I also want to throw my support behind the 'print both F and C degrees' bit. I have no issue with feet/inches/metres/centimetres conversions and with volumes and masses same story but F/C ... I can do the conversion in my head without a calculator, it just takes a minute.
One of the reasons English measures are still well known is that many measuring devices (tape measures, rulers, measuring cups, etc) and recipes in cooking still use the old measures (same with thermometers actually) (well at least in parts of Canada that's the case-not sure about elsewhere) (also many older people got used to English units before full conversions to metric happened-basically metric will probably eventually become more used but there is still attachment to the old system.
If we can get the USA to switch to metrics as the baseline, then we can switch Pathfinder over. But until that happens, we can't really do that.
What I propose is that we establish a standardized system of conversion between the two systems expressly for gaming purposes so that units are broadly interchangeable between systems of weights and measures. The biggest part of this would be the easy method I have already proposed: 1 kilo to 2 poundss, 2 meters to 5 feet, 2 kilometers to 1 mile. As I have said, real-life equivalencies aren't really the point. I realize that space is precious, but largely this would amount to an extra column if you wished to list them both in, say, a chart.
I would like to see both units listed, but I think there's a more fundamental issue that needs to be tackled for the good of the gaming community. First, as I have said, we should try to get the gaming world to adopt a standard of conversion between the two. But even more fundamentally, we should tweak what's already given in order to eliminate any sources of difficult conversion. Again, real-world equivalencies don't matter as much as these two issues:
1) That convenience in calculation is maintained on both sides of the conversion, and between them.
2) That any game-mechanical effect is the same in either case.
Temperature
I don't consider any temperature ranges outside of human experience to be relevant to the game until a standard is worked out as to how many joules it takes to do a hit point worth of damage and what the melting point of mithral is. Any place where temperatures outside the experiential range are given, it's fluff rather than crunch, and even within that range the actual numbers have no game mechanical effect. So, just list the alternate units in parenthesis.
As a side note, for those who don't have an intuitive grasp of degrees Celsius, this rhyme will help:
30 is hot,
20 is nice,
10 is cold,
0 is ice.
If your refrigerator is functioning properly, it should be between 1 and 5 degrees, and if your body is functioning properly it should be around 37 degrees.
Distance
The 5-feet to 2-meters standard covers the most vital issue just fine, and a version of it is outlined here:
http://www.wizards.com/default.asp?x=dnd/rg/20040323a
But for creatures whose footprint is less than one square, the only thing that matters is how much of a regular game square they take up. This is an issue which Skip Williams did not take up in his Going Metric column. When you get down to tiny creatures, you're dealing with a fighting space of 2.5 ft^2, which in this system converts to 1 m^2, which is 1/4 of a regular game square in either system.
The next step is 1 ft.^2, which is 1/5 of a square. This is a matter which deserves some re-thinking, because if there's any point to sub-dividing the fighting space, it's a point that is undermined by problems that are going to arise if you ever need to scale down to 1/5 of a square, and then match up the foot print of 1/4 of a square. I won't come up often, but there's no reason for it to be an issue at all. Simply scale down by a divisor of 2 each time.
That's easy enough and will disrupt nothing, but add convenience. For the rare times when you actually need to see how pixies and cats can fit into a melee with ogres, it'll save lots of headaches and as an added benefit the conversion will be more convenient.
And other distances are merely numbers designed to give a general idea of the size of something and will never be specifically important. Some are so trivial that even a consistent conversion is unnecessary. PFB 116: "The stoppered container is usually no more than 1 inch wide and 3 inches high." Those are made-up numbers of about the right proportions. Might as well make it 3 cm by 8 cm -- not an accurate conversion, but the same rightness of proportion, and the numbers are just as convenient. But for anything that might make a difference in game mechanical terms, both sides should be scalable accurately.
For the purposes of hit points of materials by thickness, the numbers of hit points per inch are made up and don't represent anything real. Optimizing this could be handled by the expedient of converting the number of hit points per inch by 3 if we want it to match with the map scale (3.33 would be more accurate, but messier), or simply by 2 to make it easier all around. Those numbers given for hit points per inch aren't meant to be scaled up to hit points per 1/60 of a square, after all. There's no equivalence to be preserved.
Some spells which specify inches are similarly not necessarily meant to make a game mechanical difference. Feel free to point out where non-map scale distance measures in any spell would make a difference in how an encounter played out in a game where the GM wasn't uncommonly fastidious. And a GM that picky will be driven to the nuthouse by the assumption that the same size coins in different metals weigh the same amount and so won't be availble to run anyway.
Volume
Spells produce volumes that don't convert well between English and metric. You get a lot of 1-foot cubes, which is close enough to 30 liters as to not matter, but if you can imagine it making a difference in map scale, then the spell description should be changed to something that works well on that scale. Instead of cubes whose sides are 1/5 of a map square, we should have cubes that are 1/4 of a map square. For liquid volumes, just treat a gallon as 4 liters. This works out to almost the right weight for water even on the 1 kg/2 lb. scale, which is the substance whose weight you're most likely to need to calculate.
Again, those numbers of units that spells produce are made up numbers that don't matter most of the time in game terms, so why not replace them with made up numbers that don't matter and are convenient on both scales?
This is preliminary in scope and open to some debate, but even if you don't put out a version of the game that includes metric equivalences, at least you could adjust certain things to make conversions easier, and from here on in design new rules with convenient conversions in mind. Paizo is in a better position to do such a service to the gaming community than anyone else.
I have never lived in the U.S, or anywhere that uses 'imperial' measures. That said, I have played D&D for twenty-eight years. I know that I am six foot tall, and I can picture five foot a lot better than I can two metres. The only scale I do not like to see in an RPG product, is Fahrenheit.
Fahrenheit baffles me, mainly because the US is the only Western country to still use that archaic system. However, due to the ineffienciency of the British government, we're still using a curious mix of Imperial and Metric measurements.
For the record, there are eight stone in one hundredweight (112 lbs), and thus the Imperial ton is 2,240 lbs. The Metric tonne is always spelt with two 'n's and an 'e', otherwise you're not accurately referring to the Metric measurement.
For the record, there are eight stone in one hundredweight (112 lbs), and thus the Imperial ton is 2,240 lbs.
Oh, so you are speaking about the long hundredweight here - don´t forget to mention the short hundredweight and the corresponding short ton ;-)
Stefan
I don't think adding dual measurements for temperature (the rest of the imperial units I can deal with and judging from the posts here, so can most other posters) would take up much space.