Have a look at this http://www.metric-conversions.org/le...-to-inches.htm Unbelievable! People were programming it, they store it on some server, and they need a computer fit enough for Moon landing calculations for such a simple task. What can be simpler? Just take size in mm, double it twice and shift decimal point two positions. I.e. 100 mm =100x2=200x2=400 - makes 4 inches. It's not precise, but blazingly quick and good enough to say that 100 mm aperture and 4 inch aperture is the same thing. In reverse: 5.2 in = 520/2=260/2=130 - roughly 130 mm. (132.08 mm precisely) Mistake is (132.08-103)/132.08x100 = 1.6% Another good one - do you remember PI number? Yes, 3.14. The next 4 digits? Here it is - 355/113. Mistake in 7th digit after the decimal point. Have fun! Michael

Ha, didn't know that one (about the doubling) I always used to divide by 25. Many kids use rulers in school that are 30cm/12in in length and from that I remember that there are 25mm (actually, 24.5, if I remember correctly) to the inch.

Pi = 22/7 .... close enough for government work.

Originally Posted by RichardJBartlett Ha, didn't know that one (about the doubling) I always used to divide by 25. Many kids use rulers in school that are 30cm/12in in length and from that I remember that there are 25mm (actually, 24.5, if I remember correctly) to the inch. actually its 25.4 or there abouts. Another easy way from inches to metric units is simply divide by 4 then multiply by 10 for cm or 100 for mm and the reverse for conversion from metric; not exact but very close.

Re: Millimeters to inches Hello KISSers Wouldn't a solar power calculator be more suitable??? Or a slide-rule??? Addition??? Multiplication??? 150mm 150 x 2... 300 x 2... 6 inches! 150 x 4...??? 6 inches! 100 x 4... 4 inches! 80 x 2... 160 x 2... 3.2 inches 80 x 4... 3.2 inches!!! KISSoff This isn't Rocket Science...it's Astrophysics JFD Oberewerk BT-70-45 Oberwerk Mirror Mount working on signature