# How to measure length and area in ImageJ (with the right units)

> Measure length and area in ImageJ: set the scale, draw lines or area selections (polygon, wand, threshold), press Measure, and get µm and µm², not pixels.

Published 2026-11-06 by LabCanvas team. https://www.labcanvas.com/blog/measure-length-area-imagej

![ImageJ measuring a synthetic SEM image of nanorods at 2.5 nm per pixel: three rods measured with straight lines (810.9, 585.5 and 850.2 nm in the Length column) and a fourth outlined with the wand (Area 17,050.0 nm²) in the Results table](https://www.labcanvas.com/blog/imagej/measure-length-area-imagej-cover.webp)

To measure length in ImageJ, set the scale (**Analyze › Set Scale…**), draw a line along the object with the **Straight**, **Segmented** or **Freehand Line** tool and press Ctrl M (**Analyze › Measure**): the Results table gives the **Length**. To measure area, outline the object with the **Rectangle**, **Oval**, **Polygon** or **Freehand** tool, or click it with the **Wand**, and **Measure** gives the **Area**. Without a scale both come out in pixels; with one, lengths scale with the pixel size and areas with its square:

$$\text{length} = \text{length in pixels} \times \text{pixel size}$$

$$\text{area} = \text{area in pixels} \times \text{pixel size}^2$$

This guide goes through every step in ImageJ and Fiji and explains the units, with a worked example on an SEM image of nanorods.

## In ImageJ (or Fiji): length and area step by step

These commands are part of ImageJ 1.x, so they are the same in plain ImageJ and in Fiji.

### 1. Set the scale first

Check **Image › Properties…**: if the unit is pixel, the image has no scale. Draw a line along a stage micrometer or a scale bar of known length and use **Analyze › Set Scale…**, or type the known pixel size there. The full steps, with a calibration image to practise on, are in [how to set the scale in ImageJ](https://www.labcanvas.com/blog/imagej-scale-bar-set-scale). Do this before measuring: measurements already in the Results table do not change when you set the scale afterwards.

### 2. Length: straight, segmented and freehand lines

1. Choose the **Straight Line** tool and drag from one end of the object to the other. Zoom in (the + key) to place the ends precisely; hold Shift for an exactly horizontal or vertical line. The status bar shows the length as you drag.
2. Press Ctrl M (**Analyze › Measure**). A straight line gives **Length** and **Angle** (degrees from the horizontal), plus the grey values along the line if those are ticked in Set Measurements.
3. For a curved object, right-click the line tool and choose **Segmented Line** (click along the object, double-click to finish) or **Freehand Line** (drag along it). **Edit › Selection › Fit Spline** turns a segmented line into a smooth curve through its points, which follows a neurite or a fibre better than straight segments.
4. To keep many lines, press T after each one to add it to the **ROI Manager** (**Analyze › Tools › ROI Manager…**), then click **Measure** in the ROI Manager for one row per line.

The line width (double-click the line tool, or **Image › Adjust › Line Width…**) changes how many pixels are averaged for a profile, not the length.

### 3. Area: draw it, trace it with the wand, or threshold it

- **Draw it**: **Rectangle**, **Oval**, **Polygon** (click each corner; double-click, or click the first point, to close) or **Freehand** (drag around the object). Then Ctrl M.
- **Trace it with the wand**: choose the **Wand** tool and click inside the object; it outlines the connected pixels of similar value. Double-click the tool to set its **Tolerance** and **Mode** (Legacy, 4-connected or 8-connected). On a thresholded image the wand traces the thresholded object under the click, which is the most repeatable way to outline something by hand.
- **Threshold it**: **Image › Adjust › Threshold…** highlights the object in red. Then either tick **Limit to threshold** in **Analyze › Set Measurements…** and press Ctrl M (the area of the red pixels inside the selection), or run **Analyze › Analyze Particles…** for one row per object, with **Include holes** to measure each object as its outline.

