An observatory is three shapes stacked: a round drum, a dome on top, and a slit cut through the dome from its rim to its crown. The slit is what turns a domed tower into an observatory, and it is also the part nobody gives a size for.
TL;DR: A 15-wide Minecraft observatory is a 321-block dome, 8 tall, with a 3-wide slit from rim to top that removes 33 of those blocks, leaving 288. Under it, a round wall of 40 blocks per layer and a floor inside it of 137. A drum six layers tall brings the whole build to 665 blocks.
Table of contents
Open Table of contents
The dome and its slit
Real observatories open a slit about a quarter of the dome’s width. The Gemini North dome on Mauna Kea is 36 metres across with a 10-metre slit (Gemini Observatory fact sheet, read 14 October 2026), which is 28 percent. On a Minecraft dome a slit is a strip of whole blocks, and an odd-width dome has a centre column, so the natural slit is 3 wide, centred on it.
The dome comes from the dome generator set to Hollow. The slit is a 3-wide strip taken out of one side, from the bottom ring to the top. MC Shapes is a free, browser-based set of Minecraft shape generators (circle, oval, sphere and dome) that outputs block-by-block blueprints with exact block counts; it does not export schematic files.
| Width | Dome | Height | Slit removes | Dome with slit |
|---|---|---|---|---|
| 13 | 245 | 7 | 33 | 212 |
| 15 | 321 | 8 | 33 | 288 |
| 17 | 405 | 9 | 37 | 368 |
| 21 | 653 | 11 | 49 | 604 |
A 15-wide dome is 321 blocks and 8 tall; a 3-wide slit takes out 33 of them and leaves 288. A 13-wide dome loses the same 33 and keeps 212. A 21-wide dome is 653 blocks, loses 49 to the slit, and keeps 604.
At 15 wide, a 3-wide slit is 20 percent of the dome, a little narrower than Gemini’s 28. A 5-wide slit on the same dome would be 33 percent and starts to look like a dome with a bite out of it. Three is the width I would start with.
The drum underneath
The dome sits on a round wall the same width. At 15 wide that wall is 40 blocks per layer, from the circle generator in Hollow with the thin border; at 21 wide, 56.
Six layers of drum is room for a door, a ring of windows and a ceiling high enough for a telescope. The floor inside a 15-wide drum is the solid circle that fits within the wall, 13 wide and 137 blocks.
| Part | 15 wide | 21 wide |
|---|---|---|
| Dome with slit | 288 | 604 |
| Drum, 6 layers | 240 | 336 |
| Floor | 137 | 293 |
| Total | 665 | 1,233 |
A 15-wide observatory is 665 blocks: 288 in the dome, 240 in the drum and 137 in the floor. A 21-wide one is 1,233, nearly twice as much, for a room with a little over twice the floor.
Closing the slit
A real slit has a shutter. In Minecraft the usual way is trapdoors laid over the opening from the inside, which you open and close by hand, or slabs you remove. A slit that opens by itself needs pistons; minecraft.how’s observatory guide points to piston extenders and observers for a working roof and calls them not simple builds (read 14 October 2026).
Leaving the slit open is fine for looks. It does let rain in, so light the floor with lanterns rather than leaving the room dark.
Where the observatory goes
Put it high. On a tower the dome reads against the sky, and the climb is half the point. A round tower as a base is the same ring repeated layer after layer; the round tower guide covers widths and stairs, and an observatory dome sits on any tower of the same width.
For a dome you can see through rather than a slit you look out of, a glass dome is the other way to build it, with its glass counts.
What stays hand work
MC Shapes has no observatory mode; Plotz offers an observatory generator if you want the whole model drawn at once (its own page, read 14 October 2026). Here the dome, the drum and the floor come from two generators, and the slit is cut by hand: count three blocks across the centre column of each ring and leave them out, from the bottom ring to the top.
The telescope, the shutter, the stairs and the windows are hand work, and everything is full blocks. For the ring-by-ring building order, the dome tutorial is the method.
A 15-wide observatory is 665 blocks with its slit, drum and floor. The dome generator gives the dome at any width, ring by ring, and the circle generator gives the wall and the floor.