# Crop, rotate, resize and convert images in ImageJ (without changing your data by accident)

> How to crop, rotate, resize, zoom and convert to 8-bit or grayscale in ImageJ, and what each command does to pixel values and the scale, measured.

Published 2026-11-06 by LabCanvas team. https://www.labcanvas.com/blog/imagej-basic-image-editing

![ImageJ 1.54f rotating the synthetic nuclei sample by 15 degrees with Enlarge image and Preview ticked, above a Results table that compares the area, mean, minimum, maximum and pixel width after crop, rotate, resize, scale and 8-bit conversion](https://www.labcanvas.com/blog/imagej/imagej-basic-image-editing-cover.webp)

To crop in ImageJ, drag a rectangle and choose **Image › Crop**; to rotate, use **Image › Transform › Rotate 90 Degrees Right** (lossless) or **Rotate…** for any angle; to resize, use **Image › Adjust › Size…** or **Image › Scale…**; to convert to grayscale, **Image › Type › 8-bit**; to zoom, press **+** and **−**. Crop, quarter-turn rotations and zoom keep every pixel value; free rotation, resizing and 8-bit conversion change them, and resizing also changes the pixel size.

This page does each command once on the same image and measures before and after (**Analyze › Measure**), so you can see exactly what changes. The numbers are from ImageJ 1.54f.

> **What you need:** ImageJ 1.x or Fiji (the same menus in both). Any image with a known pixel size; we use our synthetic nuclei sample (a generated fluorescence image, 512 × 512 pixels, 16-bit, 0.325 µm per pixel). About 10 minutes.

## Before you start: keep the original

Each of these commands changes the image in its window. Two habits keep your data safe:

- **Duplicate first.** **Image › Duplicate…** (Ctrl Shift D) makes a copy to edit; the original stays untouched in its own window.
- **Never save over the raw file.** Use **File › Save As › Tiff…** with a new name. **File › Revert** (Ctrl Shift R) goes back to the saved file if you have not saved yet.

## Edit an image, step by step

### Step 1: Measure the image as it is

Choose **Analyze › Set Measurements…** and tick **Area**, **Mean gray value** and **Min & max gray value**. Then choose **Analyze › Measure** (Ctrl M) with nothing selected. ImageJ measures the whole image and writes one row in the Results table: this is your baseline.

![ImageJ with the nuclei sample and a Results table row: Area 27688.961, Mean 1224.124, Min 316, Max 6232](https://www.labcanvas.com/blog/imagej/imagej-basic-image-editing-step1-measure.webp "Step 1: the line above the image gives the size in µm and pixels and the bit depth 1. Measure gives the baseline 2: area 27,688.961 µm², mean 1224.124, min 316, max 6232.")

The area is 166.4 × 166.4 µm, because 512 pixels × 0.325 µm = 166.4 µm. If your image shows "pixels" instead of µm, set the scale first (see [how to set the scale in ImageJ](https://www.labcanvas.com/blog/imagej-scale-bar-set-scale)).

### Step 2: Draw the crop box

Click the **Rectangle** tool, the first icon in the toolbar. Drag a box over the part you want to keep. Drag the small handles to adjust it; hold Shift for a square. To type exact numbers, use **Edit › Selection › Specify…**.

![ImageJ nuclei image with a yellow 256 by 256 pixel rectangle selection in the middle](https://www.labcanvas.com/blog/imagej/imagej-basic-image-editing-step2-rectangle.webp "Step 2: a 256 × 256 pixel rectangle 1 over the centre of the image. The yellow outline is the selection; nothing has changed yet.")

### Step 3: Image › Crop

Choose **Image › Crop** (Ctrl Shift X). Everything outside the box is removed. Measure again.

![ImageJ after Image › Crop: a 256 × 256 pixel image of 83.20 × 83.20 microns and a second Results row with area 6922.240](https://www.labcanvas.com/blog/imagej/imagej-basic-image-editing-step3-crop.webp "Step 3: the cropped image is 83.2 µm wide and still 0.325 µm per pixel 1. Its area is 6,922.240 µm² 2, exactly 256 × 256 × 0.325². The mean rises to 1334.922 only because the crop holds a different mix of nuclei and background; no pixel value was changed.")

