Thales Theorem Calculator
In a triangle ABC, if D is on [AB] and E is on [AC] with (DE) parallel to (BC), then AD/AB = AE/AC. Enter three lengths to find the fourth.
Enter three values and leave the fourth field blank.
Pourquoi utiliser cet outil ?
The Thales theorem lets you calculate an unknown length from a proportionality ratio between parallel segments. Widely used in school geometry but also for indirect measurements in the field, this tool instantly calculates the missing length from the other three, with no risk of getting the formula wrong.
Why it's free
A discreet ad banner covers hosting costs — that's the whole business model. No account to create, no usage limit, no paid tier unlocking the real result. Calculations run in your browser, so your numbers, dates and text never leave your device.
How our tools work