How to measure the area of a room with only a stopwatch and a spool of thread?

You can make the coil oscillate on a thread equal to the length of the room l1. Count the number of complete oscillations n1 in time t1. Find the oscillation period:
T1 = t1 / n1.
Find the length of the pendulum thread l1 from the period T1:
T1 = 2 * pi * root (l1 / g);
T12 = (4 * pi2 * l1) / g;
l1 = gT12 / 4 * pi2;
Similarly, find the width of the room l2.
Find room area:
S = l1 * l2;
If it is difficult to perform vibrations on long threads, then you can use a thread for the pendulum, which is an integer number of times shorter than the measured length or width.