extrusion width in Prusa's default 0.4 mm nozzle profiles (Prusa's own number for its profiles). Use your profile's extrusion width if it differs. Measure at least three times in the middle of the walls and average. Prusa warns cheap calipers may not be accurate enough for one wall.

7. Save the result to that filament's profile, not to the printer, so the next spool starts from its own value.

Confirm it is fixed

Print a small box with a flat solid top. The top should be smooth to the touch with no ridges or piled-up edges and no gaps between lines. Then measure a simple part of known size with calipers; outside dimensions should be close to the model and holes should not be undersized. See Dimensional Accuracy for a full sizing test.

Why it happens

The slicer plans each line as a strip of plastic of a set width and height, works out its volume, and converts that volume into a length of filament to push. Anything that makes the real push larger than the plan adds plastic that has nowhere to go. It piles up where lines meet, bulges walls outward and shrinks holes.

Filament diameter is a squared term, so small differences matter more than they look:

Flow ratio is the slicer's correction for everything it cannot know about your filament and hardware, which is why Prusa and OrcaSlicer both treat it as a per-filament value rather than a printer setting. It should be a small correction: OrcaSlicer notes the test starts from the profile's value because most profiles are already close. A flow value far from the default is a sign something else is wrong, such as a diameter mismatch or a stacked firmware flow change.

Over-extrusion and under-extrusion are the two ends of the same setting, and one test catches both. If you see gaps rather than bulges, go to Gaps, Thin Lines and Weak Walls, and fix hardware there before touching flow.