Matter of Metrics

I used to measure the skies, now I measure the shadows of Earth.

Johannes Kepler

Outer space, even in a fantasy setting, is really, really big. But it still needs to be measured somehow. Spelljammer officially uses a variant of the Imperial measurement system (inches, feet, yards, miles) to describe the distance between objects in space. I suspect this was done for simplicity.

A common real world unit used in planetary science is the astronomical unit, or AU. This is defined as the average radius of earth’s orbit, which is roughly 93 million miles. The primary fantasy worlds of Toril, Oerth and Krynn all have positions relative to their stellar primary of roughly 100 million miles. This is slightly more than one AU. A typical spelljamming vessel travels at 100 million miles per day. I suspect the writers did this to keep things simple and to keep math at a manageable level. A measurement based on 100 is easier to work with than one based on 93. So apparently the Spelljammer analogue to an AU is a distance of 100 million miles. I’ll call this a Reach, which can be abbreviated with a capital R, or the Greek letter rho (which looks like a P).

One Reach = 100 million standard miles, which is the average distance between Toril, Oerth, Krynn and their primaries.¹ One Reach is also shorthand for how far a vessel can travel per day, at spelljamming speed.

If someone has a different idea, I’d love to hear it. I made this up on the fly.

This talk about a fictional unit of measurement was fun, but my real purpose was to provide some distance conversion tables. I use a program called AstroSynthesis to store my database of Spelljammer world information. It is very metric oriented, which is a nuisance given the Imperial-centric nature of the official Spelljammer materials. If there is a way to switch AstroSynthesis from Imperial to Metric, I haven’t found it yet.

When recording the size of planets, I needed to convert miles to kilometers. I simply chose to take the provided distance in miles and multiply it by 1.6, to convert to kilometers. I rounded it to the nearest tenth to keep the math easier. I recommend a calculator or spreadsheet.

This table is for planetary sizes. The original tables use miles. If you need kilometers, multiply by 1.6. This table is more useful if you, like me, have to start with kilometers.

Size Class	miles		kilometers
A	0-10	1d10	0-160	1d10 * 1.6
B	10-100	1d10 * 10	160-1600	1d10 * 16
C	100-1000	1d10 * 100	1600-16000	1d10 * 160
D	1000-4000	1d4 * 1000	1600-6400	1d4 * 1600
E	4000-10000	1d6 * 1000 + 4000	6400-16000	1d6 * 1600 + 6400
F	10000-40000	1d4 * 10000	16000-64000	1d4 * 16000
G	40000-100000	1d6 * 10000 + 40000	64000-160000	1d6 * 16000 + 64000
H	100000-1000000	1d10 * 100000	160000-1600000	1d10 * 160000
I	1000000-10000000	1d10 * 1000000	1600000-16000000	1d10 * 1600000
J	10000000 or more	arbitrary	16000000 or more	arbitrary

This larger table is for the planetary movement table. I’ve frequently stumbled with the placement and movement table, mostly regarding the scale. So I came up with this to assist. It has worked for me, but it isn’t perfect. The last entry, measuring a light year, is largely for laughs. It also provides some perspective on just how large space really is, and illustrates the difference between interplanetary distance and interstellar distance.

Ring		Miles (millions)	Kilometers (millions)	AU	Reach (P)
Inner	1	20	32	0.215	0.2
	2	40	64	0.430	0.4
	3	60	96	0.645	0.6
	4	80	128	0.860	0.8
	5	100	160	1.075	1
	6	120	192	1.290	1.2
	7	140	224	1.505	1.4
	8	160	256	1.720	1.6
	9	180	288	1.935	1.8
	10	200	320	2.151	2
	11	220	352	2.366	2.2
	12	240	384	2.581	2.4
	13	260	416	2.796	2.6
	14	280	448	3.011	2.8
	15	300	480	3.226	3
	16	320	512	3.441	3.2
	17	340	544	3.656	3.4
	18	360	576	3.871	3.6
	19	380	608	4.086	3.8
Outer	1	400	640	4.301	4
	2	800	1280	8.602	8
	3	1200	1920	12.903	12
	4	1600	2560	17.204	16
	5	2000	3200	21.505	20
	6	2400	3840	25.806	24
	7	2800	4480	30.108	28
	8	3200	5120	34.409	32
	9	3600	5760	38.710	36
	10	4000	6400	43.011	40
	11	4400	7040	47.312	44
	12	4800	7680	51.613	48
	13	5200	8320	55.914	52
	14	5600	8960	60.215	56
	15	6000	9600	64.516	60
	16	6400	10240	68.817	64
	17	6800	10880	73.118	68
	18	7200	11520	77.419	72
	19	7600	12160	81.720	76
	20	8000	12800	86.022	80
	21	8400	13440	90.323	84
	22	8800	14080	94.624	88
	23	9200	14720	98.925	92
	24	9600	15360	103.226	96
1 Lightyear		58786.25	94058	63241	58786

I couldn’t get the numbers to wrap correctly in this table. Some of the numbers were as big as space itself. I was able to copy/paste the table directly into a spreadsheet and everything lined up fine. I would recommend doing something similar, especially if you want to print it. It’s much easier to read that way.

And to drive home the space is big thing, for a ship to travel from Sol to Alpha Centauri at spelljamming speed, a distance of 4.25 light years, it would take about 684 years. If my math is correct, and it is questionable. At any rate, that’s am absurdly long trip.

Space is difficult to measure, but not impossible.

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[1] In the original Spelljammer material, Oerth is in a geo-centric star system (which I personally don’t follow). But even so, the average distance between Oerth and Liga, the local star, is 1 Reach. (One-hundred million miles.)

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Image credit: www.unoks.com, The Role of Navigation Tools in Maritime History, viewed 19 October 2026.

Quote from www.goodreads.com, viewed 19 October 2025.