How to calculate movement within an area

Question

Related:

• How does the UA Tunnel Fighter fighting style's reaction attack interact with the Sentinel feat's speed-reduction effect?
• Can a Tunnel Fighter Polearm Master hit the same incoming enemy twice?

The Tunnel Fighter feat from UA has the following wording:

As a bonus action, you can enter a defensive stance that lasts until the start of your next turn. While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5 feet while within your reach.

What exactly is meant by moving "more" than 5 feet?

Would either of the two following paths be considered moving "more than 5 feet", assuming the player was holding a weapon with reach?

• Path 1: Enter range, move 5 feet, exit range
• Path 2: Enter range, move another 5 feet within range
• Path 3: Enter range, immediately exit range

┌───┬───┬───┰───┬───┐
│MAX-RANGE->┃   │   │
├───┼───┼───╂───┼───┤
│   │   │ .------P_1│
├───┼───┼─|─╂───┼───┤
│ME │   │ `-->F │   │
├───┼───┼───╂───┼───┤
│   │ F <--------P_2│
├───┼───┼───╂───┼───┤
│   │   │ .------P_3│
┝━━━┿━━━┿━|━╃───┼───┤
│   │   │ F │   │   │
└───┴───┴───┴───┴───┘

PURELY looking at the diagram and adding up the lengths of the lines, you could calculate the sum of lines WITHIN my range as being:

• Path 1: 2.5 + 5 + 2.5 = 10
• Path 2: 2.5 + 5 = 7.5
• Path 3: 2.5 + 2.5 = 5

But I don't know if this is strictly correct. I suspect that partial values may only be calculated before, or after, movement.

If movement is calculated before the step is taken, then stepping into my reach won't count as movement within my reach, but stepping out will count as 5 feet. This makes Path 2 safe to travel.

If movement is calculated after the step is taken, then stepping into my reach will count as moving 5 feet within my reach, but stepping out won't count as any. This makes Path 2 dangerous.

If movement is only calculated when it is entirely (start+finish) within reach, then all paths are safe.

There's a lot of different ways to spin this. Is there any official ruling on how to calculate movement within an area?

Answer 1

When looking at distances using a square grid reference then decimals and portions of the squares are not used. Based on page 192 PHB: Variant: Playing on a Grid, it would count that each square moved counts as 5 feet:

Rather than moving foot by foot, move square by square on the grid. This means you use your speed in 5-foot segments.

Given that Tunnel Fighter is clear that it triggers with movement in excess of 5 feet while WITHIN 5 feet of the character then sum of the paths you show above would be:

Path 1: 10 feet

Path 2: 10 Feet

Path 3: 5 feet

Note: I am also counting diagonal of the squares as 5 feet as suggested on p.192 of the PHB. If you use the variant of diagonals from p.252 of DMG then I would say the feat only triggers for path 2.

DMG p.252 Optional Rule: Diagonals:

When measuring the range or moving diagonally on a grid the first diagonal square counts as 5 feet, but the second diagonal square counts as 10 feet.

Answer 2

I don't think there was ever an official ruling or errata, and I don't think there ever will be.

However, the language "moves more than 5 feet while within your reach" seems fairly clear to me in the first place.

What exactly is meant by moving "more" than 5 feet?

"More" means greater than. Oxford Languages defines more as "a greater or additional amount or degree."

• Is 4 more than 5? No.
• Is 5 more than 5? No.
• Is 6 more than 5? Yes.

In cases 1 and 2 you calculated that the target has indeed moved "more than 5 feet while within reach". So in cases 1 and 2, Tunnel Fighter triggers. In case 1 it actually triggers on the second move, then again on the third, you could choose to use your reaction at either step.

In case 3 they moved "exactly 5 feet". Since 5 is not more than 5, Tunnel Fighter does not trigger.