FDM Line Width Calculator

Line width is the actual width of plastic deposited on the bed by an FDM printer. It is rarely equal to the nozzle diameter. Slicers let you set it anywhere from about 100% to 150% of the nozzle bore depending on the goal. Getting this right has more impact on print quality than most people realize.

The practical rule:

w = 100–125% of nozzle diameter

For a 0.4 mm nozzle this means setting line width somewhere in the 0.40–0.50 mm range. Going below 100% forces the slicer to underextrude relative to the nozzle bore, which leaves visible gaps between adjacent extrusions. Going above about 150% asks the hotend to push plastic significantly wider than the nozzle opening, which usually shows up as poor surface finish, inconsistent extrusion, and occasional under-extrusion at the start of perimeters.

The cross-section equation (stadium / oblong):

A = (w − h) × h + π × (h / 2)²

This is the shape every modern slicer assumes for a deposited line: a flat rectangle in the middle (w − h wide, h tall) with two semicircular caps on the sides (radius h / 2). The h cap on the bottom flattens against the previous layer or build plate; the h cap on top is the rounded top surface.

Where w and h sit in practice:

• w = line width (mm), the lateral extent of the bead on the bed
• h = layer height (mm), the vertical extrusion thickness
• A = mm² of plastic deposited per mm of toolpath travel

Worked example, common settings: 0.4 mm nozzle, 0.45 mm line width, 0.20 mm layer height:

A = (0.45 − 0.20) × 0.20 + π × (0.10)² A = 0.25 × 0.20 + π × 0.01 A = 0.050 + 0.0314 A = 0.0814 mm² per mm of line

Connecting to flow rate: The volumetric flow rate Q the hotend must supply equals A × print speed. At 60 mm/s print speed with the example above:

Q = 0.0814 × 60 = 4.88 mm³/s

That is well under the 10–20 mm³/s most direct-drive hotends can handle, so this combination prints reliably. Pushing print speed to 200 mm/s gives Q = 16.3 mm³/s, which is right at the limit for stock hotends and needs a high-flow nozzle (Volcano, CHT, Mosquito) for sustained operation.

Filament feed speed conversion: 1.75 mm filament has cross-section π × 0.875² = 2.405 mm². So:

feed speed (mm/s) = (A × print speed) / 2.405

For the example above at 60 mm/s print speed: feed speed = 4.88 / 2.405 = 2.03 mm/s of filament. Extruder steppers easily handle this; that is why this setup is rock-solid.

When to use wider lines (0.5–0.6 mm from a 0.4 mm nozzle):

• Infill: wider lines mean fewer total extrusions for the same coverage, much faster
• Bridging: a wider, thinner extrusion spans gaps better with less sagging
• Hard-to-stick first layers: wider first-layer width (often 110–120%) creates more contact area with the bed and improves adhesion

When to use narrower lines (0.35–0.4 mm from a 0.4 mm nozzle):

• Fine detail in perimeters where surface quality matters more than speed
• Tall thin features (text, small overhangs) that would otherwise be merged into thick walls
• Tree supports, where minimal contact area matters

Adaptive line width (Arachne, variable width): Modern slicers (PrusaSlicer, OrcaSlicer, SuperSlicer) now offer variable line width inside a single perimeter to fill thin or odd-shaped features without leaving gaps. The cross-section equation above still applies at each point. The width just varies along the path. This is the single biggest quality improvement in slicing software in the past five years.

Extrusion multiplier vs line width: Extrusion multiplier (flow rate %) is a scalar applied to every extrusion move. Setting it to 98% reduces all line widths and bead areas by 2%. It is a fine-tuning correction, not a substitute for calibrating steps/mm on the extruder. If your prints come out consistently thick or thin across the whole part, calibrate steps/mm. If they come out correct in dimensions but the perimeter walls measure slightly off, tweak extrusion multiplier.