The short answer
Why use Measure Area?
Getting an area off a plan by hand means reading coordinates and running the formula yourself. Here you drag three corners onto the shape and the number updates as you move them.
The three red points start at fixed spots: 25% across and 35% down, 75% and 65%, then 50% and 80%. Drag them onto the corners, or type exact values in the Start, End and Third X and Y boxes. Coordinates are pixels of the original image, even when the preview is shrunk to fit your screen. Pixels per unit starts at 1, so the first answer is in square pixels.
Area grows with the square of the scale, so a small error in Pixels per unit gets bigger. If your reference length is off by 5%, the area is off by about 10%. A photo taken at an angle makes the far side of a floor or field look shorter than it is, so shoot plans from straight above or flatten them first with Perspective Crop.
If you need the length around the triangle rather than the space inside it, Measure Perimeter uses the same three points and adds up the sides. For one straight length, such as a single wall, Measure Distance or Online Ruler is quicker because each uses only two points.
Where Measure Area fits
Garden beds
Estimate the soil or turf needed for a triangular corner bed from an aerial photo.
Floor plans
Work out the floor area of an angled room section on a scanned plan with a printed scale bar.
Geometry homework
Check a hand-worked triangle area against the shoelace result for the same three corners.
Start to finish
How to use Measure Area
- 01Tap Choose image and open the plan or top-down photo that contains the triangular area.
- 02Drag the three red points onto the corners, and type your scale in Pixels per unit if you want real units instead of square pixels.
- 03Read Triangle area in the status line, then tap Apply changes and Download result for a PNG with the triangle outlined.
What to expect from Measure Area
| 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 Area tips
- A four-sided plot is two triangles: measure each half and add the two areas.
- An error of 5% in Pixels per unit becomes about 10% in the area.
- Pick a scale reference that lies flat on the same surface as the area.
- Type exact X and Y values when corners come from a drawing with known coordinates.
Measure Area FAQ
Something missing? Ask us.
Can I measure a shape with more than three corners?
How do I get square centimeters or square meters?
Which formula does it use?
Does Measure Area upload my file?
Is Measure Area free to use?
Why did Measure Area fail or look wrong?
Related tools
More in Measurement.