Crop is safe for measurements: the pixels you keep are the original ones, with the original scale.

### Step 4: Image › Transform › Rotate…

For a quarter turn, choose **Image › Transform › Rotate 90 Degrees Right** (or Left). For any other angle, choose **Image › Transform › Rotate…**, type the **Angle** (15 here), pick **Interpolation** (Bilinear is a good default), tick **Enlarge image** so the corners are not cut off, and tick **Preview** to see it before you click **OK**. **Grid lines** draws a guide to help you straighten the image; the lines are not added to it.

![ImageJ Rotate dialog with angle 15, grid lines 1, Bilinear interpolation, Enlarge image and Preview ticked, and the rotated nuclei image behind a yellow grid](https://www.labcanvas.com/blog/imagej/imagej-basic-image-editing-step4-rotate.webp "Step 4: Angle 15 1, one grid line as a guide 2, Enlarge image 3 and Preview 4, then OK 5.")

What it does to the numbers:

| Command | Size (px) | Mean | Max | What changed |
|---|---|---|---|---|
| Original | 512 × 512 | 1224.124 | 6232 | |
| Rotate 90 Degrees Right | 512 × 512 | 1224.124 | 6232 | Nothing: pixels are only moved |
| Rotate… 15°, no Enlarge | 512 × 512 | 1123.436 | 6185 | Corners cut off and filled with 0; peaks smoothed |
| Rotate… 15°, Enlarge | 627 × 627 | 816.258 | 6195 | Bigger canvas filled with 0; peaks smoothed |

A free rotation computes every new pixel from its neighbours (interpolation), so the brightest values drop (6232 to 6185 or 6195) and the new corners are filled with 0, which pulls the mean down. Rotate to make a figure look straight; measure intensities on the original.

### Step 5: Image › Adjust › Size…

To change the number of pixels, choose **Image › Adjust › Size…**. Type the new **Width (pixels)** (256 here, half the original), keep **Constrain aspect ratio** and **Average when downsizing** ticked, and click **OK**.

![ImageJ Size dialog with width 256, height 256, Constrain aspect ratio and Average when downsizing ticked, Bilinear interpolation](https://www.labcanvas.com/blog/imagej/imagej-basic-image-editing-step5-size.webp "Step 5: Width 256 1, both boxes ticked 2, OK 3. Average when downsizing averages the pixels that merge, instead of skipping some.")

The image is now 256 × 256 pixels, and ImageJ changed the pixel width from 0.325 to 0.650 µm, so it still covers 166.4 µm and the area is still 27,688.961 µm². The mean barely moves (1224.130) but the extremes do (min 540, max 6090), because each new pixel is the average of four old ones.

**Size vs Scale.** **Image › Scale…** (Ctrl E) does the same resampling with a scale factor (0.5, or 2) and can put the result in a new window (**Create new window**), so the original is kept. We scaled by 2: 1024 × 1024 pixels at 0.1625 µm, area again 27,688.961 µm². Both commands adjust the calibration; what they cannot do is add detail. Upscaling makes more pixels, not more resolution.

**Canvas Size is different.** **Image › Adjust › Canvas Size…** adds or removes a border without resampling: the pixels and the pixel size stay as they were.

### Step 6: Image › Type › 8-bit

Choose **Image › Type › 8-bit**. The image becomes 8-bit, with values from 0 to 255. Measure again.

![ImageJ nuclei image converted to 8-bit and the Results table: mean 1224.124 before, 37.140 after, max 255](https://www.labcanvas.com/blog/imagej/imagej-basic-image-editing-step6-8bit.webp "Step 6: the image is now 8-bit 1. Area is unchanged, but the mean goes from 1224.124 to 37.140 and the range from 316–6232 to 0–255 2.")

For a 16-bit image, ImageJ maps the current **display range** (what Brightness/Contrast shows, here 458 to 5736) onto 0–255. Everything below 458 becomes 0 and everything above 5736 becomes 255. So the result depends on how the image was displayed, and bright pixels can be clipped. Measure intensities on the original 16-bit image, and convert copies for display only. **Edit › Options › Conversions › Scale when converting** controls this; leave it ticked unless you know your values already fit 0–255.