### 4. Choose the columns: Analyze › Set Measurements…

**Area** is ticked by default. The others include **Mean gray value**, **Standard deviation**, **Min & max gray value**, **Centroid**, **Center of mass**, **Perimeter**, **Bounding rectangle**, **Fit ellipse**, **Shape descriptors**, **Feret's diameter**, **Integrated density**, **Median** and **Area fraction**. Also useful: **Display label** (the image and ROI name in each row) and **Decimal places (0-9)**. For sizes, Area, Perimeter and Feret's diameter (the longest distance across an object, with MinFeret its narrowest width) are the ones to tick; [Feret diameter, circularity and solidity](https://www.labcanvas.com/blog/feret-diameter-circularity) explains the shape descriptors.

![ImageJ Results table showing the length of a calibrated line](https://www.labcanvas.com/blog/imagej/ij-measure.webp "ImageJ: with the scale set, a line over 50 divisions of the practice micrometer measures 499.889 µm in the Results table true length 500 µm.")

![ImageJ Set Measurements dialog](https://www.labcanvas.com/blog/imagej/ij-set-measurements.webp "ImageJ: Analyze › Set Measurements chooses the columns of the Results table: Area, Mean gray value, Integrated density, Perimeter, Shape descriptors, Feret’s diameter and the rest.")

### 5. Many objects at once

For more than a handful of objects, threshold them and run **Analyze › Analyze Particles…** with a size range, **Display results** and **Summarize**: every object gets its area, perimeter and Feret diameters, and the Summary window gives the count and mean size. [Analyze Particles, explained](https://www.labcanvas.com/blog/analyze-particles-imagej) goes through every option.

## Pixels vs calibrated units, and why area scales with the square

A pixel has a length (the pixel size) and an area (the pixel size squared). On an image at 2.5 nm per pixel, one pixel covers \(2.5 \times 2.5 = 6.25\ \mathrm{nm}^2\). The worked example: the synthetic SEM nanorod sample, which carries its pixel size (2.5 nm) in the file, so ImageJ 1.54f measures in nm from the start. Three rods were measured with straight lines from tip to tip, and a fourth was outlined with the wand on the thresholded image.

![ImageJ Results table with the Length of three lines and the Area of a wand outline on an SEM image of nanorods](https://www.labcanvas.com/blog/imagej/measure-length-area-imagej-cover.webp "ImageJ 1.54f on the synthetic nanorod sample, 2.5 nm per pixel from the file. Three straight lines 1 measure 810.9, 585.5 and 850.2 nm in the Length column; the wand outline of a fourth rod 2 has an Area of 17,050.0 nm² and a perimeter of 746.9 nm. For a line, ImageJ’s Area column is just its length × the 1-pixel line width; read Length.")

The same measurements, converted back to pixels:

| Measurement | In pixels | × | Calibrated |
|---|---|---|---|
| Line 1 (a nanorod's length) | about 324 pixels | 2.5 nm | 810.9 nm |
| Line 2 | about 234 pixels | 2.5 nm | 585.5 nm |
| Area of the wand-traced rod | 2,728 pixels | 6.25 nm² | 17,050.0 nm² |

The common mistake is to convert an area with the pixel size instead of its square: here that would give 6,820 nm², 2.5 times too small. Let ImageJ do it by setting the scale, and check the unit in the Results table before you copy anything out. In **Analyze Particles**, sizes are typed in calibrated units squared unless **Pixel units** is ticked.

