Similar Triangle Calculator
Find missing side lengths of similar triangles instantly. Calculate scale factors and solve geometry problems.
Enter the known side lengths for two similar triangles. You must provide at least one corresponding pair of sides (e.g., Side A and Side a) to find the scale factor.
Triangle 1
Triangle 2 (Similar)
Results
Solve Similar Triangles With Side Ratios
Enter the sides you know and this free calculator finds the missing side lengths of similar triangles using proportional ratios — plus the scale factor between the two triangles.
Great for geometry homework, construction math, and scale drawings. Results appear instantly in your browser with the proportions shown so you can follow the math.
Missing sides, solved
Give the known sides of both triangles and get every missing length calculated through the similarity ratio.
Scale factor and ratios
See the scale factor between the triangles and the exact proportions used, so the answer is checkable, not magic.
Free and private
All geometry runs locally in your browser — your numbers are never sent to a server.
How to use
- Enter the known side lengths of both triangles.
- Click Calculate.
- Read the missing sides, scale factor, and the ratios used.
Frequently Asked Questions
What are similar triangles?
Triangles with the same shape but different sizes — their corresponding angles are equal and their corresponding sides are in the same ratio.
How do you find a missing side?
Set up a proportion between corresponding sides (a/b = c/d) and solve for the unknown — the calculator does this automatically.
What is the scale factor?
The number you multiply one triangle's sides by to get the other's — it is the ratio of any pair of corresponding sides.
Do the triangles need to be drawn to scale?
No. The math works from the side lengths you enter, regardless of any drawing.
Is the calculator free?
Yes, free with no signup.
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