minecraft-java-edition
I have a very deep vertical mine shaft (I started in a mountain and hit bedrock), so it takes a long time to get up or down using my ladder. To speed up the trip down I thought "why don't I add a little pool for me to jump in?", but when testing it out the first time I died and had to rush back to my mine to collect my items before they disappeared.
How deep do I have to make the pool in order to prevent myself from taking damage when I hit the water?
Fall damage is calculated by calculating the distance fallen (in blocks).
Specifically, Falling Damage =(number of blocks fallen x ½) - 1½.
This means that you can safely drop 3 blocks at once without taking damage. Jumping gives 1½ blocks worth of upward momentum, so if you jump off another block, you can only fall one and a half blocks safely.
Ladders, water, and other blocks can mitigate this. Ladders can by cancelling your momentum (and resetting your fall distance) and water because it's water, and generally stops falling damage in any video game that fails to model surface tension!
More information available here.
When I dug a pit to bedrock, this is the technique I used after I ran out of TNT.
Dig a vertical shaft down to the bottom level (use safe techniques to avoid dropping into a cave) and place ladders to climb up.
Dig horizontally to make a 2-block-high space covering all of the horizontal area you're planning to dig out. This tells you when to stop digging later, and also checks for deep caves filled with lava. (Note that lava lakes, which can occur at any height, are still a hazard.
From the top, dig down around the edges (vertically) of the area (techniques covered below). This checks for caves intersecting the region you are digging.
Having made these cuts, you now have a better idea of what hazards the remainder contains that you might encounter while digging out.
If you are not entertained by being the proud owner of a slightly floating landmass, proceed with digging out the middle.
Caution: These techniques are optimized for simplicity, not safety. You should get a brief glimpse of any caves but you might not react fast enough.
While standing on top stand just on the edge of one block while facing another block, and aim so that your line of sight intersects both. You want to be positioned such that both blocks are supporting you, so that when you click to mine, you remove both blocks before you actually fall.
Then hold down the mouse button. You will repeatedly mine two blocks and fall one block. When you reach bottom, you will fall 3 blocks into your previously cleared space (taking no fall damage since it's less than 4).
If you have an L-shaped set of 3 columns, you can position yourself to do the same all-at-once digging trick, but there's no way to do it with a 2×2 (since your aim can't pass through all four).
Best Answer
As of version 1.4.3, only a single block is enough to prevent all fall damage.
For previous versions it was 3 blocks.