## Pitfalls

- **No scale, or the wrong one.** Check the unit in the column header before you copy a number. An image from another objective needs its own scale; in ImageJ, (G) in the title bar means a Global scale is applied to every image.
- **Ends placed on blurred edges.** Zoom in, and measure every object between the same kind of point (outer edge to outer edge, or tip to tip). For many objects, a threshold and Feret's diameter are more repeatable than lines by hand.
- **A straight line on a curved object.** It measures the chord, not the length along the curve; use a segmented line or a skeleton.
- **Area that depends on the threshold.** A higher or lower cut grows or shrinks every object, more for small ones. Pick the method once, report it, and use it on every image.
- **Projected lengths.** An image is a 2D projection: a rod tilted out of the plane, or an SEM sample viewed at an angle, looks shorter than it is.
- **Perimeter depends on resolution.** A rough outline gets longer the more finely it is sampled, so compare perimeters only between images at the same pixel size.
- **Measurements on a JPEG or a resized copy.** Measure the original file; a resized picture needs its scale set again, and compression changes the edges.

## Reporting it

A methods sentence to copy, with the parts in brackets filled in:

> Lengths and areas were measured in [Fiji (ImageJ 1.54)] on images calibrated at [0.325] µm per pixel (Analyze › Set Scale, from a stage micrometer). Lengths were measured with [straight / segmented] line selections from [tip to tip]; areas from [wand-traced / thresholded (Otsu)] outlines with Analyze › Measure. [n] objects from [N] images per condition were measured.

Cite ImageJ (Schneider et al. 2012) or Fiji (Schindelin et al. 2012). Report lengths and areas as mean ± SD with n, or show the distribution: every object as a point, grouped by biological replicate.

Next: [set the scale](https://www.labcanvas.com/blog/imagej-scale-bar-set-scale) if you have not, or measure [leaf area from a photo](https://www.labcanvas.com/blog/leaf-area-imagej). Fiji is also the place for lengths in 3D, through a Z-stack, and for macros that measure thousands of files.

## An easier way: measure length and area in LabCanvas

The same lines, outlines and columns work in the browser in [LabCanvas Image Analysis](https://www.labcanvas.com/analysis.html), with nothing to install.

1. Open the image. A calibrated TIFF opens with its scale; otherwise set it in **Image › Scale**.
2. Draw with the tool rail: **Line** (L), **Rectangle** (R), **Oval** (O), **Polygon** (G), **Freehand** (F) or the **Wand** (W). Each shape becomes an ROI in the **Measure** tab's list.
3. Choose the columns in **Set measurements** (Area, Perimeter, Feret's diameter and the rest).
4. Click **Measure**, or **Measure all ROIs**: lines give **Length** and **Angle**, areas give **Area**, each with its unit in the header. For many objects, **Count › Count particles** is Analyze Particles.

:::grid
![Length and area in calibrated units in LabCanvas Image Analysis](https://www.labcanvas.com/analysis-examples/measure.webp "Length and area in calibrated units: Three nanorods measured with the line tool, 418.3, 652.8 and 454.1 nm long, and a fourth outlined with the wand: area 14,356.3 nm², perimeter 975.8 nm 2.5 nm per pixel, from the file.")
![Nanorod length and width SEM in LabCanvas Image Analysis](https://www.labcanvas.com/analysis-examples/em-sem-rods.webp "Nanorod length and width SEM: 36 nanorods those cut by the frame left out: length 496.7 ± 154.6 nm Feret, width 53.47 ± 10.17 nm MinFeret, aspect ratio 11.51 ± 3.885.")
![Line intensity profile in LabCanvas Image Analysis](https://www.labcanvas.com/analysis-examples/profile.webp "Line intensity profile: Green and magenta values along a 28.56 µm line Line 1, 3 px wide, one curve per channel.")
:::

On the same nanorod image, ImageJ's Analyze Particles and LabCanvas **Count particles** both find 36 rods, 496.7 ± 154.6 nm long (Feret); the perimeter of a traced outline is computed as ImageJ does.

Why it is a good choice for lengths and areas:

- **Nothing to install**: it runs in the browser on any computer or tablet.
- **ImageJ's names and units**: Length, Angle, Area, Perim., Feret, with the unit in every column header, so a pixel result is hard to miss.
- **Straight into Graphs**: **Summary** adds mean, SD and n, **Send to Graphs** plots every object as a point, and the **Methods** line states what was measured and the pixel size.
- **Free with a LabCanvas account**; downloading the table as CSV is part of LabCanvas Pro. Every measurement is checked in [more than 4,000 automated tests](https://www.labcanvas.com/image-analysis-validation).

