A line needs to be exactly 500 longer. There is no boundary to extend it to, and redrawing it is not worth the trouble. That is precisely the job of the lengthen command in AutoCAD.
This article measures all three of its options on AutoCAD 2027 at the Institute, including which end of the line actually moves. For the full set of editing commands, see the AutoCAD for Construction course.
It needs no boundary, unlike the extend command
Autodesk describes it in one line on the LENGTHEN page: "Changes the length of objects and the included angle of arcs".
The difference lies in needing no boundary at all. The extend command wants another object to stop against; this one wants a number. How that other command behaves is covered in our article on the extend command.
The prompts, copied verbatim from the 2027 release.
Select an object to measure or [DElta/Percent/Total/DYnamic] <Total>:
Enter delta length or [Angle] <0.0>:
The word measure in the first line is worth noticing. Pick an object without choosing any option and the command simply reports its current length and stops, changing nothing. That makes it a quick measuring tool as well.
The delta option, the percent option and the total option on one 2000 line
We drew four identical lines, each exactly 2000 long, ran one option on each, and measured again.
| Option | Entered | Length measured |
|---|---|---|
DElta | 500 | 2500 |
DElta | −500 | 1500 |
Percent | 150 | 3000 |
Total | 1234 | 1234 |
Four rows, four round numbers, and every one of them checks with mental arithmetic. 500 onto 2000 gives 2500. Minus 500 gives 1500. A hundred and fifty percent of 2000 is 3000. Total ignores the old length altogether and sets the new one.
The second row is the one worth remembering. A negative value shortens rather than raising an error, so one command covers both directions. Autodesk describes the percent option as "Sets the length of an object by a specified percentage of its total length" and the total option as "Sets the length of a selected object by specifying the total absolute length".
Which end moves: the one you click nearest
The most practical question of all, and we measured it in both directions to be sure.
The same line running from 0 to 2000, the same 500 added, and only the pick point different.
| Clicked nearer | Left end afterwards | Right end afterwards |
|---|---|---|
| The right end | 0 | 2500 |
| The left end | −500 | 2000 |
Two rows, exactly opposite. The rule fits in one sentence: the end nearest your pick is the end that changes, and the other stays put.
Knowing that ends the business of lines growing the wrong way. To grow it to the right, click on the right half of it. It costs nothing to be deliberate about the click, and it saves an undo every time.
The rule holds for all three options, not only for the delta one. With Total the end far from your pick acts as the fixed mark, and the new length is measured from there. Seen that way the 1234 in the table above is no surprise: the line still starts where it did, and only the other end moves in.
It changes the included angle of arcs as well
Not just straight lines, and this is the part of the lengthen command in AutoCAD that most people never discover. We drew an arc with a chord of 2000 and a rise of 500, which makes the radius 1250, and then added 500.
| Measurement | Before | After |
|---|---|---|
| Included angle | 106.26 degrees | 129.1 degrees |
That number checks by hand, which is how we trust the measurement. Adding 500 of arc on a radius of 1250 must increase the angle by 500 divided by 1250 radians, or 22.9 degrees. Added to 106.26 that gives 129.2 degrees, matching what the program returned.
In other words, for an arc the number you type is the arc length, not a straight-line distance. That is exactly what you want when a curve has to come to a given developed length.
Not every object can be lengthened
We tried it on a circle and the program answered with a single line, copied verbatim.
Cannot LENGTHEN this object.
Reasonable, since a circle is closed and has no end to pull. We checked further with a proper pair of controls: the same kind of object, differing only in being closed or open.
| Object tested | Result |
|---|---|
| A closed polyline | Cannot LENGTHEN this object. |
| An open polyline | lengthened, still a three-vertex polyline |
So the rule is not about the object type but about whether it has an endpoint at all. A closed polyline has to be opened first.
Whether a polyline is open or closed, and how to merge loose lines into one, is covered in our article on polylines in AutoCAD.
Three jobs where the lengthen command in AutoCAD earns its keep
Few people remember this command, yet three tasks are quicker with it than by any other route.
Extending grid lines evenly. Grid lines have to overshoot the building by the same amount on every axis. Choose DElta, enter that overshoot once, then click each grid line in turn. No temporary boundary to draw.
Setting a bar to a scheduled length. A structural drawing often needs a bar of exactly the length in the schedule. Total sets that number directly, with no arithmetic about what is missing.
Making an arc of a given developed length. Arc length is awkward to construct geometrically. Here it is just a number, and the measurement above shows the program reads it as arc length.
What all three share is that the number already exists, in your head or in a schedule. When it does, hunting for a boundary to extend to is work you did not need to do.
There is a fourth use that is really a habit rather than a task. Because picking an object without choosing an option simply reports its length, the command doubles as the fastest length check in the program. Two keystrokes, one click, and the number appears at the prompt with nothing selected afterwards and nothing changed.
How it compares with the trim command and its neighbours
Four commands that get used in each other's places, set side by side.
| Command | Needs | Result |
|---|---|---|
LENGTHEN | a number | longer or shorter by exactly that |
| The extend command | an object as boundary | stops exactly at the boundary |
| The trim command | an object as boundary | cuts the surplus away |
| The stretch command | a crossing window | moves a whole end by a vector |
The rule for choosing: when a line already exists to stop against, extend or trim to it. When all you have is a number, use this one. The stretch command is the odd one out of the four, because it moves whatever falls inside a crossing window rather than changing a length as such; reaching for it to lengthen one line usually drags along neighbours you did not intend to touch.
And when you only want to know a length rather than change it, pick the object at the first prompt. It is a perfectly good quick measure, quicker than opening the properties palette. For the distance between two arbitrary points, use the distance command, aliased to DI.
Both aliases we read straight out of the 2027 alias file rather than from memory. The line for this command is LEN, *LENGTHEN and the line for the distance command is DI, *DIST. Adding aliases of your own is covered in our article on command aliases.
Three things often repeated that are not true
| Often repeated | What we measured on the 2027 release |
|---|---|
| It only lengthens; shortening needs another command | A negative value shortens at once, 2000 down to 1500 |
| Which end grows is up to the program | The end nearest the pick changes, measured in both directions |
| For an arc the number is a straight-line distance | It is arc length; the angle grew by length divided by radius |
Frequently asked questions
How does it differ from the extend command?
The extend command needs another object as its target. This one needs only a number, so it works even with nothing else nearby.
How do I shorten something?
Still the DElta option, but with a negative value. We measured a 2000 line entered as minus 500 and got exactly 1500.
Why did the line grow the wrong way?
Because you clicked the other half. The end nearest the pick point is the end that changes.
How do I set an exact length?
Use the Total option. We entered 1234 on a 2000 line and measured exactly 1234 afterwards.
Why will a circle not lengthen?
The program answers Cannot LENGTHEN this object. A closed shape has no endpoint to pull.