What Is Infill in 3D Printing? Percentages Tested on Real Parts
Infill is the lattice a 3D printer builds inside a part, between the solid outer walls and the top and bottom layers, so the part doesn't have to be printed solid. The percentage is how much of that inside space gets filled: 20% is the common default, 15-20% suits most parts, and 30-40% is for parts that get clamped, stood on or screwed into.
The percentage isn't the share of the part that is plastic. A part at 20% infill is often a third or more solid, because walls and shells are always solid. To show what the setting really changes, we sliced three parts at every common infill and wall count with the exact profile our quotes use: PLA, 0.4 mm nozzle, 0.2 mm layers, 2 walls, 5 top and 3 bottom layers, grid infill. The 20% versions match our live quotes to the hundredth of a gram.
What the inside of a 3D print looks like
Cut a print in half and you see four things:
- Walls (perimeters): 2 or more solid lines around the outline of every layer, about 0.4-0.45 mm each.
- Bottom layers: solid layers on the bed.
- Infill: the lattice in the middle.
- Top layers: solid layers that bridge over the infill and close the part.
Infill holds the walls apart, gives the top layers something to rest on and spreads load across the part. For how layers, walls and infill fit together, see layers, infill and shells explained; for the top and bottom, see top and bottom shell layers.
Test 1: a 40 mm cube from 0% to 100%
Only the infill changes. Everything else is our quote profile.
| Infill | Plastic | Print time | Weight vs 20% |
|---|---|---|---|
| 0% (hollow) | 9.2 g | 24 min | 38% |
| 5% | 14.0 g | 25 min | 58% |
| 10% | 17.3 g | 26 min | 71% |
| 15% | 20.3 g | 27 min | 84% |
| 20% (our quotes) | 24.3 g | 30 min | 100% |
| 30% | 30.7 g | 35 min | 126% |
| 40% | 37.9 g | 42 min | 156% |
| 60% | 51.4 g | 52 min | 212% |
| 80% | 65.7 g | 66 min | 270% |
| 100% (solid) | 78.9 g | 76 min | 325% |
Three things stand out:
- Even at 0%, the cube weighs 9.2 g. That's the walls and the top and bottom layers. At 0% the top layers have nothing under them and sag across 38 mm, so 0% only works for parts with no top surface.
- 20% is about 31% of the solid weight. Infill percentage and part density are different numbers.
- Every 10% adds about 6.5-7 g here. From 20% to 100%, the cube gets 3.2 times heavier and takes 2.5 times as long.
Test 2: real parts
A cube is mostly inside. Real parts are thinner, so infill matters less. Here are two parts from our catalog, a small shelf bracket (120 x 96 x 22 mm) and a phone stand (72 x 101 x 70 mm):
| Setting | Shelf bracket | Phone stand |
|---|---|---|
| 10% infill, 2 walls | 20.7 g, 35 min | 54.8 g, 77 min |
| 20% infill, 2 walls (our quotes) | 23.7 g, 39 min | 62.9 g, 95 min |
| 40% infill, 2 walls | 29.8 g, 49 min | 81.2 g, 132 min |
| 100% infill | 49.8 g, 75 min | 134.2 g, 206 min |
| 20% infill, 4 walls | 32.7 g, 47 min | 95.6 g, 119 min |
Solid roughly doubles both parts, not triples them, because their thin arms and plates are mostly walls already. With 2 walls of about 0.42-0.45 mm each side, any section thinner than about 1.7 mm has no room for infill and prints solid at any setting.
Walls or infill: where strength comes from
When a part bends, the outside surfaces take most of the stress and the middle takes very little. Infill sits in the middle. Walls sit on the outside. That's why adding walls usually buys more strength per gram than adding infill, which is also what CNC Kitchen's widely cited infill and wall tests found.
