The FILLET command in AutoCAD rounds a corner, and its alias is F. It replaces the apex of a corner with an arc, and Autodesk describes it as "Rounds or fillets the edges of two 2D objects or the adjacent faces of a 3D solid".
The command needs only one figure, the radius, so it looks simpler than its bevelling sibling. That simplicity hides a few habits that cost people time: the radius is remembered between calls, a radius of zero turns out to be the most useful setting of all, and two parallel lines make the command ignore the radius entirely. To learn modelling and documentation end to end, see the AutoCAD for construction course.
The FILLET command in AutoCAD: prompts and five options
The button sits on the Modify panel of the Home tab. Typing F is quicker.
The command's first line always reports the trim mode and the radius in force, and only then asks for the two objects. The three lines read as follows.
| What the command line prints | What it is saying |
|---|---|
Current settings: Mode = TRIM, Radius = 0.0000 | Trimming is on, and the remembered radius is zero |
Select first object or [Undo/Polyline/Radius/Trim/Multiple]: | Pick the first object, or type one of the capitalised letters to take an option |
Select second object or shift-select to apply corner or [Radius]: | Pick the second object. Holding Shift here applies a zero radius just this once |
The five options and what each is for:
| Option | What it does |
|---|---|
Undo | Reverses the previous action without leaving the command |
Polyline | Rounds every vertex of a 2D polyline where two straight segments meet |
Radius | Sets the radius for later fillets; changing it does not affect fillets already drawn |
Trim | Controls whether the two lines are trimmed to meet the ends of the arc |
Multiple | Rounds several corners without recalling the command |
Autodesk's FILLET page documents all five.
The command is not limited to two-dimensional linework. Autodesk's own description mentions "the adjacent faces of a 3D solid". For solids the default radius lives in a different variable, FILLETRAD3D, which Autodesk records as holding the radius for fillets "created with the FILLETEDGE and SURFFILLET commands".
The radius is remembered, and FILLETRAD holds it
This is the habit that most often looks like a fault. Set the radius to 400 once and every later fillet uses 400, until it is set again.
That figure lives in FILLETRAD. Autodesk records it as "Stores the current fillet radius for 2D objects", of type Real, with an initial value of 0.0000, and saved in the drawing. The variable page adds: "If you use the FILLET command to change the radius of a 2D fillet, the value of this system variable changes to match".
Saved in the drawing means the figure travels with the file. Reopen the drawing tomorrow and the radius is still what you set today; open a different drawing and it is that drawing's own figure. The simplest defence is to read the command's first line before picking, because it always reports the current value.
A radius of zero: the quickest way to close a corner
The default radius is zero, and that turns out to be the best use of the command. A fillet of radius zero draws no arc at all: it brings the two lines together at a single point and trims what sticks out.
Autodesk confirms the technique in one line: "A radius value of zero can be used to create a sharp corner". On a building plan with dozens of wall corners that do not quite close, this is the fastest tidy-up there is — faster than trimming and extending as two separate commands.
There is a shortcut that leaves your radius alone. The second prompt reads shift-select to apply corner, meaning that holding Shift while picking the second object applies a zero radius just for that corner, whatever radius is set. Autodesk describes it as "hold down the Shift key before selecting the second object or line segment of a 2D polyline to extend or trim the selected objects to form a sharp corner".
Parallel lines make the radius irrelevant
This is a habit of the command that its own help page does not mention, so it is worth recording as an observed behaviour rather than a documented rule.
Pick two parallel lines and the command reports no error, but it does not use the radius in force either. It joins them with a semicircle whose radius is exactly half the distance between them.
The habit is useful rather than merely something to avoid. To cap the end of a pipe drawn as two parallel lines, call F and pick the two lines. No measuring the gap, no halving it, and whatever radius happens to be set makes no difference.
Rounding a whole polyline, and several corners at once
The Polyline option rounds every vertex of a 2D polyline in one pick. Type P, click any side, and the command reports how many corners it rounded, for example 4 lines were filleted.
A rectangle drawn with RECTANG is a polyline, so it works straight away. Four separate lines drawn with LINE do not; join them with JOIN first.
For a run of separate segments, use Multiple. Without it the command ends after one corner, so every further corner means calling it again.
Why a fillet refuses to happen
People really do search for this, and none of the Vietnamese tutorials I read gives it a section of its own.
The first two causes are the common ones. If the radius still holds an old value, type R and set it again. If the two lines are parallel, the command still runs but gives a semicircle, which is not a fault.
The third is worth knowing because it is silent: objects on a locked layer cannot be touched by editing commands. The command line does say so, but it is easily missed by anyone watching the drawing rather than the text.
As for No trim, the arc really is added; it is the two original lines staying put that makes it look broken. Type T and choose Trim to restore it. The setting lives in TRIMMODE, and Autodesk states "The current value is stored in the TRIMMODE system variable". That variable is shared with CHAMFER, so setting No trim in one command stops the other trimming too.
Fillet or chamfer: choosing between them
The two are siblings, sharing a flyout and with nearly identical prompts.
FILLET | CHAMFER | |
|---|---|---|
| Alias | F | CHA |
| Replaces the apex with | An arc | A straight line |
| Defined by | One radius | Two distances, or a distance and an angle |
| Variable holding the figure | FILLETRAD, saved in the drawing | The method sits in CHAMMODE, not saved |
| Trim setting stored in | TRIMMODE | TRIMMODE, the same one |
| A value of zero | Squares the corner | Squares the corner |
The bevelling command has its own article, the CHAMFER command in AutoCAD. Which to use is a drafting convention: planter corners, slab edges and cast details are rounded, while step nosings and stair nosings are usually bevelled.
Frequently asked questions
Which command rounds a corner in AutoCAD?
FILLET, aliased F. The button sits on the Modify panel of the Home tab.
I wanted a sharp corner and got a rounded one. Why?
The radius still holds a value from an earlier call. Type R and set it to zero, or hold Shift while picking the second object to apply a zero radius just this once.
How do I round all four corners of a rectangle at once?
Type F, then P, then click any side. The shape must be a polyline; four separate lines need joining with JOIN first.
What happens if I pick two parallel lines?
The command reports no error but ignores the radius in force. It joins the lines with a semicircle whose radius is half the gap between them.
Why are both original lines still there afterwards?
The command is in No trim mode. Type T and choose Trim. The setting lives in TRIMMODE, which is shared with CHAMFER.
Where is the fillet radius stored?
In the FILLETRAD variable, and it is saved in the drawing rather than on the machine. Open a different drawing and you get that drawing's own figure.
Related articles
- The CHAMFER command in AutoCAD — bevelling by two distances or by a distance and an angle
- How to draw a rectangle in AutoCAD — the REC command, eight options and the settings that persist
- Layers in AutoCAD — a standard layer set, locked layers and deleting layers
- What is AutoCAD — a handbook for people working in construction