Infill vs Perimeter Strength Calculator
Whether to add infill or another wall, worked from second moment of area — a perimeter is worth about twice as much per gram.
The same part, seven ways
A perimeter is worth twice what infill is, per gram
Bending stiffness follows the second moment of area, which weights material by the square of its distance from the centre — so a wall on the outside works far harder than the same plastic sitting in the middle. On a 20 mm section with 0.4 mm lines, going from two perimeters to three buys 1.62 points of stiffness per point of material. Infill buys 0.846. A factor of 1.9 — which is why the reflex to raise infill when a part breaks is usually the wrong lever.
Head to head: four perimeters and no infill gives 50.2% of solid stiffness for 29.4% of the material. Two perimeters and 20% infill gives 42.7% for 32.3%. More stiffness, less plastic, and less time — infill is fast to print but walls are faster still per unit of strength gained.
The crossover is out of reach. There is a perimeter count beyond which another wall stops beating infill, because each new one sits closer to the neutral axis — but on a 20 mm section it's the tenth perimeter, and on a 40 mm section the seventeenth. Slicer defaults are two to four. In the range anybody actually prints in, a wall wins every time.
And infill's return is flat, not diminishing. Every 10% of infill buys the same stiffness for the same plastic, at 10% and at 90%. The received wisdom about diminishing returns above 50% is real, but it's about print time and material cost — not stiffness per gram, which never improves and never worsens. It's simply always below a perimeter.
- Orientation beats both. An FDM part is weakest between layers, and no infill percentage fixes that. If a part is breaking, the first question is which way it was printed, the second is how many walls, and infill is only the third.
- Raise top and bottom layers before raising infill if the top surface is sagging — that's the failure infill genuinely exists to prevent. Five solid top layers over 15% infill beats three over 40%, for less plastic and much less time.
- This is a bending model with two stated limits. It treats infill as smeared uniformly through the core rather than as the lattice it is, and bending isn't the only load — infill does real work in compression, in supporting top surfaces, and in absorbing impact, none of which a second moment of area captures.
How to use
- Enter the cross-section where the part is actually loaded, not its overall size.
- Compare the return on one more perimeter against ten more per cent of infill.
- Add walls until the crossover, which is further out than most people expect.
- Check the print orientation before either — it outweighs both.
Frequently asked questions
Does more infill make a 3D print stronger?
Yes, but far less efficiently than another wall does. Bending stiffness follows the second moment of area, which weights material by the square of its distance from the centre — so plastic on the outside works much harder than the same plastic in the middle. On a 20 mm section, a perimeter returns about 1.62 points of stiffness per point of material where infill returns 0.846.
Is it better to increase infill or perimeters?
Perimeters, at any setting anybody actually uses. Four walls with no infill gives 50.2 per cent of solid stiffness for 29.4 per cent of the material; two walls with 20 per cent infill gives 42.7 per cent for 32.3. That is more stiffness for less plastic and less time, which is the opposite of the instinct to raise infill when a part breaks.
Is there a point where infill beats adding walls?
Yes, and it is further out than people expect. Each new perimeter sits closer to the neutral axis than the last, so its return falls — on a 20 mm section infill finally overtakes at the tenth perimeter, and on a 40 mm section the seventeenth. Slicer defaults are two to four, so in practice the crossover is never reached.
Does infill have diminishing returns?
Not in stiffness per gram, which is flat — every ten per cent buys the same amount for the same plastic, at 10 per cent and at 90. The received wisdom about diminishing returns above 50 per cent is real but it is about print time and material cost. The reason to avoid high infill is not that it stops helping, it is that a wall would have helped more for the same plastic all along.
What infill percentage should I use?
For most functional parts, 15 to 25 per cent with more walls rather than 40 to 50 per cent with fewer. Infill exists mainly to support the top surface so it does not sag and to resist compression — neither of which needs a high percentage. If a part is failing in bending, the answer is walls; if it is failing at the top surface, the answer is more solid top layers.
Why is my 3D printed part breaking even with high infill?
Most likely the orientation rather than the settings. An FDM part is weakest between layers, and no infill percentage fixes that — a part loaded across the layer lines can fail at a fraction of the strength the same geometry has along them. Check which way it was printed first, how many walls it has second, and infill third.
How many perimeters should I use for a strong part?
Four is a reasonable default for anything functional and three for anything decorative, and going to five or six is still a better use of plastic than raising infill. The wall thickness that gives is simply the count times the line width — four perimeters at 0.4 mm is a 1.6 mm wall, which is what actually resists bending.
Does infill pattern matter as much as percentage?
For stiffness in a single direction, the percentage matters more, because what counts is how much material sits at what distance. Pattern matters for anisotropy — gyroid is close to equal in all directions and is easy to print, where grid and triangular favour their own axes. If a part is loaded one way and you know which, an aligned pattern helps; if it is loaded unpredictably, gyroid is the safe choice.
How do I calculate the weight of a 3D print?
Multiply the material volume by the filament density: PLA is 1.24 g per cubic centimetre, PETG 1.27, ABS 1.04. Converting to filament length, 1.75 mm stock has a cross-section of 2.405 square millimetres, so a kilogram of PLA is about 335 metres. That relationship is also why weighing a partly used spool is only useful if you subtract the spool.
Does a hollow part have to be weaker than a solid one?
It has to be weaker in absolute terms and it is far stronger per gram. Two walls and nothing else gives 28 per cent of solid stiffness for 15 per cent of the material — an efficiency of 1.85, where solid is 1.00 by definition. That is the entire reason for printing hollow, and it is the same reason bicycle frames and scaffold poles are tubes.
What does second moment of area mean for 3D printing?
It is the geometric term that decides how much a section resists bending, and it weights each bit of material by the square of its distance from the centre line. Doubling how far out a piece of plastic sits multiplies its contribution by four — which is why a wall matters so much more than infill, and why a taller section is stiffer than a wider one of the same weight.
Should I use solid infill for a really strong part?
Only when you have run out of wall. Solid is 100 per cent of the material for 100 per cent of the stiffness — efficiency exactly 1.00 — where every hollow configuration is above 1. If the part must be as strong as possible regardless of weight and time then solid is correct, and if it must be as strong as possible for a given weight then walls are.
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