The golden number for Metrinch coversions is 25.4. I say again, 25.4. This is as accurate as dammit (we're talking engineering measures here, not nanometer-scaled sub-atomic wavelengths) to the number of mm in an Inch. All else flows from this magic distance measure. 10" are 254mm. A foot is 12 x 25.4= 304.8mm, rounded up to 305.
To metrivert 7/16" you use the simple calculation (25.4/16) x 7 = 11.1125mm. Again, the 'magic number' divided into sixteenths by the actual number or amount of sixteenths. The same simple rule applies all other fractions, logical or illogical, poofteenths, whatever.
Therefore metriverting 5/23 one uses the sum (25.4/23) x 5 = 5.5217391304348mm, rounded to 5.5mm.
I'm no mathematician. My big brother has a Doctorate in Pure Maths. I hate Algebra. Who doesn't? I struggled with Uni-level statistics & calculus, but do a bit better @ geometry & trig. There may very well be better methods, but this tried & true conversion factor has worked successfully & accurately for me for over 50 years. 25.4. Commit it to memory. It'll stand you in good stead for the rest of your life.
As an aside the magic numbers for gallons to litres is 4.546, miles to km is 5/8 or inverted to 8/5. Neither are as accurate as the distance conversion. Merely easily learned, rule-of-thumb back of an envelope calculations. Unfortunately, I believe some parts of North America use an 'unusual', different gallon measure. The 4.546 conversion factor is for 'classic', 'imperial' or perhaps 'medieval' gallons, not American gallons. As I haven't ever had the need , nor any clue, to know exactly what an American gallon actually is other than 'different' I can't really help you there.
Tell the truth, maybe you even have different miles too! Nautical miles, aerial miles, American miles? My miles are exactly 1,609.344 M, 63,360", 5,280 feet, 1,760 yards (I think), 80 chain, 20?? furlongs etc. long. Excuse my imperfect memory. I haven't used these non-SI units for almost 55 years, having learned these by rote in Primary school.
Newton Metres to Foot Pounds & back? Sorry, don't have any easy numbers for that one! I don't even really understand it either. The Newton is the universal SI unit for force, one NM being the 'bending force', moment or leverage of one Newton applied to a bar precisely one metre long. A foot pound is.... a pound's mass @ sea-level on a lever 1' long??? Buggered if I know. Sorry.
May I add a brief plea for using SI units. They work. They're universal, easily & readily calculable, & above all else interconnected in an almost eerie or even creepy way to illustrate the 'universal' rules of the universe. Sorry, bad pun. They have an irresistable, timeless beauty in the way they seem to work together, using base ten units of calculation. All except time of course. The oddfellow.
The downside: I will never, ever be able to visualise, let alone contextualise, a Megapascal of pressure. Pounds per square inch however (another seemingly irrational medieval measure), I'm able to gauge intuitively. I think it's an age-related thing. Like most of us I suspect, I can relate to & understand most MPG figures rather than those elusive L/100 km figures in relation to fuel consumption. Even if the MPG figures ARE completely different depending on which side of the ocean you're measuring it! Yet another reason why the universality of SI measurement works, whereas nothing else does anywhere near as well!