**Colour photos to grayscale.** On an RGB image, **Image › Type › 8-bit** makes one grey channel. By default each grey value is (R + G + B) / 3. With **Edit › Options › Conversions › Weighted RGB conversions** ticked, ImageJ uses 0.299 R + 0.587 G + 0.114 B, closer to how bright the colours look. On our synthetic leaf photo the mean is 194.36 unweighted and 198.79 weighted, so say which you used. To keep the colours apart instead, use **Image › Color › Split Channels**.

### Step 7: Zoom in and out

Press **+** to zoom in and **−** to zoom out, or use **Image › Zoom › In** and **Out**. The magnifying-glass tool zooms where you click (right-click or Alt-click zooms out). **Image › Zoom › View 100%** (Ctrl 5) shows one screen pixel per image pixel. The window title shows the zoom, for example "nuclei.tif (300%)".

![ImageJ nuclei image zoomed to 300% with the navigator box in the corner and two identical Results rows](https://www.labcanvas.com/blog/imagej/imagej-basic-image-editing-step7-zoom.webp "Step 7: at 300% a small box in the corner shows which part of the image you are looking at 1. Measure gives the same numbers at 100% and 300% 2.")

Zoom never touches the data. When you click points or draw lines at high zoom, you place them more precisely, which is the only way zoom affects a measurement.

## All the commands, measured

Here is the same table from ImageJ, one row per command, with the image width and the pixel width added:

![ImageJ Results table comparing Original, Image Crop, Rotate 90 Degrees Right, Rotate 15 deg, Rotate 15 deg enlarge, Adjust Size 256, Scale x2 and Type 8-bit](https://www.labcanvas.com/blog/imagej/imagej-basic-image-editing-result-table.webp "Measured in ImageJ 1.54f on the nuclei sample. Crop and the 90° rotation keep every value; the 15° rotations add zeros and smooth the peaks; Size and Scale change the pixel width 0.650 and 0.163 µm so the area stays 27,688.961 µm²; 8-bit rescales the values to 0–255.")

| Command | Pixel values | Pixel size (scale) | Safe before measuring? |
|---|---|---|---|
| Image › Crop | Kept | Kept | Yes |
| Rotate 90 Degrees Right / Left | Kept (moved) | Kept | Yes |
| Rotate… (any angle) | Interpolated, corners 0 | Kept | Not for intensities |
| Adjust › Size… / Scale… | Resampled | Changed to match | Not for intensities or small objects |
| Adjust › Canvas Size… | Kept, border added | Kept | Yes (border is 0 or background) |
| Type › 8-bit | Rescaled from the display range | Kept | No: may clip |
| Zoom | Kept | Kept | Yes |

## How do I merge images in ImageJ?

"Merge" means two different things:

- **Channels into one colour image** (for example nuclei in blue and a protein in green): **Image › Color › Merge Channels…**, choose an image for each colour, tick **Create composite**. The values of each channel are kept. Step by step in [merge channels in ImageJ](https://www.labcanvas.com/blog/merge-channels-imagej).
- **Images side by side**: **Image › Stacks › Tools › Combine…** puts two images (or stacks) next to each other or one above the other. **Image › Stacks › Images to Stack** turns open images of the same size into one stack.

## Pitfalls

- **Converting before measuring.** 8-bit conversion and free rotation change intensities. Measure the original, then make display copies.
- **Resizing to "fit the figure".** Resample only the copy for the figure, and resize in the figure software if you can.
- **Losing the scale.** Copying an image through the clipboard, or saving as JPEG or PNG from another program, can drop the pixel size. Check **Image › Properties…** (Ctrl Shift P) after any edit.
- **Rotating a stack slice by slice.** Rotate… asks whether to process the whole stack; say yes, or the slices will no longer line up.
- **Saving as JPEG.** JPEG compression changes pixel values. Save edited images as TIFF.

## Reporting it

> Images were cropped to the region of interest in ImageJ 1.54f (Image › Crop) without resampling; all measurements were made on the original 16-bit data. Images in Figure 2 were rotated by 15° (bilinear interpolation) and converted to 8-bit for display only.

## Switch to LabCanvas: the same edits, with the raw data protected

In [LabCanvas Image Analysis](https://www.labcanvas.com/analysis.html), in the browser, display changes never touch the measured values, and the commands that do change pixels are clearly separate.