On our cube, the cost of each:
| Change from the default (2 walls, 20%) | Plastic | Print time |
|---|---|---|
| 3 walls | +2.5 g | +3 min |
| 4 walls | +4.5 g | +4 min |
| 6 walls | +8.5 g | +6 min |
| 30% infill | +6.4 g | +5 min |
| 40% infill | +13.6 g | +12 min |
Going to 6 walls costs less plastic than going to 40% infill, and puts it where bending loads are. Raise infill instead when the load is crushing rather than bending (a part that gets clamped, stood on or pressed), or when screws go into the part, since a screw needs plastic around it all the way in. For screws, see adding threads to 3D printed parts.
Which percentage to use
| Part | Infill | Walls |
|---|---|---|
| Decorative: figures, signs, ornaments | 5-15% | 2 |
| Most everyday parts: holders, organizers, stands | 15-20% | 2-3 |
| Brackets, hooks, mounts | 20-30% | 3-4 |
| Clamped, stood on, screwed into | 30-50% | 4+ |
| Small parts that must be as strong as possible | 60-100% | 4+ |
Patterns: same plastic, different time
The pattern is the shape of the lattice. At 20% on the cube, most patterns used the same plastic but not the same time:
| Pattern | Plastic | Print time | Notes |
|---|---|---|---|
| Grid (our quotes) | 24.3 g | 30 min | Two directions every layer, strong and fast |
| Cubic | 23.8 g | 30 min | 3D cubes, even strength in every direction |
| Triangles | 24.1 g | 30 min | Stiff in the layer plane |
| Gyroid | 23.6 g | 45 min | Curved, even in every direction, slower |
| Honeycomb | 27.6 g | 57 min | Strong, but the slowest here |
| Adaptive cubic | 19.5 g | 28 min | Denser near walls, sparse in the middle |
| Lightning | 10.7 g | 26 min | Only holds up the top, adds almost no strength |
For a part that has to hold up, the pattern matters much less than the percentage and the walls. Grid, cubic and gyroid are all fine choices.
What Mandarin 3D uses, and how to get a stronger part
Every Mandarin 3D quote is sliced with 2 walls, 5 top and 3 bottom layers and 20% grid infill at 0.2 mm, in PLA or PETG. That's right for most parts and keeps every quote instant. Live prices today, one piece, shipped by USPS anywhere in the US, before tax:
| Part | Price |
|---|---|
| 40 mm test cube, PLA | $10.40 |
| 40 mm test cube, PETG | $11.58 |
| 4 cubes, PLA | $15.25 ($3.81 each) |
| Small shelf bracket, PLA | $10.47 |
| Phone stand, PLA | $13.01 |
PETG is made to order in black or white and adds 3 to 5 business days.
Since the profile is fixed, make a part stronger in the model, where it works best anyway:
- Thicken the section that takes the load. Doubling the thickness of a bracket arm makes it far stiffer than any infill setting does. See wall thickness for 3D printing.
- Add ribs or a brace instead of filling the part. Our shelf bracket gets its stiffness from a diagonal brace, not from infill.
- Add the file check ($5) and tell us what the part has to hold. We review the model before printing and email you if something looks too thin.
Drop your STL, 3MF or STEP file in the box below for an instant price, or describe the part to the AI designer and say what it has to carry.
Questions
What does infill mean in 3D printing?
Infill is the lattice a printer builds inside a part, between the solid outer walls and the top and bottom layers, so the part doesn't have to be solid. The percentage is how much of that inside space gets filled.
What infill percentage should I use?
15-20% for most parts, 30-40% for parts that get clamped, stood on or screwed into, and 0-10% for decorative parts. Above 40%, adding walls is a cheaper way to get strength.
Is 100% infill the strongest?
For a given shape, yes, but it costs a lot for what it adds. Our 40 mm test cube went from 24.3 g at 20% to 78.9 g at 100%, and the print time from 30 to 76 minutes.
Are walls or infill more important for strength?
Walls, for most parts. Bending loads are carried by the outside of a part. On our test cube, going from 2 to 4 walls added 4.5 g; going from 20% to 30% infill added 6.4 g and does less for bending.