The short answer
Why use Measure Perimeter?
Measuring three sides separately and adding them invites rounding slips. Here one reading covers all three edges, and it updates as you move any corner.
Each side is the straight-line distance between two corners, worked out from the original image's pixel coordinates and divided by Pixels per unit. The three are added and shown to two decimals. To set a scale, measure something of known size first: if a 30 cm ruler spans 600 pixels, type 20 and the answer is in centimeters.
Perimeter scales in a straight line with your scale factor, so it is more forgiving than area: a 5% error in the scale gives a 5% error in the result. Lens distortion is the bigger risk. Wide phone lenses bow straight edges near the corners of the frame, so keep the shape near the middle of the photo.
Measure Area uses the same three corners and gives the space inside instead of the edge length. If you only need one side, Measure Distance or Online Ruler reads a single line between two points, which is faster and leaves less room for a misplaced corner.
Where Measure Perimeter fits
Fence runs
Estimate the fencing needed around a triangular paddock from a scaled site plan.
Trim and edging
Total the edge length of a triangular shelf or garden border before buying trim.
Map routes
Add up a three-leg walk on a map screenshot using its distance scale.
Start to finish
How to use Measure Perimeter
- 01Tap Choose image and open the plan, map screenshot or photo showing the triangle.
- 02Move the three red points onto its corners and enter Pixels per unit from a known length, such as a scale bar.
- 03Read Triangle perimeter, tap Apply changes, and use Download result to save the PNG with the closed outline.
What to expect from Measure Perimeter
| Check | What it means here |
|---|---|
| Supported input | JPEG, PNG, WebP, GIF, BMP, SVG and browser-supported AVIF images |
| Main controls | Draggable points, exact X and Y fields, and an optional pixels-per-unit scale |
| Output | The measurement on screen and a marked-up copy of the image |
| Working limit | 50 MB and 50 megapixels per image |
Measure Perimeter tips
- Set the scale from the longest known length in the picture to keep rounding small.
- Keep the triangle near the middle of the frame, away from wide-angle lens bowing.
- For a single side, Measure Distance gives one length with only two points.
- Add a margin for posts and corners when the total is for fencing or edging.
Measure Perimeter FAQ
Something missing? Ask us.
Can it measure a curved or many-sided outline?
Why does the result say units instead of cm?
Does Measure Perimeter upload my file?
Is Measure Perimeter free to use?
Why did Measure Perimeter fail or look wrong?
Related tools
More in Measurement.