Knotted cords were used by rope stretchers, royal surveyors who measured out the sides of fields (Egyptian 3ht).
The longest measured length listed in the Rhind Mathematical Papyrus is a circumference of about a Roman mile with a diameter of 9 khet.
Despite many popular claims, there is no surviving evidence that the 3-4-5 triangle, and by implication the Pythagoras' theorem, was used in Ancient Egypt to lay out right angles, such as for the pyramids.
[1] Right angles were certainly laid out accurately in Ancient Egypt;[1] their surveyors did use knotted cords for measurement;[1] Plutarch recorded in Isis and Osiris (around 100 AD) that the Egyptians admired the 3-4-5 triangle;[1] and the Berlin Papyrus 6619 from the Middle Kingdom (before 1700 BC) made statements that suggest knowledge of Pythagoras' theorem.
Therefore, the historian of mathematics Roger Cooke published that the Ancient Egyptians probably did know the Pythagorean theorem, but concludes that "there is no evidence that they used it to construct right angles".