1. **Crop**: draw a rectangle, then **Image › Stack and image › Crop to rectangle**. The pixel size is kept.
2. **Bit depth**: **Process › Background and filters › Bit depth** converts to 8-, 16- or 32-bit, scaled to the display range or not, and every Process step can go to a new image.
3. **Zoom**: **+**, **−**, **0** (fit to window) and **1** (100%).
4. **Measure** in the Measure tab; the methods sentence under the table states the pixel size used.

![Image check in LabCanvas Image Analysis](https://www.labcanvas.com/analysis-examples/check.webp "Image check: All fine: nothing clipped brightest value 3,688, even illumination the background varies by 1.06×, sharpest at slice 8 of 15.")

Why switch for this job:

- **An image check** on every image warns when a file is a JPEG, a picture taken from a figure, or has clipped pixels, which are the usual signs that someone already edited it.
- **Brightness and contrast never change the data**: measurements read the raw 8-, 16- or 32-bit values.
- **Figures with the scale bar**: Add to a figure places the image as shown, scale bar included.

Free rotation and resizing are not in Image Analysis; for a figure, the Pixel editor crops and rotates images inside the figure itself. ImageJ remains the tool for resampling data and for batch conversions.

[Open Image Analysis](https://www.labcanvas.com/analysis.html), free with a LabCanvas account (downloading files is part of Pro).

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(7), 671–675. [doi:10.1038/nmeth.2089](https://doi.org/10.1038/nmeth.2089)

## Frequently asked questions

### How do I crop an image in ImageJ?

Pick the Rectangle tool (the first icon in the toolbar), drag a box over the part you want to keep, then choose Image › Crop (Ctrl Shift X). The pixel values and the pixel size stay the same; only the field of view gets smaller. Duplicate the image first (Image › Duplicate) if you still need the full frame.

### How do I rotate an image in ImageJ?

For quarter turns, use Image › Transform › Rotate 90 Degrees Right or Left: nothing is resampled, so every pixel value is kept. For any other angle, use Image › Transform › Rotate…, type the angle, choose the interpolation and tick Enlarge image to keep the corners. That rotation interpolates new pixel values, so measure intensities on the unrotated image.

### How do I resize an image in ImageJ without losing the scale?

Image › Adjust › Size… changes the image in place and Image › Scale… (Ctrl E) makes a resized copy; both adjust the pixel size so that distances and areas in µm stay correct. In our test, halving the nuclei image to 256 × 256 changed the pixel width from 0.325 to 0.65 µm and left the total area at 27,688.961 µm².

### How do I convert an image to grayscale in ImageJ?

Choose Image › Type › 8-bit. For an RGB photo, ImageJ averages R, G and B ((R+G+B)/3) unless Edit › Options › Conversions › Weighted RGB conversions is ticked, which uses 0.299R + 0.587G + 0.114B. For a 16-bit image, 8-bit conversion maps the current display range to 0–255, so values outside it are clipped; measure intensities before converting.

### How do I zoom in ImageJ?

Press + to zoom in and − to zoom out (Image › Zoom › In and Out), or click with the magnifying-glass tool. Image › Zoom › View 100% (Ctrl 5) shows one screen pixel per image pixel. Zooming only changes the view: measurements are identical at any zoom.

### How do I merge two images in ImageJ?

To overlay channels in colour, use Image › Color › Merge Channels… and pick an image for each colour. To put images side by side, use Image › Stacks › Tools › Combine…; to make a stack from open images of the same size, Image › Stacks › Images to Stack.

## Related links

- [How to set the scale in ImageJ](https://www.labcanvas.com/blog/imagej-scale-bar-set-scale): Check the pixel size before and after you resize
- [Merge channels in ImageJ](https://www.labcanvas.com/blog/merge-channels-imagej): Overlay channels in colour, step by step
- [Check for saturated pixels in ImageJ](https://www.labcanvas.com/blog/imagej-histogram-saturation): Why 8-bit conversion can clip your data
- [Measure length and area in ImageJ](https://www.labcanvas.com/blog/measure-length-area-imagej): Measure, in calibrated units
- [Image Analysis: the complete guide](https://www.labcanvas.com/blog/image-analysis-complete-guide): Every tool in the browser app