How do i convert : Full scale to 20.3 scale?
I do it like this…full scale in feet times 12…divide by 20.3…gives you actual inches in 20.3 scale.
An alternative, take the prototype measurement in feet, and multiply by 15. That gives you the dimension in millimeters. (1:20.3 is 15mm per foot)
I’ve got a collection of various metric rulers where I’ve taken a fine-tipped Sharpie and divided up the metric side into 15mm increments. Gives me a bunch of 1:20.3 scale rules.
Later,
K
Manfred,
Try this variation on Randy’s method. Feet times 12 to get feet converted to inches prototype. Add the prototype inches and then divide by 20.3, resulting in actual inches required.
ex. 12’-4 1/2" prototype converts as follows:
12 feet time 12 equals 144 inches, add 4 1/2" equals 148 1/2".
1418 1/2 inches divided by 20.3 equals 7.31527 inches or 7 5/16".
This method works no matter what scale yo are working in. Just change the 20.3 to what ever scale you are working to. HO = 87, S=64 and so on.
Hope this explains it clearly.
Bob C.
Isn’t the 3 just indicating the gauge of the track, not the scale of the train?
I have always heard 1.20 on 3
Matt
No, 1:20.3 is the scale.
I should say the scale for narrow gauge!
45mm (G track width) represents 36" (Narrow gauge track width in 1:1).
36"/20.3 = 1.773399"
1.773399" * 25.4 = 45.044334 mm
Good to know, thanks!
Thanks guy’s!
I think Kevin’s tip is the easy way to figure things out!
Manfred
Hey, clear as mud.
I understand 1:20.3. Head starts spinning with On3, On30, etc.
I just stick to what I know and love!!!
Out of curiosity what is the 25.4? Is that a constant, or just what you get when dividing 45… by the 1.77…
25.4 is the number of millimeters in an inch.
Danged metric system! 
You can always wander over to the Scale Converter page here…
Easier, order one of these. Any two scales you want on same ruler, at a great price. I have an aluminum one and they are great quality.
http://www.therailscale.com/index.htm
Chris
Matt,
Absolutely NOT. That is the accurate scale. If what you propose were true, how would you define 1:20 standard gauge? … or 1:20 42 inch gauge?
36" = 914.4mm. And 914.4mm divided by 45mm (actual track gauge) = 20.32 to be precise.
NMRA standard defining scales and gauges may be found here http://www.nmra.org/standards/sandrp/pdf/S-1.2%202009.07.pdf . I am not sure where you got that impression as I have never heard that explanation, and I have heard a couple of doozies. This scale is also frequently referred to as 1:20, dropping the .3 for simplicity. Unless you are going to go over a model with a dial caliper, the difference between 1:20 and 1:20.32 is negligible.
I am scratching some D&RGW 6000 series flat cars in 1:20.3 or 1:20 whichever suits your fancy. I scaled up some drawings from the Narrow Gauge and Short Line Gazette, with some line weights 2" thick, not real great for accuracy. After making the scale drawings as close as I could in prototype feet and inches, I scaled them to 1:20.3. Then rounded all the weird decimal dimension to fractions in 32nds. This will be close enough for almost any modeler.
Disclaimer: Although I linked the NMRA scale/gauge page in my post, I do not necessarily support their efforts, this was simply a means to help clarify scale and gauge.
Happy railroading Matt.
Bob C.
Bob McCown said:
You can always wander over to the Scale Converter page here...
Well there you go throwing cold water on a hot thread!
Who would’a thunk? Neat feature I never knew existed!
Wow all this time- guess hear-say isn’t always right 
lol. Thanks a lot for the very informative answers guys. I do appreciate it
Matt
I remain totally thunderstruck, nay, blitzed, at the lack of awareness of the Metric system that generally pervades the US - despite the fact that 99.9999& of ALL vehicles on your roads made since the middle seventies are likely to be put together, and held together, by metric standard fastenings.
EVERY single ‘white-good’ is also metric, as is EVERYTHING that comes to the USA by ship. Every plane that flies is also metric, as is every TV that you watch. All our little trains are metrically held together, and every optic you look through when you go hunting, in spite of the fact that you still hold on to the 1" diameter scope tube. Poor old Zeiss actually have to make a range of scopes specially for you because of this.
The UK and the USA are the last bastions of feet and inches, and parts thereof, and although the UK ‘went metric’ back in 1971, you would never know it. Our gas is sold in litres, but we ignore that, and just say ‘fill’erup’ and call out vehicle economy in mpg and not litres per hundred kilometres. Food is sold by metric weight, but we ignore that, too, and by by the size and not by the weight in grams or kilos. And road distances are measured in miles and yards…
Long may this confusion prosper.
tac, ig & The Imperial Measures Boys
Matt Z said:
Isn't the 3 just indicating the gauge of the track, not the scale of the train?I have always heard 1.20 on 3
Matt
Not at all! The correct (accurate to 3 decimal places) scale factor is 1:20.32.
The 1:20.32 numerical scale factor has nothing to do with describing the gauge of either the prototype or model track.
That (highly accurate) 1:20.32 scale factor is arrived at by using 15mm=1 foot or 12 inches (per Kevin’s post above). That was originally derived from the use of 45mm gauge model track to represent 3’ gauge prototype track.
Most of us round off to the significant places depending on what item is to be scaled. As an example, I usually use the rounded scale factor “1:20” when converting building dimensions. When writing about the scale and for higher accuracy in determining scale dimensions, I use the rounded scale factor of “1:20.3.” The full scale factor “1:20.320” is only used for building scale watch parts!!
Happy (Scale Model) RRing,
Jerry
For TAC:
Having lived in the UK and Germany during the 1970s, as well as working in engineering and science all my life, I became very comfortable with expressing the speed of my trains in that old English standard:
“Barleycorns/fortnight!”
Happy (Well Measured) RRing,
Jerry
tac said:
The UK and the USA are the last bastions of feet and inches, and parts thereof, and although the UK 'went metric' back in 1971, you would never know it. Our gas is sold in litres, but we ignore that, and just say 'fill'erup' and call out vehicle economy in mpg and not litres per hundred kilometres. Food is sold by metric weight, but we ignore that, too, and by by the size and not by the weight in grams or kilos. And road distances are measured in miles and yards...Long may this confusion prosper.
tac, ig & The Imperial Measures Boys
Yes TAC and then there is Canada. Here we did a “soft conversion” - probably refering to the head of whoever came up with the scheme. Food is regularly advertised with the Imperial weight as the main price base. Except for the 2 liter jumbo bottles all else in the soft drink realm lists “funky” milli liter measures which are a correct equivalent of the Imperial measures. Same goes for cereals. And on and on it goes.
SWMBO gets fairly upset about all this, but hasn’t written the letter to the editor, yet, just threatened to.
Usually I just chuckle … until the next time I’m looking for 1/8" plywood and the stuff on the shelf measures just shy of 3mm - which is perfect for that other job, but not what I need right now.
Actually there is a word for this mishmash condition = learning resistance using heritage as the justification.
PS I always find these threads very entertaining.
And BTW any scale they throw at you divide (1:x) or multiply (x:1) the original measurement by the scale factor. The scale factor doesn’t care if it’s metric or inch.