One limitation: the **Line** tool draws straight lines only, with no segmented or freehand line yet. Measure a curve in straight pieces, use the **Branching and skeleton** assay for filaments, or bring ImageJ's segmented lines over with **Import…** (.roi, RoiSet.zip). [Open Image Analysis](https://www.labcanvas.com/analysis.html) to measure your own images. ImageJ and Fiji are independent open-source projects; LabCanvas is not affiliated with them.

## References

- Schneider CA, Rasband WS, Eliceiri KW (2012). NIH Image to ImageJ: 25 years of image analysis. *Nature Methods* 9, 671–675. [doi:10.1038/nmeth.2089](https://doi.org/10.1038/nmeth.2089)
- Schindelin J, Arganda-Carreras I, Frise E, et al. (2012). Fiji: an open-source platform for biological-image analysis. *Nature Methods* 9, 676–682. [doi:10.1038/nmeth.2019](https://doi.org/10.1038/nmeth.2019)

## Frequently asked questions

### How do I measure length in ImageJ?

Set the scale first (Analyze › Set Scale…), then draw a line along the object with the Straight Line tool and press Ctrl M (Analyze › Measure). The Results table gives the Length in your unit and the Angle of the line. Zoom in to place the ends precisely.

### How do I measure a curved line in ImageJ?

Right-click the line tool and choose Segmented Line (click along the curve, double-click to finish) or Freehand Line (drag along it), then press Ctrl M. Edit › Selection › Fit Spline turns a segmented line into a smooth curve through its points. A straight line between the ends of a curved object underestimates its length.

### How do I measure area in ImageJ?

Outline the object with the Rectangle, Oval, Polygon or Freehand selection tool, or click it with the Wand, and press Ctrl M: the Results table gives the Area in your unit squared, if the scale is set. For many objects, threshold the image (Image › Adjust › Threshold…) and run Analyze › Analyze Particles….

### Why does ImageJ give my measurements in pixels?

Because the image has no scale: it was saved as JPEG or PNG, taken as a screenshot, or the scale was never set. Set it with Analyze › Set Scale…, from a stage micrometer, a scale bar or the known pixel size, and measure again. Image › Properties… shows the current unit and pixel size.

### How do I convert an area from pixels to µm²?

Multiply by the pixel size squared, not by the pixel size. At 0.5 µm per pixel, one pixel covers 0.25 µm², so 1,000 pixels are 250 µm². Lengths are multiplied by the pixel size itself: 100 pixels are 50 µm.

### What is the difference between Area and Area fraction in ImageJ?

Area is the size of the selection (or the whole image) in calibrated units squared. Area fraction is the percentage of the pixels in that selection that are thresholded (highlighted in red), or that are not zero when there is no threshold. Use Area fraction for percent coverage, such as the stained area of a section.

## Related links

- [How to set the scale and add a scale bar in ImageJ](https://www.labcanvas.com/blog/imagej-scale-bar-set-scale): Do this first, with a calibration image to practise on
- [Measure leaf area with ImageJ](https://www.labcanvas.com/blog/leaf-area-imagej): Area from a photo with a ruler
- [Feret diameter, circularity, solidity](https://www.labcanvas.com/blog/feret-diameter-circularity): Shape descriptors explained
- [Analyze Particles in ImageJ, explained](https://www.labcanvas.com/blog/analyze-particles-imagej): Every option in the dialog
- [Microscope pixel size calculator](https://www.labcanvas.com/tools/microscope-pixel-size-calculator): µm per pixel from the camera or a micrometer
- [Image Analysis: the complete guide](https://www.labcanvas.com/blog/image-analysis-complete-guide): Every tool and setting in the app, with examples