Tanjiro's blade cuts in at best
maybe half a centimeter into Rui's neck. Now, I'd say that Rui is a child maybe in the 10-15 years old range, and here's a list of
neck circumferences based on age. So let's assume that Rui's neck is the smallest at a circumference for 10 inches, which translates to 25.4 cm. Dividing by 2π (since 2πr is the circumference), we get a radius of about 4.04 cm. If Tanjiro's blade cut in one centimeter, you can then draw a
chord through the circle that's one centimeter from the edge of the neck.
So if we draw radii to the two points where the chord touches the circle, we get a triangle with a 3.54 cm height and two sides of 4.04 cm. Then we can calculate from there the chord length by drawing the radius down the center of the triangle to create two congruent right triangles, and then apply the Pythagorean theorem. (4.04 cm)^2 - (3.54 cm)^2 = (0.5*(chord length))^2. Chord length comes out to about 3.9 cm. (Already we can see that 0.5 cm is a vast overestimation because Tanjiro doesn't cut so deeply into Rui's neck that the cut's length is nearly that of the radius of Rui's neck, but let's keep going and say that the end result is a low end.)
So the total triangle area is half the base multiplied by the height, so about 6.89 cm^2. The arc created by the radii involves trigonometry, but the short of it is that we can take the arcsin of half of 3.9 cm divided by the radius of 4.04 cm to get half of the arc's angle, and then double it to get the total arc's angle, and then use the angle to determine the fraction of the circle. The arc comes out to around 57.6 degrees, and using the radius we can determine that the area of the arc is 8.21 cm^2.
Subtracting the triangle from the arc to get the area of the section cut by Tanjiro, we get that he cut about 1.32 cm^2. The total area of the neck would be 51.34 cm^2, which is about 39 times that. It would mean that Giyu is striking with at minimum 39 times the force that Tanjiro is cutting.