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I see that there are |spell=in and |spell=on options, but not |spell=out. Have there not been queries? An example taken from Kilometre § Kilometre records:

Some sporting disciplines feature 1000 m (one-kilometre) races (the existing phrase)

Some sporting disciplines feature ((convert|1000|m|km|0|adj=on)) → 1,000-metre (1 km) races

Some sporting disciplines feature ((convert|1000|m|km|0|adj=on|spell=in)) → one-thousand-metre (1 km) races

Some sporting disciplines feature ((convert|1000|m|km|0|adj=on|spell=on)) → one-thousand-metre (one-kilometre) races

Some sporting disciplines feature ((convert|1000|m|km|0|adj=on|spell=out)) → 1,000-metre (1 km)^* races (this is the unsupported case)

Would it make sense to add this logical extension? In the example above the expression used by the editor could use this, but whether the work of supporting this logical extension of the existing functionality is justified I do not know. —Quondum 15:22, 5 April 2018 (UTC)

my 2c: the second value (bracketed) is only secondary, and so never requires spelling. What are the cases (article writing style) for spelling? - DePiep (talk) 16:05, 5 April 2018 (UTC)

In this particular case, is the conversion really necessary? Is there anyone in the entire world who doesn't understand there are 1000 m in a km? If there are, would they benefit from having this conversion? Kendall-K1 (talk) 18:13, 5 April 2018 (UTC)

Yeah (and I note that the more useful convert into yd,ft is missing). Also, in the domain of sports spelling is not common. - DePiep (talk) 19:14, 5 April 2018 (UTC)

I forget exactly what the issues were that led to no spell=out, but convert has to deal with stuff like this:

• ((convert|8|km|0|spell=In)) → Eight kilometres (5 mi)
• ((convert|8|km|0|spell=In|order=flip)) → Five miles (8 km)

I probably decided against handling spell=out + spell=Out because I couldn't think of a case where it would be useful, and the complexity didn't seem worthwhile. Johnuniq (talk) 07:49, 6 April 2018 (UTC)

Once more & again, Johnuniq shows that he is ahead of my quests! - DePiep (talk) 21:03, 8 April 2018 (UTC)

Since the output spelling is supported for both input and output (albeit in restricted combinations), my guess is that any such complexities are purely related historical implementation. Anyhow, since your previous reasoning probably still applies and there does not seem to be a specific call for it, I am not pushing for it. I was simply noting it as a minor oddity, which in a fresh implementation might have been simpler to support than not. —Quondum 14:25, 6 April 2018 (UTC)

I know this has been discussed before, but nothing has come of it. If I knew Lua, I would program it myself after a discussion, but I don't, so I'll propose it here. I would like to see a unit (c) for the speed of light in a vacuum. This would be used for noting the relative velocities of astronomical or orbital objects. For example, "Asteroid X is moving at 600,000 m/s (.2% c) relative to the Earth." or "At perihelion, comet X travels at approximately 960,000 km/h (0.0008895 c)." A simple equation, in decimal or percentile would be greatly appreciated. What does everyone think? -- Veggies (talk) 00:20, 12 April 2018 (UTC)

Previous discussion: Dec 2017 and some at Mar 2015.

Please link to sections in a couple of articles where the unit would be used. I haven't recently examined the previous discussions but it appears there was a thought that having a conversion may lead to inappropriate use, that is, for a medium other than vacuum. Perhaps there should be a discussion at a suitable wikiproject first. Johnuniq (talk) 02:01, 12 April 2018 (UTC)

It would be for astronomical/orbital mechanics such as Helios (spacecraft). -- Veggies (talk) 12:26, 12 April 2018 (UTC)

As per DePiep's comment in the first discussion linked just above by Johnuniq, example uses would be helpful to understand whether this makes sense, and how to implemented if it does. I do not see an example at the linked Helios (spacecraft). I echo Johnuniq's request: please give examples of specific intended constructions, as well as specific contexts (or disciplines) in which c is used as a unit in a way that makes its use in ((convert)) sensible. —Quondum 13:30, 12 April 2018 (UTC)

I'm also not sure it makes sense - in physics, while c is used as a velocity all the time (particularly in particle physics), it's pretty rare to convert that to another unit outside of physics homework problems. If you're doing calculations involving lightspeed (such as relativistic time dilation), you almost always end up using c-normalized units anyway (e.g., Planck constant times c normalized to 1240 eV nanometers, rather than using the SI-normalized value 6.626*10^-34 J s) throughout the calculation. You might eventually want to convert a result into SI units, but it's rare that the velocity itself is what you need (more likely energy, distance or time). For anything relativistic, the SI-based velocity is essentially always within a few percent of 3*10^8 m/s.

For space probes, nothing approaches meaningful fractions of c; Helios at 70,220 m/s only reached 0.02% of c (slightly over 1/5000th of c), so it doesn't really make sense to to express that velocity in fractions of c, other than to say it didn't even get close. Tarl N. (discuss) 15:45, 12 April 2018 (UTC)

We have some precedent; for example ((val)) seems to recognize it, e.g. 5 eV/c² and 0.99999 c. Not that this indicates that ((convert)) should support it, though. —Quondum 21:45, 12 April 2018 (UTC)

Yes c is used no doubt, but what we need here is a demonstration of useful conversion. - DePiep (talk) 22:59, 12 April 2018 (UTC)

Two possible examples of conversion have been given in the first post of this thread. Would it be reasonable for ((convert)) to provide this? If it is stylistically reasonable and useful, I would say 'Yes'. Should the template design make a call on what is reasonable? I'd say 'No, it is outside the scope of a template; it is for the article editors to decide'. The only question to answer is the one you pose: Is it sufficiently useful to warrant implementing? I am not yet convinced that it is. Isolated conversions are less trouble if done by hand than implementing in a template. ((convert)) is already evidently monstrously complex because it tries to do too many things; adding more might not be a good idea. This example might not add complexity (only a unit), but the italics on 'c' might be something new. And if the effort of adding a feature is more than manually adding all the uses, why bother? —Quondum 16:22, 13 April 2018 (UTC)

That's a good point. -- Veggies (talk) 16:27, 13 April 2018 (UTC)

I think Tarl N. above made a nice point, that if a conversion with c is made, it is a rough percentage ("to get the idea, order of magnitude)". I guess (working hypothesis) that ~none of our physics articles do use a m/s conversion today. - DePiep (talk) 16:48, 13 April 2018 (UTC)

At Gevninge helmet fragment#Description, the first sentence currently starts "Eight centimetres (3 in) wide and five (2 in) tall". The first portion ("Eight centimetres (3 in)") currently uses the convert template, but I couldn't find a way of getting the template to work for the second part. Is there a parameter that will omit the first unit ("five centimetres (2 in) tall") but keep the second? Thanks, --Usernameunique (talk) 12:57, 28 April 2018 (UTC)

You could try with disp=out and keep the five in the text : Eight centimetres (3 in) wide and five (2 in) tall -- WOSlinker (talk) 13:25, 28 April 2018 (UTC)

In the US, a "ton" is a short ton. The convert template refers to a "ton" (t) as a metric ton. Perhaps it should be changed so that short ton or metric ton is always explicitly specified.45.46.252.14 (talk) —Preceding undated comment added 14:14, 29 April 2018 (UTC)

A discussion is going on at MOS:UNIT#Decide_on_“l”_vs_“L”_for_litre_(liter). - DePiep (talk) 15:42, 29 April 2018 (UTC)