3D Print Strength vs Infill Estimator

Extra walls usually add more strength per gram than extra infill, because the outside of a part carries most of a bending load. Enter your part size and up to four slicer settings to compare the filament used, a relative strength index and a relative print time. The strength numbers come from a simple, stated model: use them to compare settings, not as test results.

Published October 30, 2026

1. Part and filament

Sizes in mm. The part is treated as a solid block (or plate) of this size, bent across its height.

2. Settings to compare

3D print strength vs infill

Comparison

SettingFilament1.75 mm2.85 mmBendingTensionGain per extra gTime
Bending and tension are % of a solid part of the same size (bending strength and stiffness come out the same in this model). Gain per extra g: bending points gained per gram added over A. Time is relative to A.
Bending index (% of solid)
Filament (g)

    Walls × Infill Map

    Bending index (% of solid) and grams for each combination, with setting A's top/bottom layers and pattern. Read across a row to add infill, down a column to add walls. The outlined cell is the strongest that is no heavier than A.

    Filament Length ↔ Weight

    g per metre = π × (d/2)² × 1000 mm × density ÷ 1000. Type in either box.

    Filament Densities

    Materialg/cm³g/m 1.75m/kg 1.75g/m 2.85m/kg 2.85
    Typical published densities; brands vary by a few per cent and filled filaments more. Your spool's datasheet is best.

    How the Strength vs Infill Estimate Works

    In bending, the plastic far from the middle of a part does most of the work, and walls and top and bottom skins are exactly there. Infill sits near the middle and is a lattice, not solid plastic, so each gram of infill adds less strength than a gram of wall.

    This tool turns that into numbers with a deliberately simple model, so you can see the trade-off for your own part. It is an estimate for comparing settings. Real strength also depends on layer adhesion, the load direction, holes, temperature and print quality, which the model does not include.

    The Model

    1. Shell and core
    wall tw = walls × line width
    skins = top (bottom) layers × layer height
    core = (L − 2tw)(W − 2tw)(H − ttop − tbottom)
    volume = (L·W·H − core) + core × infill %
    The shell (walls plus skins) is solid plastic. Infill fills its percentage of the core. Perimeter overlap and gap fill are ignored, so slicer estimates may differ by a few per cent.
    2. Weight and length
    grams = volume (cm³) × density
    metres = grams ÷ (π (d/2)² × density)
    With d in mm the cross-section in mm² × 1000 mm gives cm³ per metre. 1.75 mm PLA at 1.24 g/cm³ is 2.98 g/m, about 335 m per kg; 2.85 mm is 7.91 g/m, about 126 m per kg.
    3. Bending index
    Ecore = infill % × kpattern (solid = 1)
    I = Iouter − (1 − Ecore) · Icore (about the neutral axis)
    bending index = (I / c) ÷ (Isolid / (H/2))
    The section is W × H, bent across the height with layers flat. The neutral axis moves toward the thicker skin if top and bottom differ. Tension index = effective area ÷ solid area, which is close to the weight ratio.
    4. Time index
    t ∝ Vwalls ÷ 1.0 + Vskins ÷ 1.3 + Vinfill × pattern ÷ 1.8 + layer overhead
    Walls print slowest, skins faster and sparse infill fastest (relative volumetric speeds 1.0, 1.3 and 1.8, typical of default profiles). Honeycomb and gyroid slow the infill down. Only the ratio between settings means anything; the slicer's estimate is the real time.

    Infill Pattern Factors (Approximate)

    k is how well the infill carries load compared with the same weight of solid plastic. A lattice is much less efficient than solid material (lattice theory gives roughly k = 1/3 for an ideal stretch-dominated lattice, and less for a bending-dominated one). The values are approximate, picked to follow the general trend of published infill strength tests, where most patterns land within about ±20% of each other at the same density. Load direction changes them a lot.

    Patternk (strength)Time factorNotes
    Lines / rectilinear0.351.00Strong along the lines, weak across them.
    Grid0.401.05Crosses itself every layer; the nozzle can catch at high infill.
    Triangles0.451.05Stiff in the plane.
    Cubic0.401.05Fairly even in all directions.
    Gyroid0.401.15Even in all directions; curved moves print a little slower.
    Honeycomb0.501.50Strong but slow.
    Concentric0.301.05Best for flexible parts; weak between rings.

    Worked Example

    A 100 × 20 × 10 mm PLA bar, 0.2 mm layers, 0.45 mm line width, gyroid
    • A: 2 walls, 4 top, 4 bottom, 15%. Walls 0.9 mm, skins 0.8 mm, core 98.2 × 18.2 × 8.4 = 15,013 mm³. Volume = 20,000 − 15,013 + 0.15 × 15,013 = 7,239 mm³ = 7.24 cm³, so 8.98 g (3.01 m of 1.75 mm). Bending index 49.3% of solid.
    • B: same but 40% infill. 13.63 g (+4.65 g), bending 54.7%: 1.2 points per extra gram.
    • C: 4 walls, 15% infill. 10.80 g (+1.82 g), bending 54.3%: 2.7 points per extra gram, more than twice as good, and nearly as strong as B for 2.8 g less.
    • D: 6 walls, 5 top, 5 bottom, 10%. 12.47 g, bending 64.1%: stronger than B and still lighter.
    • The walls × infill map shows 5 walls at 10% infill (10.92 g) beats 2 walls at 40% (13.63 g) while using 20% less filament.

    Practical Tips

    • Add walls before infill for parts that bend, twist or take screws. 3 to 4 walls at 15 to 25% infill is a common strong, economical setting; going above about 40% infill adds weight and time for little gain.
    • Top and bottom skins matter in bending just as walls do: aim for at least 0.8 to 1 mm of skin (4 to 5 layers at 0.2 mm).
    • Orient the part so the load runs along the layers, not across them. Layer adhesion is often the weakest direction and is not in this model.
    • Wider lines are stronger and faster: a 0.5 mm line with a 0.4 mm nozzle makes thicker walls in the same number of passes.
    • Around holes and inserts, walls are what hold. See the Heat-Set Insert Hole Size tool for the wall needed, and Hole & Fit Compensation for sizes. Big solid parts shrink more; see Shrinkage & Scale.
    • Need real numbers? Print test bars with each setting and break them. For metal alternatives, compare densities in the Metal Properties Finder.

    Frequently Asked Questions

    For most parts, walls. In bending and twisting the outer plastic carries most of the load, so extra walls add more strength per gram than extra infill. In this tool's example bar, going from 2 to 4 walls adds about twice as much bending strength per extra gram as going from 15% to 40% infill.

    About 335 m. A metre of 1.75 mm filament holds π × 0.875² × 1000 = 2,405 mm³, which is 2.98 g of PLA at 1.24 g/cm³. ABS at 1.04 g/cm³ gives about 400 m per kg, and 2.85 mm PLA about 126 m per kg.

    At the same density, published tests put most patterns within about 20% of each other, and the winner depends on the load direction. Honeycomb and triangles are often near the top but print slower; gyroid and cubic are good all-round choices. Changing the pattern matters far less than adding walls.

    A solid part is the strongest and stiffest, but it is also the heaviest and slowest, and thick solid sections warp and shrink more. Many walls with moderate infill give most of the strength for much less filament. Use 100% for small, highly loaded parts such as gears.

    It is a comparison tool, not a test. It uses simple beam theory on a block with solid walls and skins and an infill whose stiffness is set by approximate pattern factors. It leaves out layer adhesion, holes, stress concentrations and print quality. Use it to rank settings, then test the part that matters.

    Strength and time figures are estimates from a simplified model with approximate pattern factors. They are for comparing settings only; test critical parts.