Flow ratio calculator for OrcaSlicer tests and wall-thickness measurements
What it calculates
OrcaSlicer's YOLO tests add the best block's modifier to the current ratio (0.98 + 0.01 = 0.99); its legacy 2-pass test multiplies: old × (100 + modifier) ÷ 100 [1]. For a single-wall cube, Prusa's formula is line width ÷ measured wall thickness [2]; this calculator multiplies that by your current ratio so it works from any starting value.
Three ways to arrive at a flow ratio
Flow ratio (OrcaSlicer, Bambu Studio) and extrusion multiplier (PrusaSlicer) are the same idea: a per-filament factor that scales how much plastic the slicer asks for. Too low leaves gaps and weak layers; too high leaves rough, ridged surfaces and parts that come out oversized [1]. The calculator covers the three common tests.
OrcaSlicer YOLO (Archimedean chords)
OrcaSlicer’s current recommended method prints a set of blocks with a spiral-and-arc top pattern, each labelled with a modifier. The Recommended version spans −0.05 to +0.05 in 0.01 steps; the Perfectionist version spans −0.04 to +0.035 in 0.005 steps [1]. Choose the block with the smoothest top surface, no gaps between the arcs and the least visible line between the inner spiral and outer arcs, then:
new flow ratio = old flow ratio ± modifier
OrcaSlicer’s example: 0.98 with a best block of +0.01 gives 0.99 [1].
OrcaSlicer 2-pass (legacy)
The older method prints nine blocks in pass 1 and ten blocks in pass 2, with modifiers from −9 to 0 in the second pass [1]. Here the modifier is a percentage:
new flow ratio = old flow ratio × (100 + modifier) ÷ 100
OrcaSlicer’s example chain: 0.98 with +5 in pass 1 gives 1.029; then −6 in pass 2 gives 0.96726 [1]. Enter both modifiers and the calculator chains them.
Single-wall cube (calipers)
Prusa’s precise method prints a cube in vase mode, so each wall is one extrusion wide, measures each wall three or more times, and averages the result. Prusa’s formula is extrusion multiplier = extrusion width ÷ average measured wall thickness, using 0.45 mm as the width for its 0.4 mm nozzle profiles [2]. That formula assumes the cube was printed at a multiplier of 1. The calculator multiplies by your current value instead, so it also works when you start from 0.95 or 1.05:
new = old × expected width ÷ measured width
Prusa cautions that cheap calipers may not be accurate enough to measure a single perimeter reliably [2]. A 0.02 mm reading error on a 0.45 mm wall is a 4% error in the result, which is more than the correction you are trying to make.
Before you trust the number
- Calibrate the extruder first on printers where you control it (rotation distance and e-steps). Flow ratio should fix one filament, not a wrong extruder setting.
- Print the test at the temperature and speed you will actually use. OrcaSlicer bases the YOLO test on the selected filament’s current flow ratio, so select the right filament before generating it [1].
- Save the result in the filament profile, not the process profile, so it follows the spool.
- If the top surface still shows ridges or holes after a good flow result, look at top-layer settings, pillowing and over-extrusion causes rather than pushing the ratio further.
The flow rate calibration guide covers the full procedure for OrcaSlicer, PrusaSlicer and Bambu Studio.
Questions
My Bambu printer has a 'Flow calibration' checkbox. Is that the same thing?
No. OrcaSlicer's wiki warns Bambu Lab users not to select the Flow calibration option when running its flow ratio test [1]. That printer option is a separate automatic routine; the flow ratio test measures the filament profile's ratio.
What range of values is normal?
Prusa's guide says adjusted extrusion multipliers usually fall between 0.9 and 1.1 [2]. A result well outside that points to a different problem, such as the extruder steps, the filament diameter or a partial clog.
Sources
- Flow Ratio Calibration. OrcaSlicer Wiki. Slicer documentation. YOLO (Recommended, Perfectionist) and 2-pass methods, formulas and worked examples. Accessed October 12, 2026.
- Extrusion multiplier calibration. Prusa Knowledge Base. Manufacturer documentation. Precise method: vase-mode cube, extrusion width ÷ average measured wall thickness; typical range 0.9–1.1. Accessed October 12, 2026.