Prone: standing up and attacker distance

SRD 5.2.1 · Conditions

At the table

1. Move by crawling, or spend half your Speed, rounded down, to right yourself. You cannot right yourself if Speed is zero.
2. Apply Disadvantage to your attack rolls.
3. An incoming attack has Advantage if its attacker is within five feet; otherwise it has Disadvantage.

Example decision

A Prone character has Speed zero because they are also Grappled. Can they spend half of zero feet and stand for free?

No; Prone explicitly prevents righting yourself when Speed is zero.

The zero-Speed restriction directly prevents that shortcut. End or otherwise resolve the effect holding Speed at zero before using the normal standing option. With Speed 30 and no such restriction, standing would cost 15 feet of movement. The amount of movement left on the turn is not the same thing as the Speed value used to calculate this cost.

Watch for this mistake

Do not classify the incoming-attack adjustment solely as melee versus ranged. The condition uses attacker distance. Other attack rules still apply and may add their own Advantage or Disadvantage.

Ask the table

What is current Speed, and how far away is this particular attacker?

Source rule

While you have the Prone condition, you experience the following effects. Restricted Movement. Your only movement options are to crawl or to spend an amount of movement equal to half your Speed (round down) to right yourself and thereby end the condition. If your Speed is 0, you can’t right yourself. Attacks Affected. You have Disadvantage on attack rolls. An attack roll against you has Advantage if the attacker is within 5 feet of you. Otherwise, that attack roll has Disadvantage.

Source: SRD 5.2.1, 186. Online guide.