Image Measurement Tool
Measure lengths and angles on a photo. Set one known distance to show real-world lengths; angles use the photo geometry and need no length scale.
- 1Load image→
- 2Set scale→
- 3Measure
Measuring on photos: a practical guide
How the scale works, what to use as a reference, and how much error to expect. For a step-by-step walkthrough with a worked photo, read how to measure an object in a photo.
How the scale works
A photo has no real-world size of its own, only pixels. When you mark a reference and type its length, the tool divides that length by the reference’s pixel distance to get a scale. Every later measurement is its pixel distance multiplied by that scale. Example: a bank card’s 85.60 mm edge spans 428 px, so the scale is 0.2 mm per pixel, and an edge that spans 1150 px is 230 mm long. Angles compare directions between points, so they need no scale at all.
Good reference objects for photos
Pick a reference that lies in the same plane as the thing you measure and is about as long. A tape measure or ruler in the shot is best; otherwise use an object whose size is fixed by a standard.
| Reference | Standard size | Best for |
|---|---|---|
| Bank card (ISO/IEC 7810 ID-1) | 85.6 × 54.0 mm | Small objects, product shots |
| A4 sheet | 210.0 × 297.0 mm | Desk items, documents, parcels |
| US Letter sheet | 215.9 × 279.4 mm | Desk items, documents, parcels |
| US license plate | 304.8 × 152.4 mm | Car parts, garages, driveways |
| EU license plate (long format) | 520.0 × 110.0 mm | Car parts, garages, driveways |
| US modular brick (face) | 193.7 × 57.2 mm | Walls and façades |
| UK brick (face) | 215.0 × 65.0 mm | Walls and façades |
| German NF brick (face) | 240.0 × 71.0 mm | Walls and façades |
Plate and brick sizes are nominal. Brick faces vary by country and by maker, and mortar joints add about 10 mm; measure several bricks and divide.
Why a longer reference is more accurate
Each click lands within a pixel or two of the true edge. That fixed error matters less on a long reference: the same slip is 1% of a 100 px reference but 0.1% of a 1000 px one, and the scale error carries into every measurement. Zoom in before placing endpoints and choose the longest reference available.
| Reference length in the photo | Scale error from a 1 px slip | From a 5 px slip |
|---|---|---|
| 100 px | ± 1.00 % | ± 5.00 % |
| 250 px | ± 0.40 % | ± 2.00 % |
| 500 px | ± 0.20 % | ± 1.00 % |
| 1000 px | ± 0.10 % | ± 0.50 % |
| 2000 px | ± 0.05 % | ± 0.25 % |
What affects accuracy
- Same plane: the reference and the object must be the same distance from the camera. A card held 10 cm in front of a box makes the box look smaller than it is.
- Square-on view: point the camera straight at the surface. Tilting it foreshortens one direction, and this tool does not correct perspective.
- Distance and zoom: step back and zoom in (or use a 2× lens) rather than shooting close with a wide lens. Perspective changes less across the frame.
- Lens distortion: wide-angle phone lenses bend straight lines near the edges. Keep the reference and the object near the center of the frame.
- Resolution and focus: use the original photo, not a compressed copy, and make sure the edges you click are sharp.
- Reference length: an error in the length you type carries into every result in the same proportion. Measure the reference yourself if you can.
Scans and screenshots: pixels per millimeter
A flatbed scan is already square-on and free of perspective, so it is the most accurate input for this tool. If you know the scan resolution, the table gives the scale; enter it by marking a known distance, or check the result against a ruler placed on the glass. Screenshots do not carry a physical scale, so they always need a reference in the image.
| Scan resolution | One pixel equals | Pixels per cm |
|---|---|---|
| 72 DPI | 0.3528 mm | 28.3 px |
| 96 DPI | 0.2646 mm | 37.8 px |
| 150 DPI | 0.1693 mm | 59.1 px |
| 300 DPI | 0.0847 mm | 118.1 px |
| 600 DPI | 0.0423 mm | 236.2 px |
| 1200 DPI | 0.0212 mm | 472.4 px |
From angles to slopes
The angle tool also reads slopes: mark a point on the horizontal, the corner, then a point on the slope. Roofs are described as rise per 12 units of run and ramps or paths as a percent grade. The table converts between the three.
| Roof pitch | Angle | Grade |
|---|---|---|
| 1/12 | 4.8° | 8.3 % |
| 2/12 | 9.5° | 16.7 % |
| 3/12 | 14.0° | 25.0 % |
| 4/12 | 18.4° | 33.3 % |
| 5/12 | 22.6° | 41.7 % |
| 6/12 | 26.6° | 50.0 % |
| 7/12 | 30.3° | 58.3 % |
| 8/12 | 33.7° | 66.7 % |
| 9/12 | 36.9° | 75.0 % |
| 10/12 | 39.8° | 83.3 % |
| 12/12 | 45.0° | 100.0 % |
A photo angle is only reliable when the slope is photographed side-on. For building work, confirm with a level or an inclinometer.
What people measure on photos
- Furniture and rooms: check whether a sofa or shelf fits, using a door frame or a tape measure in the shot.
- Product listings: estimate an item’s size from a seller photo that shows a hand, a coin or a ruler.
- Floor plans and maps: set the scale from the printed scale bar, then measure rooms or distances.
- Microscope and lab images: set the scale from the scale bar and measure features in the same units.
- Crafts and sewing: measure a pattern piece or embroidery from a photo of it lying on a cutting mat.
- DIY and repairs: photograph a part next to a ruler to order a replacement, or read an angle on a bracket.
How to measure on a photo
- Choose a photo to load it. On desktop, you can also drag it onto the page or paste with Ctrl/Cmd + V.
- Click two points on a distance you already know and type its real length — this sets the scale.
- Use Length to click two points. Angle works with or without a length scale: click three points, corner first.
- Fine-tune any endpoint by dragging it, review results in the side list, and export an annotated PNG.