HOW TO CALIBRATE FLOW RATE (SINGLE-WALL CUBE METHOD)

Flow rate (called 'extrusion multiplier' in PrusaSlicer / OrcaSlicer / Bambu, or 'flow' in Cura) is the per-filament scalar applied to the slicer's volume calculation. Default 100% (or 1.0). If your e-steps are correctly calibrated and the filament diameter is correct, flow rate should be 95-100% for most materials. It exists because individual filament brands and colours vary slightly in actual diameter and density. A spool that measures 1.74 mm produces slightly different volume per mm than one measuring 1.78 mm, even with everything else equal. Flow rate compensates per filament without changing the underlying e-step calibration.

Calibrate when: opening a new filament brand or type, walls measure consistently thicker or thinner than expected, surface looks slightly bumpy (over-extruded) or shows gaps (under-extruded) despite e-step calibration. Don't bother when: you're on a brand you've already calibrated for. Flow rate is per-filament, not per-printer - if you've calibrated PolyMaker PolyTerra PLA Black on this printer, that value transfers to every print of the same filament.

1. Verify e-steps first. Flow rate calibration assumes e-steps are correct. If they aren't, you'll be compensating for a deeper miscalibration. Run how-to-calibrate-e-steps if unsure. 2. Verify filament diameter. Measure with calipers in 4-5 spots, average. Enter exact value in slicer's filament profile (e.g., 1.74 instead of generic 1.75). 3. Set up the test print: - Download or model a single-wall calibration cube (search Printables for 'single wall cube' or 'flow calibration cube'). - Slice with: 1 wall, 0% infill, 0 top layers, 0 bottom layers, 0.20 mm layer height (or your normal layer height). - Set flow rate to 100% (default) for the first run. 4. Print the cube. Should be a hollow open-top box with single-perimeter walls. 5. Measure wall thickness with calipers. - Measure each wall in 3 spots (top, middle, bottom). Average per wall, then average all four walls. - Compare to your slicer's line width setting (typically 0.4 mm for a 0.4 nozzle, 0.42 mm if you bumped line width 105%). 6. Calculate new flow rate: ``` new_flow_rate = current_flow_rate x (target_line_width / measured_wall_thickness) ``` Example: target 0.40 mm, measured 0.42 mm. New flow = 100% x (0.40 / 0.42) = 95.2%. 7. Update flow rate in filament profile. - Cura: Material - Flow. - PrusaSlicer: Filament Settings - Filament - Extrusion multiplier. - OrcaSlicer / Bambu Studio: Filament - Flow Ratio. 8. Reprint the cube and measure. Should now be within +/- 0.02 mm of target. If not, repeat the calculation with the new flow as starting point. 9. Save the value to the filament profile. Future prints automatically use this calibrated flow.

Calibrating flow before e-steps. E-steps is the underlying calibration; flow is the per-filament fine-tune. Calibrate e-steps first, flow second. Measuring with the print still hot. Plastic shrinks slightly as it cools. Wait until cool before measuring. Measuring at corners. Corner geometry is unreliable - measure mid-wall, away from corners. Setting flow below 90% to fix problems. If flow needs to drop below 90% to fit, the underlying problem is e-steps, filament diameter, or temperature - not flow. Investigate. Calibrating once and applying to all filaments. Each filament needs its own value. Different brands of the same material can vary by 5-10%. Setting flow above 105% to compensate for under-extrusion. Same as above - investigate root cause (clog, low temp, slow extruder) instead of masking with flow.

Calibrate e-steps first - see how-to-calibrate-e-steps. After flow, calibrate Pressure / Linear Advance per filament - see how-to-calibrate-pressure-advance. For ongoing under- or over-extrusion that calibration didn't fix, see under-extrusion or over-extrusion.