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Similar Triangles Calculator
Find the missing side of a similar triangle from a pair of matching sides, with the scale factor and area ratio.
Missing side of a similar triangle
Matching side b: —
Scale factor: —
Area ratio: —
How to use the Similar Triangles Calculator
- Enter the two known sides of the first triangle.
- Enter the length from the second triangle that matches side a.
- Read the missing side, scale factor and area ratio in the result panel.
Features
- Finds a missing side from similar triangles
- Shows the scale factor between the shapes
- Shows the ratio of the two areas
- Runs locally, no uploads
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Frequently Asked Questions
Common questions about the free online similar triangles calculator.
Enter two matching sides of the first triangle and the matching side of the second, and the
missing side appears.
Two triangles are similar when their matching angles are equal and their matching sides are
all in the same ratio.
It is the ratio between a side of the second triangle and the matching side of the first
triangle.
Multiply the area of the first triangle by the square of the scale factor to get the area of
the second.
Yes. Side a of the first triangle must match side a of the second, so check the matching
angles first.
Yes. Any positive number works, including decimals such as 2.5 and values copied from a
fraction.
It is 9, because the area ratio is the scale factor multiplied by itself.
The triangles are congruent, so every matching side is the same length and the area ratio is
1.
No. The calculation runs entirely in your browser and nothing you type is sent anywhere.
Yes. It is completely free, unlimited and needs no account or sign-up.
Last updated: October 10, 2026