# Talk:Approximate conversion of units

## SI to US but what of UK?[edit]

There's no mention of the Imperial system here at all. Why is that? It's time to change this. Jimp 15Jul05 ... Done. Jimp 16Jul05

## Name Change[edit]

Would *Approximate conversion between English and SI units* not be a better name than *Informal conversion of common units*? Jimp 16Jul05 ... Done. Jimp 23Aug05 Better still: *Approximate conversion between English and metric units*. The litre is not SI. I'm changing it again. Jimp 1Dec05 You'd think I could make up my bloody mind wouldn't you but I'm now thinking that *Approximate conversion of units* would be even better still. This is more succinct. The fact that the conversions are between English and metric units as opposed to any others is not really important nor completely accurate. There are no articles which deal with approximate conversions to, from or between other types of units. I don't see the need for the creation of such an article. Such conversions would probably be better included here rather than on another page. Also some conversions are within the metric system or between U.S. & imperial units. Calories to Joules, for example, is a conversion within the metric system just as U.S. pints to imperial pints is a conversion from one set of English units to another. We have an article *Conversion of units*. This will be a closer parallel with that article having the same title except for the addition of *Approximate*. Jimp 06:32, 19 October 2006 (UTC)

## What's the point?[edit]

What is the point of this article? Wouldn't a list of actual conversions be better, and the user could decide exactly how far off they wanted to be by choosing the number of decimal places to use? How is this better? --Eliasen 00:10, 19 July 2006 (UTC)

### Exact is much better[edit]

Why not just give exact conversions?

### An answer[edit]

Yes, exact is much better. Why not give exact conversions? Yes, why not? They're given elsewhere. The point of the article is to give some good approximations which are easy to use. Take the following approximation.

- 13 in. ≈ 33 cm

This is a relatively simple ratio and it's good to within 0.07%. Note that neither 33⁄13 nor 13⁄33 can be expressed as a terminating decimal. This leads us to the following useful approximation.

- 1 m ≈ 3 ft. 3 in.

This is still good to within 1%. Okay, let's try using an approximation arrived at simply by rounding up one decimal place.

- 1 in. ≈ 2.5 cm

This has an error greater than 1.5%. Useful simple approximations are not necessarily obvious from simply looking at a table of exact conversions ... of course some of them are and those could possibly be trimmed from the article. Jimp 07:28, 17 October 2006 (UTC)

## Convenience not accuracy[edit]

Ortolan88 who created this article made the following comment in the edit summary.

“ | new table, clip and carry wherever you go, please expand, but remember, convenience, not accuracy | ” |

I agree with him. The article has expanded immensely. However, you might wonder how convenient or useful some of the entries are. I plan to do some trimming down of the article. One thing I'll be doing is getting rid of duplication in the form of "1 in. ≈ 25 mm = 2.5 cm = 1⁄4 dm", we don't need the the value in mm, cm & dm one will do. Jimp 03:23, 20 October 2006 (UTC)

## Indication of accuracy[edit]

Ortolan88 used the following terminology to indicate whether the approximate value was greater or less than the original value.

“ | 3 cm just over 1 inch (just over, round down conversion)
10 cm 4 inch (just under, round up conversion) 1 meter 1 yard (just over) ... 1 km half a mile (just over, round down conversions) 3 miles 5 km (just under, round up conversions) |
” |

```
3 cm is about 18% over 1 in.
10 cm is about 1.6% under 4 in.
1 m is about 9.4% over 1 yd.
1 km is about 24% over 1⁄2 mi.
3 miles is about 3.4% under 5 km.
```

So Ortolan88's *just* could be anything from 1.6% to 18%. This wasn't all that helpful really and was later deleted. However, whether the approximation is over or under could be useful information. It would also be useful to know how accurate the approximation is.

We could give the percentages as I've done here but this might be a bit of an overkill. So instead I propose the following.

```
Let O = the original unit
Let A = the approximation
Let F = O⁄A
If 125% ≥ F > 111% label the approximation as "well over".
If 111% ≥ F > 101% label the approximation as "over".
If 101% ≥ F > 100% label the approximation as "just over".
If 100% > F ≥ 99% label the approximation as "just under".
If 99% > F ≥ 90% label the approximation as "under".
If 90% > F ≥ 80% label the approximation as "well under".
```

Of course, I've left a few gaps viz. F = 100%, F > 125% & F < 80%. If F = 100%, then this is exact not approximate thus doesn't really belong here. I also propose that if F > 125% & F < 80%, the approximation not be included here on grounds of its not being accurate enough. Jimp 05:00, 20 October 2006 (UTC)

Why make the upper bound 125% i.e. 25% over whilst the lower bound is 80% i.e. 20% under? Suppose we made the upper bound 120% in a (misguided) effort to match the 80% lower bound i.e. we include an approximation only if it's no more than 20% over and no less than 20% under. Then consider the following very rough approximation.

- 1 m ≈ 4 ft.

One metre is about 18% under four feet. Now consider the following approximation.

- 1 ft. ≈ 1⁄4 m

One foot is about 22% over 1⁄4. We would have to omit the second whilst we include the first. This would make no sense as the first is no more accurate than the second nor is it any less accurate.

125% is equivalent to the ratio 5:4. 80% is equivalent to the ratio 4:5. These correspond to approximations of the same level of accuracy. Similarly I propose 111% and 90%. Actually 1111⁄9% would be better but 111% should do. Also 1011⁄99% would be the ideal match for 99% but now we're getting a bit pedantic. Jimp 06:34, 20 October 2006 (UTC)

## Pointless[edit]

This article is pointless, because nothing is given to show just how accurate these approximations are. Four different approximations are given for fluid ounces, and I don't know which is the closest. Why do we even need to approximate for people? They can just take the actual values and round as needed. Night Gyr (talk/Oy) 00:40, 24 December 2006 (UTC)

- An indication of accuracy would be a good addition. I'd been considering this but haven't got around to it yet. At present there are three different approximations from imp fluid ounces to metric, three from US fl oz to metric arranged from more to less accurate. This is in general how I'd tried to arrange these approximations though a note that this is the case would be useful. We don't need to approximate for people but on the same token do we need to write an encyclopædia? Many are not as mathematically inclined as you or I may be. Rounding is not the only way of producing an approximation and is often not the best. For example, π ≈ 22⁄7 but you don't arrive at this by rounding the decimal expansion of π. Jimp 01:24, 18 April 2007 (UTC)