Curve
A true semicircle. The rise is always half the span.
Size
A round arch's rise is always half its span, so there is nothing to set.
Bridge
Block
Stone Bricks. Blocks that fall or are not a full cube are left out, so nothing here can drop out of the arch as you build it.
Materials
The commands are relative to where you stand, one per run of blocks, so the arch lands in front of you with its feet at your level.
On Bedrock, open the .mcpack to import it, turn it on under the world's Behavior Packs (cheats have to be on), stand where its north-west corner should go and run /structure load mcmaps:round_arch_25x18x3 ~ ~ ~. On Windows a structure block can also import the .mcstructure file.
Why the curve matters
An arch stands up because the blocks above it push sideways into each other rather than straight down. Minecraft does not simulate any of that, so the only thing the curve changes in game is whether it looks right. It is worth getting anyway, because the eye knows.
A round arch is the safe answer and the one that needs no decisions: its rise is half its span by definition. The moment you want something flatter than a semicircle, you want a segmental arch rather than a squashed round one, and the moment you want something taller you want a catenary or a pointed arch. Setting the rise on a round arch is the usual mistake, and this tool hides that control on purpose.
Building it by hand
Place both legs first, all the way to the springing, then work up the curve from each side at the same time. Keep the elevation open beside you: it shows the whole arch dimmed with the course you are on at full strength, which is the only thing you need once you are standing on scaffolding halfway up. The cutaway in the 3D view does the same job from the other side, hiding everything above you so you can see where the next block lands.
Frequently Asked Questions
How do you build a curved arch in Minecraft?
One course at a time from both feet inwards, and the hard part is knowing which block steps in on each row. This tool draws the elevation with the course you are on picked out, so you place that row, press Up, and place the next. Build both legs first, then work up the curve evenly from each side so the two halves meet at the crown.
Which arch shape should I use?
A round arch for anything Roman or medieval: its rise is always half its span, which is why it looks right without any thought. A segmental arch for a road bridge, because it is shallow. A pointed arch for gothic builds. A catenary is the shape a hanging chain makes turned upside down, so it is the arch that carries its own weight with no bending at all, and it looks subtly better than a parabola at a tall rise.
How wide can an arch be?
This goes to 192 blocks of span and 96 of rise, which is well past what anyone builds by hand. Past about 60 blocks the schematic or the function file is the sensible route, because the block count climbs faster than your patience.
What is the deck for?
It turns an arch into an arch bridge: a flat roadway across the top, with posts every four blocks carrying it down onto the curve. That is how a real arch bridge works, and it is also the part people usually forget, which is why their bridge looks like an arch with a plank balanced on it.
Can I paste it in on Bedrock?
Yes. The Bedrock pack (.mcpack) holds the arch as a structure: import it, turn it on for the world, stand where its north-west corner should go and run the /structure load command the tool shows. On Windows a structure block can also import the .mcstructure file directly.
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