Hmm, has anyone noticed this aspect of mechanical pencil refills?
Pentel Ain refills (just to pick a particular brand) are all the same price.
And all are 60mm in length.
But they contain:
0.3mm – 20 pieces
0.4mm – 30 pieces
0.5mm – 40 pieces
0.7mm – 40 pieces
0.9mm – 36 pieces
What gives? Thinner lead costs more? I would think one would get more pieces of thin lead per dollar.
If you calculate volume = length * pi * radius * radius
Then single pieces of lead are:
0.3mm = 60mm * 3.14 * 0.15mm * 0.15mm = 4.24mm3
0.4mm = 60mm * 3.14 * 0.20mm * 0.20mm = 7.54mm3
0.5mm = 60mm * 3.14 * 0.25mm * 0.25mm = 11.78mm3
0.7mm = 60mm * 3.14 * 0.35mm * 0.35mm = 23.08mm3
0.9mm = 60mm * 3.14 * 0.45mm* 0.45mm = 38.16mm3
That’s right – a 0.9mm lead is 9 times the volume of a 0.3mm lead! (Which makes sense as the volume is proportional to the square of the three-times-larger radius.)
So as to how much lead is in a Pentel Ain box:
0.3mm: 20 * 4.24mm3 = 84.80mm3
0.4mm: 30 * 7.54mm3 = 226.20mm3
0.5mm: 40 * 11.78mm3 = 471.20mm3
0.7mm: 40 * 23.08mm3 = 923.20mm3
0.9mm: 36 * 38.16mm3 = 1373.76mm3
That’s right – for the same price, a 0.9mm user gets 16 times more lead in a box than a 0.3mm user!